[烛光生存狂]-空姐电击事件-【翻译国外博文】危险的山寨充电器

rollei120

国外DX Ken Shirriff有篇关于山寨充电器的分析博客，这里翻译给大家，也做为解释iphone电击事件的一个详细技术分析贴。原文在这里：http://www.righto.com/2012/03/inside-cheap-phone-charger-and-why-you.html

我之前还有篇他的译文：拆解苹果绿点，有兴趣可以去看看：http://www.shoudian.org/thread-292986-1-1.html

-------------------------------------------------------------------------------

马爱伦事件的几点思考

据报道，一位中国女性在使用充电着的iphone时，不幸被电击身亡。如果她使用的是那种廉价的山寨充电器，那么这就验证了我这篇文章所说理论观点。要知道，充电器里面有着340V的高压直流电，这么高的电压足以致人于死地。那种廉价的山寨充电器里，340V高压和输出端之间的隔离距离往往不到1mm，完全不符合安全标准里的推荐距离。这种充电器一旦发生短路，致命的高压电就会被传递到USB线上，如果使用者这时接触到了电路，且站在潮湿的地板，或者同时接触到连到大地的金属物，那么人就有可能被电死！充电器如果是工作在潮湿的环境里（例如潮湿的浴室），电路内非常潮湿，那么短路会变得异常容易。正版的苹果充电器（或者是其他有品牌的正规充电器），是会严格遵循安全法规的。如果这次的电击事件，使用的是正规品牌的充电器，那么这是难以置信的。我看看是否有专家能出来解释一下，这次事件是不是正品苹果充电器的原因，因为假冒的充电器在外观上确实和真的没什么区别。我也听到过一些观点，认为房子的布线才是真凶，但这种充电器是典型的不接地设备，我看不到房间布线在这次事件中能起到什么作用。当然了，这些观点都是推测，也可能手机和充电器都和这次事件无关。

注：这篇博客是作者2012年3月写的，上面的话是作者看到新闻报道后才加的。有些网站在引用他的这篇文章时，都误认为作者是在知道电击事件后才写的。

近来我写了篇科普的文章《电脑电源的历史》，导致了我对苹果/三星/黑莓和其他公司销售的迷你充电器的兴趣，在这些一寸见方的USB充电器里面到底是怎样设计的？也是出于科学上的兴趣，我在eBay上用2.79美元购买了一个廉价的无牌迷你充电器，并拆解开来。一个复杂的充电器，销售价只是几美元？外观上很像苹果的正版充电器，但价格便宜许多。但拆开来看里面，最重要的安全设置，被完全省掉了，这会存在被340V电击的隐患。另外，山寨充电器里产生的干扰，会导致手机的触摸屏失灵。我建议还是多花些钱，买个有品牌的正规充电器。

(170K)

(224K)

(264K)

(201K)

Tuesday, March 13, 2012

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.

A one-inch USB charger designed for the iphone4

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]

Inside a USB phone charger

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.

Internals of a USB phone charger

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 circuit board inside a tiny USB charger

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.

新用户注册返回首页
首页上一页下一页尾页页次1/1 转到第页