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Oscillator (RAMP1, or called RTCT)
In CM6800T, fRTCT=4xfpwm=4xfpfc fRTCT=272Khz,
fpwm=68Khz 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
t
RAMP
DEADTIME
The dead time of the oscillator is derived from the
following equation:
t
= C
x R
x In
RAMP
T
T
at VREF = 7.5V:
t
= C
x R
x 0.51
RAMP
T
T
The dead time of the oscillator may be determined using:
t
=
2.5V
DEADTIME
3.64mA
The dead time is so small (t
operating frequency can typically be approximately by:
1
fRTCT =
t
RAMP
Ct should be greater than 470pF.
Let us use 1000PF Solving for R
standard components values, C
5.88kΩ
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 CM6800T 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
voltage-mode operation.
2010/08/03
Rev. 1.2
−
V
1.25
REF
−
V
3.75
REF
x C
= 686.8 x C
T
T
>> t
) that the
RAMP
DEADTIME
yields 5.88K. Selecting
T
= 1000pF, and R
T
capable
of
current-mode
Champion Microelectronic Corporation
CM6800T
EPA/85+ PFC+PWM COMBO CONTROLLER
Design for High Efficient Power Supply
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. DCI
, which provides cycle-by-cycle current limiting, is
LIMIT
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 V
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 I
stage over-current protection.
No voltage error amplifier is included in the PWM stage of the
CM6800T, 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 V
percent duty cycle for input voltages below around 1.8V.
PWM Current Limit (DCILIMIT)
The DC I
pin is a direct input to the cycle-by-cycle current
LIMIT
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
=
T
CM6800T'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.
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 V
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.
or
(Turbo-Speed PFC+Green PWM)
DC
input is used for output
LIMIT
to command a zero
DC
is less than its nominal
FB
will be compared.
19
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