AN-6982

APPLICATION NOTE

© 2010 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.0 • 6/8/10

2

Functional Description

Widely used operation modes for the boost converter are

continuous conduction mode (CCM) and boundary

conduction mode (BCM). These two descriptive names

refer to the current flowing through the energy storage

inductor of the boost converter, as depicted in Figure 2.

The inductor current in CCM is continuous; while in BCM,

the new switching period is initiated when the inductor

current returns to zero, which is at the boundary of

continuous conduction and discontinuous conduction

operations. CCM PFC is commonly used for high-power

applications above 300W since the inductor current has a

small ripple and higher power factor can be obtained than

BCM operation. Due to the reverse-recovery current of the

output diode, using a high-speed diode with a small reverse

recovery current is crucial to achieve high efficiency and

low EMI.

Figure 2. CCM vs. BCM Control

Current and Voltage Control of PFC

As shown in Figure 3, the FAN6982 employs two control

loops for power factor correction: a current-control loop

and a voltage-control loop. The current-control loop shapes

inductor current, as shown in Figure 4, such that voltage

during one switching cycle:

0

()

S

T

LCS

MO

M

S

I Rdt

I

R

T

1

⋅=

⋅

∫

(1)

function of input current of IAC pin; and voltages of the

VRMS and VEA pins are calculated as:

2

10.5 (

- 0.7)

(- 0.7)

EA

MO

AC

MAX

RMS

EA

V

II

VV

×

=×

(2)

Figure 3. Current and Voltage Control

Feedback Circuit

M

MO

CS

R

I

R

=⋅

Figure 4. Operation Waveforms of CCM PFC

The voltage-control loop regulates PFC output voltage

using an internal error amplifier such that the FBPFC

voltage is same as the internal reference of 2.5V. Note that,

constant to obtain pure sinusoidal reference for the input

current shaping. Because there is always twice the line

frequency ripple in the PFC output voltage, a narrow

bandwidth should be used for the output voltage-control

loop to minimize the line frequency ripple. Otherwise, the

control loop tries to remove the output voltage ripple,

changing the error amplifier output voltage as shown in

Figure 5, which causes distortion of the input current.