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Over Voltage Protection Crowbar Circuit

P

panfilero

Jan 1, 1970
0
I'm needing an over-voltage protection circuit to put at the output of a switching regulator that takes in 100V input and drops it to 24V... I'd like to protect the output from going over 28V.

Here's the 3 options I was considering: For all the options the point is to blow my 5A fast acting fuse that's at the input, so I'm trying to decide on a way to short my output

1. A Zener - I'm thinking these get rather bulky for higher current handling, and they're not very accurate

2. A TVS - Probably better than the Zener option as far as current handling and bulkiness... I think TVS kills Zener for this application

3. An SCR - I could trigger an SCR off of a LM431 to get an accurate trip voltage and I think I could probably find a decently sized SCR that could handle the current required to make the fuse pop

Anybody have any thoughts or recommendations for OVP circuits like this... I'm likeing the SCR option right now

much thanks!
 
I'm needing an over-voltage protection circuit to put at the output of a switching regulator that takes in 100V input and drops it to 24V... I'd like to protect the output from going over 28V.

Here's the 3 options I was considering:  For all the options the point is to blow my 5A fast acting fuse that's at the input, so I'm trying to decide on a way to short my output

1.  A Zener - I'm thinking these get rather bulky for higher current handling, and they're not very accurate

2.  A TVS - Probably better than the Zener option as far as current handling and bulkiness... I think TVS kills Zener for this application

3.  An SCR - I could trigger an SCR off of a LM431 to get an accurate trip voltage and I think I could probably find a decently sized SCR that could handle the current required to make the fuse pop

Anybody have any thoughts or recommendations for OVP circuits like this.... I'm likeing the SCR option right now

much thanks!

why not something like this: http://www.analog-innovations.com/SED/OverAndReverseVoltageProtection.pdf
no need for fuses then

-Lasse
 
J

Joerg

Jan 1, 1970
0
panfilero said:
I'm needing an over-voltage protection circuit to put at the output
of a switching regulator that takes in 100V input and drops it to
24V... I'd like to protect the output from going over 28V.

Here's the 3 options I was considering: For all the options the
point is to blow my 5A fast acting fuse that's at the input, so I'm
trying to decide on a way to short my output

1. A Zener - I'm thinking these get rather bulky for higher current
handling, and they're not very accurate

2. A TVS - Probably better than the Zener option as far as current
handling and bulkiness... I think TVS kills Zener for this
application

3. An SCR - I could trigger an SCR off of a LM431 to get an accurate
trip voltage and I think I could probably find a decently sized SCR
that could handle the current required to make the fuse pop

Anybody have any thoughts or recommendations for OVP circuits like
this... I'm likeing the SCR option right now

much thanks!


#3 is the only option I ever consider. Personally I like to have the SCR
right at the circuit side of the fuse. If the fuse were at the input and
the SCR were at the output you'd have the upper FET and the inductor in
the path. This will slow down the fuse tripping and there is a chance
that the FET grenades before the fuse trips, something that is generally
not desired.
 
P

panfilero

Jan 1, 1970
0
I'm needing an over-voltage protection circuit to put at the output of a switching regulator that takes in 100V input and drops it to 24V... I'd like to protect the output from going over 28V.



Here's the 3 options I was considering: For all the options the point is to blow my 5A fast acting fuse that's at the input, so I'm trying to decide on a way to short my output



1. A Zener - I'm thinking these get rather bulky for higher current handling, and they're not very accurate



2. A TVS - Probably better than the Zener option as far as current handling and bulkiness... I think TVS kills Zener for this application



3. An SCR - I could trigger an SCR off of a LM431 to get an accurate trip voltage and I think I could probably find a decently sized SCR that could handle the current required to make the fuse pop



Anybody have any thoughts or recommendations for OVP circuits like this... I'm likeing the SCR option right now



much thanks!

Thanks for the input, can anyone tell me why the SCR is a better option than the TVS? I was considering using one of these guys...

http://www.littelfuse.com/products/tvs-diodes/surface-mount.aspx
 
J

Joerg

Jan 1, 1970
0
panfilero said:
Thanks for the input, can anyone tell me why the SCR is a better option than the TVS? I was considering using one of these guys...

http://www.littelfuse.com/products/tvs-diodes/surface-mount.aspx


The SCR is much faster. Assume this scenario: The FB resistor comes
unglued for some reason, gradually. Now the voltage slowly rises, the
TVS begins to conduct, gets hot, gets hotter, starts to smell, smoke
arises, a smoke alarm goes off, your lab begins to become drenched in
sprinkler water, the bell rings at the local fire station, engine
company 27 roars down the road with wailing sirens ...

The SCR, in contrast, can be control by a "snap action" circuit. Voltage
exceeds a certain threshold ... WHAMMO ... fuse blows, the day is saved.
 
M

mike

Jan 1, 1970
0
I'm needing an over-voltage protection circuit to put at the output of a switching regulator that takes in 100V input and drops it to 24V... I'd like to protect the output from going over 28V.

Here's the 3 options I was considering: For all the options the point is to blow my 5A fast acting fuse that's at the input, so I'm trying to decide on a way to short my output

1. A Zener - I'm thinking these get rather bulky for higher current handling, and they're not very accurate

2. A TVS - Probably better than the Zener option as far as current handling and bulkiness... I think TVS kills Zener for this application

3. An SCR - I could trigger an SCR off of a LM431 to get an accurate trip voltage and I think I could probably find a decently sized SCR that could handle the current required to make the fuse pop

Anybody have any thoughts or recommendations for OVP circuits like this... I'm likeing the SCR option right now

much thanks!

Is this a dedicated supply where you KNOW what it's gonna be hooked to
in EVERY case?
There are times when it's not a good idea for the output to short.
Like when you're charging a battery.
 
J

Joerg

Jan 1, 1970
0
Tim said:
What Joerg said.

And what Joerg said: you can't blow the fuse on the input side by putting
a TVS on the output side unless you go through the FET and whatnot. With
an SCR you can sense overvoltage on the output and blow a fuse on the
input.

And what Mike said about a crowbar on the output.

And finally -- if I'm remembering correctly, the TVS's that you cite
_are_ zener diodes -- they're just optimized for TVS service. If you're
dead set on using them, remember that they're _transient_ voltage
suppressors -- Joerg's example of things slowly going to hell and the
output rising isn't a transient any more.

Isn't it possible to make a switching supply such as this with a topology
and a transformer with a turns ratio that limits the output voltage to
some specific fraction of the input? It's probably not economical unless
it's a honkin' big supply, but it would give you a supply that
"naturally" can't go above your 28V with a 100V input.

Sounds like the OP is using a buck. A sync buck would give at least some
level of protection. A white-knuckle event is when the upper FET welds
through. Eventually, depending on its control architecture, the lower
FET will come on and then one can only hope that the fuse opens. A
non-synchronous buck would not protect at all, the output would head
straight up to Vin and most likely leave a path of destruction in
anything connected.

Belts-and-suspenders would be to sense the output and trip a beefy SCR
right at the fuse.
 
Yup, a well-sized SCR is probably the best at producing a low-drop short
when called upon, and surviving the event. Maybe that is not critical
in your design.

Please remember to put some ferrites in series with the SCR gate
drive.

I once made a power supply for a 100 W RF transmitter. When used with
an indoor antenna, the RF signal could get into the protection circuit
and trigger the SCR and blow the fuse.
 
L

legg

Jan 1, 1970
0
Is this a dedicated supply where you KNOW what it's gonna be hooked to
in EVERY case?
There are times when it's not a good idea for the output to short.
Like when you're charging a battery.

With the battery attached, the overvoltage condition would be
difficult to detect, until the battery was toast. A current regulator
failure in this condition would likely blow the fuse without external
aid.

If it's a back-up battery, it should be connected into the circuit in
such a manner that source failure (including the shorted condition)
doesn't interfere with the intended back-up function.

There ARE issues with scr failure, when improperly sized for large
storage capacity and supply/fuse combinations. Parts intended for
crowbar applications have I^2t ratings that are considerably larger
that jellybean devices. MCR67,68 and 69 were examples of parts
designed for this use. The latter two are still available.

RL
 
LOL
very long time ago, my first job, we were making
big controlled rectifiers for the army, used to test their transmitters.
We got a call that the voltage was not stable, went way up,
and killed their transmission equipment.
We went there, and sure enough, voltage went way up,
measured some things, case was not grounded, electronics got the RF,
bad things happened.
After grounding the cases things worked normally.

The dimensions of the human body match quite well with 1/2 or 1/4
wavelengths at VHF frequencies, thus the radiation limits for humans
are quite low at VHF frequencies.

However, at HF frequencies, the limits are quite higher, so in reality
much higher levels are allowed, so in places into which people are
allowed to freely enter, any equipment should also tolerate these
field strengths.

The situation gets worse, when long wires are attached to the device,
such as speaker cables in an audio amplifier or power supply wires,
forming antennas much longer than the human body.

For this reason audio amplifiers and DC power supplies needs some
filtering in the external wires (or at least in the feedback
circuitry).
 
M

mike

Jan 1, 1970
0
With the battery attached, the overvoltage condition would be
difficult to detect, until the battery was toast. A current regulator
failure in this condition would likely blow the fuse without external
aid.

If it's a back-up battery, it should be connected into the circuit in
such a manner that source failure (including the shorted condition)
doesn't interfere with the intended back-up function.

There ARE issues with scr failure, when improperly sized for large
storage capacity and supply/fuse combinations. Parts intended for
crowbar applications have I^2t ratings that are considerably larger
that jellybean devices. MCR67,68 and 69 were examples of parts
designed for this use. The latter two are still available.

RL

I spent a significant portion of my career trying to train that kind
of thinking out of engineers.

Some engineers leap directly to telling you why your example is wrong.
Many times, I've had to go back and fix their designs.

The ones you want working on your project use the example to consider
what real users might do with the product and come up with a
more robust design.

I don't remember the exact one, but I once had a commercial bench
power supply that
I wanted to use for charging batteries. Turns out, if you turned
off the power switch before disconnecting the battery, it made
a LOT of smoke.
 
F

Fred Abse

Jan 1, 1970
0
#3 is the only option I ever consider. Personally I like to have the SCR
right at the circuit side of the fuse. If the fuse were at the input and
the SCR were at the output you'd have the upper FET and the inductor in
the path. This will slow down the fuse tripping and there is a chance that
the FET grenades before the fuse trips, something that is generally not
desired.

Seconded.
 
F

Fred Abse

Jan 1, 1970
0
I'm needing an over-voltage protection circuit to put at the output of a
switching regulator that takes in 100V input and drops it to 24V... I'd
like to protect the output from going over 28V.

Here's the 3 options I was considering: For all the options the point is
to blow my 5A fast acting fuse that's at the input, so I'm trying to
decide on a way to short my output

Put a fuse at the *output* and use the usual SCR/zener circuit.

Don't rely on the PSU input fuse. Too uncertain.
 
F

Fred Abse

Jan 1, 1970
0
Just use the SCR to clear a fuse. Just make sure your PCB traces are up
to the pulse current. I have measured ~100A while clearing a 1A fuse...
makes a teeny little flash when expiring ;-)

I agree. The biggest I've done was for a 100-amp bought-in power supply. I
used fabricated copper busbars for that. It made a satisfying thump on
test.
 
F

Fred Abse

Jan 1, 1970
0
The SCR gate should indeed be driven hard by some schmitt-trigger sort of
thing. A wimpy gate drive (say, a zener from anode to gate) can result in
a slow-spreading current concentration somewhere in the SCR die, and
damage the SCR.

Not my experience. A big SCR driven by a sensitive one (like C106) with
just an A-G zener is extremely robust. The only faults I've encountered
with that sort of circuit (both mine and others') have been age-related
false tripping at greater than ten years.
Moto used to make a specific crowbar driver chip for this application...
can't recall the part number. OnSemi may still make it.

I've used them (once). Bitches to keep stable.
 
F

Fred Abse

Jan 1, 1970
0
I wanted to use for charging batteries. Turns out, if you turned off the
power switch before disconnecting the battery, it made a LOT of smoke.

Very common. Bench power supplies should *never* be used to charge
batteries without a suitably rated diode in series.

I've had it done to my bench supplies when I've been away. I better not
find out who it was...
 
J

Joerg

Jan 1, 1970
0
Jim said:
Sounds like a couple of amateurs when it comes to computing I^2*t >:-}

Huh?

This works, and fast, done it many times. One has to make sure that the
SCR triggers with gusto and is big enough.
 
J

Joerg

Jan 1, 1970
0
Jim said:
I misconscrewed you as the one saying "inductor"... or maybe not >:-}

That was my point, ideally you don't want to have the inductor in the path.
 
L

legg

Jan 1, 1970
0
I spent a significant portion of my career trying to train that kind
of thinking out of engineers.

Some engineers leap directly to telling you why your example is wrong.
Many times, I've had to go back and fix their designs.

The ones you want working on your project use the example to consider
what real users might do with the product and come up with a
more robust design.

I don't remember the exact one, but I once had a commercial bench
power supply that
I wanted to use for charging batteries. Turns out, if you turned
off the power switch before disconnecting the battery, it made
a LOT of smoke.

I don't follow your reasoning.

Is it the battery back-up, overvoltage protection method, or the power
supply design you're questioning? The subject is crowbar ovp methods.
You're point was the ovp condition occurring in a battery charging
situation. I think was addressed.

Power supply compatibility in parallel redundant or backup
applications is a subject in itself. Misapplication of products, or
their immunity to such conditions can also be an interesting
discussion, along with the sad state of the equipment and the
amazement or frustration of the misapplicator.

Rest assured that you won't see a product that wasn't immune to the
application of external voltages occupy or originate in my work area.
This is, in fact, a crude test method to confirm the function of OVP
circuitry, though alternative methods exist that may perform the same
function automatically and with reduced hardware, in both design and
in production test.

I do not, however, assume this capacity in any equipment that is
unfamiliar to me, until proven otherwise.

The crowbar method of protection isn't always used, and it's external
application in any black box situation should probably be evaluated.

Your battery charging or back-up situation isn't exactly a black box,
so you should be able to get your mind around it, with a little
effort.

There's no reason why a commercial bench power supply should smoke on
application of external voltage, within it's normal output compliance,
save the naivete of it's designer, or the complaisance of it's
purchaser.

RL
 
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