Electronic – Protecting a switching regulator in a 12v automotive system

One of the main differences between a TVS and a Varistor is that the voltage tolerance of a TVS is much tighter than a Varistor. They also don't appear to have the "wear" factor that Varistors have.

Varistors that are repeatedly subjected to transients appear to have their clamping voltage lowered as a result of the stress. Eventually, they become conductive at the operating voltage and then fail catastrophically. That is: they get very hot, then turn black, then eventually catch fire.

Many of the better AC power bars that have Varistor clamps within also have a thermal cutoff device mounted between two Varistors. The thermal cutoff device disables the power bar when a Varistor fails.

In terms of what happens to a transient suppression device when it is exposed to voltages in excess of their rated clamp voltage: they get hot. The will, in fact, get hot enough to fail.

When a Varistor fails because of over-temperature, it generally becomes restively-conductive. That is, it turns black, then catches fire. Your over-current device may or may not operate, depending on how low the resistance of the Varistor has dropped.

When a Tranzorb fails because of over-temperature, it generally becomes a short-circuit. This causes sufficient current to flow to cause your over-current device to open.

Would a design using either PPTC or automotive circuit breakers be feasible?

OK I'm going to answer and say "No", PTC resettable fuses just won't cut the mustard. I'm basing this on a search of Farnell. I narrowed down the search by telling Farnell's search engine to look for PTC fuses that were rated at 60V or more. From memory, 60V is a decent enough margin in automotive applications to allow electronics to survive i.e. a 60V rating gave something reliability in the automotive environment.

Once narrowed down, the largest fuse available was 3.75 amps and this barely eats into the rangle of 3A to 30A specified in the question. Given also that these types of fuse seem to hit the end stops at 85C I would also consider them to be "fragile" in these applications.

Passive aging, thermal shock and humidity aging is something else that makes them look unsuitable but I'm no expert here. This specification gave no indication of how they cope with vibration either and, I'm sure this is important. Also, on page 1 of the data sheet (normally where all the intentional lies occur), "Automotive" was not listed or implied as an application.

My search is not exhaustive so I would encourage you to use my methodology to see if something looks like it might be suitable.