New High Voltage SiC diodes are not marking the end of BCM Boost PFC controllers?

[treez]

Why haven’t new high voltage SiC diodes meant large reductions in the sales of BCM Boost PFC control chips? After all, the main advantage of the BCM Boost PFC is its lack of reverse recovery problems. But new SiC diodes massively reduce such problems when Boost PFC’s are done in CCM.

A CCM Boost PFC doesn’t need an extra winding in the Boost inductor, which the BCM Boost PFC needs.

 

[dick_freebird]

Design-in cycle = ???
Production release cycle = ???
Desirability of a 4V Vf = "not very".

 

[treez]

Thanks,
this one has a low Vf..........c4d02120a

https://www.wolfspeed.com/downloads/dl/file/id/75/product/52/c4d02120a.pdf

..also this one... IDH04G65C5

https://www.infineon.com/dgdl/IDH04G65C5_Final_Datasheet_v_2_1.pdf?folderId=db3a30431ddc9372011ed0010fda1bd3&fileId=db3a304339dcf4b1013a033ff5d95947

 

[Easy peasy]

20 & 38A max forward current, respectively, for half sine wave surge rating is a limiting factor for those diodes - if you turn off and then on it can go bang due to inrush - compare to Si diodes ...

 

[treez]

Thanks, i see your point, it makes you wonder why SiC diodes are used in CCM PFCs at all.
I even went to an interview at a quite big electronics company making medical stuff and the guy's question was..."tell me about the SiC diodes that you would use in a CCM Boost PFC"

I can see that there would need to be a circuit to do "OFF then ON again" sensing...probably this would be easiest using a microcontroller.

Though surely you could short out the SiC diodes to inrush with a regular Si diode connected from Boost input to Output?...

 

[dick_freebird]

I'm sure a SiC diode could be made with a higher Ipk/Isteady
margin. But it's not as easy when you operate at >3X the Vf
(so power density 3X) and crowd the heat into a smaller
region as well (more of it shed at the junction, less in the
drift region / bulk). They'd probably have to work at it and
there's about 50 years' less product differentiation been
done, yet.

If you "short" the SiC diode with a silicon rectifier, have
you not just made the SiC device irrelevant?

 

[treez]

If you "short" the SiC diode with a silicon rectifier, have
you not just made the SiC device irrelevant?

Well, i mean like in the boost PFCs where you get the diode connected from input to output, and its just always reverse biased when the boost gets powered up.

 

[treez]

With this "bypass" diode, there wouldnt be significant inrush in the boost diode

 

[treez]

So since the Boost PFC diode will never see inrush (when inrush is bypassed as discussed), then why are BCM Boost PFC controllers so popular?

I mean, BCM Boost PFCs need an extra winding in the Boost inductor….why bother with this faff when you can just do CCM Boost PFC instead? Another point is that BCM Boost PFC’s suffer from very high frequency operation when on light load…this results in unnecessarily high switching losses, since BCM Boost PFC’s do nothing to help turn-OFF switching losses….

Also, when the load lightens, BCM Boost PFCs get more noisy and loose PFC operation quicker than CCM PFC’s…this is because the signal from the “signal” winding on the BCM Boost inductor gets very oscillatory and so the zero crossing point is often not distinct enough to be picked out by the ZCD comparator.

All this is a big downer for BCM Boost PFC.

So why do it?..why not CCM Boost PFC instead?

 

[Easy peasy]

Actually BCM boosters can be made very quiet - and use std Si diodes optimised for low on drop ( Trr = 50/75nS ) due to the low volt turn on of the fet ( diode already largely biased off) the main drawback is the ripple current in the caps feeding the power ckt - we have made dual interleaved BCM boosters to 1.5kW and they work straight off the bat ( 180 - 265 Vac )
One needs to keep the max freq to reasonable bounds, and ensure current balance in the 2 // stages -

 

[treez]

I must inject though, that when you compare today’s SiC diodes Vf with UF diodes Vf, they are getting closer......

IDH10G120CS SIC diode (1200V)
https://datasheetspdf.com/pdf-file/969774/Infineon/IDH10G120C5/1

RFN5TF8S UF diode (800V)
https://fscdn.rohm.com/en/products/databook/datasheet/discrete/diode/fast_recovery/rfn5tf8s.pdf

The "standard" Si diodes are of course masters of low Vf, but they have trr , as you know, of 500us to 2us...and so wouldnt be risked even with BCM boost PFC?

 

[Easy peasy]

You need to understand all the aspects of DCM or BCM boost ...