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Problem with 400V peak when the transformer in SMPS push-pull is turned on

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hi every body, I design a SMPS push-pull the problem is when the transformer is switched a peak appear ... a least 400V, what can I do to avoid that ? change frequency ?
 

Re: SMPS

Try a snubber,or clamp.
 

Re: SMPS

what are the characteristic to consider when designing a snubber circuit ?
 

SMPS

do you have a RCD block?
 

Re: SMPS

See here:
**broken link removed**
AND HERE
Push-pull example.
http://www.maxim-ic.com.cn/pdfserv/en/an/AN3835.pdf

The bottom trace is the drain source voltage? Where did you get the transformer, that’s quite the spike for what looks like a 30 or 40V input (If I’m reading your scope shot right). Or is this a high current design? . What’s your Fet rated for? How much power is the converter delivering?

Words of caution if you elect to use a simple RC snubber don’t get to aggressive. Get it down to 25% or so below the max Vds of your FET. If you slow down the turn off to much your FET will dissipate excess power. Check for commutation overlap.
 

Re: SMPS

My power supply was around 15 volt, but the main idea is to take the current directly to the wall, rectified it and output 60V 5A I don't know what you mean by high current, but for me it's high current. I measure the peak current and it was near 10 Amp, at the same time as the (blue) drain to source voltage (you were right max). The peaks was measured at 10 Amp, but the amp meter on my power supply did pass over 2 amp, that's why I think my transformer is oscillating. My Fet's are IRF840 500V 5A
 

Re: SMPS

MOSFET are often drive by a transformer, can I do so with a fast opto-coupler and diodes ( to step down the voltage across the opto and to cancel the "kick-back" of the transformer)
 

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