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Unusual Vdrain waveform

Posted by: juinbeh on Fri, 07/31/2009


I'm working on a 200W power supply with the TOP261YN.
At no load, I could get the desired output voltage of 22V.
But at half load, the output voltage reduced to 13V (sometimes 15V).

When I probed at Vdrain, it seems that it's not switching down to GND level. I've checked my probe's ground clip, it has good connection. Has this anything to do with the reduced output voltage?

Your comment is appreciated.



Submitted by Jiangming Yao on Mon, 08/03/2009

From your description, I understand that your output requirement is 22V and 200W. You are using TOP261YN with half frequency. Could you please also give us more details such as the input voltage range, transformer core, number of turns and primary inductance? Have you got any warning when you designed your power supply with our design software PI Expert of PIXLs Designers?

Please change the time scale to show only one or two switching cycles so that we can see more details. If possible, please also provide the drain current waveform. You may have to replace the trace to the drain pin with a piece of wire so that you can use current probe.


Submitted by juinbeh on Mon, 08/03/2009

Hi Scotty,

I have since found the cause of the waveform. It's the probe that I used. The waveform appears to be normal with another probe.

To answer your question, PI Expert didn't give any warning with the design.

Thanks for your reply.