Arfa Daily escreveu:
"Dotinho" wrote in message
oups.com...
Its okay sam, but the PWM have some protections, and if output voltage
was more than 6.5V it disable every thing, with or without load..

the tranformer are malfunction, and i can't get another becouse don't
produce anymore, this power supply is very old, and i have to repair
it, decrease the output tension, and i dont know how..

there is no regulation, just exist a small feedbak for output
protecion, if voltage was superior to 6.5V it disable everything.
thats all..

The shematic ir very simple, it rectifier 220VAC from nets, to 315VDV
wih 4 diods and 2 big capacitors, than the tranformer is conected in
serie with MOSFET, pulsing that 315VDC to tranformer, and the there is
3 output, one at 5VDC with 18A and another 24V with 0.5A...

The problem is the tranformer, the mosfet is directly connected to PWM
with one wire, there is no regulation!!

How can i reduce the power changing the shema??

thanks


I would suggest that you are missing something somewhere. OK, so the MOSFET
is connected to the PWM with one wire, but I would be 99% sure that this is
where your trouble lies. Something somewhere is generating that PWM, and it
is likely doing it with the wrong mark space ratio. I would be very
surprised if a switcher is producing a 5v rail specced to 18 amps, without
some kind of basic feedback. Usually, a 5v rail in older kit, with that sort
of current capability, is supplying a bunch of 'straight' or 'LS' 74 series
logic. That stuff is critical of its supply voltage to around +/- 0.2v.

I can't actually think of a failure mode for the transformer, which would
cause it to produce excess output. In fact, SMPS transformers are incredibly
reliable across the board. Since they've become really popular over the last
25 - 30 years, I can only think of one item of equipment that had the
transformer fail regularly, and that was an early DB sat receiver. In that
case, the primary used to fail with shorted turns, which led to catastrophic
failure of the MOSFET and input fuse.

How is the PWM generated in the first place, and are you absolutely certain
that there is not an optoisolator hiding somewhere ? How does the
overvoltage shutdown circuit work ? Are you sure that it does not also serve
as the basic control loop, and only has the shutdown function as a last
resort if control is lost ?

Arfa


ya ya, i know that. perhaps i should send the shmatic? is that okay?

dotinho