CCM Boost PFC with synchronous rectifier is a rare beast? - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: June 14, 2020, 01:29:52 pm
content: Hi,We wish to do CCM Boost PFC. We wish to use a SiC FET for the Sync Fet. 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 14, 2020, 01:57:54 pm
content: Because they do..?https://www.wolfspeed.com/downloads/dl/file/id/824/product/194/c3m0120090d.pdf4.4-4.8V, 24ns, 115nC.Which is actually surprisingly good, considering SiC tends to recombine slowly compared to Si. And you can always put a SiC schottky across it, if you don't mind the increased capacitance.Yes, in DCM you need a zero cross detect to turn it off, otherwise you're forcing CCM and the efficiency tanks.Tim

author: ocset
author_rank: Super Contributor
timestamp: June 14, 2020, 02:56:32 pm
content: The "combo JFET" solution sounds liek the Bees Knees.There is certainly a great lack of App Notes decrying SiC FET sync Rects for CCM Boost PFC.I wonder if its something to do with di/dt limitations on the SiC FETs?QuoteThanks, Yes, But using CCM Boost with SiC diode, you would think that the efficiency woudlnt tank quite as much...just the losses due to higher Vf?BCM Boost PFC is widely decryed as being for <250W, and best below 150W_________----------------_______________---------------_________________________I must admit, when you compare today’s SiC diodes Vf with UF diodes Vf, they are getting very close, and I would say the days of the advantageous BCM Boost PFC are gone?....just use CCM Boost PFC with SIC diodes, and have no fear of reverse recovery…and enjoy much lower peak current.Would you agree, that we are about to consign BCM boost PFC to the Switch Mode Museums?IDH10G120CS SIC diode (1200V)https://datasheetspdf.com/pdf-file/969774/Infineon/IDH10G120C5/1RFN5TF8S UF diode (800V)https://fscdn.rohm.com/en/products/databook/datasheet/discrete/diode/fast_recovery/rfn5tf8s.pdf

author: ocset
author_rank: Super Contributor
timestamp: June 14, 2020, 09:06:54 pm
content: Thanks, yes, the problem as you know, is that the BCM Boost PFC cannot do anything to reduce Switch_OFF switching losses...and when going into lighter load....there are higher than wanted turn_OFF switching losses due to the increased frequency at light load.BCM Boost PFC controller datasheets almost never deliver this bad news to us.

author: poorchava
author_rank: Super Contributor
timestamp: June 14, 2020, 09:50:12 pm
content: Why do u need sync rectification? I mean if you shoot for best efficiency, then classical boost is not that good, since you have losses on the input bridge. And in order to mitigate those, there are bridgeless topologies such as totem-pole, dual boost etc, which are often made synchronous. This is probably why classical boost with SR is a rare beast. The drive for such SR FET is also not trivial to design since the it has to be floating and in order to achieve best efficiency dv/dt is very high.On the flip side, I've built a 3kW PFC unit that is single phase, CCM and the heating is not that bad. IIRC the efficiency was above 96...97%.

author: ocset
author_rank: Super Contributor
timestamp: June 15, 2020, 01:11:53 pm
content: QuoteThanks, thats interesting, i appreciate that the dv/dt is high in the Boost PFC, but it is also high on other topologies which use say a pulse transformer gate drive.....what would make things worse with a Boost PFC rather than say a hi side drive on a two Transistor forward , say?