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Solving reverse recovery problem in LLC converter


Aug 3, 2009
Aug 3, 2009
The attached LTspice simulation shows how to totally solve reverse recovery problems in the LLC converter! It shows an LLC with a series diode and parallel SiC diode…..and as a comparison an LLC with just normal Si FETs in the leg.

The switching frequency is taken down into the capacitive region ……the “series diode LLC” suffers absolutely zero diode reverse recovery!!! can see that there are high current spikes , but doing the calculation, you can see that these spikes correspond only to what you expect to see due to the sudden discharge of the FET stray capacitance….not reverse recovery.

Whereas the LLC with no series diode suffers from quite utterly horrendous reverse recovery current spiking…FET killers !!! many of those reverse recovery spikes does it take to kill a FET?....probably just one!.........and even if the FET doesn’t die…it will be weakened such that it will soon die.

Maybe it is thought we can simply avoid the capacitive region by setting a minimum switching frequency……but this only works if your load never deviates from maximum….eg overload or short or startup, or restart, or return from temporary brownout etc etc…..when that happens the LLC resonant frequencies change such that our “minimum frequency” is no longer well set…..and we stray into the capacitive region anyway……..i wonder does anyone yet believe that they can detect the first LLC hard switch of a series of them?…, detect it before it actually it happens…so you can avoid switching on at all…..i very much doubt anyone has done this….though there is an Infineon App note where work toward that has been done….maybe they succeeded?

But anyway…you can eradicate the problem by using the series standard diode as shown here (nice low Vf) and parallel SiC diode. And as a bonus, you get to pick the lowest rds(on) FET that you like!!! more worrying that the rdson isn’t going to be big enough to raise vds sufficiently during ON time so as to sweep out minority carriers in the intrinsic diode……the intrinsic diode is not going to conduct because your series diode has blocked it!

You now have an LLC converter, that, as long as you have current sense transformer/comparator primary side, is just as tame as a little DCDC buck converter!

And you loose no efficiency…because you can now choose the lowest rdson FET.


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