A dozen USB chargers in the lab: Apple is very good, but not quite the best

When you buy a USB charger, how do you know if you're getting a safe, high-quality charger for your money? You can't tell from the outside if a charger provides silky-smooth power or if it is a dangerous charger that emits noisy power that cause touchscreen malfunctions[1] and could self-destruct. In this article, I carefully measure the performance of a dozen different chargers, rate their performance in multiple categories, and determine the winners and losers.

The above picture shows the twelve chargers I analyzed.[2] The charger in the upper-left is the cube-shaped Apple iPhone charger. Next is an oblong Samsung adapter and a cube Samsung adapter. The Apple iPad power adapter is substantially larger[3] than the iPhone charger but provides twice the power. The HP TouchPad power charger has an unusual cylindrical shape. Next is a counterfeit iPhone charger, which appears identical to the real thing but only costs a couple dollars. In the upper right, the Monoprice iPhone charger has a 30-pin dock connector, not USB. The colorful orange charger is a counterfeit of the Apple UK iPhone charger. Next is a counterfeit iPad charger that looks just like the real one. The Belkin power adapter is oval shaped. The KMS power supply provides four USB ports. The final charger is a Motorola Charger.

Summary of ratings

The chargers are rated from 1 to 5 energy bolts, with 5 bolts the best. The overall rating below is the average of the ratings in nine different categories, based on my measurements of efficiency, power stability, power quality, and power output. The quick summary is that phone manufacturers provide pretty good chargers, the aftermarket chargers are worse, and $2 counterfeit chargers are pretty much junk. Much to my surprise, the HP TouchPad charger (which isn't sold any more) turned out to have the best overall score. The counterfeit iPhone charger set a new low for bad quality, strikingly worse than the other two counterfeits.

	Model	Overall rating
Apple iPhone	Apple A1265
Samsung oblong	Samsung travel adapter ETA0U60JBE
Samsung cube	Samsung travel adapter ETA0U80JBE
Apple iPad	Apple 10W USB Power Adapter A1357
HP TouchPad	Hewlett Packard LPS AC/DC Adaptor P/N 157-10157-00
Counterfeit iPhone	Fake Apple A1265 "Designed by California"
Monoprice	Monoprice Switching Mode Power Supply MIPTC1A
Counterfeit UK	Fake Apple A1299
Counterfeit iPad	Fake Apple 10W USB Power Adapter A1357
Belkin	Belkin UTC001
KMS	KMS-AC09
Motorola	Motorola AC Power Supply DC4050US0301

Inside a charger

These chargers cram a lot of complex circuitry into a small package, as you can see from the iPhone charger below. (See my iPhone charger teardown for more details.) The small size makes it challenging to make an efficient, high-quality charger, while the commoditization of chargers and the demand for low prices pressure manufacturers to make the circuit as simple as possible and exclude expensive components, even if the power quality is worse. The result is a wide variation in the quality of the chargers, most of which is invisible to the user, who may believe "a charger is a charger".

Inside the iPhone charger

Internally a charger is an amazingly compact switching power supply that efficiently converts line AC into 5 volt DC output. The input AC is first converted to high-voltage DC. The DC is chopped up tens of thousands of times a second and fed into a tiny flyback transformer. The output of the transformer is converted to low-voltage DC, filtered, and provided as the 5 volt output through the USB port. A feedback mechanism regulates the chopping frequency to keep the output voltage stable. Name-brand chargers use a specialized control IC to run the charger, while cheap chargers cut corners by replacing the IC with a cheap, low-quality feedback circuit.[4]

A poor design can suffer several problems. If the output voltage is not filtered well, there will be noise and spikes due to the high-frequency switching. At extreme levels this could damage your phone, but the most common symptom is the touchscreen doesn't work while the charger is plugged in.[1] A second problem is the output voltage can be affected by the AC input, causing 120 Hz "ripple".[5] Third, the charger is supposed to provide a constant voltage. A poor design can cause the voltage to sag as the load increases. Your phone will take longer to charge if the charger doesn't provide enough power. Finally, USB chargers are not all interchangeable; the wrong type of charger may not work with your device.[6]

Counterfeits

Counterfeit chargers pose a safety hazard as well as a hazard to your phone. You can buy a charger that looks just like an Apple charger for about $2, but the charger is nothing like an Apple charger internally. The power is extremely bad quality (as I will show below). But more importantly, these chargers ignore safety standards. Since chargers have hundreds of volts internally, there's a big risk if a charger doesn't have proper insulation. You're putting your phone, and more importantly yourself, at risk if you use one of these chargers. I did a teardown of a counterfeit charger, which shows the differences in detail.

I've taken apart several counterfeit chargers and readers have sent me photos of others. Surprisingly, the counterfeit chargers I've examined all use different circuitry internally. If you get a counterfeit, it could be worse or better than what I've seen.

How do you tell if a charger is counterfeit? The fakes are very similar; it's hard for me to tell, even after studying many chargers. There's a video on how to distinguish real and fake chargers through subtle differences. You can also weigh the charger (if you have an accurate scale), and compare with the weights I give above. The easiest way to get a genuine Apple charger is fork over $29 to an Apple store. If you buy a $2 "Original Genuine Apple" charger on eBay shipped from China, I can guarantee it's counterfeit. On the other hand, I've succeeded in buying genuine used chargers from US resellers for a moderate price on eBay, but you're taking a chance.

The following picture shows a counterfeit charger that burned up. The safety issues with counterfeits are not just theoretical; when hundreds of volts short out, the results can be spectacular.

Photo by Anool Mahidharia. Used with permission

Indicated charger type

A device being charged can detect what type of charger is being used through specific voltages on the USB data pins.[6] Because of this, some devices only work with their own special chargers. For instance, an "incorrect" charger may be rejected by an iPhone 3GS or later with the message "Charging is not supported with this accessory".[7]

There are many different charger types, but only a few are used in the chargers I examined. A USB charger that follows the standard is known as a "dedicated USB charger". However, some manufacturers (such as Apple, Sony, and HP) don't follow the USB standard but implement their own proprietary charger types. Apple has separate charger types for 1 amp (iPhone) and 2 amp (iPad) chargers. HP has a special type for the HP TouchPad.

The point is that USB chargers are not interchangeable, and devices may not work if the charger type doesn't match what the device expects. The table below shows the type of charger, the current that the label claims the charger provides, the current it actually provides, and the charger type it indicates to the device.

The types of the counterfeit chargers are a mess, as they advertise one power level, actually supply a different power level, and have the charger type for a third level. For example, the counterfeit iPhone charger is advertised as supplying 1 amp, but has the 2A charger type, so an iPad will expect 2 amps but not obtain enough power. On the other hand, the counterfeit iPad charger claims to supply 2 amps, but really only supplies 1 amp and has a 1A type.

	Charger type	Label	Measured current	Weight
Apple iPhone	Apple 1A charger	5V 1A	1.79A	23.0g
Samsung oblong	dedicated USB charger	5V 0.7A	.80A	33.1g
Samsung cube	dedicated USB charger	5V 1A	1.17A	23.2g
Apple iPad	Apple 2A charger	5.1V 2.1A	2.3A	67.5g
HP TouchPad	HP TouchPad charger	5.3V 2.0A	2.4A	54.8g
Counterfeit iPhone	Apple 2A charger	5V 1A	.94A	18.8g
Monoprice	Apple dock	5V 1A	1.22A	67.8g
Counterfeit UK	dedicated USB charger	5V 1A	.57A	29.4g
Counterfeit iPad	Apple 1A charger	5.1V 2.1A	1.2A	43.4g
Belkin	Apple 1A charger	5V 1A	1.27A	43.0g
KMS	Apple 2A charger	5V 2.1A	3.4A	99.5g
Motorola	dedicated USB charger	5.1V .85A	.82A	38.6g

Efficiency

People often wonder how much power their charger is wasting while it's idle, and if they should unplug their charger when not in use. I measured this "vampire" power usage and found the chargers varied by more than a factor of 20 in their idle power usage. The Samsung oblong charger came in best, using just 19 mW; this was so low compared to the other chargers that I measured it again a different way to make sure I hadn't made an error. On the other extreme, the fake iPhone charger used 375 mW. The Apple iPhone charger performed surprisingly badly at 195 mW. If plugged in for a year, this would cost you about 21 cents in electricity, so it's probably not worth worrying about.[8] In the following table, I use the official charger Star Rating System (yes, there actually is such a thing).[9][10]

I also measured efficiency of the chargers under load.[11] One of the benefits of switching power supplies over simpler linear supplies is they are much more efficient at converting the input power to output. The chargers I measured all did pretty well, with 63% to 80% efficiency. The HP charger was the winner here.

	Vampire	milliwatts	Efficiency	Percent
Apple iPhone		195		74
Samsung oblong		19		76
Samsung cube		86		77
Apple iPad		62		78
HP TouchPad		91		80
Counterfeit iPhone		375		63
Monoprice		78		72
Counterfeit UK		103		63
Counterfeit iPad		95		66
Belkin		234		66
KMS		179		69
Motorola		59		75

The chargers up close

Apple iPhone and counterfeit

The above photo shows a real iPhone charger (left) and a counterfeit (right); the two chargers are almost identical, down to the green dot. If you look closely, the genuine one says "Designed by Apple in California", while the counterfeit has the puzzling text "Designed by California". The counterfeit also removed the "Apple Japan" text below the plug. I've seen another counterfeit that says "Designed by Abble" (not Apple). I assume the word "Apple" is removed for legal or trademark reasons, since the word "Apple" is often (but not always) missing from counterfeits.

Samsung oblong

I call this charger the Samsung oblong charger, to distinguish it from the Samsung cube charger.

Samsung cube

The Samsung cube charger is shaped very similarly to the Apple iPhone charger. Internally, however, it turns out to be entirely different.

Apple iPad and counterfeit

The photo above shows a real iPad charger (left) and a counterfeit (right). The counterfeit has almost identical text, but without "Designed by Apple in California. Assembled in China", "Listed" under UL, and the manufacturer "Foxlink". Inexplicably this sanitization left "TM and © 2010 Apple Inc".

The above photo shows a real iPad charger on the left and a fake iPad charger on the right, with the plug removed. The most visible difference is the real charger has a round metal grounding post, while the fake has plastic. (The US plug isn't grounded, but in other countries the lack of ground in the counterfeit could pose a safety hazard.)

HP TouchPad

The HP TouchPad charger has a very unusual cylindrical shape, which is striking if perhaps not practical. The charger twists apart, allowing the plug to be replaced for different countries. (It took me weeks to discover this feature.)

Monoprice

The Monoprice charger isn't a USB charger, but instead has a 30-pin iPhone dock connector attached. It is a relatively large charger.

Counterfeit UK

This charger is a counterfeit of the Apple UK iPhone charger. They've removed Apple from the text, but left Emerson Network Power, which I'm sure is not the actual manufacturer. The genuine Apple UK charger can be distinguished by a serial number inside the USB connector.

Belkin

The Belkin charger eschews the minimal design styling of most chargers, with a roughly oval cross section, curves and ribs, and a cover over the USB port.

KMS

The KMS charger is unusual in providing 4 USB ports. It also gives off a blue glow while in use. The plug can be removed and replaced for use in different countries, similar to the iPad and HP TouchPad chargers. I couldn't find any UL safety approval on this charger, but I did find a report of one catching fire.

Motorola

The Motorola charger has the lowest listed power output, 850mA. The back of it has a holographic sticker (like a credit card), which may ward off counterfeiters, even though it's unlikely for anyone to counterfeit this charger. I wonder though why Apple doesn't use holograms or other anti-counterfeiting techniques, given the large number of counterfeit Apple chargers being sold.

Delivery of advertised power

Each charger has an advertised power output, but some chargers produce considerably more and some produce much less. Your device will take longer to charge, if the charger can't put out enough power. This table shows each charger's ability to deliver the rated power, based on my measurements of maximum power. While most chargers meet or exceed the power rating, there are some exceptions.

The counterfeit chargers perform extremely poorly, putting out a fraction of the expected power. Charging your device with one of these chargers will be a slow, frustrating experience. In particular, the counterfeit UK charger only produces a third of the expected power. Although the label claims the charger works on 100-240 volts, it's clearly not designed to work on US power.

The iPad is a surprise, putting out less power than expected. Despite being nominally a 10 watt charger, the label says it provides 5.1V and 2.1A, which works out to 10.7 watts. However, the maximum power I measured is 10.1 watts (4.4 volts at 2.3 amps, as shown in the Power section below). Since the measured power is slightly less than advertised, it only gets four bolts.

	Rating	Label	Watts from label	Measured watts
Apple iPhone		5V 1A	5.0	6.0
Samsung oblong		5V 0.7A	3.5	4.0
Samsung cube		5V 1A	5.0	5.5
Apple iPad		5.1V 2.1A	10.7	10.1
HP TouchPad		5.3V 2.0A	10.6	11.4
Counterfeit iPhone		5V 1A	5.0	2.7
Monoprice		5V 1A	5.0	5.7
Counterfeit UK		5V 1A	5.0	1.7
Counterfeit iPad		5.1V 2.1A	10.7	5.9
Belkin		5V 1A	5.0	5.6
KMS		5V 2.1A	10.5	10.9
Motorola		5.1V .85A	4.3	4.3

Power quality

In this section, I measure the quality of the power produced by the different chargers. I analyze it for voltage spikes, high frequency noise, and line-frequency ripple. The following table summarizes the results in three categories. Spikes indicates extremely brief large voltage spikes in the output, while Noise indicates high-frequency noise in the output, and Ripple indicates low-frequency (120 Hz) fluctuations in the output.[12]

	Spikes	Noise	Ripple
Apple iPhone
Samsung oblong
Samsung cube
Apple iPad
HP TouchPad
Counterfeit iPhone
Monoprice
Counterfeit UK
Counterfeit iPad
Belkin
KMS
Motorola

The following oscilloscope traces show the output signal (yellow) and frequency spectrum (orange). The left images provide high-frequency information on the output voltage. The right images show the low-frequency information on the output voltage.[13]

The desired voltage graph is a flat, thin yellow line indicating totally smooth power. However, some factors mess this up. First, any ripple from the power line will show up as 5 sinusoidal peaks in the first (high-frequency) yellow line. High-frequency noise will widen the yellow line. Voltage spikes will appear as vertical spikes in the yellow line.

The plots also show the frequency spectrum in orange, from 0 at the left to 230 kHz at the right. The desired graph would have the orange spectrum near the bottom of the screen. Thus, the power quality exponentially gets worse as the orange line gets higher. The left (high frequency) spectrum generally shows noise at the switching frequency of the charger (and harmonics). The right (low frequency) spectrum typically shows spikes at multiples of 120 Hz, caused by ripple from the 60 Hz power.[5]

Apple iPhone

The ripple is clearly visible as the waves in the yellow trace on the left and as the spikes (at 120 Hz and 240 Hz) in the orange trace on the right.

The iPhone charger performs extremely well at filtering out spikes and noise, the best of the chargers I measured. Apart from the 120 Hz spikes, the noise spectrum (orange) is flat and very low. The power quality is so good, I checked the results several times to make sure I wasn't missing something.

Samsung oblong

The Samsung charger's output has a lot more noise than the iPhone charger. This is visible in the thickness and jaggedness of the yellow output curves. The orange frequency spectrum on the left shows large peaks at harmonics of the switching frequency. The 120 Hz spike on the right is a bit lower than the iPhone charger, so the ripple filtering is a bit better.

Samsung cube

The Samsung cube charger shows some noise in the output (yellow). The frequency spectrum shows wide peaks at multiples of the the switching frequency, about 90kHz. There's some ripple.

Apple iPad

The iPad charger almost eliminates the ripple; only a small blip is visible in the orange spectrum on the right. The noise level is low, although appreciably worse than the iPhone.

HP TouchPad

There's no ripple visible in the HP charger spectrum on the right. The overall noise level is good.

Counterfeit iPhone

The output from this counterfeit charger is a wall of noise. In order to fit the waveform in the display, I had to double the scale on the left and increase it by a factor of 5 on the right, so the yellow curve is actually much worse than it appears. On the left, note the huge ripple with massive high-frequency noise on top. This output is not something you want to feed into your phone.

Monoprice

The output from this charger is very noisy, as you can see from the thickness of the yellow line. Note that the frequency spectrum (left) has very tall but narrow spikes at harmonics of the 28kHz switching frequency, showing a lot of high-frequency noise. On the positive side, there is hardly any ripple.

Counterfeit UK

This charger has very bad output. The large degree of ripple is visible in the waveform (yellow, left) and the very large spikes in the spectrum (orange, right). The thickness of the yellow waveform shows the large amount of high-frequency noise, which is also visible in the very high peaks in the spectrum (orange, left).

Counterfeit iPad

This counterfeit charger has so much noise in the output that I had to double the scale on the left to get it to fit. Note the very large spikes in the output (yellow). The spectrum (orange, left) is much higher everywhere, indicating noise at all frequencies. Surprisingly, it has only a moderate amount of ripple; the manufacturer seems to have done at least one thing right.

Belkin

The Belkin charger does well at eliminating ripple, but has a lot of noise otherwise. The spectrum (orange, left) shows large peaks. The yellow output is wide, showing a lot of noise, combined with many large voltage spikes of about 1/3 volt.

KMS

The KMS charger has fairly good output, with a small peak in the spectrum (orange, left) at the switching frequency. It has no detectable ripple. However, it has many large voltage spikes in the output, over half a volt, as can be seen on the right.

Motorola

The Motorola charger has a lot of spikes in the output (yellow) . The spectrum (orange, left) shows high frequency noise at the switching frequencies. There's a moderate amount of ripple (yellow, left and orange, right).

Summary

The quality of the output power is radically different between chargers. The counterfeit chargers are uniformly bad, with hardly any effort at filtering the output. The other chargers vary in quality with the iPhone charger setting the standard for noise-free power, but surprisingly poor filtering of ripple. The power quality is a key factor that affects the performance of chargers; spikes and noise are known to interfere with touchscreens.[1]

Power curve

In this section I look at the voltage and current output by the charger as the load increases. The first rating is Voltage Sag, which is the undesired drop in output voltage as the load increases. The second rating is Current Sag, which shows how the current fluctuates as load increases. Finally, Regulation shows the overall stability of the output from the charger.

	Voltage sag	Current sag	Regulation
Apple iPhone
Samsung oblong
Samsung cube
Apple iPad
HP TouchPad
Counterfeit iPhone
Monoprice
Counterfeit UK
Counterfeit iPad
Belkin
KMS
Motorola

The graphs in this section need a bit of explanation, which is provided in the diagram below. The voltage/current load curve shows the performance of the charger under different loads. Each point on the curve shows the current (X axis) and voltage (Y axis) produced by the charger under a particular load condition. Follow the yellow curve clockwise from the upper left to the lower left to see the effect of increasing load. The upper left point of the curve shows the voltage produced by the charger when there is no load on the charger. As the load increases, the charger is supposed to keep a constant voltage and increase the current (i.e. horizontal line), until it reaches the maximum power (upper right). If the load continues increasing, the charger switches to a constant current mode, dropping the voltage while continuing to provide the maximum current (i.e. vertical line).[14] At the lower right, the charger has reached its shutdown point due to excessive load, and rapidly drops to no output in the lower left corner to avoid damage.

[16]

Apple iPhone

The output from the Apple iPhone charger is surprisingly non-constant under load. The charger starts off with 5.2 volts with no load, dropping to 4.6 volts as the load increases, resulting in the downwards slope of the top yellow line. As the load increases, the current keeps increasing, resulting in the slope of the right yellow line. Note however that the yellow line is relatively thin, so the regulation is pretty good at each point.

Note that because this charger has a high current output, this chart has a different current (horizontal) scale than most of the charts to fit the whole trace in the image. Stretch it horizontally to compare with other graphs.

Samsung oblong

For this charger, the voltage is approximately flat, except for a bump under no load (upper left) which is probably a measurement artifact. The vertical yellow line shows the current stays nearly constant as the load increases. The charger shows good voltage and current stability under changing load. The yellow line is a bit wider than the iPhone charger, showing a bit less regulation for a fixed load.

Samsung cube

The voltage curve sags slightly under load. The right hand curve shows the current stays stable, but the line is moderately wide, showing a bit of weakness in regulation.

Apple iPad

Similar to the iPhone charger, the iPad charger shows a lot of voltage sag. The voltage is about 5.1 V unloaded, dropping to 4.4 volts and 2.3 A (10.1 W) at the corner. Unlike the iPhone charger, the iPad charger has pretty good current stability. The regulation is solid, as shown by the narrowness of the yellow trace. Note the scale change due to the high current output.

I'm puzzled by the steep voltage sag on both the iPhone and iPad charger. Since the designers of the Apple charger went to a great deal of effort to build a high quality charger, I conclude they must not consider voltage sag worth worrying about. Or, more interestingly, maybe they built this sag as a feature for some reason. In any case, the chargers lose points on this.

HP TouchPad

The charger has some voltage sag, but the current (vertical) is nice and constant. The yellow line is relatively thin, showing good regulation. Note the scale change due to the high current output.

Counterfeit iPhone

This counterfeit charger shows extremely poor regulation, as shown by the very wide yellow line. It's hard to fit a voltage-current curve to this picture. The amount of power supplied by this charger seems almost random.

Monoprice

The Monoprice charger shows reasonably straight voltage and current lines showing good constant voltage and current outputs. The vertical line shows some width and noise, suggesting the regulation isn't totally stable.

Counterfeit UK

For this charger, the upper line doesn't get very far, showing that this charger doesn't output much current. My suspicion is that it was only tested with 240 volts so it performs poorly with 120 volts, even though the label says it takes 100 to 240 volts. The width of the yellow line shows very poor regulation.

Counterfeit iPad

The output of this counterfeit charger is so poorly regulated that it's hard to tell exactly what's happening with the voltage and current. It looks like the voltage is roughly constant underneath all the noise.

Belkin

The Belkin charger shows voltage sag as the current increases. In addition, the output is fairly noisy.

KMS

The KMS charger shows a lot of voltage sag as the load increases. In addition, the output is all over the place, showing very poor regulation, more like what I'd expect from a counterfeit charger. Note the scale change due to the high current output.

Motorola

The Motorola charger shows a bit of voltage sag, but good current stability. The regulation is good but not perfect, as shown by the width of the yellow line. (The gaps in the vertical line are just measurement artifacts.) Note that the maximum current output of this charger is fairly low (as advertised).

Conclusions

So what charger should you spend your hard-earned money on? First, make sure the charger will work with your phone - for instance, newer iPhones only work with certain chargers. Second, don't buy a counterfeit charger; the price is great, but it's not worth risking your expensive device or your safety. Beyond that, it's your decision on how much quality is worth versus price, and I hope the data here helps you make a decision.

P.S. How about some teardowns?

My previous iPhone charger and fake charger teardowns were surprisingly popular, but if you were hoping for teardowns on the full set of chargers, you'll need to wait for a future blog post. I haven't torn the chargers apart yet; if I need to take more measurements, I don't want to have just a pile of parts. But I do have some preview pictures to hold you over until my teardown article.

The above picture shows the internals of a counterfeit Apple iPhone cube charger. The two boards stack to form the compact cube shape. This charger blatantly tries to pass as a genuine Apple charger; unlike the "Designed by California" charger, this one exactly copies the "Designed by Apple in California" text from the real charger. Note the very simple circuitry[4] - there are no components on the other side of the board, no controller IC, and very little filtering. Also look at the terrible mounting of the transistor on the front right; clearly the build quality of this charger is poor. Finally, note the overall lack of insulation; this charger wouldn't meet UL safety standards and could easily short out. But on the plus side, this charger only cost a couple dollars.

The above $2 charger is notable for its low-profile design; it's about as thin as you can make a charger and still fit the power prongs and the USB port. The transformer is very short to fit into this charger. Like the previous charger, it uses a very simple circuit,[4] has little filtering, and almost no safety insulation.

Finally, the above pictures show the internals of the Samsung cube charger, which has circuit boards packed with tiny components and is much more advanced than the counterfeits (although slightly less complex than the Apple charger). Despite being very similar to the Apple charger on the outside, the Samsung charger uses an entirely different design and circuitry internally. One interesting design feature is the filter capacitors fit through the cut-out holes in the secondary circuit board, allowing the large filter capacitors to fit in the charger.

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Notes and references

[1] For an explanation of how the noisy output from cheap chargers messes up touchscreens, see Noise Wars: Projected Capacitance Strikes Back.

[2] The charger selection may seem slightly eccentric; it is based on chargers I had previously acquired, chargers I could obtain at a reasonable price, chargers supplied by Gary F. and Anthony H. (thanks, guys!), and some counterfeit chargers for comparison.

[3] TI has an interesting new design for a 10 watt inch-cube charger. With this design a tablet charger could be as small as the iPhone charger.

Photo of PMP8286 10W cube charger used with permission from Texas Instruments.

[4] The cheap chargers all use a "ringing choke converter" circuit, which coincidentally is the same power supply topology used by the Apple II. These chargers use an extremely simple feedback mechanism in place of the control IC in higher-quality chargers. See a comic-book explanation or a technical explanation for details.

[5] Since the input AC has a frequency of 60 Hertz, you might wonder why the ripple in the output is 120 Hertz. The diode bridge converts the 60 Hz AC input to 120 Hz pulsed DC, as shown in the diagram below. The pulses are smoothed out with filter capacitors before being fed into the switching circuit, but if the filtering isn't sufficient the output may show some 120 Hz ripple.

Image by WdWd, used under CC BY 3.0

[6] The chargers use specific voltages on the data pins to indicate the charger type to the device being charged. Because of this, an "incorrect" charger may be rejected by an iPhone with the message "Charging is not supported with this accessory".[7] Under the USB standard, a charger should short the two data pins together to indicate that it's a "dedicated" charger and not a real USB device. However, companies such as Apple, HP, and Sony have their own proprietary nonstandard techniques. The following table summarizes the voltages that appear on the D+ and D- lines for different chargers, and how the D+ and D- lines are configured internally.

Charger type	D+ voltage	D- voltage	D+/D- shorted	D+ pullup (kΩ)	D+ pulldown (kΩ)	D- pullup (kΩ)	D- pulldown (kΩ)
dedicated USB	float	float	yes	none	none	none	none
Apple .5A	2	2	no	75	49.9	75	49.9
Apple 1A	2	2.7	no	75	49.9	43.2	49.9
Apple 2A	2.7	2	no	43.2	49.9	75	49.9
HP TouchPad 2A	2.8	2.7	yes	250	300	n/a	n/a
Sony	3.3	3.3	no	5.1	10	5.1	10

Most of this data is based on Maxim USB Battery Charger Detectors, Adafruit's The mysteries of Apple device charging, TouchPad's USB Cable, XDA forum (Samsung), and TPS2511 USB Dedicated Charging Port Controller and Current Limiting Power Switch datasheet. The Apple 2A (i.e. iPad) information is a new result from my measurements. For details on USB charging protocols, see my references in my earlier posting.

Amusingly, semiconductor manufacturers have recently introduced chips that allow chargers to sequentially pretend to be different proprietary chargers until they trick the device into accepting the charger. It seems crazy that companies (such as Apple) design incompatible chargers, and then chip companies invent schemes to work around these incompatibilities in order to build universally compatible chargers. Two example chips are the TI TPS 2511 chip, and SMSC's USC1001 controller, which pretends to be nine different charger types.

[7] If you've wondered why some chargers cause the iPhone to give a "Charging not supported with this accessory" error, Silicon based annoyance reduction made easy describes how devices use proprietary protocols to limit the chargers they will work with.

[8] For the efficiency analysis I use 12 cents / kilowatt-hour as a typical residential energy price, which I got from US Energy Information Administration table 5.3.

[9] The official no-load charger star ratings are discussed at Meeting 30 mW standby in mobile phone chargers.

[10] There are many standards for energy consumption; see 5 W Cellular Phone CCCV (Constant Current Constant Voltage) AC-DC Adapter. For Energy Star ratings, a 5W charger must have under .5W no-load consumption, and 63% efficiency under load. A 10W charger must have under .75W no-load consumption, and 70% efficiency.

[11] Because switching power supplies use power in irregular waveforms, I used a complex setup to measure power consumption. I measured the AC input voltage and current with an oscilloscope. The oscilloscope's math functions multiplied the voltage and current at each instant to compute the instantaneous power, and then computed the average power over time. For safety and to avoid vaporizing the oscilloscope I used an isolation transformer. My measurements are fairly close to Apple's[15], which is reassuring.

You might wonder why I didn't just use a Kill A Watt power monitor, which performs the same instantaneous voltage * current process internally. Unfortunately it doesn't have the resolution for the small power consumptions I'm measuring: it reports 0.3W for the Apple iPhone charger, and 0.0W for many of the others. Ironically, after computing these detailed power measurements, I simply measured the input current with a multimeter, multiplied by 115 volts, and got almost exactly the same results for vampire power.

[12] The spike, noise, and ripple measurements come from the oscilloscope traces. The Spikes measurement is based on the maximum peak-to-peak voltage on the high frequency trace (the low frequency trace yields almost identical results). The Noise measurement is based on the RMS voltage on the high-frequency trace, and Ripple is based on the maximum dB measured in the low-frequency spectrum. These measurements appear on the right in the traces.

[13] In the power quality section, the high-frequency (left) images show 40 milliseconds of the waveform in yellow, and the frequency spectrum up to 234 kHz in orange. The low-frequency (right) images show 1 second of the output voltage in yellow and the frequency spectrum up to 600 Hz in orange. Because the frequency spectrum is measured in dBm, it is logarithmic; every division higher indicates 20 dB which is 10 times the voltage and 100 times the power.

[14] The chargers use a design called constant-voltage, constant-current (CVCC), since they provide a constant voltage (and increasing current) up to the maximum load and then a constant current (and decreasing voltage) if the load continues to increase. [15] The Apple 3GS Environmental Report gives some efficiency measurements for the Apple USB Power Adapter. It lists 0.23W no-load power and 75% efficiency. These values are reasonably close to my measurements of 0.195W no-load consumption and 73.6% efficiency.

[16] Measuring these curves was a bit tricky. I used a NTE2382 power MOSFET transistor as a variable load, manually varying the gate bias to generate the load curve. The transistor needed a large heat sink to dissipate 10 watts. A more complex dynamic load circuit is described here, but the simple circuit was sufficient for me.

The graphs were generated using the X-Y mode on the oscilloscope, with the load voltage as Y and the current as X. I used a .12Ω current sense resistor to measure the load current. This works out to 1/6 amp load current per division for the 20mV/div traces (most of them), and 5/12 amp load current per division for the 50mV/div traces (the high-current devices).

Note that increasing load corresponds to a decreasing resistance across the output: the upper left has infinite resistance (no load), the lower left has zero resistance (short circuit), and the resistance decreases in between. Since the power (in watts) is voltage * current, the maximum power is in the upper right corner, approximately 4W in this case. The load resistance can be computed by Ohm's law, e.g. middle of the upper curve: 5 V / .4 A = 12.5Ω, upper right corner 5 V / .8 A = 6.25 ohms. Middle of the right hand curve: 2.5 V / .8 A = 3Ω, overload point = .5 V / .8 A = .6Ω.

[17] Most of these chargers aren't made by the companies that sell them, and there are some interesting facts about the manufacturers. The manufacturers of the chargers can be looked up from the UL certification number. The oblong Samsung is made in China by Korean RFTech, a manufacturer of mobile phone products. The Samsung cube is made in China by Korean power supply manufacturer Dong Yang E&P. The HP charger is made by Foxlink, who also makes the iPad charger for Apple. The counterfeit chargers are made by anonymous Chinese manufacturers, despite what they claim on the labels. The Monoprice is made by Golden Profit Electronics (formerly ShaYao Electric Factory Three - no word on what happened to factories One and Two). The Belkin charger is manufactured by the obscure company Mobiletec of Taiwan. The KMS charger doesn't give any clues as to the manufacturer, and I can't identify KMS as a company. The Motorola charger is built by Astec (now part of Emerson Network Power). Interestingly, Astec's big break was manufacturing power supplies for the Apple II, as I discuss in my article on the Apple II power supply.

Apple uses a dizzying variety of manufacturers for their chargers. The iPhone charger (A1265) is made by Flextronics, the UK charger (A1299) is made by Emerson Network Power (except the one I have is counterfeit), the iPad charger (A1357) is made by Foxlink Technologies, and the Magsafe (ADP-85) charger (not discussed in this article) is made by Delta Electronics. The A1385 iPhone charger often comes with the iPhone 5 and looks identical to the A1265 I measured, but is manufactured by Emerson Network Power instead of Flextronics. I am told that by using multiple manufacturers, Apple has more negotiating leverage, since they can easily switch manufacturers at any time if they're not happy with the price or quality.

Confusingly, Foxlink (Taiwan), Foxconn (Taiwan), and Flextronics (Singapore) are all manufacturers for Apple with similar names. Foxlink (the name for Cheng Uei Precision Industry) and Foxconn (the name for Hon Hai Precision Industry) are entirely independent companies aside from the fact that the chairmen of both companies are brothers and the companies do a lot of business with each other (statement, Foxlink annual report). Foxconn is the company with continuing controversy over employee treatment. Foxconn and Flextronics are the world's #1 and #2 largest electronics manufacturing companies according to the Circuits Assembly Top 50.

Apple iPhone charger teardown: quality in a tiny expensive package

This article is now available in Vietnamese Bộ sạc iPhone của Apple.

Disassembling Apple's diminutive inch-cube iPhone charger reveals a technologically advanced flyback switching power supply that goes beyond the typical charger. It simply takes AC input (anything between 100 and 240 volts) and produce 5 watts of smooth 5 volt power, but the circuit to do this is surprisingly complex and innovative.

How it works

The iPhone power adapter is a switching power supply, where the input power is switched on and off about 70,000 times a second in order to get the exact output voltage required. Because of their design, switching power supplies are generally compact and efficient and generate little waste heat compared to simpler linear power supplies.

In more detail, the AC line power is first converted to high voltage DC[1] by a diode bridge. The DC is switched on and off by a transistor controlled by a power supply controller IC. The chopped DC is fed into a flyback[2] transformer which converts it into low voltage AC. Finally, this AC is converted into DC which is filtered to obtain smooth power free of interference, and this power is output through the USB jack. A feedback circuit measures the output voltage and sends a signal to the controller IC, which adjusts the switching frequency to obtain the desired voltage.

The side view above shows some of the larger components. The charger consists of two circuit boards, slightly under one inch square each.[3] The top board is the primary, which has the high voltage circuitry, and the bottom board, the secondary, has the low voltage output circuitry. The input AC first passes through a fusible resistor (striped), which will break the circuit if there is a catastrophic overload. The input AC is converted to high-voltage DC, which is smoothed by the two large electrolytic capacitors (black with white text and stripe) and the inductor (green).

Next, the high voltage DC is chopped at high frequency by a MOSFET switching transistor, which is the large three-pinned component in the upper left. (The second transistor clamps voltage spikes, as will be explained below.) The chopped DC goes to the flyback transformer (yellow, barely visible behind the transistors), which has low voltage output wires going to the secondary board below. (These wires were cut during disassembly.) The secondary board converts the low voltage from the transformer to DC, filters it, and then feeds it out through the USB connector (the silver rectangle in the lower left). The gray ribbon cable (just barely visible on the lower right under the capacitor) provides feedback from the secondary board to the controller IC to keep the voltage regulated.

The picture above shows the flyback transformer (yellow) more clearly, above the USB jack. The large blue component is a special "Y" capacitor[4] to reduce interference. The controller IC is visible above the transformer on the top of the primary board.[5]

The circuit in detail

The primary

The primary circuit board is packed with surface mounted components on both sides. The inner side (diagram above) holds the large components while the outer side (diagram below) has the controller IC. (The large components were removed in the diagrams, and are indicated in italics.) Input power is connected to the corners of the board, goes through the 10Ω fusible resistor, and is rectified to DC by the four diodes. Two R-C snubber circuits absorb EMI interference created by the bridge.[6] The DC is filtered by the two large electrolytic capacitors and the inductor, producing 125-340V DC. Note the thickness of the circuit board traces connecting these capacitors and other high-current components compared to the thin control traces.

The power supply is controlled by an 8-pin STMicrosystems L6565 quasi-resonant SMPS controller chip.[7] The controller IC drives the MOSFET switching transistor which chops the high voltage DC and feeds it into the primary winding of the flyback transformer. The controller IC takes a variety of inputs (secondary voltage feedback, input DC voltage, transformer primary current, and transformer demagnetization sensing) and adjusts the switching frequency and timing to control the output voltage through complex internal circuitry. The current sense resistors let the IC know how much current is flowing through the primary, which controls when the transistor should be turned off.

The second switching transistor, along with some capacitors and diodes, is part of a resonant clamp circuit that absorbs voltage spikes on the transformer. This unusual and innovative circuit is patented by Flextronics.[8][9]

The controller IC needs DC power to run; this is provided by an auxiliary power circuit consisting of a separate auxiliary winding on the transformer, a diode, and filter capacitors. Since the controller IC needs to be powered up before the transformer can start generating power, you might wonder how this chicken-and-egg problem gets solved. The solution is the high-voltage DC is dropped to a low level through startup power resistors to provide the initial power to the IC until the transformer starts up. The auxiliary winding is also used by the IC to sense transformer demagnitization, which indicates when to turn on the switching transistor.[7]

The secondary

On the secondary board, the low voltage AC from the transformer is rectified by the high-speed Schottky diode, filtered by the inductor and capacitors, and connected to the USB output. The tantalum filter capacitors provide high capacitance in a small package.

The USB output also has specific resistances connected to the data pins to indicate to the iPhone how much current the charger can supply, through a proprietary Apple protocol.[10] An iPhone displays the message "Charging is not supported with this accessory" if the charger has the wrong resistances here.

The secondary board contains a standard switching power supply feedback circuit that monitors the output voltage with a TL431 regulator and provides feedback to the controller IC through the optocoupler. A second feedback circuit shuts down the charger for protection if the charger overheats or the output voltage is too high.[11] A ribbon cable provides this feedback to the primary board.

Isolation

Because the power supply can have up to 340V DC internally, safety is an important issue. Strict regulations govern the separation between the dangerous line voltage and the safe output voltage, which are isolated by a combination of distance (called creepage and clearance), and insulation. The standards[12] are somewhat incomprehensible, but roughly 4mm of distance is required between the two circuits. (As I discuss in Tiny, cheap, dangerous: Inside a (fake) iPhone charger, cheap chargers totally ignore these safety rules.)

You might expect the primary board to have the dangerous voltages and the secondary board to have the safe voltages, but the secondary board consists of two areas: the hazardous area connected to the primary board, and the low-voltage area. The isolation boundary between these areas is about 6mm in the Apple charger and can be seen in the above diagram. This isolation boundary ensures that dangerous voltages cannot reach the output.

There are three types of components that cross the isolation boundary, and they must be specially designed for safety. The key component is the transformer, which provides a way for electrical power to reach the output without a direct electrical connection. Internally, the transformer is extensively insulated, as will be shown below. The second component type is the optocouplers, which send the feedback signal from the secondary to the primary. Internally, the optocoupler contains a LED and a photo-transistor, so the two sides are connected only by light, not by an electrical circuit. (Note the silicone insulation on the secondary side of the optocouplers to provide extra safety.) Finally, the Y capacitor is a special type of capacitor[4] that lets EMI (electromagnetic interference) escape between the high-voltage primary and the low-voltage secondary.

The above picture shows some of the isolation techniques. The secondary board (left) has the blue Y capacitor. Note the lack of components in the middle of the secondary board, forming an isolation boundary. The components on the right of the secondary board are connected to the primary board by the gray ribbon cable so they are at potentially high voltages. The other connection between the boards is the pair of wires from the flyback transformer (yellow) delivering the output power to the secondary board; these were cut to separate the boards.

Schematic

I've put together an approximate schematic showing the charger circuit.[13] Click for a larger version.

These circuits are very small

Looking at these pictures, it's easy to lose track of how very small these components are, and how the charger crams all this complexity into one inch. The following slightly magnified picture shows a quarter, a grain of rice, and a mustard seed to give a size comparison. Most of the components are surface-mount devices which are soldered directly to the printed circuit board. The smallest components, such as the resistor pointed out in the picture, are known as "0402" size since they are .04 inches by .02 inches. The larger resistors to the left of the mustard seed handle more power and are known as "0805" size since they are .08 x .05 inches.

Transformer teardown

The flyback transformer is the key component of the charger, the largest component, and probably the most expensive.[14] But what's inside? I took apart the transformer to find out.

The transformer measures roughly 1/2" by 1/2" by 1/3". Inside, the transformer has three windings: a high voltage primary input winding, a low voltage auxiliary winding to provide power to the control circuits, and a high-current low voltage output winding. The output winding is connected to the black and white wires coming out of the transformer, while the other windings are connected to the pins attached to the bottom of the transformer.

The outside of the transformer has a couple layers of insulating tape. The second line appears to start with "FLEX", for Flextronics. Two grounded strands of wire are wrapped around the outside of the transformer to provide shielding.

After removing the shielding and the tape, the two halves of the ferrite core can be removed from the windings. Ferrite is a rather brittle ceramic material, so the core broke during removal. The core surrounds the windings and contains the magnetic fields. Each core piece is roughly 6mm x 11mm x 4mm; this style of core is known as EQ. The circular center section is very slightly shorter than the ends, creating a small air gap when the core pieces are put together. This 0.28mm air gap stores the magnetic energy for the flyback transformer.

Underneath the next two layers of tape is a 17-turn winding of thin varnished wire, which I think is another shield winding to return stray interference to ground.

Underneath the shield and another two layers of tape is the 6-turn secondary output winding that is connected to the black and white wires. Note that this winding is heavy-gauge wire, since it is feeding the 1A output. Also note that the winding is triple-insulated, which is a UL safety requirement to ensure that the high voltage primary remains isolated from the output. This is one place where cheap chargers cheat - they just use regular wire instead of triple-insulated, and also skimp on the tape. The result is there's not much protecting you from high voltage if there's an insulation flaw or power surge.

Under the next double layer of tape is the 11-turn heavy gauge primary power winding, that powers the controller IC. Since this winding is on the primary side, it doesn't need to be triple insulated. It's just insulated with a thin layer of varnish.

Under the final double layer of tape is the primary input winding, which is 4 layers of approximately 23 turns each. This winding receives the high voltage input. Since the current is very low, the wire can be very thin. Because the primary has about 15 times as many turns as the secondary winding, the secondary voltage will be 1/15 the primary voltage, but 15 times the current. Thus, the transformer converts the high voltage input to low voltage, high current output.

The final picture shows all the components of the transformer; left to right shows the layers from the outside tape to the innermost winding and bobbin.

Apple's huge profit margins

I was surprised to realize how enormous Apple's profit margins must be on these chargers. These chargers sell for about $30 (if not counterfeit), but that must be almost all profit. Samsung sells a very similar cube charger for about $6-$10, which I also disassembled (and will write up details later). The Apple charger is higher quality and I estimate has about a dollar's worth of additional components inside.[14] But it sells for $20 more.

Apple's 2008 charger safety recall

In 2008, Apple recalled the iPhone chargers due to a defect that the AC prongs could fall off the charger and get stuck in an outlet.[15] The faulty chargers had the prongs attached with what was described as little more than glue and "wishful thinking".[15] Apple replaced the chargers with a redesigned model indicated by the green dot marking shown above (which counterfeit chargers inevitably imitate).

I decided to see what safety improvements Apple made in the replacement charger, and compare with other similar chargers. I tried pulling out the prongs of the Apple charger, a Samsung charger, and a counterfeit charger. The counterfeit prongs came out with a tug with pliers, as there's basically nothing anchoring them but friction. The Samsung prongs took a lot of pulling and twisting with pliers, since they have little metal tabs holding them in place, but eventually they came out.

When I moved on to the Apple charger, the prongs didn't budge, even with my hardest pulling with pliers, so I got out the Dremel and ground through the case to find out what was holding the prongs. They have large metal flanges embedded in the plastic of the case, so there's no way a prong can come loose short of the destruction of the charger. The photo shows the Apple plug (note the thickness of plastic removed from the right half), the prong from the counterfeit charger held in only by friction, and the Samsung prong held in by small but sturdy metal tabs.

I'm impressed with the effort Apple put into making the charger more safe after the recall. They didn't just improve the prongs slightly to make them more secure; clearly someone was told to do whatever it takes to make sure there's absolutely no way the prongs could possibly come loose again under any circumstances.

What makes Apple's iPhone charger special

Apple's power adapter is clearly a high-quality power supply designed to produce carefully filtered power. Apple has obviously gone to extra effort to reduce EMI interference, probably to keep the charger from interfering with the touchscreen.[16] When I opened the charger up, I expected to find a standard design, but I've compared the charger to the Samsung charger and several other high-quality industry designs,[17] and Apple goes beyond these designs in several ways.

The input AC is filtered thorugh a tiny ferrite ring on the plastic case (see photo below). The diode bridge output is filtered by two large capacitors and an inductor. Two other R-C snubbers filter the diode bridge, which I've only seen elsewhere in audio power supplies to prevent 60Hz hum;[6] perhaps this enhances the iTunes listening experience. Other chargers I disassembled don't use a ferrite ring and usually only a single filter capacitor. The primary circuit board has a grounded metal shield over the high-frequency components (see photo), which I haven't seen elsewhere. The transformer includes a shield winding to absorb EMI. The output circuit uses three capacitors including two relatively expensive tantalum ones[14] and an inductor for filtering, when many supplies just use one capacitor. The Y capacitor is usually omitted from other designs. The resonant clamp circuit is highly innovative.[9]

Apple's design provides extra safety in a few ways that were discussed earlier: the super-strong AC prongs, and the complex over-temperature / over-voltage shutdown circuit. Apple's isolation distance between primary and secondary appears to go beyond the regulations.

Conclusions

Apple's iPhone charger crams a lot of technology into a small space. Apple went to extra effort to provide higher quality and safety than other name-brand chargers, but this quality comes at a high cost.

If you're interested in power supplies, please take a look at my other articles: tiny, cheap, dangerous: Inside a (fake) iPhone charger, where I disassemble a $2.79 iPhone charger and discover that it violates many safety rules; don't buy one of these. Also take a look at Apple didn't revolutionize power supplies; new transistors did which examines the history of switching power supplies. To see Apple's adapter disassembled, check out videos created by scourtheearth and Ladyada. Finally, if you have an interesting charger lying around that you don't want, send it to me and maybe I'll write up a detailed teardown of it.

Also see comments on Hacker News.

Notes and references

[1] You might wonder why the DC voltage inside the power supply is so much higher than the line voltage. The DC voltage is approximately sqrt(2) times the AC voltage, since the diode charges the capacitor to the peak of the AC signal. Thus, the input of 100 to 240 volts AC is converted to a DC voltage of 145 to 345 volts internally. This isn't enough to be officially high voltage but I'll call it high voltage for convenience. According to standards, anything under 50 volts AC or 120 V dc is considered extra-low voltage and is considered safe under normal conditions. But I'll refer to the 5V output as low voltage for convenience.

[2] The Apple power supply uses a flyback design, where the transformer operates "backwards" from how you might expect. When a voltage pulse is sent into the transformer, the output diode blocks the output so there is no output - instead a magnetic field builds up. When the voltage input stops, the magnetic field collapses causing voltage output from the transformer. Flyback power supplies are very common for low-wattage power supplies.

[3] The primary board measures about 22.5mm by 20.0mm, while the secondary board is about 22.2mm by 20.2mm. [4] For more information on X and Y capacitors, see Kemet's presentation and Designing low leakage current power supplies.

[5] For clarity, some insulation was removed before taking the pictures in this article. The Y capacitor was covered with black heat shrink tubing, there was tape around the side of the circuit, the fusible resistor was covered with black heat shrink tubing, and there was a black insulating cover over the USB connector.

[6] Snubber circuits can be used to reduce 60 Hz hum generated by the diode bridge in audio power supplies. A detailed reference on R-C snubbers for audio power supply diodes is Calculating Optimum Snubbers, and a sample design is An Audio Amplifier Power Supply Design.

[7] The power supply is controlled by the L6565 quasi-resonant SMPS (switched-mode power supply) controller chip (datasheet). (To be sure, the chip could be something else, but the circuit exactly matches the L6565 and no other chip I examined.)

To improve efficiency and reduce interference, the chip uses a technique known as quasi-resonance, which was first developed in the 1980s. The output circuit is designed so when the power is switched off, the transformer voltage will oscillate. When the voltage hits zero, the transistor switches back on. This is known as Zero Voltage Switching because the transistor is switched when there is essentially no voltage across it, minimizing wasted power and interference during switching. The circuit remains on for a variable time (depending on the power required), and then switches back off, repeating the process. (See Exploring quasi-resonant converters for power supplies for more information.)

One interesting consequence of quasi-resonance is the switching frequency varies depending on the load (with 70kHz as a typical value). Early power supplies such as the Apple II power supply used simple variable-frequency circuits to regulate the power. But in the 1980s, these circuits were replaced by controller ICs that switched at a fixed frequency, but varied the width of the pulses (known as PWM). Now, advanced controller ICs have gone back to variable frequency controls. But in addition, super-cheap knockoff power supplies use variable frequency circuits almost identical to the Apple II. So both high-end and low-end chargers are now back to variable frequency.

It took me a long time to realize that the "FLEX01" marking on the controller IC indicates Flextronics, and the X on the chip was from the Flextronics logo: . I assume the chip has these markings because it is being manufactured for Flextronics. The "EB936" marking on the chip could be Flextronics' own part number, or a date code.

[8] I thought Flextronics was just an electronics assembler and I was surprised to learn that Flextronics does a lot of innovative development and has literally thousands of patents. I think Flextronics should get more credit for their designs. (Note that Flextronics is a different company than Foxconn, which manufactures iPads and iPhones and has the controversy over working conditions).

The picture above is from Flextronics Patent 7,978,489: Integrated Power Converters describes an adapter that looks just like the iPhone charger. The patent itself is a grab bag of 63 assorted claims (spring contacts, EMI shields, thermal potting material), most of which are not actually relevant to the iPhone charger.

[9] Flextronics Patent 7,924,578: Two Terminals Quasi Resonant Tank Circuit describes the resonance circuit used in the iPhone charger, which is shown in the following diagram. Transistor Q2 drives the transformer. Transistor Q1 is the clamp transistor, which directs the voltage spike from the transformer into resonance capacitor C13. The innovative part of this circuit is that Q1 doesn't need special drive circuitry like other active clamp circuits; it is self-powered via the capacitors and diodes. Most charger power supplies, by contrast, use a simple resistor-capacitor-diode clamp which dissipates the energy in the resistor.[18]

Later Flextronics patents extend the resonance circuit with even more diodes and capacitors: see patents 7,830,676, 7,760,519, and 8,000,112

[10] Apple indicates the charger type through a proprietary technique of resistances on the USB D+ and D- pins. For details on USB charging protocols, see my earlier references.

[11] One puzzling feature of the Apple charger is the second feedback circuit monitoring the temperature and output voltage. This circuit on the secondary board consists of a thermistor, a second 431 regulator, and a few other components to monitor the temperature and voltage. The output is connected through a second optocoupler to more circuitry on other side of the secondary board. Two transistors are wired in a SCR-like crowbar latch that will short out the auxiliary power and also shut down the controller IC. This circuit seems excessively complex for this task, especially since many controller ICs have this functionality built in. I could be misunderstanding this circuit, because it seems that Apple unnecessarily took up space and expensive components (maybe 25 cents worth) implementing this feature in such a complex way.

[12] Note the mysterious "For use with information technology equipment" on the outside of the charger. This indicates that the charger is covered by the safety standard UL 60950-1, which specifies the various isolation distances required. For a brief overview of isolation distances, see i-Spec Circuit Separation and some of my earlier references.

[13] Some notes on the components used: On the primary board, the JS4 package is two diodes in a single package. The input diodes labeled 1JLGE9 are 1J 600V 1A diodes. The switching transistors are 1HNK60 600V 1A N-channel MOSFETs. The values of many of the resistors and capacitors are indicated through standard SMD three-digit markings (two digits and then a power of ten, giving ohms or picofarads).

On the secondary board, the "330 j90" capacitor is a Sanyo POSCAP tantalum polymer 300mF 6.3V capacitor (j indicates 6.3V and 90 is a date code). 1R5 indicates a 1.5uH inductor. GB9 is a AS431I low cathode current adjustable precision shunt regulator, and 431 is a regular TL431 regulator. SCD34 is a 3A 40V schottky rectifier. YCW is an unidentified NPN transistor and GYW is an unidentified PNP transistor. The Y capacitor labeled "MC B221K X1 400V Y1 250V" is a 220pF Y capacitor. The "107A" capacitor is a 100 µF 10V tantalum capacitor (A indicates 10V). The optocouplers are PS2801-1. (All these component identifications should be considered tentative, along with the schematic.)

[14] In order to get a rough idea of how much the components in the charger cost, I looked up the prices of some components on octopart.com. These prices are the best prices I could find after a brief search, in quantities of 1000, attempting to match the parts accurately. I have to assume Apple's prices are considerably better than these prices.

Component	Price
0402 SMD resistor	$0.002
0805 SMD capacitor	$0.007
SMD transistor	$0.02
fusible resistor	$0.03
1A 600V (1J) diode	$0.06
thermistor	$0.07
Y capacitor	$0.08
3.3uF 400V electrolytic capacitor	$0.10
TL431	$0.10
1.5uH inductor	$0.12
SCD 34 diode	$0.13
2801 optocoupler	$0.16
1HNK60 transistor	$0.22
USB jack	$0.33
100uF tantalum capacitor	$0.34
L6565 IC	$0.55
330uF tantalum polymer capacitor (Sanyo POSCAP)	$0.98
flyback transformer	$1.36

A few notes. Flyback transformers are generally custom and prices are all over the place, so I don't have much confidence in that price. I think the POSCAP price is high because I was looking for the exact manufacturer, but tantalum capacitors are fairly expensive in general. It's surprising how cheap SMD resistors and capacitors are: a fraction of a penny.

[15] Apple's safety recall of chargers was announced in 2008. Blog reports showed that the prongs on the charger were attached only by 1/8" of metal and some glue. Apple Recalls iPhone 3G Power Adapters in Wired provides more details.

[16] Low-quality chargers interfere with touchscreens, and this is described in detail in Noise Wars: Projected capacitance strikes back. (Customers also report touchscreen problems from cheap chargers on Amazon and other sites.)

[17] There are many industry designs for USB AC/DC converters in the 5W range. Sample designs are available from iWatt, Fairchild, STMicroelectronics, Texas Instruments, ON Semiconductor, and Maxim.

[18] When a diode or transistor switches, it creates a voltage spike, which can be controlled by a snubber or clamp circuit. For a lot of information on snubbers and clamps, see Passive Lossless Snubbers for High Frequency PWM Conversion and Switchmode Power Supply Reference Manual.

Tiny, cheap, and dangerous: Inside a (fake) iPhone charger

Thoughts on the death of Ma Ailun

According to reports, a woman in China was tragically electrocuted using her iPhone while it was charging. This seems technically plausible to me if she were using a cheap or counterfeit charger like I describe below. There's 340 volts DC inside the charger, which is enough to kill. In a cheap charger, there can be less than a millimeter separating this voltage from the output, a fraction of the recommended safe distance. These charger sometimes short out (picture), which could send lethal voltage through the USB cable. If the user closes the circuit by standing on a damp floor or touching a grounded metal surface, electrocution is a possibility. If moisture condenses in the charger (e.g. in a humid bathroom), shorting becomes even more likely. Genuine Apple chargers (and other brand-name chargers) follow strict safety regulations (teardown) so I would be surprised if this electrocution happened with a name-brand charger. Since counterfeits look just like real chargers, I'll wait for an expert to determine if a genuine Apple charger was involved or not. I've read suggestions that the house wiring might have been to blame, but since chargers are typically ungrounded I don't see how faulty house wiring would play a role. I should point out that since there are few details at this point, this is all speculation; it's possible the phone and charger weren't involved at all.

I recently wrote a popular article on the history of computer power supplies, which led to speculation on what's inside those amazingly small one-inch cube USB chargers sold by Apple, Samsung, RIM, and other companies. In the interest of science, I bought a cheap no-name cube charger off eBay for $2.79, and took it apart. It's amazing that manufacturers can build and sell a complex charger for just a few dollars. It looks a lot like a genuine Apple charger and cost a lot less. But looking inside, I found that important safety corners were cut, which could lead to a 340 volt surprise. In addition, the interference from a cheap charger like this can cause touchscreen malfunctions. Thus, I recommend spending a few dollars more to get a brand-name charger.

The no-name charger I bought is just over an inch in length, excluding the Eurpopean-style plug. The charger is labeled "FOR iphone4. Input 110-240V 50/60Hz Output 5.2V 1000mA, Made in China." There are no other markings (manufacturer, serial number, or safety certifications). I opened up the charger with a bit of Dremel-ing. One surprise is how much empty space is inside for a charger that's so small. Apparently the charger circuit is designed for a smaller US-style plug, and the extra space with a European plug is unused. Since the charger accepts 110 to 240V input, the same circuit can be used worldwide.[1]

The power supply itself is slightly smaller than one cubic inch. The picture below shows the main components. On the left is the standard USB connector. Note how much room it takes up - it's not surprising devices are moving to micro-USB connectors. The flyback transformer is the black and yellow component; it converts the high-voltage input to the 5V output. In front of it is the switching transistor. Next to the transistor is a component that looks like a resistor but is an inductor filtering the AC input. On the underside, you can see the capacitors that filter the output and input.

The power supply is a simple flyback switching power supply. The input AC is converted to high-voltage DC by a diode, chopped into pulses by the power transistor and fed into the transformer. The transformer output is converted to low voltage DC by a diode, filtered, and fed out through the USB port. A feedback circuit regulates the output voltage at 5 volts by controlling the chopping frequency.

Detailed explanation

In more detail, the power supply is a self-oscillating flyback converter, also known as a ringing choke converter.[2] Unlike most flyback power supplies, which use a IC to control the oscillation, this power supply oscillates on its own through a feedback winding on the transformer. This reduces the component count and minimizes cost. A 75 cent controller IC[3] would be a huge expense for a $2.79 power supply, so they used a minimal circuit instead.

The above picture shows the circuit components; the red boxes and italics indicate components on the other side. (Click for a larger picture.) Note that most of the components are tiny surface-mounted devices (SMD) and are dwarfed by the capacitors. The green wires supply the input AC, which is filtered through the inductor. The high-voltage 1N4007 (M7) input diode and the 4.7µF input capacitor convert the AC input to 340 volts DC.[4] The MJE13003 power transistor switches the power to the transformer at a variable frequency (probably about 50kHz). The transformer has two primary windings (the power winding and a feedback winding), and a secondary winding. (The transformer and inductor are also known as "the magnetics".)

On the secondary (output) side, the high-speed SS14 Schottky diode rectifies the transformer output to DC, which is filtered by the 470µF output capacitor before providing the desired 5V to the USB port. The two center pins of the USB port (the data pins) are shorted together with a blob of solder, as will be explained below.

A simple feedback circuit regulates the voltage. The output voltage is divided in half by a resistor divider and compared against 2.5V by the common 431 voltage reference device. The feedback is passed to the primary side through the 817B optoisolator. On the primary side, the feedback oscillation from the feedback transformer winding and the voltage feedback from the optoisolator are combined in the 2SC2411 control transistor. This transistor then drives the power transistor, closing the loop. (A very similar power supply circuit is described by Delta.[5])

Isolation and safety

For safety reasons, AC power supplies must maintain strict isolation between the AC input and the output. The circuit is divided into a primary side - connected to AC, and a secondary side - connected to the output. There can be no direct electrical connection between the two sides, or else someone touching the output could get a shock. Any connection between the two sides must go through a transformer or optoisolator. In this power supply, the transformer provides isolation of the main power, and the optoisolator provides isolation of the feedback of the secondary voltage.

If you look at the picture, you can see the isolation boundary indicated as a white line on the circuit board crossing the circuit board roughly horizontally, with the primary side on top and the secondary side below. (This line is printed on the board; I didn't add it to the picture.) The circles on the line that look like holes are, in fact, holes. These provide additional isolation between the two sides.

The UL has complex safety specifications on how much distance (known as "creepage" and "clearance") there must be between the primary and secondary sides to prevent a shock hazard.[6] The rules are complicated and I'm no expert, but I think at least 3 or 4 mm is required. On this power supply, the average distance is about 1 millimeter. The clearance distance below R8 on the right is somewhat less than one millimeter (notice that white line crosses the PCB trace to the left of R8).

I wondered how this power supply could have met the UL standards with clearance less than 1 mm. Looking at the charger case more closely, I noticed that it didn't list any safety certifications, or even a manufacturer. I suddenly realized that purchasing the cheapest possible charger on eBay from an unknown manufacturer in China could actually be a safety hazard. Note that this sub-millimeter gap is all that's protecting you and your phone from potentially-lethal 340 volts. I also took the transformer apart and found only single layers of insulating tape between the windings, rather than the double layers required by the UL. After looking inside this charger, my recommendation is to spend a bit more on a charger, and get one that has UL approval and a name-brand manufacturer.

Another issue with super-cheap chargers is they produce poor-quality electrical output with a lot of noise that can interfere with the operation of your phone. Low-cost ringing choke adapters are known to cause touchscreen malfunctions because the screen picks up the electrical interference.[7] In noticed several cost-saving design decisions that will increase interference. The charger uses a single diode to rectify the input, rather than a four-diode bridge, which will produce more interference. The input and output filtering are minimal compared to other designs.[8][9] There's also no fuse on the AC input, which is a bit worrying.

USB charging protocols

You might think USB chargers are interchangeable and plugging a USB device into a charger is straightforward, but it turns out that it's a mess of multiple USB charging standards,[10][11][12] devices that break the rules,[13] and proprietary protocols used by Sony and Apple.[14][15][16] The underlying problem is that a standard USB port can provide up to 500mA, so how do chargers provide 1A or more for faster charging? To oversimplify, a charger indicates that it's a charger by shorting together the two middle USB pins (D+ and D-). Proprietary chargers instead connect different resistances to the D+ and D- pins to indicate how much current they can provide. Note that there are a few unused resistor spots (R2, R3, R8, R10) connected to the USB port on the circuit above; the manufacturer can add the appropriate resistors to emulate other types of chargers.

Advances in AC power adapters

Early power adapters were just an AC transformer producing low-voltage AC, or add diodes to produce DC. In the mid 1990s, switching power supplies became more popular, because they are more compact and more efficient.[17] However, the growing popularity of AC adapters along with their tendency to waste a few watts when left plugged in ended up costing the United States billions of dollars in wasted electricity every year.[3] New Energy Star standards[18] encouraged "green" designs that use milliwatts rather than watts of power when idle. These efficient controllers can stop switching when unloaded, with intermittent bursts to get just enough power to keep running.[19] One power supply design actually achieves zero standby power usage, by running off a "supercapacitor" while idle.[20]

The semiconductor industry continues to improve switching power supplies through advances in controller ICs and switching transistors. For simple power supplies, some manufacturers combine the controller IC and the switching transistor into a single component with only 4 or 5 pins. Another technology for charger control is CC/CV, which provides constant current until the battery is charged and then constant voltage to keep it charged. To minimize electromagnetic interference (EMI), some controllers continuously vary the switching frequency to spread out the interference across a "spread spectrum".[21] Controllers can also include safety features such as overload protection, under voltage lockout, and thermal shutdown to protect against overheating,

Conclusions

Stay away from super-cheap AC adapters built by mystery manufacturers. Spend the extra few dollars to get a brand-name AC adapter. It will be safer, produce less interference, and your device's touchscreen will perform better.

Notes and references

[1] Switching power supplies often take a "universal" input of 110V to 240V at 50/60 Hz, which allows the same supply to conveniently work on worldwide voltages. Because a switching power supply chops up the input into variable slices, the output voltage can be independent of the input voltage over a wide range. (This also makes switching power supplies more resistant to power brownouts.) Of course, designing the circuit to handle a wide voltage range is harder, especially for power supplies that must be very efficient across a wide range of voltages. To simplify the design of early PC power supplies, they often used a switch to select 120V or 240V input. Through a very clever doubler circuit, this switch converted the input bridge into a voltage doubler for 120V input, so the rest of the circuit could be designed for a single voltage. Modern power supplies, however, are usually designed to handle the whole voltage range which both avoids the expense of an extra switch, and ensures that users don't put the switch in the wrong position and destroy something.
[2] A comic-style explanation of flyback converters and ringing choke converters is at TDK Power Electronics World.
[3] The cost of idle AC adapters is given as $3.5 billion to $5.4 billion for 45 TWhour of wasted electricity in the US. The article discusses solutions, and mentions that an efficient controller IC costs 75 cents. (Note that this is a huge cost for an adapter that sells for $2.79.) Dry up avoidable leakage, EDN, Feb 1999, p96-99
[4] The DC voltage is approximately sqrt(2) times the AC voltage, since the diode charges the capacitor to the peak of the AC signal. Thus, a 240V AC input will result in approximately 340V DC inside the power supply. Because of this usage of the AC peak, only a small portion of the AC input is used, resulting in inefficiency, known as a bad power factor. For larger power supplies, power factor correction (PFC) is used to improve the power factor.
[5] The schematic of a ringing choke converter similar to the one I examined is in Analysis and Design of Self-Oscillating Flyback Converter, Delta Products Corporation.
[6] Safety Considerations in Power Supply Design, Texas Instruments, provides a detailed discussion of safety requirements for power supplies. Also see Calculating Creepage and Clearance Early Avoids Design Problems Later, Compliance Engineering. An online calculator for the UL 60950-1 clearance and creepage requirements is www.creepage.com.
[7] Cypress Semiconductor compared flyback converters and ringing choke converters; and ringing choke converters are significantly cheaper but very noisy electrically. Poor touchscreen performance is blamed on noisy aftermarket low cost chargers. Noise Wars: Projected Capacitance Strikes Back, Cypress Semiconductor, Sept 2011.
[8] Power Integrations has multiple designs and schematics for Cell Phone Charger and Adapter Applications.
[9] Power Integrations has a detailed design for a 5W cube charger based on the LinkSwitch-II controller. This circuit fits two circuit boards into the inch cube, which is pretty impressive. 5 W Cube Charger Using LinkSwitch-II and PR14 Core
[10] The official USB charging specification is Battery Charging v1.2 Spec.
[11] The updated USB standards that allow high-current charging are described in USB battery-charger designs meet new industry standards, EDN, Feb, 2008. In summary, a charger shorts D+ and D- to indicate that it can provide 1A, compared to a regular USB port that provides up to 500mA.
[12] An up-to-date discussion of USB charging is given in The Basics of USB Battery Charging: a Survival Guide, Maxim Application Note 4803, Dec 2010. This discusses the USB Battery Charging Specification, and how USB detects different power sources: SDP (standard computer USB ports), CDP (high-current computer USB ports with up to 1.5A), and DCP (power adapters).
[13] A guide to USB power that discusses the difference between what the USB standard says and what is actually done is "What your mom didn't tell you about USB" in Charging Batteries Using USB Power, Maxim Application Note 3241, June 2004. In particular, USB ports do not limit current to 500mA, and might provide up to 2A. Also, USB ports generally provide power even without any enumeration.
[14] Ladyada reverse-engineered Apple chargers to determine how the voltages on the USB D+ and D- pins controls the charging current. Minty Boost: The mysteries of Apple device charging. Also of note is the picture of the internals of a official Apple iPhone 3Gs charger, which is somewhat more complex than the charger I disassembled, using two circuit boards.
[15] Maxim MAX14578E/MAX14578AE USB Battery Charger Detectors. This datasheet has details on the proprietary D+/D- protocols used by Apple and Sony chargers, as well as standard USB protocols.
[16] Developing cost-effective USB-based battery chargers for automotive applications, EE Times, Feb 2011. This article describes the different types of USB charging ports and how to implement them. It mentions that Blackberry uses the USB Battery Charging 1.0 spec, Motoroloa uses the 1.1 spec, phones in China use the YDT-1591 spec, and Apple uses a proprietary protocol.
[17] Power supply technologies, Journal of Electronic Engineering, 1995, p41 reported AC adapters and chargers for portable computers, cameras, and video equipment are moving from "dropper" transformers to switching supplies.
[18] Energy Star added star ratings in 2010 for no-load power consumption, randing from 0 stars for chargers that use more than .5W idle power, to 5 stars for chargers that use under 30mW. The article also discusses constant-current/constant-voltage (CC/CV) chargers that provide constant current while charging the battery and then constant voltage to keep the battery charged. Meeting 30 mW standby in mobile phone chargers.
[19] A green power AC adapter design driven by power requirements, EDN Power Technology, Aug 2004, p25-26. This article describes how to build a highly-efficient AC adapter using "burst mode" during low load, and minimizing EMI interference through spread spectrum techniques.
[20] Watt Saver for a Cell Phone AC Adaptor describes an AC adapter reference design that uses a 1 Farad super capacitor to power the controller without any AC usage when there is no load.
[21] The Fairchild FAN103 PWM controller is designed for charger applications. It uses frequency hopping to spread out the EMI spectrum - the switching frequency varies betwen 46kHz and 54kHz. When there's no load, the controller switches into "Deep Green" mode, dropping the switching frequency to 370Hz, getting just enough power to keep running.