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With the circuit below:

enter image description here

Q2 and Q4 burn down when 15A is injected into the circuit. Q4 burns even if it were never activated.

The gate voltage control is 0 or 12V to activate a particular channel.

I've tried to add protection components (zener / res on the gate), but it still burns down.

I don't really get why. The circuit is referenced to the ground with Q3 and Q6 forward diod make sure the voltage drop is reasonable, which would create a dV of 1-2V. The shunt itself would make a dV of less than 0.1V which would give a Vgs of ~10V in the worst case which is plenty enough to drive the mos:

enter image description here enter image description here

Is there something I am missing on this circuit?

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  • \$\begingroup\$ What is the Voltage applied to Q2 and Q4 drains? This is crucial to understanding what is happening What heatsinking are you using on the MOSFETs? \$\endgroup\$ – Russell McMahon Jul 24 at 8:35
  • \$\begingroup\$ I start to think it might be due to PCB layout and connectors introducing higher than expected voltage offset which makes the gate being driven at lower voltage than it should. I've added optical isolation and see if that solves the issue. \$\endgroup\$ – Damien Jul 25 at 9:35
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What is the Voltage applied to Q2 and Q4 drains?
This is crucial to understanding what is happening.

What heatsinking are you using on the MOSFETs?

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  • \$\begingroup\$ In the test when it burned was about 50VDC when the gate is low. it goes to I*5mOhm + mosfet Vds worse case. \$\endgroup\$ – Damien Jul 25 at 9:33

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