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| 型號： | NCP1927DR2G |
| 廠商： | ON Semiconductor |
| 文件頁數(shù)： | 18/24頁 |
| 文件大?。?/td> | 531K |
| 描述： | IC CTLR PFC/FLYBACK 16-SOIC |
| 標準包裝： | 2,500 |
| 系列： | * |
NCP1927
http://onsemi.com
18
Transient Load Detection
Due to the low bandwidth of the regulation loop, fast load
transients may result in output voltage over and undershoots.
Overshoots are limited by the overvoltage protection (see
OVP section). To control the undershoots, an internal
comparator monitors the ratio between V
PFB
and V
REF
.
When it is lower than V
OLOW
/V
REF
(95.5% typical),
I
PControl(boost)
(240 mA typical) is connected to the PControl
pin to speed up the charging of C
PControl
. This has the effect
of increasing the EA gain by a factor of approximately 13.
The transient load detection circuit is disabled during the
startup sequence of the PFC stage to prevent it from
interfering with the operation of the softstart circuit.
On Time Control
Since the NCP1927 is designed to control a CrM boost
converter, the switching pattern consists of constant on
times and variable off times. The on time is set via an
external capacitor (C
t
) connected to the PCT pin. At the
beginning of each switching cycle, C
t
is charged linearly by
I
PCT(charge)
(210 mA typical). An internal comparator
monitors the voltage on the PCT pin (V
PCT
) and compares
it to an internal regulation limit set by V
PControl
. The internal
limit is determined by shifting V
PControl
down by a voltage
equal to one diode drop (0.6 V typical) to account for the
offset of the control voltage range. Once this level is
exceeded, the drive is turned off. C
t
is then discharged within
t
CPCT(discharge)
(maximum 500 ns) and held low until the
beginning of the next switching cycle. This sequence is
shown in Figure 21.
Figure 21. On Time Generation
PControl
PCT
+
PWM
PDRV
I
PCT(charge)
t
on
V
PControl
0.6 V
t
on
V
DD
PDRV
V
PControl
Ct
V
PCT
Since V
PControl
varies with the RMS line voltage and
output load, this naturally satisfies Equation 6. If the values
of compensation components are sufficient to filter out the
bulk capacitor voltage ripple, the on time remains constant
over the entire ac line cycle.
The maximum on time of the controller occurs when
V
PControl
is at its maximum value. Therefore, C
t
must be
sized to ensure that the required on time can be achieved at
maximum output power and minimum input voltage. The
maximum on time is calculated using Equation 11.
t
on(MAX)
+
C
t
@ V
PCT(MAX)
I
PCT(charge)
(eq. 11)
where V
PCT(MAX)
= 5 V (typical) and I
PCT(charge)
= 210 mA
(typical).
Combining Equation 11 with Equation 6, results in
Equation 12.
C
t
+
2 @ P
out
@ L @ I
PCT(charge)
h @ Vac
LL
2
@ V
PCT(MAX)
(eq. 12)
Where, Vac
LL
is the minimum ac rms input voltage.
Off Time Control
The off time varies with the instantaneous line voltage and
is adjusted every cycle so that the inductor is demagnetized
before the next switching cycle begins. The inductor is
demagnetized once its current reaches zero. When this
happens, the drain voltage begins to drop. This is detected
by sensing the voltage across an inductor auxiliary winding.
This winding, commonly known as a zero crossing detection
(ZCD) winding, provides the NCP1927 with a scaled
version of the inductor voltage. Figure 22 shows a typical
ZCD winding arrangement.
Figure 22. ZCD Winding Implementation
While the switch is on, a negative voltage appears at the
PZCD pin. When the switch turns off, the ZCD voltage
swings positive, arming the ZCD detector. The ZCD voltage
remains positive until the inductor current falls to zero and
the inductor is demagnetized. The voltage then drops to 0 V
and triggers the ZCD detector to begin the next switch cycle.
The arming threshold of the ZCD detector is typically 1.4 V
(V
ZCD(rising)
) and the triggering threshold is typically 0.7 V
(V
ZCD(falling)
).
The PZCD pin is internally clamped to V
CL(POS)
(typically 10 V) and V
CL(NEG)
(typically 0.7 V). A resistor
in series with the PZCD pin is required to limit the current
into the pin and prevent it from exceeding 3 mA at V
CL(POS)
or 2 mA at V
CL(NEG)
. Figure 23 shows typical ZCD
waveforms.
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