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
14
Getting Start:
To start evaluating CM6800T from the exiting CM6800 or
ML4800 board, 6 things need to be taken care before doing
the fine tune:
1.) Change RAC resistor (on pin 2, IAC) from the old value to
a higher resistor value between 4.7 Mega ohms to 8 Mega
ohms.
2.) Change RTCT pin (pin 7) from the existing value to
RT=5.88K ohm and CT=1000pF to have fpfc=68 Khz,
fpwm=68Khz, fRTCT=272Khz for CM6800T.
3.) Adjust all high voltage resistor around 5 mega ohm or
higher.
4.) VRMS pin (pin 4) needs to be 1.14V at VIN=80Vac and to
be 1.21V at VIN=80VAC for universal input application
from line input from 80VAC to 270VAC.
5.) At full load, the average Veao needs to around 4.5V and
the ripple on the Veao needs to be less than 250mV when
the light load comparator are triggerred.
6.) Soft Start pin (pin 5), the soft start current has been
reduced from CM6800’s 20uA to CM6800T’s 10uA.Soft
Start capacitor can be reduced to 1/2 from your original
CM6800 capacitor.
Functional Description
CM6800T is designed for high efficient power supply for both
full load and light load. It is a popular EPA/85+ PFC-PWM
power supply controller.
The CM6800T consists of an average current controlled
continuous/discontinuous boost Power Factor Correction
(PFC) front end and a synchronized Pulse Width Modulator
(PWM) back end. The PWM can be used in either current or
voltage mode. In voltage mode, feed-forward from the PFC
output bus can be used to improve the PWM’s line regulation.
In either mode, the PWM stage uses conventional trailing edge
duty cycle modulation, while the PFC uses leading edge
modulation. This patented leading/trailing edge modulation
technique results in a higher usable PFC error amplifier
bandwidth, and can significantly reduce the size of the PFC
DC buss capacitor.
The synchronized of the PWM with the PFC simplifies the
PWM compensation due to the controlled ripple on the PFC
output capacitor (the PWM input capacitor). In addition to
power factor correction, a number of protection features have
been built into the CM6800T. These include soft-start, PFC
over-voltage protection, peak current limiting, brownout
protection, duty cycle limiting, and under-voltage lockout.
Power Factor Correction
Power factor correction makes a nonlinear load look like a
resistive load to the AC line. For a resistor, the current drawn
from the line is in phase with and proportional to the line
voltage, so the power factor is unity (one). A common class of
nonlinear load is the input of most power supplies, which use a
bridge rectifier and capacitive input filter fed from the line. The
peak-charging effect, which occurs on the input filter capacitor
in these supplies, causes brief high-amplitude pulses of current
to flow from the power line, rather than a sinusoidal current in
phase with the line voltage. Such supplies present a power
factor to the line of less than one (i.e. they cause significant
current harmonics of the power line frequency to appear at
their input). If the input current drawn by such a supply (or any
other nonlinear load) can be made to follow the input voltage in
instantaneous amplitude, it will appear resistive to the AC line
and a unity power factor will be achieved.
To hold the input current draw of a device drawing power
from the AC line in phase with and proportional to the input
voltage, a way must be found to prevent that device from
loading the line except in proportion to the instantaneous line
voltage. The PFC section of the CM6800T uses a boost-mode
DC-DC converter to accomplish this. The input to the converter
is the full wave rectified AC line voltage. No bulk filtering is
applied following the bridge rectifier, so the input voltage to the
boost converter ranges (at twice line frequency) from zero volts
to the peak value of the AC input and back to zero. By forcing
the boost converter to meet two simultaneous conditions, it
is possible to ensure that the current drawn from the power line
is proportional to the input line voltage. One of these conditions
is that the output voltage of the boost converter must be set
higher than the peak value of the line voltage. A commonly
used value is 385VDC, to allow for a high line of 270VAC
rms
.
The other condition is that the current drawn from the line at
any given instant must be proportional to the line voltage.
Establishing a suitable voltage control loop for the converter,
which in turn drives a current error amplifier and switching
output driver satisfies the first of these requirements. The
second requirement is met by using the rectified AC line
voltage to modulate the output of the voltage control loop. Such
modulation causes the current error amplifier to command a
power stage current that varies directly with the input voltage.
In order to prevent ripple, which will necessarily appear at the
output of boost circuit (typically about 10VAC on a 385V DC
level); from introducing distortion back through the voltage
error amplifier, the bandwidth of the voltage loop is deliberately
kept low. A final refinement is to adjust the overall gain of the
PFC such to be proportional to 1/(Vin x Vin), which linearizes
the transfer function of the system as the AC input to voltage
varies.
Since the boost converter topology in the CM6800T PFC is
of the current-averaging type, no slope compensation is
required.
More exactly, the output current of the gain modulator is given
by:
