4.1 Digital Power Factor Correction

The inductor current (IAC), input rectified AC voltage (VAC), and DC Output Voltage (VDC) are used as feedback signals to implement the digital PFC. These signals are scaled by hardware gains set by internal/external differential Op amp gains, and are input to the analog channels of the ADC module.

The PFC algorithm uses three control loops: the voltage control loop, current control loop, and the voltage feed forward control loop.

The voltage compensator uses the reference voltage and actual output voltage as inputs to compute the error and compensate for the variations in output voltage. The output voltage is controlled by varying the average value of the current amplitude reference.

The current amplitude reference is calculated digitally by computing the product of the rectified input voltage, the voltage error compensator output, and the voltage feed-forward compensator output.

The rectified input voltage is multiplied to enable the current reference to have the same shape as the input voltage wave shape. The current signal should match the rectified voltage as closely as possible to have a high power factor.

The voltage feed-forward compensator is essential for maintaining a constant output power for a given load because it compensates for variations in the input voltage. Once the current reference is computed, it is fed to the current compensator. The output of the current compensator determines the duty cycle of the PWM pulses. The boost converter can be driven either by the Output Compare module or the PWM module.

Refer to application note AN1106, Power Factor Correction in Power Conversion Applications Using the dsPIC®DSC (DS01106), for information about the system design and digital implementations of this control method.