PIC16F1503 based MPPT controller for photovoltanic system

Once I found a really simple MPPT circuit on Chris' website: http://www.freewebs.com/acselectronics/buildppt.html I decided to build something inspired by his work. I always wanted to build a switching regulator based on a general purpose simple (and cheap!) microcontroller instead of specialized switching controller. I choose PIC16F1503 because it has got build in two comparators, PWM (maybe not very sophisticated but fully functional) and CWG - new peripheral from Microchip which can be used to generate dead-band time between two outputs.

My first goal was to build something as “prove of concept” so it is not a real MPPT controller because it doesn't measure a PV cell current, but it adjusts load of the PV cell to make sure it works with given output voltage. Here is circuit diagram:

It's actually simple step-down converter with P-channel MOSFET (I decided to use P-channel because it allows use of common ground to measure input and output voltage). Load of this circuit will be a lead-acid battery so I added D3 to make sure that circuit will not discharge battery. D3 introduces lose of efficiency so right now I'm thinking about other fancy circuit which will do that better (with lower voltage loss across itself).

Here is an idea how microcontroller is configured to control switching regulator with its peripherals:

This is straight-forward implementation of switching regulator but implemented in a microcontroller, after initialization of FVR (Fixed Voltage Reference), PWM, comparator and CWG (complementary waveform generator) it works and software can do something else. And it will do: I plan to implement perturb and observe algorithm to search for maximum power condition of PV cell. I plan to introduce some changes to Circuit: I have to add PV cell current measurement. I also have to add another PWM with RC filter as a source of voltage of comparator. PIC16F1503 allows me to use a DAC output as a input signal for comparator but DAC is 5-bit only, it is to small resolution for effective MPPT tracking.

Another improvement will be second MOSFET instead of D4 to increase efficiency. Due to a simplicity of MOSFET driver circuit off-time is longer than I want so I will search for some improvement, maybe totem-pole circuit?

Here is a waveform on pin 6 of microcontroller (channel 2) and gate of MOSFET (channel 1), it is clearly visible that there is much to improve, especially switch-off time.  

Here is picture of a circuit itself during early tests:

It's built on one-side substrate to minimize costs, why should I order PCB in a fab if I know that circuit will anyway need changes?

Here is PCB design, as usually I have to show off how nice PCBs I draw. I used SMD components wherever it was convenient. I even managed to design whole PCB on one layer without a single wire-bridge.  

I will update this post or I will write the new one when I will build next version of this circuit with mentioned improvements.

The software "as it is"

#include <pic.h>

__CONFIG(FOSC_INTOSC & WDTE_OFF & PWRTE_ON & MCLRE_ON & CP_OFF);


void main(void)
{
    OSCCONbits.IRCF=13; //4MHz

    ANSELA &= ~(1<<2);
    TRISA &= ~(1<<2);   //konfigurj LEDa
    PORTA &= ~(1<<2);   //zgas LEDa


    PWM1CON=0;
    PR2=9;           //40 us
    PWM1DCH=7;
    PWM1DCL=1<<7;


    TMR2IF=0;
    T2CONbits.T2CKPS=1; //prescaler /4
    T2CONbits.TMR2ON=1;
    PWM1CONbits.PWM1OE=1;
    PWM1CONbits.PWM1EN=1;
    TRISC &= ~(1<<5);


    FVRCONbits.CDAFVR=1; //4.096V
    FVRCONbits.FVREN=1;  //enable



    ANSELA |= (1<<7);    //comparator
    TRISC |= (1<<3);
    CM1CON1bits.C1PCH=2; //positive input to FVR (1.024V)
    CM1CON1bits.C1NCH=3; //ngative input to C2IN3-
    CM1CON0bits.C1ON=1; //on


    TRISC &= ~(1<<4);  //konfiguruj wyjscie do sterowania mosfetem
    PORTC &= ~(1<<4);
    CWG1CON0bits.G1EN=1;    //wlacz
    CWG1CON0bits.G1OEB=1;   //w?acz wyjscie
    CWG1CON0bits.G1POLB=1;  //inwersja na wyjsciu
    CWG1CON1bits.G1ASDLB=2; //zero w auto shot down
    CWG1CON1bits.G1IS=2;    //na wejsciu jest PWM
    CWG1CON2bits.G1ARSEN=1; //auto restart enabled
    CWG1CON2bits.G1ASDSC1=1; // comparator 1 

    while(1)
    {

        
        if(CMOUTbits.MC1OUT)
            PORTA |= (1<<2);  //wlacz leda
        else
            PORTA &= ~(1<<2); //zgas LEDa
    }
}