19 Apr
2012
19 Apr
'12
2:53 p.m.
The output
side cotnains the output rectifiers, smoothing capacitors,=20
crowbar, ay aditional regualtors, and the voltage feedback circuit. The=
=20
chopper drive cirucit could be on either side of
the barrier, depenedin=
g=20
on the design of the PSU.
Tony, I'm repairing switch mode PSUs for at least 20 years now :-) from a couple of posts here, and I have found it ebtter
to satart from
the basics.
> The three
wire cable from left bottom to upper rigt is going to the
> transormer behind the sheet metal on the top pcb, it looks to me like
> the drains from 2 FETs (on the heat sink) and 330V ... (don't know t=
his
=20
My guess is that this is the centre-tapped primary of the chopper=20
transformer and that you have a push-pull chopper circuit (not unusual)=
.=20
Yes, think this too. from that it looks like the 'isolation barrier'
is really the division
between the 2 PCBs.
There is no SMPS controller in there, that Unit is
build from diskrete
Wellm there is, it;s just not a single chip...
There is something to drive the chpper transitors and vry the drive based
on the output voltage, but is;ts discrete transsitors and Op-amps. And
that makes life difficult. You cna use a SMPSU control IC( let's say an
SG3524) in essentiually one way. But you can use trnasistors and op-amps
in many ways, so figuring out what is going on is harder. But you know this.
I would still try to work out just how the resitor and its FET are used.
If necessary, trace out the circuit from the mains input through the
rectifier to the capacitros and choppers. Then trace out the supply to
the control circuitry (e.g. the power pins on that 358). Hopefully
something will make sense.
-tony