Why is nobody  mandating  SiC FETs with LLC converters? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: March 31, 2022, 10:03:18 am
content: Pages 13, 14 and 15 of the following show that in typical LLC converters using typical offtheshelf LLC controller IC’s, capacitive mode destruction (due to severely ruinous reverse recovery current) simply cannot be avoided….https://www.infineon.com/dgdl/Infineon-EVAL_3KW_2LLC_P7_47-ApplicationNotes-v01_00-EN.pdf?fileId=5546d4625b3ca4ec015b3e58e44f1048The way to avoid it, as they say, is to put in a current transformer, and measure the phase shift between the resonant capacitor current, and the MOSFET gate signals. As soon as the phase gets to zero, then the “next” MOSFET should not be switched ON, and the currently ON MOSFET should be switched OFF.Another way to solve the problem of severe reverse recovery though, is simply to add low Vf diodes in series with the FETs, and SiC diodes in pllel to both fet and low Vf diode, as in the attached, would you agree?(LTspice and JPG schem attached)Another way would be to use SiC FETs....i wonder why the app note didnt suggest this?The only other way to avoid “Disastrous reverse recovery”in an LLC is to use normal FETs, but switch at fixed frequency of the upper resonant frequency, and simply use on/off control, or burst mode, to regulate the output voltage….with switching at the fixed frequency, you simply never see the sudden change in switching frequency which can result in DRR...do you agree?

author: JohnG
author_rank: Frequent Contributor
timestamp: April 04, 2022, 04:45:50 pm
content: Ok, I'll bite...I would take a very long look at SiC or GaN FETs for this application.SiC MOSFETs have reverse recovery, but it is much less than Si MOSFETs of comparable R_on and it doesn't get as bad with increasing temperatures. You can use antiparallel SiC Schottky diodes and if you keep the inductance low, they will work. However, they add capacitances so for a lot of applications, it's a wash.GaN FETs (at least those based on HEMTs, which is all commercial ones to date) do not exhibit reverse recovery because they do not have a PN junction diodes as part of the structure.Both GaN and SiC have high reverse voltage drop, so you don't want to have long deadtimes if you can avoid it. Also, the problem with reverse recovery usually occurs at light load when there is not enough current to commutate the bridge and you get hard switching.With Si MOSFETs, there are two main problems, and these get really bad with superjunction MOSFETs. First, the reverse recovery charge is usually terribly high. Second, the capacitive output charge Qoss is extremely high due to the superjunction structure. In hard switching, these two charges flow through the forward conducting FET in the bridge during the transition and punish it. A secondary problem is that the shape of the Cgd vs Vds curve is such that the turn-off delay can be extremely long and this makes it a challenge to get precise control of deadtime. This becomes a bigger problem at light load because the switching frequency is closer to its maximum.I would venture to guess that the app note did not suggest SiC because the publisher did not have SiC MOSFETs nor GaN FETs on the market when the app note was published in 2017. In addition, they have relatively recent significant investment in superjunction MOSFETs (12" fabs, etc.), so I would further speculate that they would like to recoup as much of that as possible and are not in a big hurry to see an industry-wide switch to wide bandgap semiconductors. Just MHO.John

author: Faringdon
author_rank: Super Contributor
timestamp: March 15, 2023, 09:14:23 pm
content: Thanks....Here is a GaN solution as you discuss....So, Fear the potential disaster mode in LLC of reverse recovery during short cct etc etc....this can destroy an LLC...solution..GaN FET driver for LLChttps://www.onsemi.com/pdf/datasheet/ncp51820-d.pdf...200V/ns CMTI !!!!....This totally solves the problem, with no need for the infineon software as described above, and no need for the "series diode".Just use GaN FETs with a parallel SiC diode !!!There are even GaN FETs which come in 2-on-a-package which the above driver could easily drive.What do you think?.......long hail the 500kHz LLC converter running off 400V post PFC bus?Its either st.com, onsemi or Navitas which actually do two 650V bridge GaN FETs on a chip