Mosfet conducting in reverse recovery? - Page 1
Forum section: electronics / projects

author: eTobey
author_rank: Super Contributor
timestamp: January 01, 2025, 09:40:03 pm
content: I send a PWM signal burst, to drive the Highside mosfet while the lower has a current flowing from source to drain, that is caused by the inductor (if current was flowing).Mosfet:https://www.vishay.com/docs/76233/sir180dp.pdfDriver:https://www.infineon.com/dgdl/Infineon-6EDL7141-DataSheet-v01_20-EN.pdf?fileId=8ac78c8c8e7ead30018ec7bd6aac46f3The first signal (of the burst) of the HS gate looks very good (red trace at the arrow)...... but as soon as a current is flowing it gets distorted, and the voltage at the ground at the LS mosfet increases. Green trace is GND at LS mofet.It seems to me, that the diode of the LS mosfet is conducting current in reverse (reverse recovery). Cant think of anything else.If this is the case, i have a problem: The gate is already driven with the lowest current. Adding a resistor is no option, since this driver (6EDL7141) has a current source.Could i just add an additional Cgd capacitor, or another one of the two other intrinsic?

author: uer166
author_rank: Super Contributor
timestamp: January 01, 2025, 09:48:43 pm
content: You found the hard (switching, hehe), way that body diodes in Si FETs suck for reverse recovery, and can often conduct many times more the forward current of the diode, with a really sharp recovery to boot.There's really not many things you can do here unless you change stuff/parts, consider:Soft switching or QR topology. E.g. if it's a buck, run in DCM or CRM/BCMChange FETs to SiC FETs (or GaN) with an order of magnitude less recoveryThe problem is that by the time you slow down the HS FET enough to where the LS diode recovery is a little less jarring, it's running *so* slowly that losses become untenable. You can try adding miller capacitance (Cgs) though to make it a linear turn on/off ramp.Edit: on a second look it seems that this might be a phase of a BLDC driver, or a brushed motor or something. This is an interesting edge case since SiC kind of doesn't make sense, and you obviously can't run in DCM with such massive inductance.You've blanked out the FET PNs, which is what's causing the issue, what are they?.

author: eTobey
author_rank: Super Contributor
timestamp: January 01, 2025, 10:09:05 pm
content: Quote from: uer166 on January 01, 2025, 09:48:43 pmPNs ? Product number?https://www.vishay.com/docs/76233/sir180dp.pdf

author: inse
author_rank: Super Contributor
timestamp: January 01, 2025, 10:11:24 pm
content: One annotation regarding your measurement setup: with the current grounding condition it will be hard to reliably measure high speed signals. Connecting the ground leads “all over the place“ will generate ground loops that pick up noise and distort your measurement.Connect the probes as short as possible as close as possible to a single GND point.What is this pointy oval PCB thing, partially covered in heat shrink?

author: uer166
author_rank: Super Contributor
timestamp: January 01, 2025, 10:15:28 pm
content: Quote from: eTobey on January 01, 2025, 10:09:05 pmYe, this really doesn't look that bad, especially since this is actually a pretty soft recovery (Ta < Tb). How's your layout? Are there low-side shunts? Is the hot loop low inductance with a good amount of ceramic bypass caps?

author: uer166
author_rank: Super Contributor
timestamp: January 01, 2025, 10:17:26 pm
content: Quote from: inse on January 01, 2025, 10:11:24 pmYeah, I don't see any low inductance probing GND clips here, you'd definitely wanna start with good probing. Even a cheap low-Z homemade probe out of coax would work well here for LS probing.

author: eTobey
author_rank: Super Contributor
timestamp: January 01, 2025, 10:24:01 pm
content: Quote from: inse on January 01, 2025, 10:11:24 pmIts not that easy to put them all onto one gnd. And i do not really "meaure", since i already learned that measuring can be quite a task. I "look" at those signales more or less.I do not consider these signals as high speed, at least at portions of the traces. The steeper the edges the less i trust those values.I disconnected one GND lead, and the signal did not look much different, so i guess its kinda accurate? (See "ND at mosfet")

author: eTobey
author_rank: Super Contributor
timestamp: January 01, 2025, 10:34:46 pm
content: Quote from: uer166 on January 01, 2025, 10:15:28 pmIts 1uF. pink layer is just below.

author: eTobey
author_rank: Super Contributor
timestamp: January 01, 2025, 10:38:15 pm
content: Quote from: inse on January 01, 2025, 10:11:24 pmThese are my DIY Sensepeek´s. The one with the white bands, is holding my probe.

author: uer166
author_rank: Super Contributor
timestamp: January 01, 2025, 10:42:02 pm
content: Quote from: eTobey on January 01, 2025, 10:34:46 pmLikely much too small to provide enough current sink and source during that transient. Are you measuring the bus voltage at the half bridge?Do some quick math regarding current over time into/from that cap and see if it's enough capacitance to appreciably absorb the RR current.This might not be "high speed" in the classical sense, but the di/dt of this is quite substantial regardless, it can easily couple enough field into your long ground leads to screw up the measurement.

author: eTobey
author_rank: Super Contributor
timestamp: January 01, 2025, 10:50:59 pm
content: Quote from: uer166 on January 01, 2025, 10:42:02 pmIts measured right at the very mosfet.Quick math? i havent got a clue how to calculate really.If the voltage voltage near the big cap does not drop too much, then is guess its not a big problem??

author: David Hess
author_rank: Super Contributor
timestamp: January 02, 2025, 01:56:30 am
content: In lower voltage applications a schottky diode can be placed in parallel with the power MOSFET to prevent conduction through the body diode.In high voltage applications, a silicon diode can be placed in series with the drain, and then another silicon diode placed in parallel with the series connected diode and power MOSFET. This will also prevent the MOSFET's body diode from conducting.

author: eTobey
author_rank: Super Contributor
timestamp: January 02, 2025, 06:06:21 am
content: Quote from: David Hess on January 02, 2025, 01:56:30 amBut then the power is lost in those two diodes.

author: eTobey
author_rank: Super Contributor
timestamp: January 02, 2025, 06:13:18 am
content: Quote from: uer166 on January 01, 2025, 09:48:43 pmI have thought about this:I think Cgs is not the right place: As the mosfet begins to conduct, the source will rise, and thus this capacitor will fill the gate to a certain amount, just in the very wrong moment. Doesnt it?Also:The driver has a neat feature, that allows me to have 3 steps of gate current (the last one beeing max). It can be fine tuned.

author: David Hess
author_rank: Super Contributor
timestamp: January 02, 2025, 06:48:42 am
content: Quote from: eTobey on January 02, 2025, 06:06:21 amPower is lost in the series diode since it increases the forward voltage drop of the switch. The parallel diode takes the place of the MOSFET body diode so reduces loss.