My Book power supply, part two

21.09.2010 12:40

Some time ago I was writing about a MyBook power supply that went bad. It turned out that bad soldering wasn't the culprit there because the external hard drive kept causing problems. It would turn off at random times, sometime restarting immediately and sometimes staying off for good. For the sake of posterity I'm posting a description of my second (failed) attempt at repairing it.

I took the power brick apart again and started poking around. Indeed I could see with an oscilloscope that the switcher would sometimes turn off and the output voltage would fall to zero. Sometimes it started again after a few moments and sometimes it would stay dead for several minutes. Shaking and stressing the circuit board didn't reproduce the problem and neither did heating and cooling. So I considered it unlikely that it is a mechanical defect.

The topology of the circuit is consistent with a flyback converter. The controller IC is in a 6-pin SMD package marked 63813. Although I can't find the specific chip, the circuit around it looks very much like the typical application for the SG6858, which is also available in 6-pin SSOT-6 (identification of the IC was wrong in my previous post). For reference, here's my (incomplete) sketch of the circuit:

MyBook power supply partial schematic

I started on the secondary side since its easier and safer to measure, but I soon discovered that everything seemed to be in order. Most importantly, the current through the feedback optocoupler decreased after the output voltage already dropped when the bug occurred. This pointed to a problem on the primary side.

On the primary side I first focused on the Vdd supply for the controller IC (pin 3). This voltage is supplied by 1 MΩ resistors at start-up and later by an auxiliary winding of the transformer. When the PSU wasn't working the supply voltage went between 10 and 15 V in a saw-tooth. This is consistent with the hysteresis of the under-voltage protection of the controller IC, which means the IC was turning on, depleting the charge in the 10 μF capacitor and turning off until the capacitor again got enough charge through the 1 MΩ resistors. So the signal must have got blocked somewhere between the control circuit in the IC and the gate of the MOSFET.

Both the 100 Ω resistor and the diode D9 measured fine and didn't fail when heated, cooled or mechanically stressed. However it did appear that doing the same to the 1 kΩ resistor in the current-sense network (which was nearby) increased the likelihood of the bug.

At this point however something must have failed completely. After I replaced the 1 kΩ resistor the controller chip went dead for good and I can get no response from the circuit at all. It's possible I shorted something while replacing the tiny SMD resistor although I find that unlikely - I closely inspected everything after the fact and could see no solder bridges.

So, this circuit is going into the spare-parts bin and the external disk will get a new generic power supply from the nearest shop.

Posted by Tomaž | Categories: Analog

Comments

Same thing happened to me, just ordered external box from dealextreme for about 20$.

Sse ena zrtev znanosti :-) Malo je verjetna okvara 1 kOhm upora. Jaz bi prvo otkacil kondenzator za njim ker tvoje vezje je nestabilno osciliralo. Bodi pozoren pri 220V AC/DC!

http://www.onsemi.com/pub_link/Collateral/HB206-D.PDF All you always wanted to know about PSU but didn't dare to ask :-)

MMM, I don't think I ever had a question I didn't dare to ask. However this power supply handbook you found seems like a useful reference. I'll keep it handy for any future projects.

Posted by Tomaž

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