How to Design and Tune a PID Controller for STM32 Buck Converter

Introduction

Designing and tuning a PID (Proportional-Integral-Derivative) controller for a buck converter is essential to ensure that your output voltage remains stable under varying load conditions. In this tutorial, we will walk through the process of configuring a PID controller for an STM32-based buck converter with a 48V input and a 5V output. This guide will cover prerequisites, required parts/tools, detailed steps, troubleshooting tips, and a concise conclusion.

Prerequisites

• Basic knowledge of PID control theory
• Familiarity with STM32 microcontrollers
• Experience in programming with C/C++
• Understanding of buck converter operation
• Access to an oscilloscope and multimeter

Parts/Tools

• STM32 microcontroller board (e.g., STM32F4 series)
• Buck converter module (rated for 48V input and 5V output)
• Power supply (48V)
• Load resistor (for testing)
• Development environment (e.g., STM32CubeIDE)
• Oscilloscope
• Multimeter

Steps

1. Set Up the Hardware
   1. Connect the buck converter to the STM32 board.
   2. Ensure the input is connected to a 48V power supply.
   3. Connect a load resistor to the output of the buck converter.
   4. Verify connections with a multimeter to ensure proper voltage levels.
2. Initialize the STM32 Project
   1. Open STM32CubeIDE and create a new project for your STM32 microcontroller.
   2. Configure the GPIO pins for PWM output and ADC input.
   3. Set up the timer for PWM generation.
   4. Enable the necessary peripherals for ADC readings.
3. Implement the PID Algorithm
   1. Define PID constants: Kp, Ki, Kd.
   2. Write the PID control function:

   
   void PID_Controller(float setpoint, float measured) {
       static float integral = 0;
       static float previous_error = 0;
       float error = setpoint - measured;
       integral += error;
       float derivative = error - previous_error;
       float output = Kp * error + Ki * integral + Kd * derivative;
       previous_error = error;
       
       // Set PWM duty cycle based on output
       SetPWMDutyCycle(output);
   }
           

   3. Call the PID_Controller function within the main loop, passing the desired output voltage (5V) and the measured output voltage.
4. Tune the PID Parameters
   1. Start with Kp set to a low value (e.g., 1.0) and Ki and Kd set to zero.
   2. Gradually increase Kp until the system starts to respond, but not oscillate.
   3. Add Ki to eliminate steady-state error; increase it slowly.
   4. Finally, introduce Kd to dampen any oscillations.
5. Test the System
   1. Monitor the output voltage using an oscilloscope.
   2. Vary the load and observe how the output voltage reacts.
   3. Adjust PID parameters as necessary based on performance.

Troubleshooting

• Output Voltage is Unstable:
  • Check your PID parameters; consider reducing Kp or adjusting Ki and Kd.
  • Inspect hardware connections for loose wires or poor soldering.
• Output Voltage is Too High/Low:
  • Verify the setpoint value and ensure it matches the desired output voltage.
  • Check if the ADC is correctly calibrated and reading the expected values.
• System is Non-Responsive:
  • Ensure the microcontroller is powered and running the correct firmware.
  • Check the PWM output configuration and ensure it is functional.

Conclusion

In this tutorial, we outlined the steps necessary to design and tune a PID controller for an STM32-based buck converter. Proper configuration of the hardware, implementation of the PID algorithm, and careful tuning of the control parameters are crucial for achieving stable output voltage. With practice, you can refine your PID parameters to optimize performance under various load conditions.