Introduction to H Bridges and Their Applications

An H bridge is an electronic circuit that enables a voltage to be applied across a load in either direction. It is commonly used in robotics and other applications to allow DC motors to run forwards or backwards.

The term “H bridge” is derived from the typical graphical representation of such a circuit. It consists of four switching elements, with the motor at the center, forming an H-like configuration.

Key Components of an H Bridge Circuit

The main components of a typical H bridge circuit include:

1. DC power supply
2. Four switching elements (transistors, MOSFETs, etc.)
3. Diodes for protection against back EMF
4. Control circuit to manage the switching elements

Applications of H Bridges

H bridges find use in a wide range of applications, such as:

• Robotics: Controlling DC motors for wheel drive, arm movement, etc.
• Automotive: Controlling windshield wipers, electric windows, and other DC motors
• Industrial automation: Controlling actuators, valves, and other mechanical systems
• Consumer electronics: Controlling motors in appliances, toys, and other devices

Designing a 100A 12V H Bridge

Specifications

For this project, we will design an H bridge with the following specifications:

• Supply voltage: 12v dc
• Maximum current: 100A
• Motor type: DC brushed motor

Choosing the Right Components

Switching Elements

For a high-current application like this, MOSFETs are the most suitable switching elements. They offer low on-state resistance, fast switching speeds, and can handle large currents.

We’ll use N-channel MOSFETs for the high-side switches and P-channel MOSFETs for the low-side switches. This configuration simplifies the gate drive requirements.

Some suitable MOSFETs for this project include:

• IRFB7530: 100V, 195A, N-channel
• IRF4905: -55V, -74A, P-channel

Gate Drivers

To ensure proper switching of the MOSFETs, we need gate drivers that can provide the necessary voltage and current. For this project, we can use the following gate drivers:

• IR2110: High and low side driver, 500V/2A peak output
• TC4427: Dual high-speed power MOSFET driver, 1.5A output

Protection Diodes

To protect the MOSFETs from voltage spikes caused by the motor’s back EMF, we need to include protection diodes. Schottky diodes are a good choice due to their fast switching speed and low forward voltage drop.

Some suitable Schottky diodes include:

• MBR20100CT: 100V, 20A
• STPS30SM100S: 100V, 30A

Circuit Design

Here’s a simplified schematic of the 100A 12V H bridge circuit:

            +12V
             │
             ├──────────────────┬────────────────┐
             │                  │                │
             │             ┌────┴────┐     ┌────┴────┐
             │             │  IRFB   │     │  IRFB   │
             │             │  7530   │     │  7530   │
             │             └────┬────┘     └────┬────┘
             │                  │                │
             │                  ├────────────────┤
             │                  │                │
             │                  │     Motor      │
             │                  │                │
             │                  ├────────────────┤
             │                  │                │
             │             ┌────┴────┐     ┌────┴────┐
             │             │  IRF    │     │  IRF    │
             │             │  4905   │     │  4905   │
             │             └────┬────┘     └────┬────┘
             │                  │                │
             └──────────────────┴────────────────┘
                                │
                               GND

The gate drivers and protection diodes are not shown in this simplified schematic.

PCB Layout Considerations

When designing the PCB for the H bridge, consider the following:

1. Use wide traces for high-current paths to minimize resistance and heat generation.
2. Place the MOSFETs close to each other to reduce parasitic inductance.
3. Use a ground plane to provide a low-impedance return path for the high currents.
4. Include thermal vias and a heatsink for the MOSFETs to dissipate heat effectively.

Controlling the H Bridge

Microcontroller Selection

To control the H bridge, we need a microcontroller that can generate the necessary PWM signals and handle the required I/O. Some suitable options include:

• Arduino Uno: 8-bit AVR, 16 MHz, 14 digital I/O pins
• STM32F103: 32-bit ARM Cortex-M3, 72 MHz, 37 digital I/O pins
• ESP32: 32-bit Tensilica LX6, 240 MHz, 25 digital I/O pins

PWM Generation

To control the speed and direction of the motor, we need to generate PWM signals for the high-side MOSFETs. The microcontroller should have at least two PWM channels for this purpose.

The duty cycle of the PWM signals determines the average voltage applied to the motor, which in turn controls its speed. By varying the duty cycle, we can achieve speed control.

To change the direction of the motor, we need to reverse the polarity of the voltage applied to it. This can be done by switching the PWM signals between the two high-side MOSFETs.

Safety Features

To ensure safe operation of the H bridge, consider implementing the following features:

1. Current limiting: Monitor the motor current and shut down the H bridge if it exceeds a predetermined threshold.
2. Over-temperature protection: Use temperature sensors to monitor the MOSFETs and shut down the H bridge if they overheat.
3. Soft start: Gradually increase the PWM duty cycle during startup to prevent sudden current spikes.
4. Emergency stop: Include a hardware or software emergency stop feature to quickly disable the H bridge if needed.

Testing and Verification

Before using the H bridge in a real application, it’s crucial to thoroughly test and verify its operation. Here are some steps to follow:

1. Visual inspection: Check for any soldering issues, short circuits, or damaged components.
2. Continuity test: Use a multimeter to ensure there are no shorts between the power supply, ground, and motor terminals.
3. Power-on test: Apply a low voltage (e.g., 3.3V) to the H bridge and verify that the MOSFETs switch correctly using an oscilloscope.
4. Motor test: Connect a motor to the H bridge and test its operation at various speeds and directions.
5. Current measurement: Measure the motor current during operation to ensure it stays within the designed limits.
6. Temperature monitoring: Use a thermal camera or temperature sensors to monitor the MOSFETs’ temperature during extended operation.

Troubleshooting Common Issues

MOSFETs Not Switching

If the MOSFETs are not switching correctly, check the following:

1. Gate drive voltage: Ensure the gate drivers are providing the correct voltage levels to the MOSFET gates.
2. MOSFET connections: Verify that the MOSFET source, drain, and gate pins are connected correctly.
3. PWM signals: Check that the PWM signals from the microcontroller are reaching the gate drivers.

Motor Not Rotating

If the motor is not rotating, consider these possible causes:

1. Power supply: Ensure the power supply is providing the correct voltage and current.
2. Motor connections: Verify that the motor is connected correctly to the H bridge output.
3. Mechanical issues: Check for any mechanical obstructions or damage to the motor.

Overheating

If the MOSFETs or motor are overheating, consider the following:

1. Current limiting: Ensure the current limiting feature is working correctly.
2. Heatsinking: Verify that the MOSFETs are properly heatsunk and that there is adequate airflow.
3. PWM frequency: Increasing the PWM frequency can help reduce heating in the MOSFETs.

Conclusion

Building a 100A 12V H bridge requires careful component selection, circuit design, and PCB layout considerations. Proper control and safety features are essential for reliable operation.

By following the guidelines provided in this article, you should be able to design, build, and test a robust H bridge for your high-current DC motor applications.

Remember to always prioritize safety and thoroughly test your design before using it in a real-world application.

FAQ

1. Q: Can I use this H bridge design for motors with different voltage ratings?
   A: This design is optimized for 12V motors. For motors with different voltage ratings, you may need to modify the component selection and power supply accordingly.

2. Q: What is the maximum continuous current this H bridge can handle?
   A: The maximum continuous current depends on factors such as the MOSFETs’ thermal management and the PCB’s current-carrying capacity. With proper heatsinking, this design should be able to handle up to 100A continuous current.

3. Q: Can I use this H bridge with brushless DC motors?
   A: No, this H bridge design is intended for use with brushed DC motors. Brushless DC motors require a different type of motor controller, such as a three-phase inverter.

4. Q: How do I tune the current limiting feature?
   A: The current limiting threshold can be set by adjusting the value of the current sense resistor and the comparator reference voltage. The exact values will depend on your specific application and the desired current limit.

5. Q: What is the expected efficiency of this H bridge design?
   A: The efficiency of the H bridge depends on factors such as the MOSFET on-state resistance, switching frequency, and motor current. With proper component selection and PCB design, efficiencies of 90% or higher are achievable at full load.