Over Voltage Protection Crowbar Circuit

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

 

#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.

 

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.

 

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.

 

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.

 

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.

 

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

 

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).

 

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.

 

Seconded.

 

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

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

 

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.

 

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.

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

 

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...

 

Hey, we have the absolute majority in the house now

 

Huh?

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

 

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

 

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