CM6807
10-PIN Green-Mode PFC/PWM Combo CONTROLLER for High Density AC Adapte
r
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 CM6807, 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 CM6807 try to achieve:
Therefore, equation (6) becomes:
I
d
=
I
d
×
t
off
T
sw
=
I
d
×
d
'
=
I
d
×
(1
−
d
)
(7)
Combine equation (7) and equation (5), and we get:
I
d
×
d
=
'
(
d
'
)
2
×
V
out
R
e
(8)
∴
I
d
=
∴
I
d
=
d
'
×
V
out
R
e
V
out
t
off
×
R
e
T
sw
From this simple equation (8), we implement the PFC control
section of the CM6807.
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.
R
e
=
V
in
I
in
(1)
I
l
=
I
in
(2)
Equation 2 means: average boost inductor current equals
to input current.
∴
V
in
×
I
l
≈
V
out
×
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:
V
out
V
in
=
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
×
V
out
R
e
(5)
Also, the average diode current can be expressed as:
I
d
=
1
T
sw
∫
T
off
0
I
d
(
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.
2010/04/20 Rev1.0
Champion Microelectronic Corporation
Page 9
CHAMP [ CHAMPION MICROELECTRONIC CORP. ]
