Switch Comparison: MOSFET (Metal Oxide Semiconductor Field Effect Transistor)
MOSFETs are very similar to switches. In the diagram above, note the similarities of the switch and their MOSFET equivalents. We'll continue the discussion centering around N-Channel (vs P-Channel) MOSFETs because they are more commonly used. Note that an arrow pointing into the Gate is an N-Channel MOSFET.
Note: Explanation is over "enhancement mode" MOSFETs which can be considered to be "normally open" (deactivated), whereas "depletion mode" MOSFETs are "normally closed" (deactivated). The enhancement type MOSFET is more common.
Unidirectional Control
In the diagram above, the switch (left) is closed to provide a path to ground causing the motor to spin in one direction. Likewise, an activated MOSFET (right) provides a path to ground causing the motor to spin in the same direction.
The waveform above is measured between the motor and switch or the motor and MOSFET.
One of the problems with both circuits is that an electric motor is highly inductive and very similar to an inductor (Diode Section). When the motor is de-energized (Switch opened/MOSFET deactivated), it creates a huge spike that is enough to create a spark across the switch contacts or exceed the Vds (drain-source) rating of a MOSFET. Both device lifetimes will be shortened dramatically.
Note: MOSFET is "active" (ON) GRAY.
By placing a diode in a reverse-biased position and in parallel to the motor, the spike can be dramatically reduced. During the off transitions of the MOSFET, the inductive kickback is suppressed to a safe forward-biased voltage of 0.7V (Diode Section).
Above is the new and improved waveform with the motor's inductive kickback suppressed to a safe level that does not exceed the Vds rating of the MOSFET.
A better way to suppress the inductive kickback of the motor is to use a MOSFET in parallel to short out the motor. This method dumps energy back into the motor. The upper MOSFET is ONLY ON when the lower MOSFET is OFF.
Note: The alternating action of the upper and lower MOSFETs occur during a PWM signal. At 0% duty cycle or 0 speed, both MOSFETs are OFF. At 100% duty cycle or Max speed, only the lower MOSFET is ON.
Bidirectional Control (H-Bridge)
Above, we have an H-Bridge. An H-Bridge is just a bunch of switches, in this case MOSFETs, arranged in an "H" configuration. An H-Bridge provides bidirectional control. When Q1 & Q4 are activated, the DC motor turns in one direction. To reverse the motor direction, the voltage is reversed on the motor leads by deactivating Q1 & Q4 and activating Q2 & Q3.
Here are a couple of ways to introduce a PWM signal to control the speed of the motor. Instead of modulating (PWM) both the upper and lower MOSFETs, we can activate the lower MOSFET and modulate the upper MOSFET (pictured left). The same can be true by activating the upper MOSFET and modulating the lower MOSFET (pictured right).
Remember the inductive kickback during the off transition of the motor. Where does the energy go? If you look at the picture on the left, during the off transition of Q1 the energy is suppressed using the "intrinsic" diode on Q2. Since Q4 is always ON, the intrinsic diode on Q2 is essentially in parallel with the motor.
The picture on the right is very similar, however the energy now is suppressed using the intrinsic diode on Q3 during the OFF transition of Q4.
Instead of using the intrinsic diode on the MOSFET to suppress the inductive kickback, we can leave Q4 ON, and alternate Q1 and Q2. When Q2 and Q4 are ON simultaneously (motor is shorted), energy is dumped back into motor and the MOSFETs run cooler.