Can be a high side diver reliable powered by bootstrap with SiC at 50-100kHz?  - Page 1
Forum section: electronics / projects

author: Miyuki
author_rank: Frequent Contributor
timestamp: August 27, 2021, 01:22:10 pm
content: Hi folks,I know it cannot be predicted from the table.But can high side driver work reliably with simple bootstrap and few clamps when output is fast SiC mosfets switching at 50-100kHzI expect to see some crazy oscillation at the switching node, so can use a relatively high charging resistor and very fast diodeAnd withAC/capacitor coupled gatedrive to avoid the need for a negative supplyAll the papers show fancy solutions with bipolar power for each driver, I'm just curious if this all can be just omitted

author: uer166
author_rank: Super Contributor
timestamp: August 27, 2021, 03:07:22 pm
content: Bootstrap: sure, AC coupled gate? Probably not, it's akin to a floating gate. The DC bias matters, so maybe you can make some circuit to bias it correctly, similar to transformer-driven gates that have a DC restore diode.

author: Miyuki
author_rank: Frequent Contributor
timestamp: August 27, 2021, 04:00:26 pm
content: it shouldn't float randomly

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 27, 2021, 04:11:23 pm
content: Some newer SiC support conventional (0/15V or so) input, maybe not needed at all.Tim

author: Miyuki
author_rank: Frequent Contributor
timestamp: August 27, 2021, 07:43:28 pm
content: Quote from: T3sl4co1l on August 27, 2021, 04:11:23 pmI know Infineon advertise them this way, but when I compare them to one I have chosen from GeneSiCThey cost almost two times more.And I am a little nervous about the turn of performance compared to -5/15V drive0/15V one IMW120R220M1H CoolSiC have VGS(th) = 4.5V with just 3.6V@175°C plus relative high RG,int 22Ω-5/15V have G3R350MT12D have VGS(th) = 2.7V with just 2.05V@175°C with RG,int just 2.5ΩAnd body diode is way faster at G3R350MT12D, that IMW120R220M1H really sacrificed other parameters for simple gate drive, so giving double switching losses in datasheet parameters (body diode recovery charge causing turn on loss)

author: uer166
author_rank: Super Contributor
timestamp: August 27, 2021, 09:37:34 pm
content: Quote from: Miyuki on August 27, 2021, 07:43:28 pmAnecdotal, but I'm trying outhttps://unitedsic.com/datasheets/DS_UF3SC065030D8S.pdfwith unipolar 15V drive with no issues so far. Of course they cost some coin, but also almost 1/10th of your Rdson.

author: Miyuki
author_rank: Frequent Contributor
timestamp: August 28, 2021, 03:49:36 pm
content: Quote from: uer166 on August 27, 2021, 09:37:34 pmThe problem is the 1200V variants have way worse parameters and 900V versions are not available.And how it behaves? They have just -5V values in the datasheet.

author: uer166
author_rank: Super Contributor
timestamp: August 28, 2021, 05:28:11 pm
content: Quote from: Miyuki on August 28, 2021, 03:49:36 pmBehaves perfectly fine! I suppose the application matters: it's a boost converter where this FET is on low-side and a SiC diode on high-side. There is much less (I suppose almost none?) issues with dv/dt turn-on and miller current going into gate in this topology vs. say a hard-switched half-bridge. I never tried it in half-bridge config so it's possible you'll need the -5V there to keep the FETs off when the other one switches. I remember taking some Vgs traces, and while the turn-on is ring-y with Rg=2.2Ohm, the turn-off is clean.From my notes:Fall time of Vds (FET turn-on) with Rg=2.2: 11ns.Rise time of Vds (FET turn-off) with same Rg: 14ns. Of course here the rest of the circuit and the high-side diode matter more.The swing was ~400V in above tests.

author: Miyuki
author_rank: Frequent Contributor
timestamp: August 28, 2021, 07:25:23 pm
content: On what current?

author: uer166
author_rank: Super Contributor
timestamp: August 28, 2021, 07:28:01 pm
content: Quote from: Miyuki on August 28, 2021, 07:25:23 pm~12A peak. It would be helpful if you showed us your topology since all this gate drive business depends on that.

author: Miyuki
author_rank: Frequent Contributor
timestamp: August 28, 2021, 07:50:21 pm
content: I have half bridge legs, nothing specialPowered by rectified 400V mains so I need 900V or 1200V rated transistorsOutput is LC filtered sine at about 6A