Switching supply expertise?

From: Dave Dunfield <dave04a_at_dunfield.com>
Date: Sun May 30 08:50:48 2004


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.

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

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

>From what I can tell:

 - Mains power goes to full-wave bridge rectifier and filter caps to produce
   HV DC supply.

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

 - 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).


 (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?

dave04a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools: www.dunfield.com
com             Vintage computing equipment collector.
Received on Sun May 30 2004 - 08:50:48 BST

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