參數(shù)資料
型號(hào)： NCP1927DR2G
廠商： ON Semiconductor
文件頁(yè)數(shù)： 16/24頁(yè)
文件大?。?/td> 531K
描述： IC CTLR PFC/FLYBACK 16-SOIC
標(biāo)準(zhǔn)包裝： 2,500
系列： *
NCP1927
http://onsemi.com
16
PFC CONTROLLER
The PFC stage operates in critical conduction mode
(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
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.
Figure 17. Schematic and Waveforms of an Ideal CrM Boost Converter
Diode Bridge
+

L
Diode Bridge
+

L
+
The power switch is ON
The power switch is OFF
Critical Conduction Mode:
Next current cycle starts as
soon as the core is reset.
Coil
Current
+
The power switch being about zero, the input voltage
is applied across the coil. The coil current linearly
increases with a (V
in
/L) slope.
The coil current flows through the diode. The coil voltage is (V
out
 
V
in
) and the coil current linearly decays with a (V
out
  V
in
)/L slope.
I
L
V
out
V
in
V
drain
(V
out
  V
in
)/L
I
L(peak)
I
L
V
in
V
drain
V
in
/L
V
out
V
in
If next cycle does not
start then V
drain
 rings
towards V
in
+
time
time
When the switch is closed, the inductor current increases
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
drain
) begins to drop. If the next
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.
t
on
+
2 @ P
out
@ L
h @ Vac
2
(eq. 6)
where P
out
 is the output power, L is the boost inductor
inductance and h is the system efficiency.
A plot of the MOSFET on/off time over an ac line cycle
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
L(peak)
) to
follow the ac line voltage. The NCP1927 implements
constant on time CrM control in a costeffective and robust
manner.
Figure 18. Inductor Waveform During CrM Operation
V
in(peak)
I
L(peak)
I
in(peak)
V
in
(t)
I
L
(t)
I
in
(t)
ON
OFF
MOSFET
time
time
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