CM6800T
(Turbo-Speed PFC+Green PWM)
http://www.championmicro.com.tw
EPA/85+
PFC+PWM COMBO CONTROLLER
Design for High Efficient Power Supply
2010/08/03
Rev. 1.2
Champion Microelectronic Corporation
15
Dynamic Soft PFC (patent pending)
Besides all the goodies from CM6800A, Dynamic Soft PFC
is the main feature of CM6800T. Dynamic Soft PFC is to
improve the efficiency, to reduce power device stress, to ease
EMI, and to ease the monotonic output design while it has the
more protection such as the short circuit with power-foldback
protection. Its unique sequential control maximizes the
performance and the protections among steady state, transient
and the power on/off conditions.
PFC Section:
Gain Modulator
Figure 1 shows a block diagram of the PFC section of the
CM6800T. The gain modulator is the heart of the PFC, as it is
this circuit block which controls the response of the current
loop to line voltage waveform and frequency, rms line voltage,
and PFC output voltages. There are three inputs to the gain
modulator. These are:
1. A current representing the instantaneous input voltage
(amplitude and wave-shape) to the PFC. The rectified AC
input sine wave is converted to a proportional current via a
resistor and is then fed into the gain modulator at I
AC
.
Sampling current in this way minimizes ground noise, as is
required in high power switching power conversion
environments. The gain modulator responds linearly to this
current.
2. A voltage proportional to the long-term RMS AC line voltage,
derived from the rectified line voltage after scaling and
filtering. This signal is presented to the gain modulator at
VRMS. The gain modulator’s output is inversely proportional
to V
RMS
2
. The relationship between V
RMS
and gain is
illustrated in the Typical Performance Characteristics of this
page.
3. The output of the voltage error amplifier, VEAO. The gain
modulator responds linearly to variations in this voltage.
The output of the gain modulator is a current signal, in the
form of a full wave rectified sinusoid at twice the line
frequency. This current is applied to the virtual-ground
(negative) input of the current error amplifier. In this way the
gain modulator forms the reference for the current error loop,
and ultimately controls the instantaneous current draw of the
PFC from the power line. The general formula of the output of
the gain modulator is:
I
mul
=
2
RMS
AC
V
0.7V)
-
VEAO
I
(
×
x constant (1)
Gain=Imul/Iac
K=Gain/(VEAO-0.7V)
I
mul
= K x (VEAO – 0.7V) x I
AC
Where K is in units of [V
-1
]
Note that the output current of the gain modulator is limited
around 100
μ
A and the maximum output voltage of the gain
modulator is limited to 100uA x 7.75K
≒
0.8V. This 0.8V also
will determine the maximum input power.
However, I
GAINMOD
cannot be measured directly from I
SENSE
.
I
SENSE
= I
GAINMOD
-I
OFFSET
and I
OFFSET
can only be measured
when VEAO is less than 0.5V and I
GAINMOD
is 0A. Typical
I
OFFSET
is around 25uA.
IAC=20uA, Veao=6V
Gain vs. VRMS (pin4)
When VRMS below 1V, the PFC is shut off. Designer needs
to design 80VAC with VRMS average voltage=
1.14V.
AC
MUL
AC
OFFSET
SENSE
I
I
I
I
I
=
−
=
Gain
Selecting R
AC
for IAC pin
IAC pin is the input of the gain modulator. IAC also is a
current mirror input and it requires current input. By selecting a
proper resistor R
AC
, it will provide a good sine wave current
derived from the line voltage and it also helps program the
maximum input power and minimum input line voltage.
R
AC
=Vin min peak x 53.03K. For example, if the minimum line
voltage is 80VAC, the R
AC
=80 x 1.414 x 53.03K = 6 Mega ohm.
