PSUs: current-limiting and crowbarring - incompatible?

[[email protected]]

Hi gang,

Is it not feasible to use a crowbar circuit across a PSU's output if
that PSU has a current-limiting feature? I can imagine a situation
arising where an over-voltage causes the crowbar to close, generating a
short-circuit condition which activates the current limiting. The
output will then be stuck at not enough amps to blow the upstream fuse,
but enough current to smoke the crowbar (which is not designed to be
kept conducting for any length of time of course). Is there some way to
have both over-current and over-voltage protection working together in
harmony?
Thanks.
p.

 

[Nico Coesel]

Hi gang,

Is it not feasible to use a crowbar circuit across a PSU's output if
that PSU has a current-limiting feature? I can imagine a situation
arising where an over-voltage causes the crowbar to close, generating a
short-circuit condition which activates the current limiting. The
output will then be stuck at not enough amps to blow the upstream fuse,
but enough current to smoke the crowbar (which is not designed to be
kept conducting for any length of time of course). Is there some way to
have both over-current and over-voltage protection working together in
harmony?
Thanks.

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.

 

[Roy Lewallen]

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

 

[Tim Shoppa]

Hi gang,

Is it not feasible to use a crowbar circuit across a PSU's output if
that PSU has a current-limiting feature? I can imagine a situation
arising where an over-voltage causes the crowbar to close, generating a
short-circuit condition which activates the current limiting. The
output will then be stuck at not enough amps to blow the upstream fuse,
but enough current to smoke the crowbar (which is not designed to be
kept conducting for any length of time of course). Is there some way to
have both over-current and over-voltage protection working together in
harmony?
Thanks.

Look up "Foldback current limiting".

Tim.

 

[Tom Bruhns]

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.

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

 

[Ralph Mowery]

Hi gang,

Is it not feasible to use a crowbar circuit across a PSU's output if
that PSU has a current-limiting feature? I can imagine a situation
arising where an over-voltage causes the crowbar to close, generating a
short-circuit condition which activates the current limiting. The
output will then be stuck at not enough amps to blow the upstream fuse,
but enough current to smoke the crowbar (which is not designed to be
kept conducting for any length of time of course). Is there some way to
have both over-current and over-voltage protection working together in
harmony?
Thanks.
p.

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.

 

[Joop]

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

Joop

 

[Fred Bartoli]

Tom Bruhns a écrit :

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.

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.

 

[Fred Bloggs]

Is it not feasible to use a crowbar circuit across a PSU's output if
that PSU has a current-limiting feature? I can imagine a situation
arising where an over-voltage causes the crowbar to close, generating a
short-circuit condition which activates the current limiting. The
output will then be stuck at not enough amps to blow the upstream fuse,
but enough current to smoke the crowbar (which is not designed to be
kept conducting for any length of time of course). Is there some way to
have both over-current and over-voltage protection working together in
harmony?

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

 

[[email protected]]

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. :-/

 

[Jim Thompson]

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

 

[legg]

Hi gang,

Is it not feasible to use a crowbar circuit across a PSU's output if
that PSU has a current-limiting feature? I can imagine a situation
arising where an over-voltage causes the crowbar to close, generating a
short-circuit condition which activates the current limiting. The
output will then be stuck at not enough amps to blow the upstream fuse,
but enough current to smoke the crowbar (which is not designed to be
kept conducting for any length of time of course). Is there some way to
have both over-current and over-voltage protection working together in
harmony?

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

 

[legg]

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. :-/

You haven't missed anything.

RL

 

[You]

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.

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.

 

[legg]

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

 

[jasen]

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

From the header:

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


Bye.
Jasen