CM6800T
(Turbo-Speed PFC+Green PWM)
http://www.championmicro.com.tw
EPA/85+
PFC+PWM COMBO CONTROLLER
Design for High Efficient Power Supply
2010/08/03
Rev. 1.2
Champion Microelectronic Corporation
19
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:
fRTCT
=
DEADTIME
RAMP
t
t
1
+
The dead time of the oscillator is derived from the
following equation:
t
RAMP
= C
T
x R
T
x In
3.75
V
1.25
V
REF
REF
−
−
at VREF = 7.5V:
t
RAMP
= C
T
x R
T
x 0.51
The dead time of the oscillator may be determined using:
t
DEADTIME
=
3.64mA
2.5V
x C
T
= 686.8 x C
T
The dead time is so small (t
RAMP
>> t
DEADTIME
) that the
operating frequency can typically be approximately by:
fRTCT
=
RAMP
t
1
Ct should be greater than 470pF.
Let us use 1000PF Solving for R
T
yields 5.88K. Selecting
standard components values, C
T
= 1000pF, and R
T
=
5.88k
Ω
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 capable of current-mode or
voltage-mode operation.
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
LIMIT
, 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 V
DC
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 I
LIMIT
input is used for output
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
DC
to command a zero
percent duty cycle for input voltages below around 1.8V.
PWM Current Limit (DCILIMIT)
The DC I
LIMIT
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
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
FB
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.
