Problem with negative feedback control MCU:PIC16F73

[Mithun_K_Das]

I'm generating a PWM duel clock to run an Inverter. The all the functions are working properly. But the problem is in feedback. The output voltage should be fixed. For this I'm using negative feedback. The feedback program is:

void neg_feed()
 {
      ADCON0=0b00000001;
     //avg_pwm2 = avg_pwm1;
     adc_rd0 = ADC_Read(0);
     avg_pwm1 = adc_rd0;
     while(avg_pwm1==51)
     {
      if(fb <36 && fb>=0)
      {
       feedback = fb;
       //Delay_ms(5);
      }
      else
      {
       feedback = feedback;
       Delay_ms(10);
      }
      break;
     }
     while(avg_pwm1>51)
     {
      if(fb<36)
      {
       fb+=1;
       //Delay_ms(5);
      }
      else
      {
       fb = fb;
      }
      break;
     }
     while(avg_pwm1<51)
     {
      if(fb>1)
      {
       fb-=1;
       //Delay_ms(5);
      }
      else
      {
       fb = fb;
      }
      break;
     }
     if(fb <36 && fb>=0)
      {
       feedback = fb;
      }
      else
      {
       feedback = feedback;
       Delay_ms(10);
      }
 }

The negative feedback is working properly when load is more than 200W. But below this, voltage is fluctuating, that means the value of 'feedback' is not fixed. The main clock is in TMR0 interrupt routine. What may be the reason of this?

Thanks in advance.