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CM6802AGIP PDF预览

CM6802AGIP

更新时间: 2024-10-28 06:48:11
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虹冠电子 - CHAMP 功率因数校正控制器
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25页 814K
描述
EPA/80++ ZVS-Like PFC/PWM COMBO CONTROLLER

CM6802AGIP 数据手册

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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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