Buck with PID controlled output voltage

A voltage mode buck converter regulates voltage by comparing the output voltage to a reference voltage. It adjusts the duty cycle of its switching signal to keep the output voltage stable. This type of converter efficiently steps down voltage levels, making it useful in various electronic devices like embedded battery charger.

This example will implement a voltage mode buck converter to control the output.

!!! attention Are you ready to start ? Before you can run this example, you must have successfully gone through our getting started.

Hardware setup and requirement

The circuit diagram of the board is shown in the image below.

Figure 1: circuit diagram

The power flows from VHigh to VLow. The wiring diagram is shown in the figure below.

Figure 2: wiring diagram

Warning

You will need : - 1 TWIST - A dc power supply (20-60V) - A resistor (or a dc electronic load)

Main code structure

The main.cpp structure is shown in the image below.

Figure 3: Code structure

The code structure is as follows: - On the top of the code some initialization functions take place. - Setup Routine - calls functions that set the hardware and software - Communication Task - Handles the keyboard communication and decides which MODE is activated - Application Task - Handles the MODE, activates the LED and prints data on the serial port - Critical Task - Handles the MODE, sets power ON/OFF and tracks the V1_low_value variable with a PID

The tasks are executed following the diagram below.

Figure 4: Timing diagram

• Communication Task - Is awaken regularly to verify any keyboard activity
• Application Task - This task is woken once its suspend is finished
• Critical Task - This task is driven by the HRTIM count interrupt, where it counts a number of HRTIM switching frequency periods. In this case 100us, or 20 periods of the TWIST board 200kHz switching frequency set by default.

Control scheme

The control library is imported in platformio.ini via the line :

lib_deps=
    control_lib = https://github.com/owntech-foundation/control_library.git

We can use this library to initialize a PID control with the function :

pid.init(pid_params);

The control diagram of the PID is shown in the figure below.

Figure 5: Control diagram

Expected result

This code will control V1low and V2low voltages so that they follow a voltage_reference, you can control this reference through platformio serial monitor. The image below shows you a snippet of the window and the button to press.

Figure 6: serial monitor button

When opening it for the first time, the serial monitor will give you an initialization message regarding the parameters of the ADCs as shown below.

Figure 7: serial monitor initialization

Tip

• press u to increase the voltage
• press d to decrease the voltage
• press h to show the help menu

Here's sequence when the help menu is activated with h, the power mode is then activated with p and finally the Twist converter is put in idle with the i.

Figure 8: serial monitor working

Note

When you send p the Twist board will send you back a stream of data on the following format:

I1:V1:VREF:I2:V2:VREF:IH:VH:T1:T2:

Where: - I1 is the current in LEG1 of the LOW side - V1 is the voltage in LEG1 of the LOW side - VREF is the reference voltage set for LEG1 and LEG2vof the LOW side - I2 is the current in LEG1 of the LOW side - V2 is the voltage in LEG2 of the LOW side - VREF is the reference voltage set for LEG1 and LEG2vof the LOW side - IH is the current in LEG2 of the LOW side - VH is the voltage on the HIGH side - T1 is the temperature from the NTC thermistor in LEG1 of the LOW side - T2 is the temperature from the NTC thermistor in LEG2 of the LOW side

For instance when you retrieve this:

1.44:14.80:0.13:16.14:1.14:22.82:

It means that I1 = 1.46 A, V1 = 14.80 V and so on.

If you plot your data with a python code, you will see something similar to this:

Figure 9: result_plot