CM6800
LOW START-UP CURRENT PFC/PWM CONTROLLER COMBO
The output of the gain modulator is a current signal, in the
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 ISENSE is adequately negative to cancel this
increased current. Similarly, if the gain modulator’s output
decreases, the output duty cycle will decrease, to achieve a
less negative voltage on the ISENSE pin.
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:
I
AC× VEAO
x 1V
IGAINMOD
=
(1)
2
V
RMS
Cycle-By-Cycle Current Limiter and Selecting RS
More exactly, the output current of the gain modulator is
given by:
The ISENSE pin, as well as being a part of the current feedback
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.
IGAINMOD = K x (VEAO – 0.625V) x IAC
Where K is in units of V-1
Note that the output current of the gain modulator is limited
around 228.47 μA 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.
RS is the sensing resistor of the PFC boost converter. During
the steady state, line input current x RS = IGAINMOD x 3.5K.
Since the maximum output voltage of the gain modulator is
IGAINMOD max x 3.5K= 0.8V during the steady state, RS x line
input current will be limited below 0.8V as well. Therefore, to
choose RS, we use the following equation:
However, IGAINMOD cannot be measured directly from ISENSE
.
ISENSE = IGAINMOD-IOFFSET and IOFFSET can only be measured
when VEAO is less than 0.5V and IGAINMOD is 0A. Typical
IOFFSET is around 60uA.
RS =0.8V x Vinpeak/(2x Line Input power)
For example, if the minimum input voltage is 80VAC, and the
maximum input rms power is 200Watt, RS = (0.8V x 80V x
1.414)/(2 x 200) = 0.226 ohm.
Selecting RAC 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 RAC, 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.
PFC OVP
In the CM6800, 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 CM6800 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
17.9V with 1.5V hysteresis.
RAC=Vin peak x 7.9K. For example, if the minimum line
voltage is 80VAC, the RAC=80 x 1.414 x 7.9K=894Kohm.
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 ISENSE pin.
The negative voltage on ISENSE represents the sum of all
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.
2008/10/23 Rev. 2.1
Champion Microelectronic Corporation
Page 9
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