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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
characteristic on the PFC: as the reference voltage comes up
from zero volts, it creates a differentiated voltage on I
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
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)
ΔV
ΔV
OUT
FB
=
*
ΔV
ΔV
EAO
OUT
P
*
2.5V
IN
≈
2
V
*
ΔV
OUTDC
Z
: Compensation Net Work for the Voltage Loop
CV
GM
: Transconductance of VEAO
v
P
: Average PFC Input Power
IN
V
: PFC Boost Output Voltage; typical designed value is
OUTDC
380V.
C
: PFC Boost Output Capacitor
DC
PFC Current Loop
The
current
transcondutance
compensation is similar to that of the voltage error amplifier,
V
with exception of the choice of crossover frequency.
EAO
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.
2010/08/03
Rev. 1.2
to produce a soft-start
REF
; stability and transient
EAO
ΔV
EAO
*
ΔV
FB
*
GM
*
V
*
S
*
C
EAO
DC
amplifier,
Champion Microelectronic Corporation
CM6800T
EPA/85+ PFC+PWM COMBO CONTROLLER
Design for High Efficient Power Supply
The gain vs. input voltage of the CM6800T's voltage error
amplifier, V
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
I
Filter, the RC filter between R
SENSE
There are 2 purposes to add a filter at I
which
EAO
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
Filter to attenuate the energy.
2.) To reduce L, the Boost Inductor: The I
reduce L, the Boost Inductor: The I
reduce the Boost Inductor value since the I
behaves like an integrator before going I
input of the current error amplifier, IEAO.
The I
SENSE
Filter is between 100 ohm and 50 ohm because I
resistor can generate an offset voltage of IEAO. By selecting
R
equal to 50 ohm will keep the offset of the IEAO less
FILTER
than 5mV. Usually, we design the pole of I
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
Filter, C
FILTER
Z
CV
GMi,
I
EAO
(Turbo-Speed PFC+Green PWM)
has a specially shaped non-linearity such that
EAO
) to
FB
SENSE
Filter is a RC filter. The resistor value of the I
, will be around 381nF.
and I
:
SENSE
pin:
SENSE
SENSE
Filter To
SENSE
Filter also can
SENSE
Filter
SENSE
which is the
SENSE
SENSE
x the
OFFSET
Filter at
SENSE
SENSE
17
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