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I've designed a three-phase inverter for a hobby project that I'm working on and have had the board assembled. I'm new to this sort of stuff so learning as I go. I've attached the schematic. Initially after powering up the board, the mosfets immediately burnt out. I eventually determined that I'd used far too high a series gate resistance which prevented the transistors from turning on fast enough to prevent shoot through. I solved this by simply removing the 4K7 resistors and shorting the pads together.

After solving that, I connected my load to the outputs of the inverter. The load on each phase is a coil in series with a 100R resistor. The coils are designed for magnetically levitating toys however, I have no data sheet for these parts and I'm unsure of their exact specification (I can post a link to these parts, if that's an acceptable thing to do in this forum?). VCC is 20V so I would not expect more than 200mA of current to flow through each phase (the transistors are rated up to 8A continuous drain current I think). I used a microcontroller to generate the control signals to the gate drivers. These were the standard phased enables used in such a circuit with a period of 1200us and a duty cycle of 50%. All of this seems to work fine, with the voltage applied to each phase correctly switching between +/- Vcc.

However, during experimentation, I decided to extend the duration of the pulses to 1200ms, again with a 50% duty cycle. Again, this caused the mosfets to burn out and I'm not sure why (I'm not sure if it was both the low side and the high side that burnt out). I think that the value of the bootstrap capacitor that I've used is too small to keep the high side transistors on for such a long period and that eventually they will turn off. However, I can't understand why this transistor turning off causes an issue. It does not seem to me that this should cause excessive current to flow and I don't think that it's an issue with the current in the inductors rapidly switching off and generating large voltage spikes but I can't be sure of this.

So, I'm at a bit of a loss to understand what the issue is here. I'm hoping that someone can help explain what might be happening here and suggest improvements to this circuit. Please let me know if any further information is required.

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Andy_aka comment: Maybe a redraw of your circuit might help those on tablets and laptops: -

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  • \$\begingroup\$ Connecting HIN and LIN together is probably causing issues aswell. You're supposed to add some dead time mechanism. \$\endgroup\$ Aug 14, 2021 at 11:17

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However, I can't understand why this transistor turning off causes an issue.

Yes, it IS an issue. As the transistor turns off (slowly) it enters its linear mode of operation (gate voltage level below the ZTC point) and will immediately be subject the thermal runaway: -

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In applications like this you have to ensure that the bootstrap circuit does not sag or, you choose a MOSFET that can handle linear applications.

BTW, ZTC stands for zero temperature coefficient. Above that point (gate-source voltages higher than about 4 volts), if the device warms up, drain current falls i.e. it self-protects. Below the ZTC, if the device warms then drain current rises and the device warms more and... you get the story.

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