I'm using a MOSFET irf840. The drain has a resistor connceted to it and then to the 12V rail. The source is connected straight to ground. My MOSFET driver is connected from a micro to generate the Pwm and then the driver has the positive end connected to the gate and the negative and connected to the source. The oscilloscope is connected to the output of the micro at 5 V PWM, red colour, and the blue one is the voltage across the drain source.

When the switch is on, the zero voltage drop is correct i assume, but when the PWM turns it off, shouldn't the voltage be at 12 since the switch is effectively open? Instead it shows -0.5? Not sure whats going on here.

 

Could be the FET is oscillating, especially if you have a long lead to the gate. Try putting 100R in series with the gate very close to the FET package.

 

roro36,
You did not attach the oscilloscope output, nor a schematic.

You need to supply your gate driver IC with at least 10v in order to fully turn on an IRF840. You could power it from 12v if that is within the specifications of your gate driver IC.

The TC4420 gate driver you mentioned yesterday should work.

Keep in mind that the IRF840 has an Rds(on) of 0.85 Ohms, which is rather high. This is due to it's high Vdss rating; the N channel has to be made much thicker so that it can stand up to the high Vdss.

You might do better to look at IGBT's.
Wiki article on IGBT's: http://en.wikipedia.org/wiki/Insulated-gate_bipolar_transistor