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
17
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
REF
to produce a soft-start
characteristic on the PFC: as the reference voltage comes up
from zero volts, it creates a differentiated voltage on I
EAO
which
prevents the 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
EAO
; stability and transient
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).
deviate from its 2.5V (nominal) value. If this happens, the
transconductance of the voltage error amplifier, GMv 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)
CV
V
DC
EAO
2
OUTDC
IN
FB
EAO
OUT
FB
EAO
OUT
Z
*
GM
*
C
*
S
*
Δ
V
*
V
2.5V
*
P
Δ
V
Δ
V
*
Δ
V
Δ
V
*
Δ
V
Δ
V
≈
=
Z
CV
:
Compensation Net Work for the Voltage Loop
GM
v
:
Transconductance of VEAO
P
IN
:
Average PFC Input Power
V
OUTDC
:
PFC Boost Output Voltage; typical designed value is
380V.
C
DC
:
PFC Boost Output Capacitor
PFC Current Loop
The current transcondutance amplifier, GMi, I
EAO
compensation is similar to that of the voltage error amplifier,
V
EAO
with exception of the choice of crossover frequency.
The crossover frequency of thecurrent amplifier should be at
least 10 times that of the 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. 8.33kHz for
a 50kHz switching frequency.
The gain vs. input voltage of the CM6800T’s voltage error
amplifier, V
EAO
has a specially shaped non-linearity such that
under steady-state operating conditions the transconductance
of the error amplifier, GMv is at a local minimum. Rapid
perturbation in line or load conditions will cause the input to the
voltage error amplifier (V
FB
) to
I
SENSE
Filter, the RC filter between R
SENSE
and I
SENSE
:
There are 2 purposes to add a filter at I
SENSE
pin:
1.) Protection: During start up or inrush current conditions, it
will have a large voltage cross Rs which is the sensing
resistor of the PFC boost converter. It requires the I
SENSE
Filter to attenuate the energy.
2.) To reduce L, the Boost Inductor: The I
SENSE
Filter To
reduce L, the Boost Inductor: The I
SENSE
Filter also can
reduce the Boost Inductor value since the I
SENSE
Filter
behaves like an integrator before going I
SENSE
which is the
input of the current error amplifier, IEAO.
The I
SENSE
Filter is a RC filter. The resistor value of the I
SENSE
Filter is between 100 ohm and 50 ohm because I
OFFSET
x the
resistor can generate an offset voltage of IEAO. By selecting
R
FILTER
equal to 50 ohm will keep the offset of the IEAO less
than 5mV. Usually, we design the pole of I
SENSE
Filter at
fpfc/6=8.33Khz, 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
SENSE
Filter, C
FILTER
, will be around 381nF.
