Switching supply expertise?

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Sun May 30 12:06:38 2004

>
> Hi,
>
> Rescued an Apple clone from the trash yesterday (Ci Base48) - appears to be
> all there, but power supply is dead. I've had a bit of experience with small
> DC transformerless switchers, but not with mains power, transformer based
> switchers like this.

I've worked on a few, including the infamous supply in the 11/44 and
lived to tell the tale...


>
> Fuse was blown - replace, powered up through a 60w light bulb, the lamp
> comes on (and stays on) at full intensity - clearly excessive draw.

OK, good first move.

>
> Board bears the name "Seasonic". I have no schematics.

I'd probalby be inclimed tp draw them out, but then I don't generally
worry about doing that. It can't be that complicated

>
> >From what I can tell:
>
> - Mains power goes to full-wave bridge rectifier and filter caps to produce
> HV DC supply.

Very normal

>
> - HV DC supply goes through fuse to a single heat-sinked power transistor
> (C2979) which drives the transformer primary.

Again, normal. I assume you know that's 2SC2979. Japanese transistors
often leave the '2S' off the part number.

>
> - Secondaries from transformer are recitied by big heat-sinked diodes, then
> filtered, and monitored by a circuit, which feeds back through an opto-
> isolator to the control circuit for the power transistor driving the primary
> (presumably this is the method of voltage regulation).

Again very normal. The opto-isolator feedback loop is the voltage
regulator loop as you guessed. There may be a TL431 or clone in the
secondary side, driving the LED of the optoisolator. The reason I mention
this is that the TL431 looks like a transistor (TO92 pacakge), but
doesn't test like one. This has 'thrown' a few people...

>
> Opservations:
>
> (Powering unit through isolation transformar, variac and 60w light bulb)
>
> - With fuse removed, HV DC supply apears to work fine. Filter caps charge
> up to in the high 100's Volts. - No light from lamp indicating very little
> drain (as expected).

> - With fuse installed, lamp lights at full intensity. No voltage measured
> across HV filter caps at all.
>
> - Power transistor does not appear shorted - Removed from circuit and using
> a diode test function, it shows two normal semiconductor junctions.
>
>
> Can anyone offer suggestions on where to look next? Are there any common failure
> modes of this type of power supply design?

The common failure modes are the mains rectifier diodes and the chopper
transsitor. It appears both are good in your case.. I would now check all
components arround the primary side of the chopper transformer (a shorted
capacitor can be 'interesting', and maybe check for shorts in the diodes
on the secondary side of the PSU.

Another trick is to disconnect the base of the chopper and connect it to
the emitter. This means the chopper should be turned hard off. Try again
with the series bulb. If it lights, then you have a short in the chopper
area (maybe the transistor is breaking down or something).

-tony
Received on Sun May 30 2004 - 12:06:38 BST