How does an H-bridge circuit work? Why are all four switching transistors N-channel MOSFETs selected?

The H-bridge circuit changes the polarity of the drive load, which changes the motor's direction of rotation when its polarity is altered.

For example, in driving a DC motor, the H-bridge circuit controls the motor's direction by changing the polarity of the drive load. It can also control the motor's speed by using different PWM duty cycles.

So how does it work?

When Q1 and Q4 are conducting, Q2 and Q3 are turned off. The current flows from VM to Q1, then to Q4, and finally to ground (GND). In this state, the load (motor) rotates in one direction.

When Q2 and Q3 are conducting, Q1 and Q4 are turned off. The current flows from VM to Q3, then to Q2, and finally to ground (GND). In this state, the load (motor) rotates in the opposite direction.

If Q1 and Q2, or Q3 and Q4 conduct at the same time, a short circuit will occur because VM and GND are directly connected. Therefore, when designing MOSFET driver circuits, a dead time should be provided, which is a certain interval when the upper and lower switches conduct.

Additionally, PWM duty cycle can be used to adjust the motor's speed. The greater the duty cycle, the higher the speed, and vice versa.

Here's a common issue: When the load is inductive, such as during PWM drive, the current in Q1 and Q4 won't change abruptly when they're turned off. Therefore, a path for the current to decay is needed, especially during the braking phase, where the current needs to decrease rapidly to reduce motor speed.

This can be achieved by using flyback diodes or MOSFETs.

Flyback diodes: When Q1 and Q4 conduct, the inductor's current rises. When Q1 and Q4 turn off, and Q2 and Q3 remain off, the inductor's current flows from ground (GND) through D2, then D3, and finally to VM. However, due to the diode's forward voltage drop, it can cause heating and losses, and its current decay speed is slower.

MOSFETs for flyback: When Q1 and Q4 conduct, the inductor's current rises. When Q1 and Q4 turn off, if Q2 and Q3 are turned on, the inductor's current flows from GND through Q2, then Q3, and finally to VM, and then it discharges. Because the MOSFET's on-resistance is small, the current can flow directly into VM and then decrease rapidly.

If PWM control is used for forward motor speed adjustment, when Q1 and Q4 are re-opened, Q2 and Q3 are turned off, and the current increases again. Then Q1 and Q4 turn off, followed by Q2 and Q3, and the current continues to decrease rapidly.

Lastly, why are four NMOS transistors typically chosen instead of PMOS?

Cost is a significant factor, and there are fewer high-voltage and high-current PMOS models compared to NMOS, making NMOS transistors the preferred choice.