philipsi said:if i want to use the output of the PIC microcontroller to drive the power mosfet,what should i do to make the power mosfet fully ON so that power disspation is minimised???
the output of microcontroller seems not enough to fully turn on the power mosfet
is it necessary to place a current limiting resistor in series with the gate of the powe mosfet?
True, but turning off (or on) a power MOSFET (or power anything, for that matter) slowly is not a good idea. Peak power is dissipated during the transition time. For the protection of the transistor, this time needs to be short. In power switching, 100 milliseconds is not short.The math behind the discharge makes me smile . As it turns out, a resistor from the gate to ground would have to be... 13.6Mohm for a .1 second discharge. Any smaller value, and the MOSFET should just switch faster. Basically, grab a resistor out of your box that is larger than 10k, and put it there. It'll work fine.
The bipolar transistor is a current operated device. To drive a motor, you should supply 1/10 of the motor current to the base, or if the transistor is a darlington, 1/100 of the motor current. The mosfet is a voltage operated device. You should get the data sheet to see how much voltage is required to turn it on, but typically 10 volts will do it. Most mosfets are enhancement mode, that is, it takes a positive voltage to turn on an N type. There are a few depletion mode mosfets available, these are normally on and require a negative voltage to turn an N type off.