Power Mosfet Biasing

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:

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?

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.

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

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.

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

Stick with the load on the drain and the source grounded or option