C Discharge in to L Circuit; Is Switching FET Adequate?

[bobledoux]

I have a 100uf capacitor charged to 100 volts. It is discharged through an nFET with a pulse rating of 36 amps into a coil. The coil values are 4.7 DC ohms measured and 6700uh calculated. The duty cycle is once per minute.

How do I determine if the FET, a FQD12N20L, will handle to current?

 

[Sceadwian]

For pulse discharges like you're trying to achieve typically an SCR is used not a FET, FAR cheaper and less complicated.

 

[ronv]

When you simulate it it is about 8 amps for 2 ms, so it's ok for the FETs 10 ms safe operating area.

 

[Sceadwian]

ronv, what about Vdt Idt? Mosfets have the body diode and substrate parasitic bipolar transistor effect to deal with. There's no high frequency use here, it just SCREAMS use an SCR.

 

[bobledoux]

I'm updating an earlier design. The switching FET was a BUZ 32. My updated version needs to use a smaller unit. My FET comes in a D-Pak.

The BUZ 32 has VDS=200V; ID=9.5A, RDSon 0.4ohms.

 

[duffy]

You are dealing with about a half-joule, which gives you around 12 amps peak through the coil, and about a 2.5ms half-cycle for that RLC combo. If you look at the SOA curve (Safe Operating Area) for the FQD12N20L, that's slightly outside the lines. I would go with a bigger driver.

 

[bobledoux]

Thanks, for the efforts and replies.

 

[Sceadwian]

A simple low power low current N channel fet could easily be used to trigger an SCR. You'd only need the nfet a single resistor and the SCR. The only caveat is you haven't mentioned if this application requires the ability to terminate the discharge BEFORE it's complete, an SCR can't do that, a directly connected NFET can. Again typically in these situations the voltage of the capactior is altered to control the output pulse's total power, just easier that way.