Power Mosfet Biasing

So if I read this right you have:

+100V ===== | | | ||-+ | ||O------||-+ |/ | o----------- | .-. | | | |4k '-' | | === GND created by Andy´s ASCII-Circuit v1.24.140803 Beta

Yes? If so, then the source voltage will always be smaller than the gate voltage by some amount -- if you want to drive this thing so you get 100V across the resistor you need to bring the gate up to 110V.

You can fix this in several ways. The one that's probably the simplest conceptually is to use a P-channel MOSFET with it's source connected to

+100V; use a resistor from source to gate to keep it turned off and use a NPN transistor (through a series resistor) to turn it on. You'll have to use an NPN that can withstand over 100V, and you'll want to calculate your resistive divider so that you only have about 10-20V from gate to source with 100V on the source.

This will be very slow, and will expose the MOSFET to some heating while it is transitioning from on to off, but it'll be a very simple circuit and may meet your otherwise unspecified needs.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Sounds like you're using it as a source follower. So when you apply Vg volts to the gate, the source voltage starts to rise, which reduces Vg-s, which starts to turn off the fet! So the source voltage can't rise up to even Vg. If the source voltage did get up to 100, it would blow out the fet gate anyhow; lucky it doesn't!

Ground the source and put the load in the drain circuit.

Well, that's my good deed for the year.

John

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yes thats what i did. thank you very much for your responce. would you draw the ASCII-Circuit so I would undestand better. thanks regards

I think you are using Google to view this Usenet group, use a proper newsgroup reader like Forte FreeAgent and the ascii drawings will be fine

martin

Serious error. All shortcuts have disappeared. Screen. Mind. Both are blank.