CM6802A/B/AH/BH (Dynamic Soft PFC/Green PWM)
http://www.championmicro.com.tw
EPA/80++ ZVS-Like PFC/PWM COMBO CONTROLLER
Design for High Efficient Power Supply at both Full Load and Light Load
Oscillator (RAMP1, or called RTCT)
In current-mode applications, the PWM ramp (RAMP2) is usually
derived directly from a current sensing resistor or current
transformer in the primary of the output stage, and is thereby
representative of the current flowing in the converter’s output
In CM6802A/AH, fRTCT=4xfpwm=4xfpfc fRTCT=200Khz,
fpwm=50Khz and fpfc=50Khz when VEAO=0V, it provides
the best performance in the PC application.
In CM6802B/BH, fRTCT=2xfpwm=4xfpfc fRTCT=200Khz, stage. DCILIMIT, which provides cycle-by-cycle current limiting, is
fpwm=100Khz and fpfc=50Khz when VEAO=0V, it provides typically connected to RAMP2 in such applications. For
the best performance in the PC application.
voltage-mode, operation or certain specialized applications,
RAMP2 can be connected to a separate RC timing network to
generate a voltage ramp against which VDC will be compared.
Under these conditions, the use of voltage feed-forward from the
PFC buss can assist in line regulation accuracy and response. As
in current mode operation, the DC ILIMIT input is used for output
stage over-current protection.
The oscillator frequency, fRTCT is the similar formula in
CM6800:
1
fRTCT =
t
RAMP + tDEADTIME
The dead time of the oscillator is derived from the
following equation:
No voltage error amplifier is included in the PWM stage of the
CM6802A/B/AH/BH, as this function is generally performed on
the output side of the PWM’s isolation boundary. To facilitate the
design of opto-coupler feedback circuitry, an offset has been built
into the PWM’s RAMP2 input which allows VDC to command a
zero percent duty cycle for input voltages below around 1.8V.
V
REF −1.25
REF − 3.75
t
RAMP = CT x RT x In
V
at VREF = 7.5V:
tRAMP = CT x RT x 0.51
PWM Current Limit (DCILIMIT)
The dead time of the oscillator may be determined using:
The DC ILIMIT pin is a direct input to the cycle-by-cycle current
limiter for the PWM section. Should the input voltage at this pin
ever exceed 1V, the output flip-flop is reset by the clock pulse at
the start of the next PWM power cycle. Beside, the cycle-by-cycle
current, when the DC ILIMIT triggered the cycle-by-cycle current.
It will limit PWM duty cycle mode. Therefore, the power
dissipation will be reduced during the dead short condition.
When DCILIMIT pin is connected with RAMP2 pin, the
CM6802A/B/AH/BH’s PWM section becomes a current mode
PWM controller. Sometimes, network between DCILIMIT and
RAMP2 is a resistor divider so the DCILIMIT’s 1V threshold can
be amplified to 1.5V or higher for easy layout purpose.
2.5V
tDEADTIME
=
x CT = 593 x CT
4.216mA
The dead time is so small (tRAMP >> tDEADTIME ) that the
operating frequency can typically be approximately by:
1
fRTCT =
t
RAMP
Ct should be greater than 470pF.
Let us use 1000PF Solving for RT yields 7.75K. Selecting
standard components values, CT = 1000pF, and RT
7.75kΩ
=
PWM Brown Out (380V-OK Comparator)
The 380V-OK comparator monitors the DC output of the PFC
and inhibits the PWM if this voltage on VFB is less than its nominal
2.36V. Once this voltage reaches 2.36V, which corresponds to
the PFC output capacitor being charged to its rated boost voltage,
the soft-start begins. It is a hysteresis comparator and its lower
threshold is 1.35V.
The dead time of the oscillator determined two things:
1.) PFC minimum off time which is the dead time
2.) PWM skipping reference duty cycle: when the PWM
duty cycle is less than the dead time, the next cycle
will be skipped and it reduces no load consumption
in some applications.
PWM Section
Pulse Width Modulator
The PWM section of the CM6802A/B/AH/BH is
straightforward, but there are several points which should
be noted. Foremost among these is its inherent
synchronization to the PFC section of the device, from
which it also derives its basic timing. The PWM is capable
of current-mode or voltage-mode operation.
2009/11/02 Rev. 1.4
Champion Microelectronic Corporation
19
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