User`s guide

SMPS AC/DC Reference Design User’s Guide

FIGURE 1-3: INPUT CURRENT WAVEFORM WITH AND WITHOUT PFC

These waveforms illustrate that PFC can improve the input current drawn from the

mains supply and reduce the DC bus voltage ripple. The objective of PFC is to make

the input to a power supply look like a simple resistor. The PFC circuitry provides a

power factor that is nearly equal to unity with very low current THD (< 5%).

Figure 1-4 shows a block diagram of the AC-to-DC converter stage, which converts the

AC input voltage to a DC voltage and maintains sinusoidal input current at a high input

Power Factor.

The input rectifier converts the alternating voltage at power frequency into

unidirectional voltage. This rectified voltage is fed to the chopper circuit to produce a

smooth and constant DC output voltage to the load. The chopper circuit is controlled

by the PWM switching pulses generated by the dsPIC DSC device, based on three

measured feedback signals:

• Rectified input voltage

• DC bus current

• DC bus voltage

Input Voltage

DC Bus Output Voltage

Input Current

DC Bus Output Voltage

Input Current

Without

PFC

With PFC