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Reverse engineering: Failed crappy Chinese phone charger

Backstory

One day a loud snap sounded from the breaker panel and everything went dark. A 16-amp breaker was found tripped. Everything was dark as well, so the upstream 3x25-amp breaker was also tripped.

The first breaker was reset. After the upstream one was reset, both immediately tripped again. A spark was seen from within the breaker.

After resetting them yet again they stayed up. Whatever was the fault, it was now burned away.

A tense hour ensued, with isolating which sockets belonged to the tripped breaker and going socket by socket, device by device, and eliminating all that worked well. It is generally a bad idea to have an undiagnosed failure on a power circuit. Such one can easily wait in ambush and then become an electrical fire, if of the suitable kind.

The culprit was found - a recently obtained extra-cheap Chinese USB phone charger.

Device and damage

Reverse engineering

The schematics was extracted from the board. In some parts the markings were impossible to read; on one resistor (R2) because of missing coating on the cap (this one was measured to be 470 ohms, unmarked in the schematics), on the transistors (Q1 reads "13(something)", Q2 reads "S90(something)") because of missing parts of their faces, on the diodes (D1 reads "1N5(something)", D2 reads "1N400(something)") due to their unfavorable orientations and remains of black deposit.


Schematics, raw

Schematics, with values

Schematics, visible damage

Damage current path

The device is based on an Armstrong oscillator, or a self-exciting flyback topology. It is a very simple one-transistor circuit, where the oscillation feedback is provided by a second winding on the transformer primary.

The voltage feedback is provided by an optocoupler, which opens a second transistor that grounds the base of the first transistor, inhibiting the oscillations when the coupler's internal LED is lit. The LED is powered from secondary side, through a resistor and a Zener diode that sets the output voltage.

The board around the D1 diode allows using of either a single diode or a diode bridge module.

The C4 capacitor bridges the primary and secondary side; its role is to suppress EMI. The spacing of the traces between the C4 capacitor legs, capacitively coupling the input and output, seems to be catastrophically low. The capacitor itself is a regular kind rated for 1kV; it should be a safer Class Y type.

The parts are apparently the cheapest available.

See more about dangers of cheap power supplies here.

For more teardowns see here. Measurements here.

Damage

The charger shown signs of internal overheating - slight localized deformation of the plastic casing.

The socket it was plugged in was bearing marks of venting of hot gas carrying aerosolized fine black particulates, likely copper oxide from vaporized copper and, to smaller degree, carbon from affected organic materials. Corresponding marks were found on the plug. The hot gases vented from within, through a thin gap between the snap-in plug and the outer housing.


Scorched socket

Scorched plug

The casing shows signs of localized internal overheating.


Deformed casing

Deformed casing

Deformed casing

The failed charger was carefully pried apart. The housing consists of an outer shell made of black thermoplastic material, and the snap-in plug insert made of white material. The plug pins are connected to the electronics board with wires. The wires were found melted off near the joints; one on both ends, the other on only one.


Deformed casing with removed plug

Deformed casing with removed plug

Deformed casing with removed plug

Deformed casing with removed plug

Deformed casing with removed plug

Deformed casing with removed plug

Casing and plug

Plug

Plug

The casing is deformed, bearing signs of gross internal overheating. The most severe marks correspond to the parts on the output side, namely the C2 capacitor (found bulged) and the Zener diode. This suggests sustained output overvoltage. Smaller thermal damage signs correspond to the position of the transformer and of the Q1 transistor.


Overheat position correspondence

Overheat position correspondence

Overheat position correspondence

The exposed electronics was covered with fine deposit of black soot-like material, presumably based on copper oxide from vaporized wires and carbon from pyrolyzed organic coatings.


Inside casing

Inside casing

Inside casing

Inside casing

Inside casing

Inside casing

Taken apart

The electronics board itself was undamaged, albeit covered with black deposit.


Burned board, top

Burned board, top

Burned board, top

Burned board, top

Burned board, top

Burned board, top

Burned board, top

Burned board, top

Burned board, side

Burned board, side

Burned board, side

Burned board, side

Burned board, side

The bottom side had some traces burned away in a few locations.


Burned board, bottom

Burned board, bottom

Burned board, bottom

Burned board, bottom

The board was cleaned with soap and a toothbrush. This removed most of the black deposit.


Cleaned board, top

Cleaned board, top

Cleaned board, bottom

Cleaned board, bottom

Cleaned board, bottom

Damage in detail

The damaged parts were photographed using a stereomicroscope, in 20x and 40x magnification.

The copper traces on the circuitboard, between the D1 diode, C1 capacitor and the  transformer primary up to the R1 resistor, were vaporized at several places. The return trace was missing some of the solder mask, likely due to localized thermal shock. The edges of the vaporized traces show signs of melting, a mark of an electric arc.


Burned traces, 20x

Burned traces, 20x

Burned traces, 20x

Burned traces, 20x

Burned trace, 40x

The R3 resistor (15 ohms) had one leg severed by melting off, one end cap partially melted and part of surface charred and showing marks of an electric arc.


Burned resistor R3, 20x

Burned resistor R3, 20x

Burned resistor R3, 20x

Burned resistor R3, 40x

Burned resistor R3, 40x

Both transistors in TO-92 casings were exploded, parts of their faces missing and ends of the middle and side pin exposed.


Exploded transistor Q1, 20x

Exploded transistor Q1, 20x

Exploded transistor Q1, 40x

Exploded transistor Q1, 40x

Exploded transistor Q1, 40x

Exploded transistor Q1, 40x

Exploded transistor Q2, 20x

Exploded transistor Q2, 20x

Exploded transistor Q2, 40x

Exploded transistor Q2, 40x

Exploded transistor Q2, 40x

The transformer coil shown signs of mild overheating.


Overheated transformer, 20x

Overheated transformer, 20x

Overheated transformer, 20x

Overheated transformer, 20x

The optocoupler itself was undamaged, is shown for reference of its marking.


Optocoupler detail, 20x

Other damage, seen on non-zoomed images in earlier section, includes melted off leg of the diode D2 and inflated capacitor C3 on the low-voltage output side.

In addition, measurements uncovered shorted diode D2, and severed primary transformer winding.

The optocoupler output is shorted. Its input failed open.

Essentially the entire primary loop was destroyed.

The survivors on the primary side were C1 (not subjected to overvoltage nor overcurrent, operating within its rating), R1 (too high value to sustain enough current for dissipation of meaningful amount of energy), R2 and C2 (also too low current path, due to the capacitor).

On secondary side the D1 Schottky diode appears to be a survivor, the capacitor C3 retains at least some capacitance despite visual damage signs (bulging top), and the Zener diode and its serial resistor (R4, not marked in the schematics) also seem intact.

Probable damage mechanism

Running the power supply on idle without load led to overly high voltage on the output. This led to overheating of the output capacitor and of the Zener diode and the optocoupler, causing the severe overheating marks on the casing. Other minor signs correspond to the main switching transistor and the transformer.

The thermal damage on the casing is highest against the optocoupler. The failure chain probably started there, either by opening of the LED or shorting of the output transistor. The oscillator then lost feedback saying that the output voltage shouldn't get any higher, and started feeding the output with as much power as it could. This overheated the transformer and the Q1 transistor, which probably shorted; the overheating caused spiking of the junction, where molten aluminium from the wirebonding pads forms a hard short through the thin base region. This formed a low-impedance path through D2, transformer primary coil, Q1, and low-value R3.

This was probably the cause of the first breaker trip.

The breaker reset provided more energy, an overwhelming amount. This now dissipated virtually simultaneously in the parasitic resistance of the traces between the D1 and transformer primary, which vaporized, maybe in the return trace from the Q2, which misses some solder mask, in one of the D2 leads which melted off, in the lead of the R3 resistor which melted off, in the resistor R3 whose body bears traces of a high temperature electric arc, and in the Q1 and Q2 transistor dies, which exploded and cracked the epoxy casings.

The unswitched current through the transformer primary, during the first and/or the second event, likely caused overvoltage on the unregulated output, which destroyed the C3 capacitor and the resulting internal pressure bulged its top. (The C3 may have been also damaged earlier, during the gradual overload that overheated the optocoupler.)


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