ncp1927 ON Semiconductor, ncp1927 Datasheet - Page 16

• Manufacturer Part Number: ncp1927
• Description: Combination Power Factor Correction Controller And Flyback Controller For Flat Panel Tvs
• Manufacturer: ON Semiconductor

PFC CONTROLLER
(CrM). CrM occurs at the boundary between discontinuous
conduction mode (DCM) and continuous conduction mode
(CCM). In CrM, the driver on time is initiated when the
boost inductor current reaches zero. CrM operation is an
linearly to its peak value. When the switch opens, the
inductor current linearly decreases to zero. At this point, the
drain voltage of the switch (V
switching cycle does not start, the voltage rings with a
dampened frequency around Vin. A simple derivation of
equations (such as those found in AND8123) leads to the
result that good power factor correction in CrM operation is
achieved when the on time is constant across a single ac
cycle. Equation 6 shows the relationship between on time
and system operating conditions.
where P
inductance and h is the system efficiency.
is illustrated in Figure 18. The MOSFET off time varies
based on the instantaneous line voltage, but the on time is
constant. This causes the peak inductor current (I
follow the ac line voltage. The NCP1927 implements
The PFC stage operates in critical conduction mode
The power switch being about zero, the input voltage
is applied across the coil. The coil current linearly
increases with a (V
When the switch is closed, the inductor current increases
A plot of the MOSFET on/off time over an ac line cycle
Diode Bridge
out
V
Coil
Current
drain
is the output power, L is the boost inductor
The power switch is ON
+
−
in
/L) slope.
t
on
V
in
+
+
Figure 17. Schematic and Waveforms of an Ideal CrM Boost Converter
2 @ P
h @ Vac
drain
V
out
) begins to drop. If the next
in
/L
@ L
L
2
V
out
I
L(peak)
I
L
L(peak)
(eq. 6)
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) to
The coil current flows through the diode. The coil voltage is (V
V
16
in
) and the coil current linearly decays with a (V
Diode Bridge
ideal choice for medium power PFC boost stages because it
combines the lower 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 17.
constant on time CrM control in a cost−effective and robust
manner.
MOSFET
Figure 18. Inductor Waveform During CrM Operation
(V
V in(peak)
I in(peak)
out
I L(peak)
− V
If next cycle does not
start then V
towards V
OFF
ON
in
+
−
)/L
The power switch is OFF
V
in
in
+
drain
Critical Conduction Mode:
Next current cycle starts as
soon as the core is reset.
rings
V
in
L
V
drain
out
I
L
V in (t)
− V
time
in
I L (t)
)/L slope.
+
I in (t)
V
out
out
time
time
−
time