AN1965 Freescale Semiconductor / Motorola, AN1965 Datasheet - Page 5

• Manufacturer Part Number: AN1965
• Description: Design of Indirect Power Using the 56F800/E
• Manufacturer: Freescale Semiconductor / Motorola

To produce the rectangular input current waveform, an active PFC is placed between the rectifier and
the filtering capacitor. This is a boost inverter consisting of an inductor (L), diode (D) and chop
switcher (T). If the switcher (T) is on at any phase position of AC input voltage, the diode is
reversed-biased, thus isolating the filtering capacitor. In this period, the input supplies energy to
inductor (L). When switcher (T) is off, the filtering capacitor is charged from the inductor as well as
from the input, thus providing input current with continuous conduction.
The Pulse Width Modulator (PWM) generates reference pulses with a frequency of about 20KHz. The
duty cycle of PWM pulses can be modified in two points:
1. The average duty cycle (a period longer than one period of input voltage) depends on output load.
2. The instant duty cycle (a period less than one-half of the period of input voltage) depends on input
A rectangular waveform provides a narrow spectrum of noise and the lowest peak value of input
current because the high frequency task of inverter switching can be filtered easily.
High output load requires a longer transistor open state to support the constant output voltage.
voltage. Low input voltage requires a longer transistor open state to support the constant input
current.
Output voltage
Input current
Standard Power Supply:
Figure 3-2. Waveforms of Output Voltage and Input Current
Freescale Semiconductor, Inc.
For More Information On This Product,
Indirect Power Factor Correction
Go to: www.freescale.com
Input current
Output voltage
Power Factor Corrector:
Power Factor Correction (PFC) Theory
5