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CM6805AGIR

10-PIN Green-Mode PFC/PWM Combo CONTROLLER for High Density AC Adapter

Champion Microelectronic 

CM6805(A;B)/CM6806A

10-PIN Green-Mode PFC/PWM Combo CONTROLLER for High Density AC Adapter

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 CM6805A/CM6806A, 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 CM6805A/CM6806A try to

achieve:

Re = Vin Iin

(1)

I l = Iin

(2)

Equation 2 means: average boost inductor current equals

to input current.

∴Vin × I l ≈ Vout × I d

(3)

Therefore, input instantaneous power is about to equal to

the output instantaneous power.

For steady state and for the each phase angle, boost

converter DC equation at continuous conduction mode is:

Vout

Vin

=

1

(1 − d )

(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):

I d = (1 − d )2 ×Vout Re

(5)

Also, the average diode current can be expressed as:

∫ I d

=

1

Tsw

Toff

0

Id (t) ⋅ dt

(6)

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

assume I d (t) ~ I d . It means during each cycle or we

can say during the sampling, the diode current is a

constant.

Therefore, equation (6) becomes:

I d = I d × toff Tsw = I d × d ' = I d × (1 − d ) (7)

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

I d × d ' = (d ' )2 ×Vout Re

∴ I d = d ' ×Vout Re

(8)

∴ Id

= Vout

Re

× toff

Tsw

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

section of the CM6805A/CM6806A.

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.

2006/10/11 Rev1.0

Champion Microelectronic Corporation

Page 9

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