Closed loop voltage control of a buck converter using pic 16F876A

[biswaIITH]

hello folks i am controlling the output voltage of a buck converter using PIC16F876A. but i am not getting the desired output.

i am using a 11.05 mhz crystal oscillator..
input voltage of the buck converter is 20v
desired output voltage is 16v.
and switching frequency is 20khz

below is the code which is a simple pi controller implemented in pic16f876A

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Code C - [expand]
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float adc;
float set_ref=3;
float err_val=0,I_err=0;
float current_duty_2=50;
float Kp=1,Ki=2;
float P_term=0,I_term=0;
float Sum_err=0;
void main() {
ADCON1=0x80;
TRISA=0xFF;
TRISC.F1=0;
PWM2_Init(20000);
PWM2_Start();
PWM2_Set_Duty(current_duty_2);
while(1)
{
adc=ADC_Read(1);
err_val=set_ref-adc;
P_term=Kp*err_val;
I_err=I_err+err_val;
if(I_err>30)
I_err=30;
else if(I_err<-30)
I_err=-30;
I_term=Ki*I_err;
Sum_err=(P_term+I_term);
if(Sum_err>255)
Sum_err=255;
else if(Sum_err<0)
Sum_err=0;
current_duty_2=Sum_err;
PWM2_Set_Duty(current_duty_2);
}
}

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[udayan92]

What is the expected behaviour and what is the behaviour that is actually happening?

 

[biswaIITH]

my doubt is whether this code is right or wrong .Any improvement if required or necessary is what i am expecting...

 

[udayan92]

The code seems to be correct as per the syntax.I recommend you to program the code in the microcontroller and start observing the output behaviour.

 

[biswaIITH]

do u think it would be better if i write my own adc code instead of using simply adc_read(1) command

 

[udayan92]

I do not think it will have any big advantage over the adc_read function,which you are currently using in your program.I think you should test your code,by running it on a system.That way,you can know whether you should keep the code as it is or change it.

 

[biswaIITH]

Well i have tested the above code in a buck converter.Below are my observations over approximately 10 trials

observation-1

1) i am getting 15 v output as against 16 v which is desired but i am not getting any pulses from the mikro or the oscilloscope is nt showing.

observation-2

1) when i am opening the loop i.e. the input voltage to the buck converter is switched off, i should get pulses as set in the code before the while loop.But, i am getting a pulses which is completely different than the one set in the code

observation-3

1)no output and no pulses in closed loop

i am confused .