Negative Biasing MOSFETS gate drive to avoid false turn-ons - Page 1
Forum section: electronics / beginners

author: Hr_Satch
author_rank: Contributor
timestamp: June 12, 2025, 07:52:06 pm
content: Dear Forum,as to counter and avoid some issues with inevitable parasitic inductances,or in order to avoid or counter possible false turn-ons, shoot throughs, . . .I am playing with the idea, of Negative Biasing the Gate-drives voltages.As even manufacturers like Unitrode, Texas Instruments state in their datasheetapplication guides ;-?However, nobody seems to apply this ? Online anyways ?Maybe too complex, too much components ?But if the driver chips makers, advice such, why not give it a trySo lets take a Max. -5V below, that will limit its useable upper gate drive voltageto some +15V, because MOSFETS have a max. Vgs of +-20V.I am thinking of making this -Negative supply even small adjustable from 0 -to -5V,and taking fixed Rg gate series resistors around 10 ohms, . . .So I can possible avoid having to alter each time four gate resistors, just to try.Every H-bridges leg, A-bank, B-bank, Low-side, High side,receive this -Neg. bias thru a separating HV-diode, as per Bootstrap principle ;-)This idea sorts from the fact that even keeping PCB-tracks as short as possible,placing Drivers near the MOSFETs, etc . . .In the end you will need thick wires from the H-bridge towards the center Load,a Toroid core transformer anyways ;-?And such big beasts never can be on the board aswellAny of you have tried this already ?Any positive suggestions are much appreciated, thanks, harry.

author: AlexRG
author_rank: Regular Contributor
timestamp: June 12, 2025, 08:27:04 pm
content: The easiest way to create a negative bias on the transistor gate is to put a decoupling capacitor and diodes. The more diodes and the higher the voltage drop on them, the higher the negative bias. I tried to do this in practice with 1N4148 diodes and GaN transistors. One diode gives about -0.4V, two diodes about -0.8V, three diodes about -1.4V. In theory, you can try SiC diodes, high-voltage 600-1200V, they have the highest voltage drop.On the oscilloscope, the red line is 0, the blue arrow shows the area of negative bias.

author: David Hess
author_rank: Super Contributor
timestamp: June 12, 2025, 09:36:13 pm
content: Quote from: Hr_Satch on June 12, 2025, 07:52:06 pmUsually negative bias is just not needed. GaN transistors are more likely to use it now because they are both faster and have a more restricted Vgs.QuoteNo, the save Vgs is from +20 volts to -20 volts, and 40 volts peak-to-peak. Using a negative voltage does not restrict the positive voltage.The absolute maximum voltage rating is *not* a normal value and should not be used or reliability will be decreased. Parts are not designed to operate at their absolute maximum ratings. Usually 10, 12, or 15 volts would be used for a 20 volt absolute maximum Vgs, and 10 or 12 volts for a 15 volt absolute maximum.

author: Benta
author_rank: Super Contributor
timestamp: June 12, 2025, 10:37:16 pm
content: A solution looking for a problem...

author: MariuszD
author_rank: Regular Contributor
timestamp: June 13, 2025, 10:23:28 am
content: Negative bias is/was used for IGBT.This driver have three power pinshttps://docs.broadcom.com/wcs-public/products/data-sheets--technical-specifications/data-sheet/96/55/av02-0717en_ds_hcpl-316j_2015-03-09.pdf

author: PartialDischarge
author_rank: Super Contributor
timestamp: June 13, 2025, 10:58:24 am
content: Negative bias is widely used in high power inverters for IGBTs (to reduce the current tail and to prevent turn-on from the capacitive divider in CB and BE) and in Mosfets only for the latter.

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 13, 2025, 12:22:00 pm
content: Anecdotally I have also mostly seen bipolar gate drive used with IGBTs in higher power (like >100A, >500V) applications.It doesn't offer much for driving standard MOSFETs. If you have problems turning off MOSFETs then it's probably poor device selection, or a flaw in the driver design which can be solved without bipolar drive.I'm aware that some vendors of GaN and SiC devices recommend using bipolar gate drive. These recommendations should be taken on a case-by-case basis, don't generalize this to other devices.I work mostly with lower voltage (<100Vds) GaN devices and have never had a need for bipolar gate drive.

author: Hr_Satch
author_rank: Contributor
timestamp: June 14, 2025, 08:41:57 pm
content: Quote from: AlexRG on June 12, 2025, 08:27:04 pm