NCP1654_11 Datasheet, PDF (15/23 Pages) ON Semiconductor – Power Factor Controller for Compact and Robust Continuous Conduction Mode Pre-Converters

NCP1654
R_boL is low side resistor of the dividing resistors between
V_in and BO pin.
R_boU is upper side resistor of the dividing resistors between
V_in and BO pin.
I_L
I_CS ^CS
+
NCP1654
R_CS
+
V_CS
−
Gnd
R_SENSE ^IL
Figure 37. Current Sensing
Refer to Figure 37, sense current I_cs is proportional to the
inductor current I_L as described in (Equation 11). I_L
consists of the high−frequency component (that depends on
di/dt or inductor L) and low−frequency component (that is
I_L−50).
I_cs
+
R_SENSE
R_CS
I_L
(eq. 11)
where
R_SENSE is the sense resistor to sense I_L.
R_CS is the offset resistor between CS pin and R_SENSE.
V_in
V_out
+
R_fbL
R_fbU
6
V_fb
V_control ^VREF
5
C_P
R_Z
C_Z
- ^{±20 mA}
+ _OTA
+
V_CONTROL(min)
To Vm Pin
Figure 38. V_control Low−Pass Filtering
Refer to Figure 38, the Operational Trans−conductance
Amplifier (OTA) senses V_out via the feedback resistor
dividers, R_fbU and R_fbL. The OTA constructs a control
voltage, V_control, depending on the output power and hence
V_out. The operating range of V_control is from
V_CONTROL(min) to V_CONTROL(max). The signal used for
PFC duty modulation is after decreasing a offset voltage,
V_CONTROL(min), i.e. V_control−V_CONTROL(min).
This control current I_control is a roughly constant current
that comes from the PFC output voltage V_out that is a slowly
varying signal. The bandwidth of I_control can be
additionally limited by inserting the external type−2
compensation components (that are R_Z, C_Z, and C_P as
shown in Figure 38). It is recommended to limit f_control, that
is the bandwidth of V_control (or I_control), below 20 Hz
typically to achieve power factor correction purpose.
The transformer of V_out to V_control is as described in
(Equation 12) if C_Z is >> C_P. G_EA is the error amplifier gain.
V_control
V_out
+
R_fbL @ G_EAR_Z
R_fbL ) R_fbU
@
1 ) sR_ZC_Z
sR_ZC_Z(1 ) sR_ZC_P)
(eq. 12)
From (Equation 7) − (Equation 11), the input impedance
Z_in is re−formulated in (Equation 13).
Z_in
+
2pR_CS
@
Ǹ_{2 RMRSENSEVoutVacKBOIL}
(eq.
(V_control * V_CONTROL(min)) @ V_REFI_L*50
13)
When I_L is equal to I_L−50, (Equation 13) is re−formulated
in (Equation 14)
Z_in
+
2pR_CS
Ǹ_{2 RMRSENSEVoutVacKBO}
@ (V_control * V_CONTROL(min))
@
V_REF
(eq. 14)
The multiplier capacitor C_M is the one to filter the
high−frequency component of the multiplier voltage V_m.
The high−frequency component is basically coming from
the inductor current I_L. On the other hand, the filter
capacitor C_filter similarly removes the high−frequency
component of inductor current I_L. If the capacitors C_M and
C_filter match with each other in terms of filtering capability,
I_L becomes I_L−50. Input impedance Z_in is roughly constant
over the bandwidth of 50 or 60 Hz and power factor is
corrected.
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