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.

thanks

 

[Ken Smith]

In article <1104549057.495776.173180@z14g2000cwz.googlegroups.com>,
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.

Did you do this:

ASCII art
100V
!
G !!- D
Vgate -----!!
!!--------- To load
S

If this is what you did, then you should expect that the load will always
have less voltage on it than is on the Vgate. The MOSFET is turned on by
the voltage difference between its shource and gate.

--
--
kensmith@rahul.net forging knowledge

 

[Ali Electronics]

thank you

yes. I did that one.

I changed the drain and source voltages and it worked
I guess now the ASCII art is like:

100V
G!!S
Vgate=20v------------!!
!!-----------To
load
D
it is woking
thanks once more

 

[Ali Electronics]

yes I used that configuration.

so what should I do?


I just want to switch the 100volt on and off by changing the gate
voltage.

any idea?

 

[Ali Electronics]

I refrenced the pulse generator to the source of the mosfet, and it
worked out.

thank you very much

 

[Active8]

On 2 Jan 2005 01:24:19 -0800, Ali Electronics wrote:

I refrenced the pulse generator to the source of the mosfet, and it
worked out.

thank you very much

That's not good. The idea behind Ken option 3 is that the opto
isolates the source. You're raising the source ref to maybe

100 V - Vgs

Stick with the load on the drain and the source grounded or option
2.
--
Best Regards,
Mike

 

[Ken Smith]

In article <1104612763.275793.216730@c13g2000cwb.googlegroups.com>,
Ali Electronics <ali@vuetec.com> wrote:

yes I used that configuration.

so what should I do?


I just want to switch the 100volt on and off by changing the gate
voltage.

You have many options:

(1)
Place the load in the drain of the N MOSFET.

(2)
Use a P MOSFET and put the load in the drain and reference the drive to
the plus power. You may have to add a small NPN to the disign to shift
the drive level.
100V
!
+---------+-------
/ !
\ R1 /---/ D1
/ ^
! !
+---------+------- To gate
!
!/
--! Q1
!\ e
!
/ R2
\
/
!
GND

R1 = R2 = about 10K

D1 = a 20V zener to protect against mistakes

Q1 = a > 100V NPN transistor



(3)
Arrange it so that the gate voltage is developed referenced to the source.
The easiest to explain is to use a "photo-voltaic opto-isolator"

(4)
Gate on and off a AC (RF) voltage to something like this:


C1 D2
AC ----!!---+--->!-----+--------+----- To gate
! ! !
---D1 / /---/ D3
^ \ R1 ^
! / !
! ! !
----------+--------+------ To source


The AC (RF) input needs to be something like 20Vp-p at lets say 10KHz to
10MHz. The frequency needs to be so that at least 10 cycles will
happen in the time you want the MOSFET to switch in.

D3 is a zener to prevent overvoltages from happening.

R1 and the gate capacitance of the MOSFET controls the turn off speed.

D1 and D2 are 1N914s

C1 is so that 1/(2*PI*F*C) is about 10 times less than R1.
--
--
kensmith@rahul.net forging knowledge