Over Voltage Protection Crowbar Circuit

I've seen some Chinese power supplies (under 100 W) that used TVS devices in the intentional fail-to-short mode. That seems to work to protect the load. But, the device needs to be replaced after it is triggered.

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.

Jon

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 ;-)

Very fast. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
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I love to cook with wine.     Sometimes I even put it in the food.

switching regulator that takes in 100V input and drops it to 24V... I'd lik e to protect the output from going over 28V.

is to blow my 5A fast acting fuse that's at the input, so I'm trying to dec ide on a way to short my output

ndling, and they're not very accurate

dling and bulkiness... I think TVS kills Zener for this application

rip voltage and I think I could probably find a decently sized SCR that cou ld handle the current required to make the fuse pop

. I'm likeing the SCR option right now

why not something like this:

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.

--
Regards, Joerg 

http://www.analogconsultants.com/

switching regulator that takes in 100V input and drops it to 24V... I'd like to protect the output from going over 28V.

blow my 5A fast acting fuse that's at the input, so I'm trying to decide on a way to short my output

and they're not very accurate

and bulkiness... I think TVS kills Zener for this application

voltage and I think I could probably find a decently sized SCR that could handle the current required to make the fuse pop

likeing the SCR option right now

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...

switching regulator that takes in 100V input and drops it to 24V... I'd like to protect the output from going over 28V.

blow my 5A fast acting fuse that's at the input, so I'm trying to decide on a way to short my output

handling, and they're not very accurate

and bulkiness... I think TVS kills Zener for this application

voltage and I think I could probably find a decently sized SCR that could handle the current required to make the fuse pop

I'm likeing the SCR option right now

the TVS? I was considering using one of these guys...

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.

--
Regards, Joerg 

http://www.analogconsultants.com/

switching regulator that takes in 100V input and drops it to 24V... I'd like to protect the output from going over 28V.

blow my 5A fast acting fuse that's at the input, so I'm trying to decide on a way to short my output

and they're not very accurate

and bulkiness... I think TVS kills Zener for this application

voltage and I think I could probably find a decently sized SCR that could handle the current required to make the fuse pop

likeing the SCR option right now

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.

switching regulator that takes in 100V input and drops it to 24V... I'd like to protect the output from going over 28V.

blow my 5A fast acting fuse that's at the input, so I'm trying to decide on a way to short my output

handling, and they're not very accurate

and bulkiness... I think TVS kills Zener for this application

voltage and I think I could probably find a decently sized SCR that could handle the current required to make the fuse pop

I'm likeing the SCR option right now

the TVS? I was considering using one of these guys...

We had an incident here. The carpet cleaner guy wanted to close the garage door, saw a red box with PULL DOWN on it, so he pulled it down. It was the fire alarm, of course. In a couple of minutes we got two fire trucks and a supervisor in a car. They were very nice, helped us reset everything, no charge.

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.

Moto used to make a specific crowbar driver chip for this application... can't recall the part number. OnSemi may still make it.

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
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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.

--
My liberal friends think I'm a conservative kook. 
My conservative friends think I'm a liberal kook. 
Why am I not happy that they have found common ground? 

Tim Wescott, Communications, Control, Circuits & Software 
http://www.wescottdesign.com

switching regulator that takes in 100V input and drops it to 24V... I'd like to protect the output from going over 28V.

blow my 5A fast acting fuse that's at the input, so I'm trying to decide on a way to short my output

handling, and they're not very accurate

and bulkiness... I think TVS kills Zener for this application

voltage and I think I could probably find a decently sized SCR that could handle the current required to make the fuse pop

I'm likeing the SCR option right now

Yep. I always like my own circuits >:-}

The fuse blower design was done before MOSFET's were common. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

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.

--
Regards, Joerg 

http://www.analogconsultants.com/

[snip]

That's always been my philosophy. ...Jim Thompson

-- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

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.

On a sunny day (Fri, 30 Nov 2012 13:17:59 -0800 (PST)) it happened panfilero wrote in :

voltage and I think I could probably find a decently

Yes, I vote for SCR.

On a sunny day (Sat, 01 Dec 2012 10:23:11 +0200) it happened snipped-for-privacy@downunder.com wrote in :

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.

switching regulator that takes in 100V input and drops it to 24V... I'd like to protect the output from going over 28V.

blow my 5A fast acting fuse that's at the input, so I'm trying to decide on a way to short my output

handling, and they're not very accurate

and bulkiness... I think TVS kills Zener for this application

voltage and I think I could probably find a decently sized SCR that could handle the current required to make the fuse pop

I'm likeing the SCR option right now

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).

Was that a gate issue or a problem with the control circuitry ?:-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

switching regulator that takes in 100V input and drops it to 24V... I'd like to protect the output from going over 28V.

blow my 5A fast acting fuse that's at the input, so I'm trying to decide on a way to short my output

handling, and they're not very accurate

and bulkiness... I think TVS kills Zener for this application

voltage and I think I could probably find a decently sized SCR that could handle the current required to make the fuse pop

I'm likeing the SCR option right now

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.