why snubber capacitors on output diodes?

I did a full bridge phase shift ZVS converter many years ago. I remember u sing some fairly significant RC snubbers on the 2 output rectifiers.

I can't seem to recall the exact explanation for why they are needed. I'm guessing the leakage reactance in the transformer has something to do with it. I draw the converter with a pair of inductors on the transformer outpu t to represent the leakage and I still can't get my head around it.

regards, Bob N9NEO

Reply to
Yzordderrex
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The leakage shows up as equivalent series inductance to the diodes, which resonates with the junction capacitance when they let go (in addition to various transformer-related resonances). A good guess for an R+C snubber is R = sqrt(LL / Cjo) and C = Cjo, give or take a factor of two on each.

Tim

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Seven Transistor Labs 
Electrical Engineering Consultation 
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Reply to
Tim Williams

Diodes have stored charge that means that when reverse biased a significant reverse current passes for a finite time. When the charge is exhausted the diode current snaps to zero, causing ringing and consequential noise.

It is typical to have small capacitors such as few 100pf across each diode in a bridge to reduce this noise. I can't say I have seen RC snubbers.

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Mike Perkins 
Video Solutions Ltd 
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Reply to
Mike Perkins

Sure, you want to (sorta) critically damp the oscillation.

Reply to
krw

You don't tell at what frequency those diodes were operating, but I remember a case where a colleague of mine had a problem with diodes blowing out. When measuring the temperature we found it must have been from overheating. It appeared that the recombination current in the junction together with the hard switched reverse voltage formed a power dissipation that was too much for them, at the frequency that he was operating them. So I suggested to put a snubber over those diodes and lo and behold, it worked!

O, by the way, those were snubber diodes themselves, that needed the snubber.

Sorry if I'm confusing you now, but, could it be a similar case?

By the way, and totally O.T., but may I aske what raised your interest in this topic so suddenly, after so many years?

joe

Reply to
joe hey

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