Electronics > Projects, Designs, and Technical Stuff
Failed electronics from customer
MagicSmoker:
--- Quote from: Psi on October 07, 2019, 01:22:32 pm ---
--- Quote from: mattko on October 07, 2019, 06:45:42 am ---Gate resistor is 10 Ohms and driven from internal driver in the MCU.
--- End quote ---
Is this a specific "gate driver" built into the mcu designed to drive mosfets? or just a GPIO ?
...
--- End quote ---
Oh, I totally missed that... I sure hope the OP isn't trying to directly drive a MOSFET from an MCU GPIO pin (well, not one bigger than a 2n7000, anyway)!
Psi:
It's fine as long as the frequency is low and you have enough dead time.
mattko:
--- Quote from: Siwastaja on October 07, 2019, 06:58:24 am ---Test and verify that your overcurrent protection kicks in properly.
Measure switch node ringing (peak voltage) using high bandwidth (at least 100MHz?) oscilloscope with full load. Is it close to the MOSFET ratings?
Measure gate voltages with full load. Does it approach the limits, even for very short times? How much negative does it go? Some gate drivers die at slightly negative gate voltages, say, just -5V will kill some.
--- End quote ---
Overcurrent protection is OK, but we are not sure if in some "special cases" it can cause a problem.
Switch node voltage under normal operation is well below MOSFET rating.
Gate voltage goes to -1 V. The driver alowed voltage is down to -8 V.
--- Quote from: MagicSmoker on October 07, 2019, 12:44:33 pm ---I bet you are using one of those shitty level-shifting driver ICs that require a bootstrap charge pump for the upper MOSFET, aren't you? Those things are notoriously sensitive to high dV/dt leading to all sorts of mysterious failures at frustratingly random times.
--- End quote ---
Correct. We use a bootstrap capacitor to double the voltage to open the high side MOSFET. Is there some documentation regarding bootstrap and dV/dt problems?
--- Quote from: mattko on October 07, 2019, 06:45:42 am ---Is this a specific "gate driver" built into the mcu designed to drive mosfets? or just a GPIO ?
Do you have TVS/Zener diodes between the gates and sources to prevent Vgs spikes from exceeding the max and damaging the mosfets?
--- End quote ---
It is a gate driver in the MCU. MCU is specialized in motor control aplication.
We do not have a zener to protect Vgs over/under voltage due to cost reasons. But we have a TVS input diode in the circuit, so an overvoltage to the MOSFETs is unlikely.
--- Quote from: Psi on October 08, 2019, 01:43:22 am ---It's fine as long as the frequency is low and you have enough dead time.
--- End quote ---
Frequency is 13kHz and dead time is 400 ns. I think this is enough.
MagicSmoker:
--- Quote from: mattko on October 08, 2019, 06:43:20 am ---Correct. We use a bootstrap capacitor to double the voltage to open the high side MOSFET. Is there some documentation regarding bootstrap and dV/dt problems?
--- End quote ---
There's lots of documentation available such as this PDF from OnSemi which I pulled at random from a duckduckgo search and verified it discusses the issue: https://www.onsemi.com/pub/Collateral/AND9674-D.PDF
aiq25:
--- Quote from: mattko on October 08, 2019, 06:43:20 am ---
--- Quote from: MagicSmoker on October 07, 2019, 12:44:33 pm ---I bet you are using one of those shitty level-shifting driver ICs that require a bootstrap charge pump for the upper MOSFET, aren't you? Those things are notoriously sensitive to high dV/dt leading to all sorts of mysterious failures at frustratingly random times.
--- End quote ---
Correct. We use a bootstrap capacitor to double the voltage to open the high side MOSFET. Is there some documentation regarding bootstrap and dV/dt problems?
--- Quote from: mattko on October 07, 2019, 06:45:42 am ---Is this a specific "gate driver" built into the mcu designed to drive mosfets? or just a GPIO ?
Do you have TVS/Zener diodes between the gates and sources to prevent Vgs spikes from exceeding the max and damaging the mosfets?
--- End quote ---
It is a gate driver in the MCU. MCU is specialized in motor control aplication.
We do not have a zener to protect Vgs over/under voltage due to cost reasons. But we have a TVS input diode in the circuit, so an overvoltage to the MOSFETs is unlikely.
--- End quote ---
I worked on a couple of 12V to 120/230 Vrms inverter module projects that initially used discrete charge pump circuits for the H-Bridge to drive the AC output. They did not work well under high inrush loads. They didn't have enough drive and the frequency response needed to turn OFF the MOSFET's under high loads. Our senior/lead engineer changed to isolated gate drives (not necessary but that's what he did) and I copied his design for the modules I worked on. That solved a lot of issues we were having with the H-Bridge.
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