UCC28950: SLUSA168 REMOVING SYNCHRONOUS RECTIFICATION

Hi Samrat

First of all there should be no problem at all in replacing the SRs with diodes - the topology 'doesn't care' how the rectification is achieved.

The simplest way to replace the SRs with diodes is to simply remove the SR symbol and place the diode inthe now vacant spot.

Do make sure that the polarity of the diode is the same as the polarity of the SR you have just removed - this might be the cause of your problem.

You could experiment by simply removing the SR gate drive and shorting the SR gate to its source - this will make the SR behave like a diode.

RCD snubbers are unlikely to be the cause of your problem so I'd tend to leave them alone until you check the above.

Please let me know how you get on

Colin

hello Colin
are u sure its gate to source shorting or is it drain-gate shorting to make mosfet behave like diode?
and i got results. polarity of the diode was the problem.
i am attaching the pic of secondary side. can u just tell me the dot convention of that high frequency transformer?
thanks
samrat

Hi Samrat

Shorting the Gate to the Source of the MOSFET turns the MOSFET channel off. The MOSFET body diode from Source of Drain will still work.

If you short the Gate to Drain then - As Vds increases to the MOSFET threshold voltage then the MOSFET channel will turn on - the MOSFET will clamp the source at or near the threshold voltage. In the other direction, the body diode will operate normally.

I didn't receive the picture  - Can you attach it using the 'Insert file' ( paper clip symbol ) and re-post it .

regards

Colin

hello Colin
thanks for the guidance
samrat