Technology Remaking the World

MOSFET schematic symbols.
Plate 1

N-Channel Power MOSFET Switching Tutorial

by Lewis Loflin

Also see P-channel Power MOSFET Switch Tutorial

Here we will learn how power n-channel power MOSFETs operate. In this example I'm using enhancement mode devices. To use depletion mode MOSFETS simply reverse the circuits where an N-channel depletion mode MOSFET will use a variation of the P-channel enhancement mode circuit.

In plate 1 we have the symbols for depletion mode and enhancement mode MOSFETs - notice the dashed versus solid lines. In a depletion mode MOSFET gate voltage closes off the conductive channel from source (S) to drain (D). With an enhancement mode MOSFETs gate voltage opens the conductive channel from source to drain.

N-channel MOSFET switch.
Plate 2

In the above examples we are switching a LED on/off using power MOSFETs. In the case of the N-channel such as the IRF630 when the gate (G) is greater than 5-volts the LED cuts on. The resistor on the gate of the N-channel MOSFET is used to bleed-off the electric charge from the gate and turn off the MOSFET. The resistor can be 5K-10K.

IRF630 and IRF9630
Plate 4

The voltage difference between the gate and source will turn on the MOSFET, but must not exceed a value in the spec sheet known as Vgs. To do so will damage the device. In the case of IRF630 and IRF9630 MOSFETs that value is 20-volts.

Note the internal parasitic suppression diodes are for use with magnetic loads. Not all power MOSFETs have those so check the specifications sheets. These particular transistors are optimized for switching and not for use in audio amplifiers.

Power MOSFET H-Bridge basic circuit.
Plate 5

The largest use of these circuits is H-bridge motor controls. They are used in conjunction with N-channel MOSFET switches.

Note that Rg (or Rgs) is used to bleed the charges off the MOSFET gates or else they may not turn off.