424V on Bulk Capacitor Problem

[Shahriar]

Hi

I am somehow familiar with SMPS troubleshooting;
But today I have encountered a strange problem

It is 48v, 150W SMPS with UC2844 Controller.
The first problem was the output was low (around 42V)
I Replaced the 47uF on Vcc pin of the UC2844 and everything got OK
But still a small Vizz Sound can be heard from the SMPS

The Most strange thing I see is the voltage on the Bulk Capacitor after diode bridge (220uF, 450V)
I expect a voltage around 320V on this Cap, but the voltage I read is 424V.
I have tested the RCD Snubber circuit but didn't see any failure.
And I think the Vizz problem is somehow related to this extra high voltage.

Have u ever seen such thing?

ThanX In advance
Shahriar

[Hero999]

Have you measured the mains voltage?

Does it have an active power factor correction module? Some PSUs have a boost converter for power factor correction, before the main filter capacitor.

[Shahriar]

Dear Hero

Yeahhhh, Thats right  ;D

it has a Power factor corection module that boost the voltage to 424V
for the main PSU.
It is made by MC33262P Controller.
I Had never seen such PFC circuitry before. That was nice experience!
This is the Topology I thought:


and this is what it really was:


and this is the PFC schematic:


The Vizz sound is coming out from PFC Transformer. and it is still out there!
I have replace the 1uF Cap (C5), but no difference  :-\
Do u have any Idea?

Shahriar

[Ryan Parks]

Brilliant