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NCP1608
high frequency switching converter to regulate the input
current harmonics. Active circuits operate at a higher
frequency, which enables them to be physically smaller,
weigh less, and operate more efficiently than a passive
circuit. With proper control of an active PFC stage, almost
any complex load emulates a linear resistance, which
significantly reduces the harmonic current content. Active
PFC circuits are the most popular way to meet harmonic
content requirements because of the aforementioned
benefits. Generally, active PFC circuits consist of inserting
a PFC pre ? converter between the rectifier bridge and the
bulk capacitor (Figure 26).
Rectifiers
PFC Pre ? Converter
Converter
AC Line
+
High
Frequency
Bypass
Capacitor
NCP1608
+
Bulk
Storage
Capacitor
Load
Figure 26. Active PFC Pre ? Converter with the NCP1608
The boost (or step up) converter is the most popular
topology for active power factor correction. With the
proper control, it produces a constant voltage while
consuming a sinusoidal current from the line. For medium
power (<350 W) applications, CrM is the preferred control
method. CrM occurs at the boundary between
discontinuous conduction mode (DCM) and continuous
conduction mode (CCM). In CrM, the driver on time begins
when the boost inductor current reaches zero. CrM
operation is an ideal choice for medium power PFC boost
stages because it combines the reduced peak currents of
CCM operation with the zero current switching of DCM
operation. The operation and waveforms in a PFC boost
converter are illustrated in Figure 27.
AC Line
Diode Bridge
+
V in
+
L
I L
V drain
AC Line
Diode Bridge
+
V in
+
L
V drain
I L
+
V out
?
The power switch is ON
With the power switch voltage being about zero, the
input voltage is applied across the inductor. The inductor
current linearly increases with a (V in /L) slope.
?
The power switch is OFF
The inductor current flows through the diode. The inductor
voltage is (V out ? V in ) and the inductor current linearly decays
with a (V out ? V in )/L slope.
Inductor
Current
V in /L
I L(peak)
(V out ? V in )/L
Critical Conduction Mode:
Next current cycle starts
when the core is reset.
V drain
V out
V in
If next cycle does not start
then V drain rings towards V in
Figure 27. Schematic and Waveforms of an Ideal CrM Boost Converter
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