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where does the SCR go?

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emc2orly

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Hello!

In a switch mode power supply that you want to protect the (expensive) circuitry hanging off the output, where would one want to place an SCR crowbar for overvoltage protection?

For reference:

**broken link removed**

Do I want to have it short out the output directly (across RL) or short out Vin? The actual circuit I want to protect is only slightly different. "Vin" would be a filter cap, and there would be a fuse before the filter cap. So unfortunately, in either case the cap is going to have to dump it's energy into the SCR.
 
Well the output voltage is the voltage you want to limit the voltage on so you put the SCR in parallel with the output (ie. the load).

Putting it in parallel with Vin makes no sense since Vin is always Vin because it is a voltage SOURCE, even during a voltage spike (which will occur somewhere else anyways). ANd if Vin is greater than your load voltage (which is why you'd be using the buck converter in the first place), then if you put an SCR rated to protect your load, it will basically short out the source until the source died and it's voltage fell below the trigger voltage of the SCR...or more likely it would fry first. If you rated it to the Vin...then obviously it isn't doing anything to protect your load.

THe SCR is supposed to dissipate the energy in the cap. THat's how it works and that's the only way it could work to supress overvoltage. It won't discharge the cap all the way to 0V though. It will stop once the voltage goes below it's holding voltage which is somewhere around the trigger voltage (ideally).
 
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Okay thanks. I thought so, but it also made sense to me to have the SCR as close to the input fuse as possible.

My next question is, how do you size the SCR? The filter cap and it's ESR mean a short circuit could produce a 100A peak surge. (Not counting ESL or external circuitry). Should I just find a SCR that can handle that surge minimum? Or are there other things to look at? Output current is minimal compared to the cap current (1a).
 
emc2orly said:
Okay thanks. I thought so, but it also made sense to me to have the SCR as close to the input fuse as possible.

My next question is, how do you size the SCR? The filter cap and it's ESR mean a short circuit could produce a 100A peak surge. (Not counting ESL or external circuitry). Should I just find a SCR that can handle that surge minimum? Or are there other things to look at? Output current is minimal compared to the cap current (1a).

Yeah I have that problem too...I've never used them in any circuit yet except for the one I'm designing and I kind of just picked the biggest one I could get my hands on. But yeah, SCRs are supposed to be rated on their peak current, not continuous. Since they have essentially zero continuous current.
 
The SCR should be able to stand the maximum short circuit current in the amount of time it takes for the fuse to blow. The safe operating area and fuse time constant can normally be found on the datasheet. If you know the impedance of the wiring, then you can easilly calculate the short circuit current using ohm's law.
 
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