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Digital DC/DC Buck Converter Using Peak Current Mode Control

This example shows how to use the Comparator Subsystem (CMPSS) to regulate buck converter output voltage (BOOSTXL-BUCKCONV) using Peak Current Mode Control (PCMC) for Texas Instruments™ C2000™ processors. Using this example, you can:

  • Configure the CMPSS to monitor the inductor current sense feedback (ILFB) signal.

  • Configure the enhanced Pulse Width Modulator (ePWM) to trip on an overcurrent condition.

  • Generate code for the controller and load it on the hardware board.

  • Monitor signals and tune parameters on the host computer.

Introduction

This example uses a CMPSS, an ePWM, and ADC (Analog Digital Converter) subsystem to perform the synchronous buck operation in PCMC.

  • Each PWM cycle starts by driving the ePWM high (PWM-HI) until the peak current threshold is reached after which the PWM-HI signal is driven low for the rest of the PWM cycle.

  • The ePWM1 module triggers ADC conversions to sense the plant feedback signals at a frequency of 200kHz.

  • The ADC ISR calculates changes to the peak current limit.

  • An on-chip analog CMPSS is configured to continuously monitor the current sense feedback (ILFB) signal against set peak current limit.

  • The Control Law Accelerator (CLA) executes the PCMC control algorithm.

Prerequisites

Before you start with this example, install these MathWorks® products:

  • Instrument Control Toolbox™

  • MATLAB™

  • Simulink®

  • DSP Toolbox™

  • Embedded Coder®

Required Hardware

Available Models

Hardware Connections

Connect the Digital Power Buck Converter BoosterPack (BOOSTXL-BUCKCONV) to the Texas Instruments Piccolo F280049C LaunchPad as detailed in the following table.

Model

Digital DC/DC Buck Converter Peak Current Mode Control (PCMC) Model

To open the model, type the following command in the MATLAB® prompt:

open_system('f280049C_DCDC_Buck_PCMC');

C2000_host_read_PCMC Model

To open the model, type the following command in the MATLAB® prompt:

open_system('c2000_host_read_PCMC');

This example demonstrates the working of PCMC for a fixed input voltage of 9 V DC and an output voltage setpoint of 2 V DC. Hence a fixed slope compensation value is applicable for the above mentioned voltages is computed. For more information, refer to TI manual 'Digital Peak Current Mode Control With Slope Compensation'.

Configure and Run Digital DC/DC Buck Converter Peak Current Mode Control (PCMC) Model

1. Open the Digital DC/DC Buck Converter Peak Current Mode Control (PCMC) model. This model is configured for TI Piccolo F280049C LaunchPad hardware.

2. To run the model on other TI C2000 processors, first press Ctrl+E to open the Configuration Parameters dialog box. Then, select the required hardware board by navigating to Hardware Implementation > Hardware board.

Note: Ensure that the Digital Power Buck Converter BoosterPack is connected to the selected hardware board with correct pin mappings.

3. The following screenshots show the CMPSS configurations performed in the model. You can use the same parameter values if you want to run this example for other hardware boards.

4. Ensure that the baud rate is set to 6250000 bits/sec.

Peripheral Block Configurations

Set the peripheral block configurations for this model. Double-click on the blocks to open block parameter configurations. You can use the same parameter values if you want to run this example for other hardware boards.

ePWM block

Configure the ePWM action qualifier to generate waveforms based on digital compare events. Configure the trip signal generated by the CMPSS module to trip ePWMA to low.

CMPSS Block in Overcurrent Limit

The overcurrent logic trips the output on an overcurrent condition. Although overcurrent protection is inherent in the peak current loop, the application implements a redundant protection path using spare comparator resources on the MCU. This trip is configured for one shot trip in the ePWM. Clear the trip flags to re-enable the outputs.

CMPSS Block in CLA Task

The PCMC control algorithm is executed by CLA module, which generates an interrupt to the CPU when the algorithm is executed. The CMPSS with computed peak current threshold is updated within the CLA interrupt routine. For information on the CLA configuration, refer to Using the Control Law Accelerator (CLA).

  • The comparator monitors the inductor feedback current against the peak current threshold. It generates a trip signal for the ePWM when the current limit is met.

  • The peak current threshold for the comparator is provided by a reference DAC. Use the ramp generator to control the reference DAC values, which are synchronized to a ePWM

  • The RAMP decrement value, which defines the slope of the RAMP signal, is fixed to 7. This fixed slope might not cover the full dynamic range of the operation of the plant using PCMC.

ADC Block in PI Controller ISR

The ePWM triggers the ADC start of conversion (SOC) event to sample the output voltage. The ADC end of conversation (EOC) interrupt executes the digital control algorithm that computes the actuation value depending on the error in the system.

Generate Code for Controller and Load on Hardware Board

1. To generate the code for the model f280049C_DCDC_Buck_PCMC , press Ctrl+B or click Build, Deploy & Start.

2. Follow the build process by opening the diagnostic viewer using the link provided at the bottom of the model canvas. After you load the code on the board, a red LED blinks on the hardware board, indicating that the code is running.

Monitor Signals and Tune Parameters on Host Computer

Configure and Run Model on Host Computer

1. On the host computer, browse to Device Manager > Ports (COM & LPT) to find the COM port.

2. Set the COM ports of the following blocks in the c2000_host_read_PCMC model to match the COM port of the host computer:

  • c2000_host_read_PCMC > Serial Receive

  • c2000_host_read_PCMC > Serial Send > Serial Send1

3. Click on the Run button to run the model.

Tune Parameters

While the model runs, you can tune parameters using the following dashboard blocks:

  • Voltage Request - Change the output voltage demand. This parameter is the main request for the control loop. The controller algorithm compares the Voltage Request value with the measured output voltage and adjusts the PWM duty cycle towards achieving the output voltage.

  • P Gain - Change the proportional gain of the controller algorithm. You can change this parameter to study the robustness of the controller. Large, abrupt changes might lead to instability of the controller; apply changes smoothly.

  • I Gain - Change the integral gain of the controller algorithm. You can change this parameter to study the robustness of the controller. Large, abrupt changes might lead to instability of the controller; apply changes smoothly.

  • Over Current Limit - Change the overcurrent setpoint to be less than the current operating in the converter. For example, when the controller is regulating the Vout at 2 V, try to program the overcurrent limit to 1.2 A. This should trip the outputs to the plant as the over current condition occurred.

  • Active load - Turn the active load present on the hardware on or off. This parameter allows you to add an extra load resistor to study the effect of abrupt changes in the load circuit.

Monitor Signals

While the model runs, you can monitor the following signals on the Scope block:

  • V_FB Voltage - The measured output voltage of the system is 2 V.

  • Active load:

Troubleshooting

  • Ensure the 9 V DC power supply input voltage is stable when active load is enabled. In the test bench setup, the active load works when input voltage is set to 10 V.

  • If you face connectivity issues with the serial connection between host and target, try to disconnect and reconnect the TI Piccolo F280049C LaunchPad.

  • When the current in the converter is more than the overcurrent setpoint, the output gets tripped and the P-I controller reaches saturation. To re-enable the output the below sequence can be followed:

  • Set the overcurrent limit to a higher value such as 5.2 A.

  • Set the voltage setpoint to 0 volts and then back to 2 V.

Other Things to Try

  • Calculate the slope compensation for a different voltage setpoint, configure the comparator and run the example.

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