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Sic diodes not needed for CCM Boost PFC?

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Advanced Member level 5
Jun 13, 2021
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We are all lead to believe that we are in the realm of Sic diodes for Boost PFC.
And then Infineon make an Eval board for dualled 2.5Kw PFC’s…..

…and just use cheap Ultra-Fast diodes…….with trr worse (greater) than loads of other ultra fast diodes on the market.

They use ID60C65D1 (approx. £1.42 at 1000 pces)
…trr from 70-110ns

There are load of sub 50ns on the market, albeit more expensive……
RFS60TZ6SGC13 ($2.52 at 1000 pces)

The IGBT they use has practically no tail time (16ns)....and a vce(on) of 1.65v at 40A, 175degC.........i think we can say goodbye to FETs for high power PFC? I wouldnt be surprised if a FET wouldnt lest long in that 2.5kW PFC with that cheap UF diode?

Would you say that its because this Eval board uses an IGBT (60khz), that it is OK to use the non SIC diode?
So would you agree that high power CCM Boost PFCs are better done with "IGBT + Ultra fast diode"… opposed to "FET and SIC diode"?

If you can switch a cheap UF diode like that, in a CCM 2.5kW boost, it makes you think that doing a 2.5kW hard switching full bridge at 60kHz also wouldnt be too bad? Who needs resonant LLC converters for high power?
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possibly not the best to believe everything you read - did you not the amount of cooling air flow needed for full power ?
--- Updated ---

2 x 5.8W fans, 6800rpm each, 38 CFM each, so take 12W off the efficiency calcs right there ....
Thanks, its a 5000W PFC, so AYK, 12W is around 0.24%. Many would say they would be happy to loose that for 5000W.
I did notice that the heatsink for the two mains rect bridges was pretty much the same size as the heatsink for the two IGBTs and two Diodes. Which seemed odd because reverse recovery losses wouldnt be that bad for the rect bridges.

From the waveforms, they appear to have this working absolutely fine. The gate drive series resistors are also pretty low value.....I mean, they arent even damping the IGBT turn ON that much.

This board is sold as an EVAL board... i am pretty sure it works.

Whether it passes EMC, and whether a sic would help it pass easier, who knows.

It uses two PI200273V1 inductors (85uH) by POCO, these look quite small, for 2.5kw....but the inductor datasheet is not found anywhere on the web, and "POCO" looks like a mobile phone App merchant.

Effciency is stated 97.5% at 230VAC.....and people could check this, so you tend to think its believable.

Infineon actually sell SIC diodes... it doesnt say much for SIC that this PSU doesnt use them, and gets 97.5% efficiency.

Infineon do another demo board of a 2.5kw SIC-diode-PFC at 65khz, and it has the same efficiency as the one using the UF diode when at full power of 2500W and 230VAC....
.....well almost the same, the one with SIC has a 0.15% improvement in efficiency...that saves 375mW.......people would say it was worth avoiding the expense of SIC and put up with that extra 375mW.

Though the SIC one has EMC pass scans, so maybe the EMC is it.
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What is difficult to understand is why they didnt use a SIC diode...because that IGBT is very cheap, and would be lower loss than a FET of the same price, so why they didnt use a sic diode with it. Maybe they are pointing out that if you use an IGBT, you can get away with a cheap UF diode. Maybe places will pass EMC with SIC, then revert to cheap UF for the production units? Like you often see those offline PSUs with non-fitted common mode chokes....just wire links fitted instead.
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I believe this App Note (top link of top post) uses UF diodes instead of SiC for PFC because the use of UF diodes creates far less noise, and as such, the PCB is easier to bring up, and less controller malfunctioning is seen.

This is because when a sic diode turns off, the turn-off is a severely abrupt “brick-wall” slamming off of the current. Whereas , when a UF diode turns off, there is a smoother kind of “backswing” of the current, as the recovery phase takes place. As such, the turn off of a UF diode is softer and less noisy than sic turn-off. Would you agree?

0.15% of 2500 watts = 3.75 watts.

SiC boost diodes will always give lower losses and less RFI, but shifted a little higher in frequencies, the lower resulting losses in the mosfet/igbt, more than offset the higher forward drop in the SiC diode ...

Thanks, you're quite right, 3.75W it is.......still, for 2500W Boost PFC's being compared, one Sic the other UF.......(and thats not even top speed UF)......3.75W seems pretty small, pretty insignificant. Maybe its just me. I cant imagaine anyone ive worked for beign impressed that i suggested saving 3.75W for the customer and the company spending more money for it.

Perhaps the Sic App Note SMPS is done badly.

3.75W in a single to-220, or to-247, is a little bit of a big deal

the real savings at high power would likely be higher than 3.75W - as Si diodes perform worse as they heat up to 100 deg C....
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