NCP1927DR2G ON Semiconductor, NCP1927DR2G Datasheet - Page 18

• Manufacturer Part Number: NCP1927DR2G
• Description: Power Factor Correction ICs PFC AND FLYBACK CONTROLER
• Manufacturer: ON Semiconductor

Specifications of NCP1927DR2G

• Product Category: Power Factor Correction ICs

Transient Load Detection
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
When it is lower than V
I
pin to speed up the charging of C
of increasing the EA gain by a factor of approximately 13.
startup sequence of the PFC stage to prevent it from
interfering with the operation of the soft−start circuit.
On Time Control
converter, the switching pattern consists of constant on
times and variable off times. The on time is set via an
external capacitor (C
beginning of each switching cycle, C
I
monitors the voltage on the PCT pin (V
it to an internal regulation limit set by V
limit is determined by shifting V
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
beginning of the next switching cycle. This sequence is
shown in Figure 21.
output load, this naturally satisfies Equation 6. If the values
of compensation components are sufficient to filter out the
CPCT(discharge)
PControl(boost)
PCT(charge)
Due to the low bandwidth of the regulation loop, fast load
The transient load detection circuit is disabled during the
Since the NCP1927 is designed to control a CrM boost
Since V
PControl
Ct
PCT
PControl
(210 mA typical). An internal comparator
(240 mA typical) is connected to the PControl
(maximum 500 ns) and held low until the
V DD
varies with the RMS line voltage and
I PCT(charge)
Figure 21. On Time Generation
t
) connected to the PCT pin. At the
V PCT
PDRV
PDRV
OLOW
V PControl
V PControl − 0.6 V
PControl
PControl
t on
t
/V
is then discharged within
t
REF
is charged linearly by
PControl
. This has the effect
PCT
down by a voltage
(95.5% typical),
PWM
PFB
) and compares
+
−
. The internal
and V
http://onsemi.com
t on
REF
.
18
bulk capacitor voltage ripple, the on time remains constant
over the entire ac line cycle.
V
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.
where V
(typical).
Equation 12.
Where, Vac
Off Time Control
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.
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
(V
(typically 10 V) and V
in series with the PZCD pin is required to limit the current
into the pin and prevent it from exceeding 3 mA at V
or −2 mA at V
waveforms.
PControl
The maximum on time of the controller occurs when
Combining Equation 11 with Equation 6, results in
The off time varies with the instantaneous line voltage and
While the switch is on, a negative voltage appears at the
The PZCD pin is internally clamped to V
ZCD(rising)
ZCD(falling)
Figure 22. ZCD Winding Implementation
PCT(MAX)
is at its maximum value. Therefore, C
LL
) and the triggering threshold is typically 0.7 V
).
C
is the minimum ac rms input voltage.
t
t
CL(NEG)
on(MAX)
+
= 5 V (typical) and I
2 @ P
h @ Vac
CL(NEG)
+
. Figure 23 shows typical ZCD
out
C
LL
@ L @ I
t
I
PCT(charge)
@ V
2
(typically −0.7 V). A resistor
@ V
PCT(MAX)
PCT(charge)
PCT(MAX)
PCT(charge)
t
= 210 mA
must be
(eq. 12)
(eq. 11)
CL(POS)
CL(POS)