CM6824IS Champion Microelectronic Corp., CM6824IS Datasheet - Page 9

• Manufacturer Part Number: CM6824IS
• Description: Low Start-Up Current PFC/PWM Controller Combo
• Manufacturer: Champion Microelectronic Corp.

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 form the power line. The general for of the
output of the gain modulator is:
More exactly, the output current of the gain modulator is
given by:
Where K is in units of V
Note that the output current of the gain modulator is limited
around 228.47µA and the maximum output voltage of the
gain modulator is limited to 228.47uA x 3.5K=0.8V. This
0.8V also will determine the maximum input power.
However, I
I
when VEAO is less than 0.5V and I
I
Selecting R
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
wave current derived from the line voltage and it also helps
program the maximum input power and minimum input line
voltage.
R
voltage is 80VAC, the R
Current Error Amplifier, IEAO
The current error amplifier’s output controls the PFC duty
cycle to keep the average current through the boost
inductor a linear function of the line voltage. At the inverting
input to the current error amplifier, the output current of the
gain modulator is summed with a current which results from
a negative voltage being impressed upon the I
The negative voltage on I
currents flowing in the PFC circuit, and is typically derived
from a current sense resistor in series with the negative
terminal of the input bridge rectifier.
2002/09/20
SENSE
OFFSET
AC
I
=Vin peak x 7.9K. For example, if the minimum line
GAINMOD
I
GAINMOD
= I
is around 60uA.
GAINMOD
GAINMOD
AC
= K x (VEAO – 0.625V) x I
=
Preliminary
for IAC pin
I
-I
AC
OFFSET
cannot be measured directly from I
V
VEAO
RMS
-1
and I
AC
SENSE
2
=80 x 1.414 x 7.9K=894Kohm.
AC
OFFSET
, it will provide a good sine
x 1V
represents the sum of all
Rev. 1.1
GAINMOD
can only be measured
AC
L
is 0A. Typical
OW
SENSE
S
(1)
pin.
Champion Microelectronic Corporation
TART-
SENSE
.
U
P
C
In higher power applications, two current transformers are
sometimes used, one to monitor the IF of the boost diode. As
stated above, the inverting input of the current error amplifier
is a virtual ground. Given this fact, and the arrangement of
the duty cycle modulator polarities internal to the PFC, an
increase in positive current from the gain modulator will
cause the output stage to increase its duty cycle until the
voltage on I
increased current. Similarly, if the gain modulator’s output
decreases, the output duty cycle will decrease, to achieve a
less negative voltage on the I
Cycle-By-Cycle Current Limiter and Selecting R
The I
loop, is a direct input to the cycle-by-cycle current limiter for
the PFC section. Should the input voltage at this pin ever be
more negative than –1V, the output of the PFC will be
disabled until the protection flip-flop is reset by the clock
pulse at the start of the next PFC power cycle.
R
the steady state, line input current x R
Since the maximum output voltage of the gain modulator is
I
input current will be limited below 0.8V as well. Therefore, to
choose R
R
For example, if the minimum input voltage is 80VAC, and the
maximum input rms power is 200Watt, R
1.414)/(2 x 200) = 0.197 ohm.
PFC OVP
In the CM6824, PFC OVP comparator serves to protect the
power circuit from being subjected to excessive voltages if
the load should suddenly change. A resistor divider from the
high voltage DC output of the PFC is fed to VFB. When the
voltage on VFB exceeds 2.75V, the PFC output driver is shut
down. The PWM section will continue to operate. The OVP
comparator has 250mV of hysteresis, and the PFC will not
restart until the voltage at VFB drops below 2.50V. The VFB
power components and the CM6824 are within their safe
operating voltages, but not so low as to interfere with the
boost voltage regulation loop. Also, VCC OVP can be served
as a redundant PFCOVP protection. VCC OVP threshold is
19.4V with 1.5V hysteresis.
URRENT
GAINMOD
S
S
is the sensing resistor of the PFC boost converter. During
=0.7V x Vinpeak/(2x Line Input power)
SENSE
max x 3.5K= 0.8V during the steady state, R
S
, we use the following equation:
pin, as well as being a part of the current feedback
SENSE
PFC/PWM C
is adequately negative to cancel this
SENSE
ONTROLLER
pin.
S
= I
S
GAINMOD
= (0.7V x 80V x
CM6824
Page 9
S
x 3.5K.
C
S
OMBO
x line