Basic MOSFET Structure and Symbol
Depletion-mode N-Channel MOSFET and circuit Symbols
Enhancement-mode N-Channel MOSFET and circuit Symbols
|MOSFET type||Vgs = +ve||Vgs = 0||Vgs = -ve|
|P-Channel Enhancement||OFF||OFF||ON |
The MOSFET as a Switch
An example of using the MOSFET as a switch
In this circuit arrangement an Enhancement-mode N-channel MOSFET is being used to switch a simple lamp "ON" and "OFF" (could also be an LED). The gate input voltage VGS is taken to an appropriate positive voltage level to turn the device and the lamp either fully "ON", (VGS = +ve) or a zero voltage level to turn the device fully "OFF", (VGS = 0).
If the resistive load of the lamp was to be replaced by an inductive load such as a coil or solenoid, a "Flywheel" diode would be required in parallel with the load to protect the MOSFET from any back-emf.
The current flowing through the lamp is calculated as:
Then the power dissipated in the MOSFET will be given as:
Power MOSFET Motor Control
Simple Power MOSFET Motor Controller
As the motor load is inductive, a simple "Free-wheeling" diode is connected across the load to dissipate any back emf generated by the motor when the MOSFET turns it "OFF".
The Zener diode is used to prevent excessivegate-source input voltages.