PID loop for LED driver

ranjithshanbhag — Tue, 24 Oct 2017 14:22:01 GMT

i am working on LED 40W driver with Fly-back followed by Digital Buck topology.

Operating output Current - 300mA to 1050mA

Operating output Voltage- 28V to 54V

i am finding 100Hz ripple contents at the LED output.

Buck current ADC conversation triggered with PWM. Every 7.69uS ADC will be sampled and 8 sample average will be calculated. Further moving average calculation of 13 samples will be done using 8 samples averaged value.

PID sampling time: 200uS

PID processing time: ~50-60uS

Can any one help me how to remove the 100Hz contents at LED output?

I attached High level circuit diagram, output current waveform and PID code.

// Count that gives the minimum target current of 20mA.
#define MINIMUM_ALLOWED_TARGET_

CURRENT_COUNT (54 * 13)

uint32_t PI_output_current(uint32_t set, uint32_t actual)
{
static int32_t previous_error = 0;

   static int32_t error_accumulated = 0;
   int32_t integral = 0;
   int32_t output = 0;
   int32_t derivative;
   static uint32_t previous_set = 0;

   // Cap the target current to minimum allowable current.
   if (set < MINIMUM_ALLOWED_TARGET_CURRENT_COUNT)
   {
       set = MINIMUM_ALLOWED_TARGET_CURRENT_COUNT;
   }

if(set > 0)
{

        if(previous_set != set)
       {
           previous_set = set;
           error_ignore = set * error_ignore_percentage;      // calculate 5% of the set point. Ignore if the error is less than that.
           error_ignore = error_ignore / 100;
       }
       error = set - actual;
       if(abs(error) > error_ignore) // only if the error is not acceptable do PID
       {
           error_accumulated += error;
           integral = (error_accumulated * (ADC_PI_Sample_time / 2)) / 10;

derivative = error - previous_error;
derivative = (derivative / (ADC_PI_Sample_time / 2)) / 10;

if(integral > integral_limit)
{

integral = integral_limit;
}

           if(integral < (int32_t)(0 - integral_limit))
           {
               integral = (int32_t)(0 - integral_limit);
           }

           output = (((int32_t)(KP * error) / (int32_t)10000) +
                      ((int32_t)(KI * integral) / (int32_t)10000) +
                      ((int32_t)(KD * derivative) / (int32_t)10000));

           if(abs(output) > MAX_DIFFERENCE_OF_OUTPUT)
           {
               if(output > 0)
               {
                   output = MAX_DIFFERENCE_OF_OUTPUT;
               }
               else
               {
                   output = (0 - MAX_DIFFERENCE_OF_OUTPUT);

               }
           }
           if((output < 0) && (output_to_set < abs(output)))
           {
               output_to_set = 0;
           }
           else
           {
               output_to_set += output;
           }
       }
       previous_error = error;

   }
   else
   {
       output_to_set = 0;
       previous_error = 0;
       integral = 0;
       error_accumulated = 0;
   }

return output_to_set;
}

Re: PID loop for LED driver

jeremyzhou — Wed, 01 Nov 2017 01:56:01 GMT

Hi Ranjith Shanbhag

Thank you for your interest in NXP Semiconductor products and 

for the opportunity to serve you.

How's it going on now?

Have a great day,
TIC

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PID loop for LED driver

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