PSUs: current-limiting and crowbarring - incompatible?

I see no problem. If the crowbar is active, the voltage accross it is about 0V so power dissipation in the crowbar device is minimal. You could probably use a SOT23 trenchFET as a crowbar which would be capable of handling a few amps.

Most supplies I've seen with both crowbar and current limiting use fold-back current limiting. When the current exceeds the set limit (due to too heavy a load or to the crowbar firing), the current drops back to a level considerably lower than the limit current, and one which the crowbar can tolerate indefinitely.

Roy Lewallen

snipped-for-privacy@burridge8333.fsbus> Hi gang,

Look up "Foldback current limiting".

Tim.

Yeah, and not only the crowbar, but the supply itself can likely tolerate it better! :-) Especially if it's a linear supply. Paul: imagine a linear supply designed to deliver 24V at a couple amps. The input to the regulator part might well be nominally 30 volts or more, to accomodate low line input and drop in the regulator pass element. At high line, it might be 33V. In normal operation, the regulator might drop 9 volts at a couple amps, dissipating 18 watts. An SCR crowbar might drop 1.5V. At 2 amps, that only 3 watts, but the regulator would be dissipating over 60 watts. But with foldback limiting, the dissipation would drop in both the SCR and the regulator, staying in pretty much a constant ratio. With a switching regulator, things probably wouldn't be nearly as bad, but the dissipation may still go up in the switching parts if you short the output.

The other thing I've seen is a crowbar for fast response, and a latching shutdown fed back to the supply that holds it off until something is reset.

Cheers, Tom

Look up the Astron supplies. They will go into a current limiating mode if the over voltage protection activates. That is if there is a spike or something. Most of the time something fails (say the pass transistors) that makes the supply go into the over voltage condition and due to the failed component the supply can not go into the current limit mode.

On Fri, 20 Oct 2006 22:35:04 GMT, "Ralph Mowery" wrote: something

From the header:

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Is is always this newsreader that causes threads to break up?

Joop

Tom Bruhns a écrit :

One pb with SMPS is that when you crowbar its output the input current is lowered since you ask for lower output power. You'll never blow the fuse. Another point, depending on the SMPS structure and on the controller minimum on time is that you have pretty low output voltage when crowbared (?) which may be too low to reset the inductor current (for ex. flyback, buck). In some poorly design supplies this might lead to higher and higher switch/diode currents, inductor saturation, sill higher currents and then fireworks. I prefer latching the fault condition then inhibit the controller, with crowbar or not, which is what you recommend below.

When the hell are you going to realize that you're not cut out for the "hard" application sciences?

That's nice! I haven't posted for several months and that's the 'welcome back' I get. I'm afraid your observation is, however, correct. If I found electronic design to be a cakewalk I'd have lost interest in it decades ago. :-/

Paul, You've been gone long enough that your PLONK expired.

Damn. I'll have to fix that ;-)

...Jim Thompson

A properly designed crowbar should be able to handle the current and stored charge of the supply it's monitoring. If a crowbar fails short in the process, it's still doing it's job, but that is unlikely to happen if foldback limiting occurs.

OVP and current limiting are both possible using small-signal techniques, but the OVP's latch may require an independant method of crossing the isolation barrier for effect.

RL

You haven't missed anything.

RL

Actually, a Crowbar System ONLY needs to be able to blow the fuse, or circuitbreaker that protects the LOAD from the overvoltage condition that the Crowbar is monitoring. Protecting the supply from overcurrent draw condition is the Foldback Current Limiter's Job. Two completely different senerios.

The aim is to protect downstream components from overvoltage. Overvoltage is an indication of loss of control. To count on the presence of other limiting circuitry, completely independent of the voltage regulator, may be whistlng for it.

At one time, thyristors with large I^2t characteristics were marketed for crowbar applications. Nowadays, it might be more sensible to make use of a semiconductor circuit breaker, integrated into bus sequencing circuitry.

RL

I noticed no break... from where is the thread broken off?

Bye. Jasen