Power MOSFET with Source higher than Drain

[skywalker898]

We are building a bipolar pulsing circuit with a couple power mosfets (**broken link removed**) and we will have an MOV in place to limit overvoltage from a potential arc to the output of the pulsing circuit. The output is a grid electrode for pulsing an electron beam, but it is modeled here as a 50pF capacitor.


I'm wondering though if we need the rectifier diode shown. With the MOV voltage specified a little higher than the operating voltage of 500V, without the diode it would be possible that the source goes to a higher voltage than the drain on mosfet M1. Am I correct that an NMOS Power Mosfet will conduct with a 0.7V drop if source is higher voltage than the drain?

So the diode is probably necessary then to prevent the +500v DC supply from needing to sink current?

Thanks,
Brandon

 

[rakeshkatkuri]

hi,
what is your measured source voltage and drain voltage?

 

[skywalker898]

The drain is at 500V and the source is at 0 most of the time, but my question is more general.

For a power mosfet, if the source is higher voltage than the drain, will current conduct through?

 

[CataM]

For a power mosfet, if the source is higher voltage than the drain, will current conduct through?

In a N channel MOSFET the parasitic diode from the PN junction of Source to Drain could well conduct if source is higher than drain, but you would need to see its characteristics to avoid burning it !

 

[Warpspeed]

That 5Kv rectifier diode is going to prevent any serious power from a positive flash over reaching the upper mosfet.
Any slight reverse recovery energy in that 5Kv diode will easily be absorbed by the parasitic source drain diode within the upper mosfet.

The circuit should work fine as it is.

 

[Easy peasy]

you might need some diodes after your mains transformers to give you your +12V for gate drive....

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R1, R2 could be 1K ohm...

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If you are testing MOV's careful the max current doesn't kill the 3kV diode....

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With Rg = 0 ohms careful the dv/dt doesn't get into your uP & wiring causing issues....

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The pulsed drain current on the Cree parts is ONLY 5A, hope you have a LOT of spares and they are easy to replace....!

[as you don't have any current limiting....]

 

[rakeshkatkuri]

remember whenever you are using body diode mosfet, there will be a conduction from source to drain if source voltage is greater than drain voltage Vs>=Vd+Vth.
Vth = body diode drop
Vd = drain voltage
Vs = source voltage

if you are connecting drain to the power source, your power source will sink the current from the source if your source voltage is higher than drain

make sure that during this source to drain conduction your mosfet will get heated up depending on the current flow to the drain, to avoid this always use a diode between drain and power source