Using Vertical Fets below Thermal Equilibrium pt?

[wildswan]

Hi all. I'm designing a simple HV shunt regulator at the moment (for a tube stage) and am prevented from using lateral fets because their highest Vds seems to be 200v or so.

Also I'll be only running them at a few mA, so they will be have a negative Vgs tempco anyway.

So the plan is to use vertical fets.

Now I gather the DC feedback will take care of the Vgs decrease with temperature, but I
am worried about the phenomenon of thermal "hot spots" in VFets


Are there any tips to operate VFets safely in these conditions? Apologies if this is a silly question, as I know Nelson Pass's stuff has no problem on this front....

Apologies if this is in the wrong forum, I just thought this would be the place to ask

Cheers dudes

 

[Bob Cordell]

Hi all. I'm designing a simple HV shunt regulator at the moment (for a tube stage) and am prevented from using lateral fets because their highest Vds seems to be 200v or so.

Also I'll be only running them at a few mA, so they will be have a negative Vgs tempco anyway.

So the plan is to use vertical fets.

Now I gather the DC feedback will take care of the Vgs decrease with temperature, but I
am worried about the phenomenon of thermal "hot spots" in VFets


Are there any tips to operate VFets safely in these conditions? Apologies if this is a silly question, as I know Nelson Pass's stuff has no problem on this front....

Apologies if this is in the wrong forum, I just thought this would be the place to ask

Cheers dudes

I have used vertical power MOSFETs as pass regulators in a tube amplifier with no problem (screen voltage regulators in a Pentode amplifier with KT88s, and also for driver and input stage B+).

Hot spots should be no problem in your application as long as you stay below rated dissipation and do a decent job of heat sinking them.

Cheers,
Bob

 

[Nelson Pass]

I don't think you'll have a hot spot issue, you appear to be well
within the SOA of the part.

 

[wildswan]

Sweet thanks guys!

I feel much more comfortable now!!

Much appreciated