CM6802SAH/SBH (Turbo-Speed PFC+Green PWM)
EPA/85+ ZVS-Like PFC+PWM COMBO CONTROLLER
http://www.championmicro.com.tw
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
stage. DCILIMIT, which provides cycle-by-cycle current limiting, is
typically connected to RAMP2 in such applications. For
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.
In CM6802SAH, fRTCT=4xfpwm=4xfpfc fRTCT=272Khz,
fpwm=68Khz and fpfc=68Khz or In CM6802SBH,
fRTCT=2xfpwm=4xfpfc fRTCT=272Khz, fpwm=136Khz and
fpfc=68Khz, it provides the best performance in the PC
application.
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:
V
REF −1.25
REF − 3.75
tRAMP = CT x RT x In
No voltage error amplifier is included in the PWM stage of the
CM6802SAH/SBH, 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
at VREF = 7.5V:
RAMP = CT x RT x 0.51
t
The dead time of the oscillator may be determined using:
2.5V
tDEADTIME
=
x CT = 686.8 x CT
PWM Current Limit (DCILIMIT)
3.64mA
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
CM6802SAH/SBH’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 1.8V or higher for easy layout purpose.
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 5.88K. Selecting
standard components values, CT = 1000pF, and RT
=
5.88kΩ
The dead time of the oscillator determined two things:
1.) PFC minimum off time which is the dead time
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.
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 CM6802SAH/SBH 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. 1.5
Champion Microelectronic Corporation
19
CHAMP [ CHAMPION MICROELECTRONIC CORP. ]
