CM6800
L
OW
S
TART-
U
P
C
URRENT
PFC/PWM C
ONTROLLER
C
OMBO
Z
CI
: Compensation Net Work for the Current Loop
GM
I
: Transconductance of IEAO
V
OUTDC
: PFC Boost Output Voltage; typical designed value
is 380V and we use the worst condition to calculate the Z
CI
R
S
: The Sensing Resistor of the Boost Converter
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
SENSE
Filter, the RC filter between R
S
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
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, 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 283nF.
Figure 2. Compensation Network Connections for the
Voltage and Current Error Amplifiers
Figure 3. External Component Connections to V
CC
2008/10/23
Rev. 2.1
Champion Microelectronic Corporation
Page 11
CHAMP [ CHAMPION MICROELECTRONIC CORP. ]
