In my efforts to debug the failures of the DC-DC converter, I tried to capture a schematic of the board. I didn't have time to create a component for the SG3525A controller, but all the pin numbers are listed. I replaced the IRFP350 FETs with IRFP460 FETs which have a higher isolation voltage and larger drive current; however, they also have about twice the gate capacitance, which I didn't think of.
Any suggestions for what could blow the FETs up would be helpful (please leave in comments section).
You can get a larger image by clicking on the thumbnail above.
Links to the various parts are:
The failing conditions are when the 12V accessories are drawing a lot of current (25 amps) and the output voltage has sagged down to 12V (DC-DC tries to put out 13.9V).
I'm guessing that as the voltage sags, the output (pin 6) of the 741 op-amp goes low and turns off the LED in the opto-coupler. With the opto-coupler off, the voltage goes low on pin1 of the SG3525A (inverting input to error amp) and increases the pulse widths going to the drive FETs. If the pulse widths are too long, they cause the FETs to stay on too long and saturate the inductors, which causes a high-current spike of +144V that blows the FETs.
Supposedly, the current sense loop voltage that is adjusted by the R8 potentiometer should send a positive pulse into pin10 of the controller to shut down the FETs, but this may not be happening quickly enough.
Initially, I cringed because there were no kick-back diodes in the circuit, but I think that C7 and R16 (2 watt resistor) send any kick-back currents into the opposite inductor when the FETs let go.
Any ideas? Thanks for your input.