CM6802 Champion Microelectronic, CM6802 Datasheet - Page 11

• Manufacturer Part Number: CM6802
• Description: NO BLEED RESISTOR GREEN MODE PFC/PWM CONTROLLER COMBO
• Manufacturer: Champion Microelectronic

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Error Amplifier Compensation
The PWM loading of the PFC can be modeled as a
negative resistor; an increase in input voltage to the PWM
causes a decrease in the input current. This response
dictates the proper compensation of the two
transconductance error amplifiers. Figure 2 shows the types
of compensation networks most commonly used for the
voltage and current error amplifiers, along with their
respective return points. The current loop compensation is
returned to V
PFC: as the reference voltage comes up from zero volts, it
creates a differentiated voltage on I
PFC from immediately demanding a full duty cycle on its
boost converter.
PFC Voltage Loop
There are two major concerns when compensating the
voltage loop error amplifier, V
response. Optimizing interaction between transient
response and stability requires that the error amplifier’s
open-loop crossover frequency should be 1/2 that of the
line frequency, or 23Hz for a 47Hz line (lowest anticipated
international power frequency). The gain vs. input voltage
of the CM6802’s voltage error amplifier, V
specially shaped non-linearity such that under steady-state
operating conditions the transconductance of the error
amplifier is at a local minimum. Rapid perturbation in line or
load conditions will cause the input to the voltage error
amplifier (V
this happens, the transconductance of the voltage error
amplifier will increase significantly, as shown in the Typical
Performance Characteristics. This raises the
gain-bandwidth product of the voltage loop, resulting in a
much more rapid voltage loop response to such
perturbations than would occur with a conventional linear
gain characteristics.
The Voltage Loop Gain (S)
Z
GM
P
V
is 380V.
C
PFC Current Loop
The current amplifier, I
the voltage error amplifier, V
choice of crossover frequency. The crossover frequency of
the current amplifier should be at least 10 times that of the
2003/06/25
CV
IN
OUTDC
DC
: Average PFC Input Power
V
: Compensation Net Work for the Voltage Loop
v
: PFC Boost Output Capacitor
: Transconductance of VEAO
V
V
OUTDC
: PFC Boost Output Voltage; typical designed value
OUT
EAO
*
FB
2
P
*
REF
Preliminary
) to deviate from its 2.5V (nominal) value. If
IN
V
*
V
V
to produce a soft-start characteristic on the
OUT
2
FB
EAO
5 .
V
*
*
EAO
S
V
*
compensation is similar to that of
V
N
EAO
C
EAO
FB
O
EAO
DC
with exception of the
; stability and transient
B
*
GM
EAO
LEED
which prevents the
V
EAO
*
Z
R
CV
has a
ESISTOR
Champion Microelectronic Corporation
G
REEN
voltage amplifier, to prevent interaction with the voltage loop.
It should also be limited to less than 1/6th that of the
switching frequency, e.g. 16.7kHz for a 100kHz switching
frequency.
The Current Loop Gain (S)
Z
GM
V
380V and we use the worst condition to calculate the Z
R
2.5V: The Amplitude of the PFC Leading Modulation Ramp
L: The Boost Inductor
There is a modest degree of gain contouring applied to the
transfer characteristic of the current error amplifier, to
increase its speed of response to current-loop perturbations.
However, the boost inductor will usually be the dominant
factor in overall current loop response. Therefore, this
contouring is significantly less marked than that of the
voltage error amplifier. This is illustrated in the Typical
Performance Characteristics.
I
There are 3 purposes to add a filter at I
The I
Filter is between 100 ohm and 50 ohm because I
resistor can generate an offset voltage of IEAO. By selecting
R
than 5mV. Usually, we design the pole of I
fpfc/6, one sixth of the PFC switching frequency. Therefore,
the boost inductor can be reduced 6 times without disturbing
the stability. Therefore, the capacitor of the I
C
SENSE
CI
OUTDC
S
FILTER
FILTER
M
: The Sensing Resistor of the Boost Converter
: Compensation Net Work for the Current Loop
S
V
I
: Transconductance of IEAO
1.) Protection: During start up or inrush current
2.) To reduce L, the Boost Inductor: The I
3.) By selecting the proper Rs, it can change the PFC
V
*
SENSE
ODE
OUTDC
D
Filter, the RC filter between R
: PFC Boost Output Voltage; typical designed value is
, will be around 283nF.
L
ISENSE
equal to 50 ohm will keep the offset of the IEAO less
OFF
conditions, it will have a large voltage cross Rs
which is the sensing resistor of the PFC boost
converter. It requires the I
the energy.
also can reduce the Boost Inductor value since the
I
I
amplifier, IEAO.
Green Mode threshold. Typical value is from 50
ohm (No Skipping) to 100 ohm.
*
SENSE
SENSE
Filter is a RC filter. The resistor value of the I
2
*
PFC/PWM C
5 .
*
R
V
Filter behaves like an integrator before going
which is the input of the current error
S
D
*
I
EAO
GM
OFF
*
I
*
Z
I
I
SENSE
CI
EAO
ONTROLLER
SENSE
S
SENSE
and I
Filter to attenuate
SENSE
SENSE
CM6802
SENSE
pin:
Page 11
SENSE
Filter at
OFFSET
Filter,
:
C
Filter
OMBO
CI
x the
SENSE