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Apple 5W Adapter Knock-offs: The Colorful A1265 Tear-Down

Home Improvement

How much of this noise is directly attributable to one capacitor of questionable quality in the output filter? We could always supplement the electrolytic capacitor with multi-layer chip capacitors sporting extremely low ESR/ESL, and then measure the output noise again.

Here, I added both 10µF and 1µF capacitors to the back of the electrolytic’s pins.


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Modified Adapter Noise

Adding 11µF worth of ceramic capacitors to the adapter nearly eliminates negative ringing and cuts the positive switching transient spikes in half, leaving behind 630mVPP worth of combined ripple and noise (down from the horrendous 1.2-1.4VPP). That's a substantial improvement, but still far too noisy for comfort. Stacking capacitors can only do so much to suppress transients from fly-back current pulses.

We’d likely see Π filters (capacitor-inductor-capacitor) and perhaps even output common-mode chokes in adapters with smoother output, though I do not expect to find many (if any) in my no-name collection.


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Hitting The Bottom Of The Scale

According to the adapter’s label, its operating range begins at 100VAC where it can provide about 420mA before output voltage collapses well below 5V. Under very light load (as in 20mA output current), though, the oscillator continues operating all the way down to input voltages as low as 20VAC.


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The Upper Limit

At the other end of the scale, raising the input to 240VAC yields about 770mA before output voltage collapses again. Given almost double the output current at just over double the AC input voltage, we're looking at a near-linear relationship between input voltage and sustainable output current. Since AC input voltage directly affects the primary coil's current rise rate and the associated positive feedback to the transistor’s base driving the oscillator, this dependency was to be expected.


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Shorted Behavior

Using my ammeter to short the adapter’s output, I get a short-circuit current of 630mA as the oscillator pulls 757mW at 115V.

While continuous short circuit current may not be ideal, you can’t expect a simple oscillator circuit to do anything about it. On the plus side, output short-circuit is harmless in flyback converters, so long as the transformer doesn’t saturate. In an oscillator circuit like this one, the next switching cycle simply cannot begin until the transformer’s magnetic flux resets.

For all of its flaws in terms of output noise and regulation, this circuit may turn out to be surprisingly resilient.


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Coming Out Of Short

What happens coming out of short-circuit? It takes about 50ms for the adapter’s output voltage to creep back up to a steady 5.4V. Along the way, you can see ripples corresponding to the AC peaks every 8.33ms, courtesy of the previously mentioned relationship between input voltage and the oscillator’s operating current, which ultimately dictates output power.

Aside from the slow recovery time and horrible ~1VPP ripple+noise, there is nothing wrong or otherwise alarming here.


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Transient Response

Since this adapter is unable to supply enough output current at 115V input to do the 250mA base + ~500mA transient (in the form of a 10Ω resistor) I originally intended, I went with 100mA base + 500mA instead.

Between the 470µF output filter capacitor and 10Ω resistor, we get a 4.7ms RC time constant (τ) with the capacitor discharge settling after 5τ, as expected. Under this 600mA combined load, the oscillator only manages to keep the output at about 4.4V. Once the resistor is removed, the output voltage returns to the same pattern as before the transient was applied with no other anomalies.


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Safety Isolation Testing, Part I

With all of the performance testing done, we can now proceed to the potentially destructive safety tests. To start, we'll try figuring out which of those clearance issues between traces causes the first failure. Or maybe it'll be the non-Y EMI suppression capacitor?

Using my variac and high-voltage transformer to apply a test voltage across the adapter’s power outlet prongs and USB connector, I progressively dialed up the angry pixies and got my first result at 1kVAC between the USB connector shield and nearby primary-side optoisolator trace. That’s 480VAC short of what UL requires for non-isolated appliances.


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First Safety Test Aftermath

Those few seconds of continuous arcing were enough to burn the trace's solder mask off and sputter some burnt material in between. While this did not appear to lower the arcing voltage in my limited testing, it is still cause for concern.


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Safety Isolation, Part II

That USB shield got in the way of finding out how good the transformer’s insulation is, so we'll repeat the test after knocking the USB connector out of the way. What fails next?


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Daniel Sauvageau is a Contributing Writer for Tom's Hardware US. He’s known for his feature tear-downs of components and peripherals.