PRODUCT SPECIFICATION

FAN4810

REV. 1.0.12 9/24/03

7

Power Factor Correction

Power factor correction makes a nonlinear load look like a

resistive load to the AC line. For a resistor, the current drawn

from the line is in phase with and proportional to the line

voltage, so the power factor is unity (one). A common class

of nonlinear load is the input of most power supplies, which

use a bridge rectifier and capacitive input filter fed from the

line. The peak-charging effect, which occurs on the input

filter capacitor in these supplies, causes brief high-amplitude

pulses of current to flow from the power line, rather than

a sinusoidal current inphase with the line voltage. Such

supplies present a power factor to the line of less than one

(i.e. they cause significant current harmonics of the power

line frequency to appear at their input). If the input current

drawn by such a supply (or any other nonlinear load) can be

made to follow the input voltage in instantaneous amplitude,

it will appear resistive to the AC line and a unity power factor

will be achieved.

To hold the input current draw of a device drawing power

from the AC line in phase with and proportional to the input

voltage, a way must be found to prevent that device from

loading the line except in proportion to the instantaneous

line voltage. The PFC of the FAN4810 uses a boost-mode

DC-DC converter to accomplish this. The input to the

converter is the full wave rectified AC line voltage. No bulk

filtering is applied following the bridge rectifier, so the input

voltage to the boost converter ranges (at twice line

frequency) from zero volts to the peak value of the AC input

and back to zero. By forcing the boost converter to meet two

simultaneous conditions, it is possible to ensure that the

current drawn from the power line is proportional to the

input line voltage. One of these conditions is that the output

voltage of the boost converter must be set higher than the

peak value of the line voltage. A commonly used value is

condition is that the current drawn from the line at any given

instant must be proportional to the line voltage. Establishing

a suitable voltage control loop for the converter, which in

turn drives a current error amplifier and switching output

driver satisfies the first of these requirements. The second

requirement is met by using the rectified AC line voltage to

modulate the output of the voltage control loop. Such

modulation causes the current error amplifier to command a

power stage current that varies directly with the input

voltage. In order to prevent ripple, which will necessarily

appear at the output of the boost circuit (typically about

10VAC on a 385V DC level), from introducing distortion

back through the voltage error amplifier, the bandwidth of

the voltage loop is deliberately kept low. A final refinement

is to adjust the overall gain of the PFC such to be propor-

system as the AC input voltage varies.

Since the boost converter topology in the FAN4810 PFC is

of the current-averaging type, no slope compensation is

required.

PFC Circuit Blocks

Gain Modulator

Figure 1 shows a block diagram of the FAN4810. The gain

modulator is the heart of the PFC, as it is this circuit block

which controls the response of the current loop to line

voltage waveform and frequency, rms line voltage, and PFC

output voltage. There are three inputs to the gain modulator.

These are:

1.

A current representing the instantaneous input voltage

(amplitude and waveshape) to the PFC. The rectified AC

input sine wave is converted to a proportional current

via a resistor and is then fed into the gain modulator at

noise, as is required in high power switching power con-

version environments. The gain modulator responds lin-

early to this current.

2.

A voltage proportional to the long-term RMS AC line

voltage, derived from the rectified line voltage after

scaling and filtering. This signal is presented to the gain

takes over, to limit power dissipation of the circuit

components under heavy brownout conditions). The

illustrated in the Typical Performance Characteristics.

3.

The output of the voltage error amplifier, VEAO. The

gain modulator responds linearly to variations in this

voltage.

The output of the gain modulator is a current signal, in the

form of a full wave rectified sinusoid at twice the line

frequency. This current is applied to the virtual-ground

(negative) input of the current error amplifier. In this way

the gain modulator forms the reference for the current error

loop, and ultimately controls the instantaneous current draw

of the PFC from the power line. The general form for the

output of the gain modulator is:

More exactly, the output current of the gain modulator is

given by:

Note that the output current of the gain modulator is limited

to 500µA.

I

GAINMOD

I

AC

VEAO

×

V

RMS

2

−−−−−−−−−−−−−−−−−−−− 1V

×

=

(1)

I

GAINMOD

K

VEAO

0.625V

–

()

×

I

AC

×

=