CM6503/4

2002/07/16

Champion Microelectronic Corporation

Page 8

For steady state and for the each phase angle, boost

converter DC equation at continuous conduction mode is:

)

1

(

1

d

V

V

in

out

−

=

(4)

Rearrange above equations, (1), (2),(3), and (4) in term of

Vout and d, boost converter duty cycle and we can get

average boost diode current equation (5):

e

out

d

R

V

d

I

×

−

=

2

)

1

(

(5)

Also, the average diode current can be expressed as:

dt

t

I

T

I

off

T

d

sw

d

⋅

=

∫

)

(

1

0

(6)

If the value of the boost inductor is large enough, we can

assume

d

d

I

t

I

~

)

(

. It means during each cycle or we

can say during the sampling, the diode current is a

constant.

Therefore, equation (6) becomes:

)

1

(

'

d

I

d

I

T

t

I

I

d

d

sw

off

d

d

−

×

=

×

=

×

=

(7)

Combine equation (7) and equation (5), and we get:

sw

off

e

out

d

e

out

d

e

out

d

T

t

R

V

I

R

V

d

I

R

V

d

d

I

×

=

∴

×

=

∴

×

=

×

'

2

'

'

)

(

(8)

From this simple equation (8), we implement the PFC control

section of the CM6503.

Leading/Trailing Modulation

Conventional Pulse Width Modulation (PWM) techniques

employ trailing edge modulation in which the switch will turn

ON right after the trailing edge of the system clock. The error

amplifier output is then compared with the modulating ramp.

When the modulating ramp reaches the level of the error

amplifier output voltage, the switch will be turned OFF. When

the switch is ON, the inductor current will ramp up. The

effective duty cycle of the trailing edge modulation is

determined during the ON time of the switch. Figure 2 shows

a typical trailing edge control scheme.

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

edge control scheme.