CM6503/4

2002/07/16

Champion Microelectronic Corporation

Page 7

possible to the IC. Good bypassing is critical to the proper

operation of the CM6503/4.

VCC is typically produced by an additional winding off the

boost inductor or PFC Choke, providing a voltage that is

proportional to the PFC output voltage. Since the VCC OVP

max voltage is 19.4V, an internal shunt limits VCC

overvoltage to an acceptable value. An external clamp,

such as shown in Figure 1, is desirable but not necessary.

VC C

GN D

1N 5250B

Fig u re 1. O p tiona l V C C C la m p

This limits the maximum VCC that can be applied to the IC

while allowing a VCC which is high enough to trip the VCC

OVP. An RC filter at VCC is required between boost trap

winding and VCC.

PWMCLK (Pin 8)

PWMCLK is the PWM Clock signal for the down stream DC

to DC PWM stage. This signal will not be on until the PFC

boost output reaches steady state which means VFB has

reaches 2.45V. (Typical PFC boost output is around 380V.

Then PWMCLK will send out 50% clock signal which has

been perfectly aligned with PFCOUT signal which is leading

edge modulation.

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 in phase 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 section of the CM6503/4 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 draws

from the power line matches the instantaneous 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 385VFB, to allow

current that the converter is allowed to draw from the line at

any given instant must be proportional to the line voltage.

PFC Control: Leading Edge Modulation with Input

Current Shaping Technique

(I.C.S.T.)

The only differences between the conventional PFC control

topology and I.C.S.T. is:

the current loop of the conventional control method is a close

loop method and it requires a detail understanding about the

system loop gain to design. With I.C.S.T., since the current

loop is an open loop, it is very straightforward to implement it.

The end result of the any PFC system, the power supply is

like a pure resistor at low frequency. Therefore, current is in

phase with voltage.

In the conventional control, it forces the input current to

follow the input voltage. In CM6503, the chip thinks if a boost

converter needs to behave like a low frequency resistor, what

the duty cycle should be.

The following equations is CM6503 try to achieve:

in

in

e

I

V

R

=

(1)

in

l

I

I

=

(2)

Equation 2 means: average boost inductor current equals to

input current.

d

out

l

in

I

V

I

V

×

≈

×

∴

(3)

Therefore, input instantaneous power is about to equal to the

output instantaneous power.