I need to design a input overvoltage protection for a 45W DC-DC converter rated for a 9-24V wall wart input (so, 45W / 9V = 5A input current at max). So far I've seen two approaches (schematics below):

  1. Crowbar circuit. The idea is that the TRIAC (or SCR) in case of an over-voltage will close and therefore short the input power terminals, therefore blowing the fuse and disconnecting the device.
  2. MOSFET switch. In case of an overvoltage the MOSFET simply opens and the high voltage simply doesn't reach the device.

I am not sure which one should I use and I haven't found anything comparing these approaches to design the overvoltage protection. So, my questions are:

  1. What are the general pros and cons of each of these circuits?
  2. What are the guidelines to which circuit one should use in a particular application?
  3. Which one of these should I use in my case?
  4. Are there any other overvoltage protection circuits I am missing?

UPDATE:

In my particular situation, I am mostly trying to protect the circuitry that the DC-DC converter is powering, since it can be pretty expensive and the DC-DC converter itself. I have no idea about the typical transient voltages when the power is plugged in; mostly I am trying to protect the circuitry from an "idiot user" who might plug something over 24V in the power inlet (I've seen up to 36V common wall warts and I am sure there are ones with even higher voltage), since the DC-DC converter is not designed for it and might output anything it wants at this point.

This is not a medical device, nobody will die if the device fails and (AFAIK) nothing could cause a fire in case of input or output overvoltage except for, maybe, potential tantalum capacitors filtering the power in the downstream circuitry which I do not control; the end user does (the DC-DC converter is for a modular synth system).

I am interested both in an answer for my particular case and in a more broad/common one showing the guidelines for selecting the method of overvoltage protection in any case.

Crowbar circuit MOSFET switch over-voltage protection

  • There are pros and cons to both methods. I think it might help if you explain why you are adding over-voltage protection. What would happen if an over-voltage was applied and you didn't have protection? Would someone die as a result? Would there be a fire? Are you trying to protect downstream circuitry? What is the range of over-voltage you need to protect against? – mkeith Nov 12 at 5:07
  • One obvious con (or pro?) is that the crowbar will blow your fuse. What would have caused the overvoltage and what do you expect to be done as correction? Do you expect the circuit to power back normally after the fault has been removed? AFAIK PTC/Resetable fuses have limited voltage ratings, especially in the higher (few As) range. – Wesley Lee Nov 12 at 7:21
  • @mkeith, kinda answered your questions by editing the post. – sx107 Nov 12 at 9:59
  • Consider two different kind of devices: First (crowbar solution), a device which once has seen overvoltage is not allowed to operate anymore and needs to be replaced or at least the fuse. Second (resettable overvoltage protection), a device for which frequent overvoltage does not pose a risk of degradation or the degradation is accepted (quality?). – stowoda Nov 12 at 10:05
  • 1
    Protecting against overvoltage and transients are two different things. No matter which solution you go for, you should add a TVS diode. For 24V, it is common to use ~33V TVS. But in case the TVS is the only protection, it will simply burn up and short in case of constant overvoltage, so that alone might not be sufficient. – Lundin Nov 12 at 10:43

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