Buck Regulator Running From Car Battery Stability Problems
You're likely dealing with either voltage spikes or big dips. I would tend to agree with those who believe that it's more likely a spike vs. a dip that's causing downstream damage.

You should put a high-power rated TVS or other crowbar across the input of your circuit, to attempt to contain big spikes. A DC line filter will also help smooth out high-frequency disturbances.

Ultimately, you may need a crowbar on the output as well, just in case the other protections fail (What happens if the buck goes to 99% duty cycle or the series MOSFET fails?) that's rated big enough to blow your fuse. Um ... please tell me you have a fuse in your design...


  Another problem might also be because the regulator is running in discontinuous mode most of the time.


DCM is a good choice of operating mode for this topology, since there's only a type-2 compensation network on the voltage feedback. If the converter transitions to CCM, the added phase boost that a type-3 compensator gives is often necessary to ensure stability.