No more PolyZens :(

Pity. We need a really good resettable fuse. We tried the TI Efuse things, and blew some up. The powerpad makes them really hard to replace. No semiconductor should be connected to an external pin, much less a power input.

One current design technique here is to thermally couple a thermistor to a big DPAK power resistor and shut things down when it gets too hot.

Yeah, I'm doing something similar--back-to-back PFETs controlled by the uC, which is powered by a bulletproof +3.3V supply. It measures the voltage on the input supply and turns off if they're out of spec. This can also be done with NFETs and a level shifter for negative supplies, of course.

Irritating to have to do that though, when a PolyZen is about the same price, much faster, and much easier to use.

Cheers

Phil Hobbs

At least from the distributors I was using, the range of PolyZens to choose from was quite sparse anyway.

Perhaps someone could design a little board with a surface mount TVS and surface mount polyswitch on it, maybe right next to each other or on opposite sides of a thin board, with lots of vias in between. I wonder whether this would work nearly as well as a PolyZen. This board could be placed as a component on a bigger board, though I haven't thought much about what would be the best way to do that.

You'd probably be infringing the Tyco patents. :(

Thermal coupling between small SMT parts mounted close together on an isolated pour is pretty good. We do high-performance temperature controllers for detectors and diode lasers by putting 0603 thermistors on the cold-plate board, with one end connected to a bottom-side pour in intimate contact with a thermoelectric cooler, and a minimum-width trace on the other end.

We routinely get thermal response times below 100 milliseconds that way, which makes it easy to do fast control loops.

Thus I'd expect that for a positive supply a TVS with a thermal pour on its cathode, shared with a SMT polyfuse, might do fine, especially if there were some slots preventing lateral heat transfer in other directions.

Getting that right is an awful lot of work compared with specifying a

Cheers

Phil Hobbs

We have considered doing that on a regular PCB, thermally coupling as best we can. But we don't like the surface-mount polyfuses; we use the radial leaded ones, which seem to work much better.

The usual combination of polyfuse and TVS, thermally uncoupled, can be teased to fry the TVS. We standardize on 24 volt warts, on the theory that some yahoo plugging in some higher voltage is unlikely.

My new pulse generator output stage is difficult, more like impossible, to protect electronically, so I have a thermistor coupled to the big DPAK 50 ohm output resistor, to shut things down if it gets too hot.

I assume they are still making the polyfuse. The polyzen is the resetable fuse and zener? You'll just have to add your own zener.

George H.

The genius of the PolyZen is that the polyfuse is tightly coupled thermally with the TVS, so as the TVS heats up, it makes the polyfuse switch much faster.

It's guaranteed to switch before the TVS melts or desolders itself, which the separate approach is not. To be safe with the discrete approach, you have to use a much bigger TVS and leave space for thermal pours, etc. It's a pain.

Cheers

Phil Hobbs

OK.. I guess I just use big zeners, maybe it's a market opportunity for someone?

George H.