CM6500UN (1MHz PFC)
http://www.championmicro.com.tw
EPA/90+ ZVS-Like PFC CONTROLLER
Design for High Efficient Power Supply at both Full Load and Light Load
Power Factor Correction
Getting Start
Power factor correction makes a nonlinear load look like a
To start evaluating CM6500UN from the exiting CM6502 ,
need to be taken care before doing the fine tune:
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.
1.) Change RTCT pin (pin 7) from the existing value to
RT=27K ohm and CT=1000pF to have fpfc = fRTCT =
68Khz for CM6500UN.
2.) Adjust all high voltage resistor around 5 mega ohm or
higher first.
3.) VRMS pin (pin 4) needs to be 1.1V at VIN=80Vac right
before PFC brown out and to be 1.78V at VIN=85VAC
right before PFC brown in for universal input application
for line input from 85VAC to 270VAC.
5.) At full load, the average Veao needs to be around 4.2V and
the ripple on the Veao needs to be less than 300mV when
the light load comparator are triggered.
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
Functional Description
voltage. The PFC section of the CM6500UN 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
CM6500UN is designed for high efficient power supply for both
full load and light load. It is a ZVS-Like PFC supply controller.
The CM6500UN is an average current controlled, 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
continuous/discontinuous boost Power Factor Correction
(PFC) which uses leading edge modulation.
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 270VACrms. 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.
In addition to power factor correction, a number of protection
features have been built into the CM6500UN. These include
soft-start, PFC over-voltage protection, peak current limiting,
brownout protection, duty cycle limiting, and under-voltage
lockout.
Since the boost converter topology in the CM6500UN PFC is
of the current-averaging type, no slope compensation is
required.
More exactly, the output current of the gain modulator is given
by:
2014/11/11 Rev. 1.0
Champion Microelectronic Corporation
10
CHAMP [ CHAMPION MICROELECTRONIC CORP. ]
