The STP75NF20 is a high-power N-channel MOSFET designed for applications requiring high current and efficiency. With a voltage rating of 200V and a current handling capacity of 75A, it is ideal for driving DC motors, powering high-current devices, and other applications requiring robust switching. In this project, we’ll design a high-power DC motor speed controller that utilizes the STP75NF20 MOSFET to provide variable speed and direction control for a DC motor.

Objective

The aim of this project is to build a DC motor speed controller capable of handling motors up to 500W, with variable speed control using pulse-width modulation (PWM). This controller is designed for applications like electric bikes, robotics, conveyor belts, or any system requiring efficient motor control.

Why Use the STP75NF20?

The STP75NF20 MOSFET is a robust and reliable power switching component. It offers:

1. High Current Capability: 75A, perfect for high-power DC motors.

2. Low RDS(on)_{DS(on)}DS(on)​: 0.009Ω, reducing conduction losses and improving efficiency.

3. High Switching Speed: Suitable for PWM control.

4. Thermal Efficiency: Designed for high-power applications with proper heat sinking.

These features make it ideal for motor control applications, where high efficiency and thermal performance are critical.

Applications of the Controller

1. Electric Vehicles: Control speed and direction of DC motors in bikes or carts.

2. Robotics: Drive motors in robotic arms or mobile platforms.

3. Industrial Automation: Control conveyor belts, fans, or pumps.

4. DIY Projects: Build custom tools, toys, or gadgets requiring motorized systems.

Materials and Components

Active Components

1. STP75NF20 MOSFET (2 pcs for H-bridge configuration)

2. 555 Timer IC: For generating the PWM signal.

3. Diodes: High-current Schottky diodes (e.g., 100V, 20A) for freewheeling.

4. Gate Driver IC: IR2104 or similar, to drive the MOSFET gates.

5. Potentiometer: 10kΩ for adjusting speed.

Passive Components

1. Resistors:

1) Gate resistors (10Ω, 1/4W) for controlling gate charge current.

2) Pull-down resistors (10kΩ) for stabilizing MOSFET gates.

2. Capacitors:

1) Decoupling capacitor (0.1μF ceramic).

2) Snubber capacitor (10nF, high voltage) across MOSFET drain-source terminals.

3. Inductor: Optional, for filtering PWM noise in motor supply.

Mechanical Components

1. DC Motor: 12V or 24V, up to 500W.

2. Heat Sink and Thermal Paste: For mounting the MOSFETs to dissipate heat.

Power Supply

1. DC Power Supply: 12V–24V DC, rated for the motor's power requirements.

Tools

1. Soldering iron and solder wire.

2. Multimeter and oscilloscope.

3. Wire cutters and connectors.

Circuit Design

The motor controller is built around the STP75NF20 MOSFET in an H-bridge configuration for bidirectional motor control. It incorporates a PWM generator for speed control, a gate driver for proper MOSFET operation, and protection circuits for safe operation.

1. H-Bridge Motor Driver

An H-bridge consists of four MOSFETs arranged to control the current flow through the motor in both directions:

1. High-Side Switches: Provide positive voltage to the motor terminals.

2. Low-Side Switches: Provide ground connection for the motor terminals.

In this project, two STP75NF20 MOSFETs are used for the low-side switches due to their high current capacity, while generic N-channel MOSFETs or P-channel MOSFETs can be used for the high-side switches.

2. PWM Speed Control

A 555 timer IC is configured in astable mode to generate a PWM signal. The duty cycle of the PWM controls the effective voltage across the motor, thus regulating its speed.

Connections:

1. The PWM output is fed into the gate driver IC (IR2104), which amplifies the signal and drives the MOSFET gates.

2. A 10kΩ potentiometer adjusts the duty cycle, allowing for variable speed control.

3. Gate Driver Circuit

The IR2104 gate driver ensures that the high-side and low-side MOSFETs are driven properly:

1. High-Side Drive: Boosts the gate voltage above the supply voltage using a bootstrap capacitor.

2. Low-Side Drive: Directly drives the gates of the STP75NF20 MOSFETs.

Gate Resistors: 10Ω resistors are connected between the driver outputs and the MOSFET gates to control the switching speed.

4. Protection Features

1. Freewheeling Diodes:

Schottky diodes are connected across the motor terminals to handle back EMF during switching.

2. Snubber Circuit:

A capacitor across the MOSFET drain and source reduces voltage spikes caused by inductive loads.

3. Overcurrent Protection:

A current-sense resistor or fuse is included to prevent overcurrent damage.

Assembly Process

Step 1: Assemble the H-Bridge

1. Mount the STP75NF20 MOSFETs on heat sinks using thermal paste.

2. Wire the MOSFETs in an H-bridge configuration.

3. Connect the motor terminals to the midpoints of the H-bridge.

Step 2: Build the PWM Generator

1. Set up the 555 timer IC in astable mode with the desired frequency (1–20 kHz).

2. Connect the potentiometer to adjust the duty cycle.

3. Verify the PWM signal using an oscilloscope.

Step 3: Add the Gate Driver Circuit

1. Connect the PWM signal to the IR2104 input pins.

2. Wire the IR2104 outputs to the MOSFET gates through 10Ω resistors.

3. Add bootstrap capacitors for the high-side drivers.

Step 4: Integrate the Protection Circuit

1. Attach Schottky diodes across the motor terminals.

2. Place a snubber capacitor across each MOSFET drain-source terminal.

3. Add a fuse or current-sense resistor in series with the motor.

Testing and Calibration

Step 1: Initial Testing

1. Power the circuit with a low voltage (e.g., 12V) without connecting the motor.

2. Verify the PWM signal and gate driver outputs using an oscilloscope.

Step 2: Motor Testing

1. Connect the motor and gradually increase the PWM duty cycle.

2. Observe the motor's speed and direction. Adjust the potentiometer to verify speed control.

Step 3: Load Testing

1. Apply a load to the motor and monitor the MOSFET temperature.

2. Ensure the heat sinks remain effective under prolonged operation.

Applications

1. Electric Vehicles: Control speed and direction of small electric motors.

2. Robotics: Drive motors for robotic arms or wheels.

3. Home Automation: Control fans, pumps, or other motorized systems.

4. DIY Tools: Build motorized tools or gadgets for personal use.

Safety Considerations

1. Heat Management: Ensure proper heat sinking for the STP75NF20 MOSFETs to prevent thermal runaway.

2. High Current: Use thick wires and secure connections to handle high currents.

3. Electrical Isolation: Keep the low-voltage control circuit isolated from the high-current power stage.

Enhancements and Future Work

1. Feedback Control:

Use a tachometer or Hall sensor to monitor motor speed and implement closed-loop control.

2. Wireless Control:

Add a wireless module (e.g., Bluetooth or RF) for remote speed adjustment.

3. High-Power Version:

Use multiple STP75NF20 MOSFETs in parallel for higher current capacity.

Conclusion

This high-power DC motor speed controller project demonstrates the versatility and robustness of the STP75NF20 MOSFET in motor control applications. By integrating efficient switching, PWM speed control, and protection features, the circuit delivers reliable performance for a variety of uses. With proper assembly and testing, this project can power motors in DIY tools, vehicles, or automation systems, making it an excellent addition to any electronics enthusiast's portfolio.