Adding Short Circuit Protection

Isn't that a good thing?

If you're that worried about your design, add a over-voltage crowbar circuit. Zener,fuse,scr, and a few resistors...

Kluge in some polyfuses, if you can afford a small voltage drop.

John

What creates the five volts? Most power supply ICs include short circuit protection - called foldback current limiting. Most all power supplies also have that feature.

Meanwhile, if your design has no short circuit protection, then an overvoltage crowbar recommendation would only make things worse.

The power supply chips using MOS pass devices tend to only have thermal shutdown since there is no SOA issue with the MOS device. If so, the output oscillates under short circuit.

If he has a regulator on board, he could put the polyfuse at the input of the regulator.

Tam

That's the second time someone has suggested an "overvoltage crowbar", which sounds like the exact opposite of what the OP was seeking - I seriously doubt that a short circuit condition would cause an overvoltage!

What he needs is a _current sense_ that shuts down the supply over a certain limit. There are as many ways to do this as there are designers; I'd crank up the supply by, say, .7V, and put a current sense resistor and a couple of transistors. Of course, you take your voltage sense _after_ the current sense, and some kind of latching circuit to shut down the supply - maybe a LED to indicate "I've been shorted and shut myself down" or something.

I once did a short-circuit protector in firmware for an SCR phase-controlled, 24V 40A battery charger thatused a 68HC11. It was pretty cool watching that sucker pumping almost a kilowatt into a load, and when you short it, the supply goes "Bup" and shuts down on the next half-cycle. :-)

Of course, we didn't actually short the battery terminals - that's a whole different ball game. ;-)

Cheers Rich

The second time an overvoltage crowbar was mentioned was to disparge the OVC recommendation. The second time OVC was mentioned was the complete opposite of a recommendation.

Meanwhile, that current sense (which is standard in power supply designs) is also called foldback current limiting.

Generally when a power supply oscillates under high load, then a power supply is unstable. Power supply designers learn closed loop theory, and the poles and zeroes in Nyquist stability criteria so that the supply does not oscillate.

When you figure how to add ss protection to the new boards, put that subcircuit on a daughter board and screw it onto the ones in stock.