424V on Bulk Capacitor Problem

4 posts in this topic


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

Share this post

Link to post
Share on other sites

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.

Share this post

Link to post
Share on other sites

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?


Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now