Boost PFCs with SiC diodes will be more noisy - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: September 08, 2022, 05:26:15 pm
content: The following App Note shows a 2 x 2.5kW Boost PFC. It uses an Ultra Fast Diode instead of a SiC diode….PFC App Note:https://www.infineon.com/dgdl/Infineon-EVAL-PFC5KIKWWR5SYS-UserManual-v01_00-EN.pdf?fileId=5546d46278d64ffd0178f986f0b708d4It is believed that SiC turns off the current very abruptly, and thus creates problematic noise which can make the PFC controller malfunction. What is best, is a Si diode with a “soft” recovery. (as shown in the following)UF diode softness:https://www.power.com/sites/default/files/documents/65-Qspeed_ECN_Dec_2007.pdf…as is told,QUOTE/….improve performance by taking advantage of the performance of the latest Silicon devices, because SiC Schottky diodes have not made Silicon rectifiers obsolete./UNQUOTEHigh power PFCs with SiC can be a liability due to the malfunction inherent due to their noisy snap-off…SiC literally slams the current off “brick wall” style.Looking at the red curve on page 1 of the below….we see much reverse recovery, but a very smooth and less noisy switching transition.https://www.power.com/sites/default/files/documents/65-Qspeed_ECN_Dec_2007.pdfUF diodes, it is believed, are on the way back.Looking at the high efficiency of that 2 x 2.5kW Boost PFC, it is as efficient as Infineon's other App Notes showing Sic diodes in similar power PFCs.Looks like the marketers have been trying to sway us again?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 08, 2022, 09:27:43 pm
content: Curiously, they only point out this one,https://www.mouser.com/datasheet/2/328/lqa08tc600-1512788.pdfnot whatever the yellow curve is.Apparently they can't read very well (par for the course for journalism?..), it says "Note to component engineers: Q-Series rectifiers employ Schottky technologies in their design and construction."No leakage curves, annoyingly.I'm assuming it's a one of those pillar MOS schottky types, improved to yet-higher voltages, and probably aided by charge injection (conductivity modulation) by putting guard-ring diodes in amidst it all, or something?..The sort of things they do with these:https://www.diodes.com/assets/Datasheets/SBR20A200.pdfThey do indeed have reverse recovery, I've measured it myself. trr and Irr vary little with If or on-time, in particular it doesn't become snappy for very short on-times (doesn't seem to exhibit drift recovery). It's not just a fast PN diode, it's some kind of hybrid thing. Plenty fast enough for average work; won't be anywhere good enough with GaN say, obviously.The higher Vf means I wouldn't choose that Qspeed part over SiC. Maybe if I wasreallystrapped on budget; it seems somewhat cheaper. SiC are available from a lot of people now, too; it might not be a good idea getting single-sourced on that thing.EMI is relevant for snappy diodes; schottky are just capacitance, manage that and you're fine. They're available in DFNs now as well, up to similar ratings, or can be paralleled; I have no problems making a tight enough loop that way, and some thermal pads handles the rest, even at a couple kW. Can make a little inverter board and clamp that with heatsinks, for example.Tim

author: Faringdon
author_rank: Super Contributor
timestamp: September 10, 2022, 09:10:35 am
content: Thanks, i see your point about "snapiness".......some of the low trr UF diodes are very snappy...ie have very high di/dt.In the following.....https://www.power.com/sites/default/files/documents/65-Qspeed_ECN_Dec_2007.pdf...the red trace is obviosuly the most inefficient, but its di/dt is the best of all of them.....i woudlnt be surprised if the sic trace could cause noise which made the SMPS controller malfunction. The sic trace is less di/dt than the yellow trace, but is still higher di/dt than the red trace.I am just wondering if sic needs extra snubbing to "round out" the di/dt? ....so that the controller doesnt malfunction.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 10, 2022, 12:00:48 pm
content: They all have the same dI/dt (falling slope, left of turn-off point). Which is set by the MOSFET.d^2I/dt^2 doesn't really mean anything, but will produce transients on nearby elements, much as suddenly steering (technically, "jerk") a car causes its passengers to sway. This is easily predictable in the schottky case, as it's just loop inductances and node capacitances. There is no excess charge / peak current flow due to recovery as in the other cases. And the usual methods can be applied (geometry of the switch loop, snubbing), without need of esoterica (e.g. saturable nanocrystalline core on the diode, to delay dI/dt).Tim

author: Faringdon
author_rank: Super Contributor
timestamp: September 10, 2022, 07:16:49 pm
content: QuoteThanks, yes i agree with that.....i didnt explain well......i am referring to the "point" at which current very suddenly changes by a large anount...the yellow trace has the worst case of this, the red trace is the smoothest in this respect. The Schottky is sonewhere between the two. Its that point that worries me, its repetitive every switching cycle, and the noise from it may , i wonder, make the controller malfunction.