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Avalance rating in FET simulation

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ronv

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I have an application where I don't want to use a clamp diode across a small inductor in a FET switch. I think I can let the FET go into avalance mode safely but I would like to see it on the simulation. In the simulation the FET does not seem to go into avalance mode and the voltage just continues to rise when it is turned off. Am I missing somethig or does LTSpice not simulate the breakdown? If it is the latter could I just add a zener to the circuit or a model to simulate it?
 
The FET models in LTSpice are behavioral they don't model silicon you can't simulate avalanche directly. Using a zener set to the avalanche voltage should work fine, then all you have to do is measure the power dissipated in the zener and determine if that's safe for your FET. It's not going to be exact but it will put you in the ballpark at least, make sure you model as many parts of the resistance in the real circuit as possible as that's going to drastically effect the peak power through the zener and that's what you're really looking for.

The main problem it won't show you is the reverse recovery characteristics of the FET's parasitic diode, which could cause latchup conditions or lots of ohmic heating if the FET gets stuck in it's saturation region during high frequency switching.
 
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I did a little more looking just cause I was curious and found this. Which pretty much calls me a liar =) Although it's undocumented, you should be able to add these parameters to any Mosfet model by Control+right clicking on the mosfet element and adding a spice line for the new parameters. The nbv ,triode and the subthres variables are something you'd have to figure out how to derive values for, leaving them blank will probably simulate but not match the true avalanche characteristics of the FET.

Just goes to show you simulations are only as useful as the data you can feed it.
 
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Thanks for the input.
I looked at the page, but I'm not sure I would ever get it right so I think I'll go the diode route then check the current against the avalanche ratings.

Thanks, again ;)
 
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