CM6800
LOW START-UP CURRENT PFC/PWM CONTROLLER COMBO
ZCI: Compensation Net Work for the Current Loop
GMI: Transconductance of IEAO
ISENSE Filter, the RC filter between RS and ISENSE :
V
OUTDC: PFC Boost Output Voltage; typical designed value
There are 2 purposes to add a filter at ISENSE 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 ISENSE Filter to attenuate
the energy.
is 380V and we use the worst condition to calculate the ZCI
RS: 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.
2.) To reduce L, the Boost Inductor: The ISENSE Filter
also can reduce the Boost Inductor value since the
ISENSE Filter behaves like an integrator before going
ISENSE which is the input of the current error
amplifier, IEAO.
The ISENSE Filter is a RC filter. The resistor value of the ISENSE
Filter is between 100 ohm and 50 ohm because IOFFSET x the
resistor can generate an offset voltage of IEAO. By selecting
RFILTER equal to 50 ohm will keep the offset of the IEAO less
than 5mV. Usually, we design the pole of ISENSE 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 ISENSE Filter,
CFILTER, will be around 283nF.
Figure 3. External Component Connections to VCC
Figure 2. Compensation Network Connections for the
Voltage and Current Error Amplifiers
2008/10/23 Rev. 2.1
Champion Microelectronic Corporation
Page 11
CHAMP [ CHAMPION MICROELECTRONIC CORP. ]
