Some time ago I built a H-bridge with 5 paralleled IRFP250N and after a couple of blown-up IR2104s I came up with this driver design that was more about protecting the IC than anything else:
- HO is upper MOSFET gate
- LO is lower MOSFET gate
- Vs is half bridge output
(R2 is not connected. It was designed so I can put another chip in there. R1, R4, R5 are shorted if I recall. R17, R18, R19, R21, R22, R23 are there so I could experiment with different values. Gate resistors are directly soldered to the transistors).
It worked well, and I never had a problem with it. Switching frequency was 400 Hz with about 700 nS turn on and off times. Then I bought two 2MBI200U2A-060 modules for cheap and hooked up these drivers. When no power is applied to the half-bridge this is the waveform on the gate of the lower IGBT:
It looks good to me. Then after applying 30 V to the bridge this is what happened:
The yellow waveform is Vge and blue Vce of lower IGBT. This is without the load. There are snubbers added but there is still some ringing on the output, but this is not what I am concerned about.
The voltage on the gate suddenly rises well above the turn-on threshold and the IGBT actually starts conducting because I can see increased power consumption when this spike goes above 7 V. I tried different gate resistors and diodes as well as adding a PNP gate discharging circuit (using I think it was TIP42C paired with the second transistor connected in Darlington) and the max spike voltage was around 5 V.
When I connect the load it gets a bit better but still not pretty. With a 100 Ω resistor and diode on the gate the turn on time was about 7 μs and turn off about 1 μs but at this point having these times so large is just a loss of power.
I tried different combinations of gate resistors, diodes, and even a second totem pole driver but I can't get rid of this spike. I know I can use negative voltage at the gate but this makes the circuit so much more complicated. How can I prevent this?
I think that this has something to do with Miller capacitance but I'm not sure. The datasheet says that this module has 14 nF input capacitance, so quite high. In the end I plan on using this bridge with 150 V, 180 A running at 400 Hz and I fear that with 150 V both transistors will turn on and blow up.
Any help is appreciated.