FAN4803

PRODUCT SPECIFICATION

6

REV. 1.2.3 11/2/04

In the case of leading edge modulation, the switch is turned

OFF right at the leading edge of the system clock. When the

modulating ramp reaches the level of the error amplifier

output voltage, the switch will be turned ON. The effective

duty-cycle of the leading edge modulation is determined

during the OFF time of the switch. Figure 3 shows a leading

edge control scheme.

One of the advantages of this control technique is that it

requires only one system clock. Switch 1 (SW1) turns OFF

and Switch 2 (SW2) turns ON at the same instant to mini-

mize the momentary “no-load” period, thus lowering ripple

voltage generated by the switching action. With such

synchronized switching, the ripple voltage of the first stage

is reduced. Calculation and evaluation have shown that the

120Hz component of the PFC’s output ripple voltage can be

reduced by as much as 30% using this method, substantially

reducing dissipation in the high-voltage PFC capacitor.

Typical Applications

One Pin Error Amp

The FAN4803 utilizes a one pin voltage error amplifier in the

PFC section (VEAO). The error amplifier is in reality a cur-

rent sink which forces 35µA through the output program-

ming resistor. The nominal voltage at the VEAO pin is 5V.

The VEAO voltage range is 4 to 6V. For a 11.3M

Ω resistor

chain to the boost output voltage and 5V steady state at the

VEAO, the boost output voltage would be 400V.

Programming Resistor Value

Equation 1 calculates the required programming resistor

value.

PFC Voltage Loop Compensation

The voltage-loop bandwidth must be set to less than 120Hz

to limit the amount of line current harmonic distortion.

A typical crossover frequency is 30Hz. Equation 1, for

simplicity, assumes that the pole capacitor dominates

the error amplifier gain at the loop unity-gain frequency.

Equation 2 places a pole at the crossover frequency,

providing 45 degrees of phase margin. Equation 3 places a

zero one decade prior to the pole. Bode plots showing the

overall gain and phase are shown in Figures 5 and 6. Figure 4

displays a simplified model of the voltage loop.

Rp

VV

I

V

A

M

BOOST

EAO

PGM

=

−

=

−

=

400

50V

35

113

.

.

µ

Ω

(1)

C

Pin

RV

VEAO

C

f)

COMP

p

BOOST

OUT

=

××

×

×

× ×

∆

(2

2

π

(2)

C

W

COMP

300

11.3M

Ω

× 400V × 0.5V × 220µF × (2 × π × 30Hz)

F

2

16n

=

=

COMP

C

Figure 3. Typical Leading Edge Control Scheme.

REF

EA

–

+

–

+

OSC

DFF

R

D

Q

Q

CLK

U1

RAMP

CLK

U4

U3

C1

RL

I4

SW2

SW1

+

DC

I1

I2

I3

VIN

L1

VEAO

CMP

U2

RAMP

VEAO

TIME

VSW1

TIME