Simple H Bridge Motor Driver Circuit using MOSFET

An H bridge motor driver circuit is a fundamental component used to control the speed and direction of a DC motor. When using MOSFETs (Metal - Oxide - Semiconductor Field - Effect Transistors) to construct such a circuit, it offers several advantages like high efficiency and fast switching speed. Here's a detailed description of a simple H bridge motor driver circuit based on MOSFETs:

1. Components Required

• MOSFETs: Four MOSFETs are needed to form the H bridge structure. Typically, two N - channel MOSFETs and two P - channel MOSFETs are used. For example, common N - channel MOSFETs like the IRF540 and P - channel MOSFETs like the IRF9540 can be suitable choices. The N - channel MOSFETs are generally used for the low - side switches (connected closer to the ground), while the P - channel MOSFETs are for the high - side switches (connected to the power supply side).
• Flyback Diodes: Each MOSFET should be paralleled with a flyback diode. These diodes, often Schottky diodes, are crucial as they provide a path for the back - electromotive force (back - EMF) generated by the motor when the current is interrupted. For instance, the 1N5817 Schottky diode can be used for this purpose. The back - EMF can be significant when the motor suddenly stops or changes speed, and without the flyback diodes, it could damage the MOSFETs.
• Power Supply: A suitable DC power supply is required to power the motor. The voltage rating of the power supply depends on the motor's requirements. For example, a 12V or 24V DC power supply might be used for small to medium-sized motors.
• Resistors: Some resistors might be used for various purposes. For example, pull - up or pull - down resistors can be used to set the initial state of the MOSFET gates, ensuring proper operation and preventing any floating states. Additionally, gate resistors can be used to limit the current flowing into the MOSFET gates during the switching process to protect the gate oxide layer.

2. Circuit Diagram and Connections

• H Bridge Configuration:
  • The four MOSFETs are arranged in an "H" shape. Let's label the N - channel MOSFETs as Q1 and Q2 and the P - channel MOSFETs as Q3 and Q4. The motor is connected between the two vertical "legs" of the H. For example, one terminal of the motor is connected to the drain of Q1 and the drain of Q3, while the other terminal of the motor is connected to the drain of Q2 and the drain of Q4.
  • The source of Q1 and Q2 is connected to the ground, and the source of Q3 and Q4 is connected to the positive terminal of the power supply.
• Flyback Diodes:
  • A flyback diode is connected in parallel with each MOSFET. For example, for Q1, the cathode of the flyback diode is connected to the drain of Q1, and the anode is connected to the source of Q1. The same connection principle applies to the other three MOSFETs.
• Control Signals and Resistors:
  • The gates of the MOSFETs are used to control their on/off states. For the N - channel MOSFETs (Q1 and Q2), a control signal from a microcontroller or other control source is connected to the gate through a gate resistor (let's say a few tens of ohms to a few hundred ohms). For example, if using an Arduino, a digital output pin can be connected to the gate of Q1 through a 100Ω resistor.
  • For the P - channel MOSFETs (Q3 and Q4), the control signals are also connected to their gates through appropriate gate resistors. However, since P - channel MOSFETs require a different voltage level to turn on compared to N - channel MOSFETs (they turn on when the gate - source voltage is negative), additional circuitry or level shifting might be needed depending on the control source.

3. Working Principle

• Motor Speed Control:
  • To control the motor speed, Pulse - Width Modulation (PWM) can be used. The PWM signal is applied to the gates of the MOSFETs. By varying the duty cycle of the PWM signal (the ratio of the on - time to the total period of the pulse), the average voltage applied across the motor can be adjusted. For example, a higher duty cycle means the MOSFETs are on for a longer time during each period, allowing more current to flow through the motor and increasing its speed.
• Motor Direction Control:
  • To change the direction of the motor, the combination of MOSFETs that are turned on is changed. For example, to make the motor rotate in one direction, Q1 and Q4 are turned on simultaneously while Q2 and Q3 are kept off. This allows current to flow through the motor in one direction. To reverse the direction, Q2 and Q3 are turned on while Q1 and Q4 are turned off, reversing the current flow through the motor.

4. Precautions and Considerations

• MOSFET Ratings:
  • It's crucial to select MOSFETs with appropriate voltage and current ratings. The voltage rating should be higher than the maximum voltage of the power supply, and the current rating should be able to handle the maximum current that the motor draws. Otherwise, the MOSFETs may get damaged due to overvoltage or overcurrent.
• Heat Dissipation:
  • When the MOSFETs are conducting current, especially in high - power applications, they can generate heat. Adequate heat dissipation measures, such as using heat sinks, should be considered to ensure the MOSFETs operate within their safe temperature ranges.
• Electrical Noise:
  • The switching of MOSFETs can generate electrical noise. To minimize this, proper grounding, the use of bypass capacitors near the power supply pins of the MOSFETs, and shielding of the circuit (if necessary) can be implemented.