CCM Boost PFC with Synchronous rectification?

[treez]

Hi,

We wish to do CCM Boost PFC. We wish to use SiC FETs. Normal FETs would cause too bad reverse recovery problem with their intrinsic diode.

Can you confirm that a SiC FET has an intrinsic SiC diode? Why don’t SiC FET datasheets give details on the internal intrinsic SiC diode?

Why are there no App Notes on CCM Boost PFC with synchronous rectifiers?

We are going to delay activation of the Synchronous rectifier until we have got into constant steady maximum load, otherwise there is severe reverse current in the Synchronous FET.

 

[FvM]

Can't confirm your observations, all SiC MOSFET datasheets that I know have static and dynamic body diode specifications. Gate timing in synchronous rectifier applications will be similar to other push-pull configurations, e.g. 200 ns dead time.

 

[treez]

Thanks, yes, but isnt it strange that there does not exist anywhere on the web, an App Note concerning CCM Boost PFC with synchronous rectification?

....I have a suspicion its because one is expected to go for this new Totem pole Bridgeless PFC thing instead...(ie when one wants higher power PFC of >300W)

 

[Easy peasy]

you can use SiC mosfet (or the SiC J-fet / Si mosfet cascode ) for the upper rectifier in a so called "bridgeless" PFC design - but the losses are only marginally lower than a good SiC diode, plus you have all the added complexity of gate drive and reduced reliability.

If you look closely at the curves for reverse current when biased fully ON, the on drop, is not that flash, and the gate drive must be absolutely tight - there are two app notes out there that show such designs - but not recommended for engineers with < 15 years experience in similar power electronics work ...

We have done "bridgeless" since 2003, for high power adapters and other designs, if we were to do 3kW or higher, and the app / customer required it we would look at SiC mosfets in the upper - but on paper - the reliability goes down - and the cost goes up - even if the heatsinks are marginally smaller ...

 

[treez]

there are two app notes out there that show such designs

Thanks i presume you're talking about "bridgeless PFC " here?.....ie these " two App Notes"?

The "CCM Boost PFC with synchronous rectification" does indeed seem to be like the Loch Ness Monster.......doesnt exist....i cant think why?

 

[Easy peasy]

it does exist but not published for obvious reasons ...

 

[treez]

i take it you mean "IP protection". But this seems odd, because there are plenty of App Notes on Synchronous rectification for LLC, Full Bridge, PSFB etc etc....so its not obvious why there aren't App Notes for sync rect in PFC Boost?

However, with 400V on a Boost PFC output, its going to be pretty bad if the sync rect got turned on when it shouldnt, as the scope for high levels of reversing inductor current would be bad....as well as the problems that this would cause with high voltages occuring on the DC Bus (and on the FET) as a result, due to its lack of capacitance...due to being PFC'd.

Also is the fact that PFC stages, due to their slow feedback loop, they often go into output overvoltage shutdown, whereby the main fet is held off for relatively long intervals…….if the logic for the sync fet is to turn on whenever the mains fet goes off…then the sync fet turns on and stays on…..and disaster ensues…so extra circuitry would be needed to mitigate this……..

As we see, in the entirety of the world, there is not one single controller IC for “CCM Boost PFC with synchronous rectification”.