Can I replace SCR with power MOSFET?

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fjpompeo

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Dear sirs,

Is it possible to replace the SCR of the attached crowbar circuit with a N-channel power MOSFET?

I have no power SCRs but have some power N-MOSFET transistors.

73,

Fernando - Call sign PU2PLL
 

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Yes, I'd think so, if the MOSFET can conduct & sustain the current which will melt the (hopefully) existent fuse in front of this over-voltage-protection circuit.
 

It won't work nearly as well due to the lack of latching operation. A MOSFET will only be able to clamp to voltages above its gate threshold. It may also "chatter" quite a bit when triggered.
 

According to linear technology's comparison of mosfet vs scr as crowbar protection the mosfet outperforms the scr for protection of digital logic processors. Don't know your application so this may not be appropriate for you. If interested search "crowbar methods: mosfet vs scr".
 

Thanks for hinting to the Linear application note https://www.linear.com/solutions/4426. It should be mentioned that the comparison is presuming a latching crowbar trigger, which must be supplemented for the original circuit, as already noted my mtwieg.

Secondly, the decision criteria applied in the comparison, sub-microsecond action and low forward voltage are probably not valid for the original problem. In any case, the application note clarifies that MOSFET crowbar is an option.
 

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For low voltage applications, MOSFETs are certainly far better than SCRs (or any bipolar technology). However the OP's schematic suggests a trigger voltage around 5.5V, which is fine for SCRs.

To me, latching behavior is essential to a crowbar device (otherwise I'd just call it a clamp). A MOSFET circuit can be made to have latching behavior, but not with the basic circuit posted by the OP. In order for it to work, the bias for the triggering circuit can't come from the supply rail being shunted by the crowbar. A separate supply is necessary.
 

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