FAN4800_06 Datasheet, PDF (12/20 Pages) Fairchild Semiconductor – Low Start-Up Current PFC/PWM Controller Combos

where:
Z_CI:
Compensation network for the current loop.
GM_I: Transconductance of I_EAO.
V_OUTDC: PFC boost output voltage (typical designed
value is 380V). The equation uses the worst-
case condition to calculate the Z_CI.
R_S:
Sensing resistor of the boost converter.
2.5V: Amplitude of the PFC leading modulation
ramp.
L:
Boost inductor.
A modest degree of gain contouring is applied to the
transfer characteristic of the current error amplifier to
increase its response speed to current-loop perturba-
tions. However, the boost inductor is usually the domi-
nant factor in overall current loop response. Therefore,
this contouring is significantly less marked than that of
the voltage error amplifier. This is illustrated in Figure 8.
V_ref
The I_SENSE filter is an RC filter. The resistor value of
the I_SENSE filter is between 100Ω and 50Ω because I_OFF-
SET ^{x R}S ^{can generate an offset voltage of I}EAO^.
Selecting an R_FILTER equal to 50Ω keeps the offset of the
I_EAO less than 5mV. Design the pole of I_SENSE filter at
f_pfc/6, one sixth of the PFC switching frequency, so the
boost inductor can be reduced six times without disturb-
ing the stability. The capacitor of the I_SENSE filter, C_FIL-
TER^{, is approximately 283nF.}
V_BIAS
R_BIAS
V_CC
FAN4800
0.22μF ^15V
Ceramic ^Zener
GND
PFC
Output
16
V_EAO
1
I_EAO
15 ^VFB
2.5V
2 ^IAC
4 ^VRMS
3 ^ISENSE
3.5k
Gain
Modulator
3.5k
PFC CMP
FAN4800 Rev.02
Figure 8. Compensation Network Connection for the
Voltage and Current Error Amplifiers
There is an RC filter between R_S and I_SENSE pin.
There are two reasons to add a filter at the I_SENSE pin:
1) Protection: During start-up or in-rush current condi-
tions, there is a large voltage across R_S, 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 the I_SENSE pin,
which is the input of the current error amplifier, I_EAO.
FAN4800 Rev.03
Figure 9. External Component Connection to V_CC
1.9 Oscillator (RAMP1)
The oscillator frequency is determined by the values of
R_T and C_T, which determine the ramp and off-time of the
oscillator output clock:
f_{O S C}
=
1
t_RAMP + t_DEAD
(8)
The dead time of the oscillator is derived from the follow-
ing equation:
t_RAMP
= C_T
× R_T
×
ln
⎛
⎜
⎝
V_REF
V_REF
-1.00
-3.75
⎞
⎟
⎠
(9)
at V_REF = 7.5V and t_RAMP = C_T x R_T x 0.55.
The dead time of the oscillator may be determined using:
t_DEAD
=
2.75V
12.11mA
×
C_T
= 227 × C_T
(10)
The dead time is so small (t_RAMP>>t_DEAD) that the oper-
ating frequency can typically be approximated by:
f_OSC
=
1
t_RAMP
(11)
© 2005 Fairchild Semiconductor Corporation
FAN4800 Rev. 1.0.5
12
www.fairchildsemi.com