CM6900 Datasheet, PDF (10/19 Pages) List of Unclassifed Manufacturers – PFC/PWM COMBO w/ INRUSH CURRENT CONTROL & SEPARATED PFCOVP

CM6900
PFC/PWM COMBO w/ INRUSH CURRENT CONTROL & SEPARATED PFCOVP
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:
IGAINMOD =
IAC× VEAO
V²
RMS
x
1V
(1)
More exactly, the output current of the gain modulator is
given by:
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 228.47µA 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, 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.
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.
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.
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.
Cycle-By-Cycle Current Limiter and Selecting RS
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.
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:
RS =0.7V x Vinpeak/(2x Line Input power)
For example, if the minimum input voltage is 80VAC, and the
maximum input rms power is 200Watt, RS = (0.7V x 80V x
1.414)/(2 x 200) = 0.197 ohm.
Separated PFC Overvoltage Protection
In the CM6900, PFC OVP is using VFB2, which is separated
from VFB to sense OVP condition. The 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 VFB2 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 CM6900 are within their safe operating voltages, but not
so low as to interfere with the boost voltage regulation loop.
Also, VCCA OVP can be served as a redundant PFCOVP
protection. VCCA OVP threshold is 19.4V with 1.5V
hysteresis.
2003/04/23 Preliminary Rev. 1.2
Champion Microelectronic Corporation
Page 9