[Solved] Why does the gate driver get destroyed when fuse blows? - Page 1
Forum section: electronics / projects

author: Kryptychon
author_rank: Contributor
timestamp: June 25, 2023, 12:30:37 pm
content: Hi,I've the following H-bridge circuit which is working as expected.This is a private project to learn about power electronics and control loops.The provided schematics show the power supply railsand the actual H-bridge including the driver circuit. For testing purposes,the power rails are fed by lab PSUs. The DC link of the H-bridge isadditonally decoupled with a capaciator bank (63000uF). The DC link voltageis 30V.There are also curcuits for over temperature and over current protection whichI omitted for now as I think they're not relevant for my question / issue.For clarification I attached two images of the PCB. On the PCB there are twoH-bridges which are exactly the same. Only one side is assembled and workingin the state the images are showing.The issue: When testing the current measuring in software, I noticed, when Iprovoke a current spike which will blow the fuse by e.g. quickly changing themotor direction, occasionally, one of the gate drivers gets damaged/destroyed.I need help to understand why this happens.I vaguely concluded that in the moment the fuse interrupts the DC link, it also disconnectsthe DC link capacitance and even worse, the TVS diodes. The overall inductivity(motor, cables, traces) induces a voltage spike wich eventually destroys thegate driver.Can anyone confirm my theory and point me to the exact why the gate driver gets destroyed?Thank you very much. Any help is greatly appreciated!RegardsKryp

author: Andy Watson
author_rank: Super Contributor
timestamp: June 25, 2023, 12:44:21 pm
content: When the fuse blows I do not see anything to clamp the main power-supply to a safe level, except C5 and C9 (which will not absorb much energy).

author: Kryptychon
author_rank: Contributor
timestamp: June 25, 2023, 01:11:37 pm
content: Thanks for your response.Quote from: Andy Watson on June 25, 2023, 12:44:21 pmIf I understand you correctly, you're confirming my theory that's an induced voltage spike which has no way to go after the fuse has blown?Any idea, why only one of the two gate drivers gets destroyed in such an event?

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 25, 2023, 01:19:55 pm
content: The spike is in VS pin. Only one side of bridge blows since (I assume) at any given time there's one of either Q3 or Q4 happens to be conducting, therefore "saving" that side of bridge.Edit: Thinking about it, the spike should only happen if all MOSFET happens to be OFF at the same time (dead time maybe) during load dump.Edit2: It also happen when Q1 and Q4 off when load is dumped to the right, or when Q2 and Q3 off when load is dumped to the left.

author: Andy Watson
author_rank: Super Contributor
timestamp: June 25, 2023, 01:20:41 pm
content: Quote from: Kryptychon on June 25, 2023, 01:11:37 pmGuessing (again), I suggest that one of the gate drivers blows and turns-on the Mosfet - thereby clamping the power-supply voltage. And/or, remember that that the state of the driver bridge is not identical when the fuse blows - one side of the bridge will be at a low voltage and the other at a high voltage - giving different voltage stresses on the gate drivers.

author: DavidAlfa
author_rank: Super Contributor
timestamp: June 25, 2023, 01:52:02 pm
content: I can't see how the battery power would come into the 12V rail powering the fet drivers, but the schematic also lack any details about it.Are you sure the mosfet isn't blowing too?Normally the gate will short out, so all the power will go to the driver with virtually no current limitations through that 4R7 resistor.You might add a protection device like a TVS, so it clamps slighly over the normal gate voltage, but will add capacitance increasing the switching load, might also cause other negative effects.The idea is to overload the 4R7 resistor and blow it before anything else.

author: Kryptychon
author_rank: Contributor
timestamp: June 25, 2023, 06:07:51 pm
content: Thanks for all the responses!@ArdWarI think you're right!I can confirm this measuring the Vs pin (but without blowing the gate driver as this test method gets expensive quickly...).The spike is visible here.The control sheme is: Either Q3 or Q4 will be conducting the whole time and PWM will be applied to Q1 / Q2 respectively. So what you think makes perfect sense to me.@Andy WatsonYeah, I think that's also correct. Taking into account what ArdWar said it depends what direction the motor is turning initially. Say it turns clockwise and I suddenly request it to turn counter clockwise. For a brief moment the battery voltage is "opposed" (the term is physically not correct) to the back EMF of the motor, this creates a large current which makes the fuse blow and creates a voltage spike at either A1 or A2 (depending on the initial dicrection of the motor. As either, Q3 or Q4 will provide a path to GND the opposite point will see the voltage spike. As A1 / A2 are connected to Vs, the gate driver gets damaged. This makes sense as the datasheet gives the maximum of VS of 0.3V above VB.@DavidAlfaThe +12V rail is provided from another PSU for the tests. In the real application the battery feeds multiple DC/DC converters which will provide the different power supply rails.No MOSFET is damaged when this occurs. But it might happen when the currents will be higher. For the testing I put a 5A fuse, in the real application there will be a 20-30A fuse.I think I should add a zener to the gate drive traces anyway - just in case. Switching speed is not critical, I've some head room here (PWM is at 15kHz). On the other hand, if the gate driver gets damaged because a MOSFET has been destroyed it's either a bad design (as mine is right now) or the bridge has been pushed too hard.Addendum: Out of curiosity, I soldered a second board and tried pushing it harder and indeed, the MOSFET itself gets destroyed and takes the gate driver with it. So you're also right :-(So, what are the learnings so far?- Having the fuse placed between power supply (battery) and DC link capacitance is not ideal- Same applies for the TVS diodes: they must be placed so they're effective even when the fuse has blown- Protect the MOSFET gates with a TVS/zenerRegardsKryp

author: Andy Watson
author_rank: Super Contributor
timestamp: June 26, 2023, 02:59:29 pm
content: Does your snubber circuit have any effect? I think the body diode of the Mosfet is going to act first and take any transient or free-wheeling current.

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 26, 2023, 03:37:55 pm
content: It won't freewheel on it's own without at least one opposing leg conducting when supply is HiZ. Well, at least not without pumping VS and Vbat.I actually not even sure if my previous hypotheses was what actually happen. VS is isolated at 600V breakdown rating so logically the MOSFETs should fail much much earlier before the drivers. Unless there's other thing breaking down (eg bootstrap diode allowing VB to be negative relative to VS or something)

author: Andy Watson
author_rank: Super Contributor
timestamp: June 26, 2023, 07:29:09 pm
content: Quote from: ArdWar on June 26, 2023, 03:37:55 pmThe conduction path is provided by the body-diode of the Mosfet in the "opposing leg".Regardless, I still think the body diode(s) make the snubber circuit redundant.