tracing out schematics

[Dwight K. Elvey]

>From: ard at p850ug1.demon.co.uk
>
>> 
>> Interestingly, there's no feedback between the low-voltage side of the
>> switching transformer and the circuitry surrounding that SOC603B 6-pin
>> IC, so maybe it isn't an opto-isolator at all. All the circuitry
>
>That does not suprise me. Since there's a pulse transformer in the 
>chopper base circuit, the controller IC is on the isolated side of the 
>PSU (the isolation is performed by the pulse transformer), so there's no 
>need for any optoisolator in the voltage feedback loop.
>
>One question. Have you found how the controller IC gets its power? Is 
>there a separate (maybe linear) PSU for this?
>

Hi
 I just thought I'd note that you don't need feedback if the
transformer is just used to drive a square wave and not
used as a flyback type. It just comes down to turns ratios
for the voltage. If there is a post regulator, having a
regulated switcher is not needed.
 Most PC supplies use a flyback type and regulate the 5V line
by the switcher to maximize efficiency but I doubt HP was
concerned about that.
Dwight