Power Mosfet Biasing

[Ali Electronics]

Hello


in Circuit Maker I am using a couple of Power Mosfets (e.g. IRF624S) as
simple switches. but I cannot get the drain voltage passed from the
mosfet unless I bias it by the same voltage.


I mean for example: Vdrian= 100v DC, R(source-ground)=4k Ohms.
I think when I apply 10 or 12v to gate I should have 100v across the
Resistor. but I cannot!


can any one help me?
I can send the schematic of my circuit to you.

may be my design is totally wrong.
I want to swith on/off 100V using Cmos or TTL pulses.
any one having an idea?


thanks

 

[Tim Wescott]

Ali Electronics wrote:

Hello


in Circuit Maker I am using a couple of Power Mosfets (e.g. IRF624S) as
simple switches. but I cannot get the drain voltage passed from the
mosfet unless I bias it by the same voltage.


I mean for example: Vdrian= 100v DC, R(source-ground)=4k Ohms.
I think when I apply 10 or 12v to gate I should have 100v across the
Resistor. but I cannot!


can any one help me?
I can send the schematic of my circuit to you.

may be my design is totally wrong.
I want to swith on/off 100V using Cmos or TTL pulses.
any one having an idea?


thanks

So if I read this right you have:

+100V
=====
|
|
|
||-+
|\ ||<-
-----------| >O------||-+
|/ |
o-----------
|
.-.
| |
| |4k
'-'
|
|
===
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

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

 

[John Larkin]

On 1 Jan 2005 12:55:41 -0800, "Ali Electronics" <ali@vuetec.com>
wrote:

Hello


in Circuit Maker I am using a couple of Power Mosfets (e.g. IRF624S) as
simple switches. but I cannot get the drain voltage passed from the
mosfet unless I bias it by the same voltage.


I mean for example: Vdrian= 100v DC, R(source-ground)=4k Ohms.
I think when I apply 10 or 12v to gate I should have 100v across the
Resistor. but I cannot!


can any one help me?
I can send the schematic of my circuit to you.

may be my design is totally wrong.
I want to swith on/off 100V using Cmos or TTL pulses.
any one having an idea?


thanks

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

 

[Rich Grise]

Crossposted to sci.electronics.design,sci.electronics.basics,
follwups-to s.e.basics.

On Sat, 01 Jan 2005 12:55:41 -0800, Ali Electronics wrote:

in Circuit Maker I am using a couple of Power Mosfets (e.g. IRF624S) as
simple switches. but I cannot get the drain voltage passed from the
mosfet unless I bias it by the same voltage.

I mean for example: Vdrian= 100v DC, R(source-ground)=4k Ohms.
I think when I apply 10 or 12v to gate I should have 100v across the
Resistor. but I cannot!

You have a source-follower there. If you put your 4K in the drain and
ground the source, you will see the drain voltage fall to almost zero when
you drive the gate high. So you'd need an additional, P-channel FET to
actually switch the 100V to the output off and on. But be sure to limit
its gate voltage to less than gate-source breakdown more negative than
100V.

Good Luck!
Rich