Enter the iphone 7 portable charger ebay you see below Sorry, we just need to make sure you’re not a robot. The no-name charger I bought is just over an inch in length, excluding the Eurpopean-style plug. The charger is labeled “FOR iphone4.
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. 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. In front of it is the switching transistor. 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.
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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. 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. The green wires supply the input AC, which is filtered through the inductor. 7µF input capacitor convert the AC input to 340 volts DC.
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. 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. 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. 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. 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 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. 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. Note that this sub-millimeter gap is all that’s protecting you and your phone from potentially-lethal 340 volts. 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. In noticed several cost-saving design decisions that will increase interference.