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Implementing 5V to 3.3V PWM Buck Converter in the DSPIC33CH Curiosity Development Board

Andrew Mosqueda
Andrew Mosqueda

Introduction

The dsPIC33CH Curiosity Development Board (DM330028) has a built in buck-boost circuit that we can experiment with. Unfortunately in the time of this writing, the code for the buck-boost converter that is provided by Microchip is only written in assembly language which I found hard to comprehend. So I made my own program that is written in C language and using PID for the close loop control. Only voltage loop control is covered by this topic.  

Buck-Boost Converter Circuit

Figure 1 below is the schematic of the buck boost circuit of the DSPIC33CH Curiosity Board. The DSPIC IC can control the buck transistor (Q6) switching through the RC14_S1PWM7H connection. Q6 is a PMOS so it needs a logic 0 (Active-low) to turn it on. The output of the converter (Vout) is sensed by the DSPIC IC through RC1_VOUTFB connection as shown in Figure 2. Make sure that J13 is shorted on the board. Removing the short on J13 will decrease the gain, make the sense voltage small and will have low resolution in the ADC. Removing the short on J13 is useful for boost operation because a higher output voltage will need a lower gain sense resistors. I am connecting a 91Ω resistor as load. For a 3.3V output, the power that will be drawn by the resistor is 3.3V*3.3V/91Ω = 120 milliwatt. RC15_S1PWM7L should be set to low all the time so that the boost transistor (Q2) is always open. 


DM330028 Buck Converter Power Circuit
Figure 1 DSPIC33CH Curiosity Board Development Board Buck-Boost Converter Circuit

DM330028 Buck-Boost Output Sense Circuit
Figure 2 DSPIC33CH Curiosity Development Board Buck-Boost Output Voltage Sense Circuit

CORE SETUP

The PWM control pins in Figure 1 are connected to the Slave Core of the IC so we need to make the source code of the PWM control in the Slave Core. We also need to make a simple source code for the Master Core just to start the Slave Core.

  1. Run MPLAB X IDE (I'm using version v5.50 at the time of this writing). Connect J20 of the Curiosity board to your PC so that the MPLAB application detects what board is connected.
  2. After opening MPLAB, close any projects that are open on the left pane.

MASTER CORE

  1. Ctrl+Shift+N to open the new project dialog box. Select Standalone Project then Click Next.
    MPLAB New Project Dialog Box
  2. Select Device dsPIC33CH128MP508 and Tool Starter Kits (PKOB)-SN:BUR182571251 then click Next. Note, there is a DM330028-2 version of the Curiosity board so the device and tool will be different. Please refer to its user guide or check the actual part number of the dsPIC IC that is mounted on the board.
    MPLAB select device
  3. Select Compiler XC16 (v1.70) then click next. You may have a newer version installed.
    MPLAB Select Compiler
  4. Name the project as "master". I save it to folder "buck" then click Finish.
    MPLAB Project name and folder

SLAVE CORE

  1. Ctrl+Shift+N to open again the new project dialog box. Select Standalone Project then click Next.
  2. Again, your Curiosity board may have a different device name but take note of the last two characters ("S1") for the device which means that it is the slave core. Select device dsPIC33CH128MP508S1 and Starter Kits (PKOB)-SN:BUR182571251 then click Next.
    MPLAB select device slave
  3. Select compiler XC16 (v1.70) then click next.
  4. Name the project as "slave" and use the same project location with the master core. Click Finish.

CODE CONFIGURATION

MASTER CORE

  1. On the Projects Pane, right click on master and select "Set as Main Project".
    MPLAB set as main project
  2. Click the MCC button on the toolbox. master.mc3 file will be created in the root of the master project folder (..\master.x\master.mc3).
  3. In the device resources pane, click the + on the SLAVE CORE.
    MPLAB Device Resources Select Slave Core

    After clicking it, the SLAVE CORE should appear on the project resources pane.
    MPLAB project resources slave core
  4. In the Slave Core Pane, name the Slave as "slave". Enable Protocol A and Protocol B. Set Protocol A Direction as M->S.
    MPLAB slave core name
  5. In the Pin Manager Pane, set RC1, RC14 and RC15 as owned by the Slave Core. RC1 is the voltage feedback input. RC14 and RC15 are the PWM pins for controlling the MOSFETs of the buck-boost circuit,
    MPLAB Master Pin Manager Slave Pin Assignment
  6. In the System Module Pane, set clock output pin as "OSC2 is general purpose digital I/O pin".
    MPLAB clock output pin configuration
  7. Go back to the Slave Core Pane and click Save Master Settings button. A file named master_config.mc3 will be generated on the root of the master folder after you click the Save Master Settings button.
  8. Lastly for the master configuration, click the Generate button on the Project Resources pane. If there are warnings, review first the Notifications [MCC] pane. 
  9. After confirmation on the generate configuration, you should see a Generation complete on the text output. After that, close the configurator by clicking the MCC toolbar button.

SLAVE CORE

  1. On the Projects Pane, right click on the slave project and select Set as Main Project.
  2. Click the MCC button on the toolbox. This will generate a file named slave.mc3 to the root of the slave project folder (..\slave.x\slave.mc3).
  3. Go to the Master Core Pane then click Load Slave Settings from Master Configuration button.
  4. In the Load Master Settings dialog box, browse to the master core folder and open the master_config.mc3 file. 
  5. In the Device Resources Pane, click the + on PWM. The PWM will appear on the Project Resources Pane afterwards.
    MPLAB device resources PWM
  6. In the PWM Pane, Select PWM Generator 7, rename to PWM_BUCK, enable PWM Generator and change PWM Output Mode to Independent. Note that we can only use Generator 7 because it is the one connected to RC14 and RC15 PWM output.
    MPLAB PWM Generator Settings
  7. Change the Requested Frequency to 40KHz.
    MPLAB PWM frequency setting

    Below is the relationship of the Special Function Registers (SFR) to the Independent Edge PWM generator from the dsPIC33/PIC24 FRM High-Resolution PWM with Fine Edge Placement document.
    dsPIC33_PIC24 Independent Edge PWM Special Function Register (FSR)

    Below table defines these SFRs and abbreviations according to my understanding.
    AbbreviationDefinition
    SOCStart Of Counter
    EOCEnd Of Counter
    PGxPERPWM Generator x (generator number) Period. The PWM
    counter counts from 0 up to the value of this SFR. This value is
    calculated as the value on "PWM Input Clock Selection"
    (8MHz) divided by the value on "Requested Frequency"
    (40kHz) less 1 and is equal to 199 (0xC7).
    PGxDCPWM Generator x (generator number) Duty Cycle. The PWM
    generator output changes to 0 when the counter is equal to
    the value of this SFR. 
    PGxPHASEPWM Generator x (generator number) Phase or deadtime.
    The output of the PWM generator starts from 0 when counter
    value is 0 and changes to 1 when the counter value is equal to
    the value of this SFR.
    PWMxPulse Width Modulation x (generator number) output. PWMx
    = 1 if Counter>=PGxPHASE & Counter <= PGxDC else 0. For
    the dsPIC33 IC that we are using, this output is connected to
    the RC14 pin.
    The initialization of these SFRs will be automatically defined later on a pwm.c file generated by the MCC. 
  8. Change the ADC Trigger 1 to Trigger A Compare and Trigger A Compare time to 1ns.
    MPLAB PWM ADC Trigger Setting
  9. Change the Data Update Trigger to Duty Cycle.
    MPLAB PWM Data Update Settings
  10. In the Registers tab of the PWM pane, change the POLH to Active-low.
    MPLAB MCC Active low PWM
  11. In the Device Resources pane, click + of the ADC1 to transfer the device to the Project Resources.
    MPLAB Device Resources ADC
  12. In the ADC1 pane, enable Core1, change  Core1 channel to S1ANA1 and Trigger Source to Slave PWM1 Trigger1. Tick the Interrupt Box.
    MPLAB ADC Selected Channels Setting
  13. In the Pin Module Pane, change the name of RC1 to RC1_VOUTFB.
    MPLAB Slave Pin Module Setting
  14. Review the MCC Notifications. Usually I get the warnings like below.
    MPLAB MCC notification
  15. In the Project Resources pane, click Generate button. Click Yes to disregard the warnings and proceed generating the source files. Wait until a "Generation complete" text appears on the MPLAB Code Configurator prompt window.
  16. Close the code configurator by clicking the MCC toolbar button.

SET THE SLAVE CORE AS SECONDARY PROJECT OF MASTER CORE

  1. Set master project as the main project.
  2. Right click on the Secondaries folder of master project and click Add Secondary  Project...
    MPLAB Add Secondary project to master
  3. In the Add Secondary Project dialog window, select slave.X and click Add button.
    MPLAB Add slave to master
  4. Right-click again on the Secondaries Folder and select Properties. Tick the Build box of the properties dialog window then click OK.

ACTUAL CODING

MASTER CORE SOURCE CODE

  1. Open the main.c file on the master project's "Source Files" folder.
  2. Below the #include "mcc_generated_files/system.h", add the following codes:
    #include "mcc_generated_files/slave1.h"
    #include "mcc_generated_files/pin_manager.h"
  3. Below the SYSTEM_Initialize();, add the following codes:
    SLAVE1_Program();
    SLAVE1_Start();
  4. Below should be how the master core main.c would look like:
    MPLAB master source
  5. Close the main.c file of the master core. Click Save when asked.

SLAVE CORE SOURCE CODE

  1. Open the main.c file on the slave project's Source Files folder.
  2. Below the #include "mcc_generated_files/system.h", add the following codes:
    #include "mcc_generated_files/pin_manager.h"
    #include "mcc_generated_files/pwm.h"
    #include "mcc_generated_files/adc1.h"

    /*PID function and variable definitions*/
    void PID(void);
    volatile int16_t n16_integrator_memory;
    volatile int16_t n16_reference;
    int16_t n16_error;
    int16_t n16_proportional;

    #define target_voltage  3.3
    #define reference       666.7*target_voltage + 17.6
    #define p_gain          0.1
    #define i_gain          0.05
    The p_gain and i_gain values are the parameters that you can adjust to fine tune the gain loop response of the buck converter.
  3. Inside the int main(void), add the following codes after SYSTEM_Initialize();
    //Initialize PID variables
    n16_integrator_memory = 0;
    n16_reference = (int16_t) reference;
    //Assign PID Handler
     ADC1_SetRC1_VOUTFBInterruptHandler(PID);
  4. After the int main(void) function, add the following code:
    void PID(void)
    {
      n16_error = n16_reference - ADCBUF1;
      n16_integrator_memory = n16_integrator_memory + i_gain*n16_error;
      n16_proportional = p_gain*n16_error;
      PG7DC = n16_integrator_memory + n16_proportional;
    }
  5. The new main.c of the slave core should now look like below:
    MPLAB PID Code

RESULT

  1. Be sure to set the master project as the main project.
  2. Be sure that the Curiosity board is connected to the PC. Make sure that J13 is shorted on the board.
  3. Click the program button on the toolbar. 
    MPLAB Program Button
    Wait until the "Programming/Verify complete" is prompted on the Starter Kit on Board Pane.
    MPLAB Programming Complete
  4. Below is my hardware setup to validate the result. I use ADALM2000 as oscilloscope.
    DM330028 dsPIC33CH Buck Converter with ADALM2000 oscilloscope
  5. Below is the waveform taken from the ADALM2000 oscilloscope:
    Scopy Oscilloscope
    The mean of the output voltage is 3.22V, a little offset from the 3.3V target. It could be due to the tolerance of the sense resistors, opamp or ADC of the DSPIC IC. The reference in the source code can ba adjusted to compensate this offset. We can also see that the duty measured is 20.03%.

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