Exceeding Vmax rating of PTC Fuse

Question

I am adding over-voltage, over-current protection in my circuit. My circuit/PCB has many DC-DC converters and these converters power a processor and various other ICs. The input voltage range (that I want to allow but my circuit can withstand more than that) is 7-12V and current will be 350mA (at 12V) or 600mA approx (at 7V).

The over-voltage protection will be provided by Crow-Bar circuit and fuse. The over-current protection should be provided by fuse alone.

After reading about various PTC fuses, I got to know they have maximum Voltage Rating. After reading more, I got to know may be PTC fuses are not good choice for over-voltage protection but I want to know what will happen if we exceed it's voltage rating?

Will it open or short? I guess if it gets open-circuit after applying voltage more than it's rating than it's still useful for my circuit as it will act as PTC re-settable fuse below 60V (rating of my fuse) and as conventional fuse for voltages more than 60V.

The acceptable voltage range to my circuit/PCB (although the circuit can withstand more voltage) is 7-12V but I have selected a PTC re-settable fuse with maximum voltage rating of 60V (0ZCF0100AF2A).

After reading document of my PTC fuse, I got to know PTC fuses has to maintain it's internal temperature, then a short-circuit event at 60V (caused by crowbar circuit) will have higher resistance (and lesser current) than a short-circuit (actual short-circuit event) at 7V. This means at 7V, there will be more current flowing through my circuit (few hundred ohms) and I am not sure if it can cause some issue? Please help.

Also, if the user applies 60V and my TRIAC gets triggered (shorting power rails), then due to very high resistance of PTC fuses (at 60V), a current less than holding current of TRIAC will be flowing which may turn-off my TRIAC. I can sense that TRIAC will keep on turning-on and off. Am I correct? If yes, then how to fix it?

Answer 1

PTC has its place, but over voltage crowbar is seldom it, applications like USB overcurrent protection are more the sorts of place for those things (and even then there are usually better ways).

One trap with PTC is that it derates HARD with elevated temperature, for a design current of 600mA, I would be using a PTC not smaller then a few amps if I was using one at all, your chosen part will be on the edge of tripping at 600mA if the in case temperature gets up to 60C (Which it well might).

The other trap is in the pathetic interrupt rating that these things have, compare with a real fuse from someone like schurter.

I would pick a standard fuse of about a two amp rating, not too fast blow and size the thyristor to suit, job done.

As to the thyristor dropping out and the thing recycling, that is on your design chops, could happen, not guaranteed to, and possibly your DC/DC converters will soak up enough current to hold the thing tripped (Check this at minimum temperature as well as maximum at both extremes of supply voltage).

I don't like the things because you have to spend far too long dicking about with an environmental chamber to be confident that they will work perfectly in an application like this.