RE: ECU Burn-out problem
I had the opportunity to open up a buddy trashed V6 ECU to see what MOSFET was used, it turns out that it does indeed have a flywheel diode integrated into the device for protection against back EMF. The device is rated at 75 Amps, 60 Vdc with an Rds on of 10 mohms. The catch is the gate voltage needs to be driven at 10 V in order to get these ratings.
I don’t know what the gate drive circuit from the processor is; however if the gate is being driven at 5V (typical uP voltage) the RDS on increases substantially which in turn will cause the package power dissipation capability to be exceeded due to the increased source to drain resistance.
"My theory" is you can end up in a case of diminishing returns, the high current required during the start sequence (motor and glow plug) causes a voltage drop of the supply voltage, and the voltage drop lowers the gate drive voltage which in turn lowers the devices capability to switch high current, the device then goes into thermal runaway until ultimate destruction.
This seems to follow what Craig explained as to what was happening with his ECU’s during the start sequence.
I copied the graphs from the MOSFET data sheet, the right hand graph show the relationship of gate voltage vs. drain current.