Freescale Semiconductor M68HC08 manual Freescale Semiconductor

Control Theory

2.2.2 Digital Power Factor Concept — Hysteresis Current Control Mode

The control technique is based on hysteretic current control. The system operates in continuous conduction mode with variable switching frequency (30–100 kHz) (see Figure 2-5).

This PFC concept is designed to have the minimum of MCU performance requirements. The basic principles of the scheme are depicted in Figure 2-4. The PFC control algorithm includes two control loops, a fast one for input current control and a slow one for output voltage control. The output voltage controller is implemented digitally using the MCU. A value proportional to the required input current is modulated by the PWM and is taken as an input to the current control loop, which is realized by the analogue comparator. The comparator switches the MOSFET in order to maintain the required current value.

The desired shape of the input current is a sine wave. The generated current waveform is shown in Figure 2-8.

A hysteresis current control mode PFC concept has several drawbacks, including variable MOSFET switching frequency, non sinusoidal input current waveform and switching under current, which causes higher losses than other PFC topologies.

The input current harmonics content, however, complies with EN 61000-3-2 standard.

The advantages are simple control circuit, low MCU resources consumption, continuous conduction mode operation, and low total harmonic distortion (THD).

0

AC Line Voltage

AC Line Current

L

Zero

Crossing

AC

DC Bus

Voltage

IC ADC

+ DC BUS

Comparator

+MOSFET

AC

LINE

DC

PWM Reference Voltage

-

+

MCU

Actual

Current

Current Sensing GND

Figure 2-4. Hysteresis Current Control Mode Principle

Dimmable Light Ballast with Power Factor Correction, Rev. 1

16 Freescale Semiconductor