A MOSFET is a transistor. It
is a Metal Oxide Field Effect Transistor.
Here is an animation showing how to turn on an N-channel MOSFET:
The easiest way to understand how MOSFETs
work is to compare them with PNP and NPN transistors and show them in
similar circuits. The advantage of a MOSFET is this: It requires
very little current (almost zero current) into the gate to turn it ON
and it can deliver 10 to 50 amps or more to a load.
A zener must be added to the gate of a
MOSFET if the gate voltage comes from a supply that is above 20v.
When the gate voltage is 4v LOWER than
rail voltage, the MOSFET turns ON. The 10k resistor on the base of the
transistor is needed to prevent the base current exceeding the amount of
current needed by the transistor to deliver current to the load. However
the 10k resistor on the gate of the MOSFET is not needed. Providing the
voltage (up to 18v) on the gate rises and falls quickly, the MOSFET will
not get hot. The critical period of time is the 0v to 3v section of the
waveform as this is when the MOSFET is turning on.
The solution is shown in the diagram below. The transistor configuration will work on ANY rail voltage but the MOSFET "totem-pole configuration" will only work up to 5v. This is due to the characteristics of a MOSFET. The MOSFETs used in this arrangement have a gate-to-source characteristic of slightly more than 3v and do not turn on when the voltage across these two terminals is 3v. This means the supply can be 6v and when the input is at mid-rail, 3v will be across each gate-to-source and neither will be turned on. That's why TTL logic is limited to 5v operation. The output will be extremely close to rail-to-rail for the MOSFET configuration.
For a supply greater than 5v, a different MOSFET configuration must be used to get full rail-to-rail output. The MOSFETs must be turned on individually.
PUSH PULL USING MOSFETS
The circuit above is much more complex than
meets the eye.
Here is an audio amplifier using PUSH PULL mode to drive a speaker:
The top two transistors are in push-pull mode to turn the P-channel
MOSFET on and off very quickly. They speed up the incoming waveform and
prevent the MOSFET generating heat during the turning-on process.
Input A HIGH, Input D HIGH - forward
PWM MOTOR SPEED CONTROLLER
WHY MOSFETs FAIL
MOSFETs have very little tolerance to over-voltage. Damage to devices may result even if the voltage rating is exceeded for as little as a few nanoseconds. MOSFET devices should be rated conservatively for the anticipated voltage levels and careful attention should be paid to suppressing any voltage spikes or ringing.
High average current causes considerable thermal dissipation in MOSFET devices even though the on-resistance is relatively low. If the current is very high and heatsinking is poor, the device can be destroyed by excessive temperature rise. MOSFET devices can be paralleled directly to share high load currents.
Massive current overload, even for short duration, can cause progressive damage to the device with little noticeable temperature rise prior to failure.
If the control signals to two opposing MOSFETs overlap, a
situation can occur where both MOSFETs are switched on together.
This effectively short-circuits the supply and is known as a
shoot-through condition. If this occurs, the supply decoupling
capacitor is discharged rapidly through both devices every time a
switching transition occurs. This results in very short but
incredibly intense current pulses through both switching devices.
When switching current through any inductive load (such as a
Tesla Coil) a back EMF is produced when the current is turned off.
It is essential to provide a path for this current to free-wheel in
the time when the switching device is not conducting the load current.
A high Q resonant circuit such as a Tesla Coil is capable of storing considerable energy in its inductance and self capacitance. Under certain tuning conditions, this causes the current to "free-wheel" through the internal body diodes of the MOSFET device. This behaviour is not a problem in itself, but a problem arises due to the slow turn-off (or reverse recovery) of the internal body diode.
MOSFET body diodes generally have a long reverse recovery time
compared to the performance of the MOSFET itself.
If the MOSFET gate is driven with too high a voltage, then the gate oxide insulation can be punctured rendering the device useless. Gate-source voltages in excess of +/- 15 volts are likely to cause damage to the gate insulation and lead to failure. Care should be taken to ensure that the gate drive signal is free from any narrow voltage spikes that could exceed the maximum allowable gate voltage.
MOSFET devices are only capable of switching large amounts of power because they are designed to dissipate minimal power when they are turned on. It is the responsibility of the designer to ensure that the MOSFET device is turned hard on to minimise dissipation during conduction. If the device is not fully turned on then the device will have a high resistance during conduction and will dissipate considerable power as heat. A gate voltage of between 10 and 15 volts ensures full turn-on with most MOSFET devices.
Little energy is dissipated during the steady on and off states, but considerable energy is dissipated during the times of a transition. Therefore it is desirable to switch between states as quickly as possible to minimise power dissipation during switching. Since the MOSFET gate appears capacitive, it requires considerable current pulses in order to charge and discharge the gate in a few tens of nano-seconds. Peak gate currents can be as high as 1 amp.
MOSFETs are capable of switching large amounts of current in
incredibly short times. Their inputs are also relatively high
impedance, which can lead to stability problems. Under certain
conditions high voltage MOSFET devices can oscillate at very high
frequencies due to stray inductance and capacitance in the
surrounding circuit. (Frequencies usually in the low MHz.) This
behaviour is highly undesirable since it occurs due to linear
operation, and represents a high dissipation condition.
MOSFET devices have considerable "Miller capacitance" between their gate and drain terminals. In low voltage or slow switching applications this gate-drain capacitance is rarely a concern, however it can cause problems when high voltages are switched quickly.
A potential problem occurs when the drain voltage of the bottom
device rises very quickly due to turn on of the top MOSFET. This
high rate of rise of voltage couples capacitively to the gate of the
MOSFET via the Miller capacitance. This can cause the gate voltage
of the MOSFET to rise resulting in turn on of this device as
well ! A shoot-through condition exists and MOSFET failure is
certain if not immediate.
Rapid switching of large currents can cause voltage dips and
transient spikes on the power supply rails. If one or more supply
rails are common to the power and control electronics, then
interference can be conducted to the control circuitry.
Antistatic handling precautions should be used to prevent gate oxide damage when installing MOSFET or IGBT devices. But are very reliable once they are soldered in place.
There are many more fact and circuits using MOSFETs on the web. This discussion is only a starting-point.