ncp1927 ON Semiconductor, ncp1927 Datasheet - Page 10

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

INTRODUCTION
(PFC) and flyback controller optimized for use in Flat Panel
TVs. This device includes all the features needed to
implement a highly efficient and compact power supply. It
integrates a critical conduction mode (CrM) PFC controller
and a fixed−frequency current mode flyback controller with
proper sequencing for simplified system design.
input, an inverter enable output, a go to standby input, and
a dedicated pin for under/overvoltage protection.
SUPPLY SEQUENCING
V
and has not been latched off or shutdown. Once the flyback
controller is enabled, a soft−start timer is activated, and it
begins switching. The soft−start timer provides a ramp
signal that increases over t
ensures that the peak current gradually increases to
minimize power component stress and limit output voltage
overshoot. Frequency jittering is disabled while the
soft−start timer is running.
output (it is no longer in overload), the PFC controller can
be enabled. As soon as the PFC controller is enabled, the
error amplifier begins to source its maximum output current,
I
pin capacitor (C
charges. An internal grounding switch on the PControl pin
is turned on each time the PFC controller is disabled, and
turned off when it is enabled. This ensures that C
always fully discharged at the beginning of soft−start.
error amplifier output current, I
0 mA. Once the output is in regulation and I
the IENABLE pin is set to V
V
capacitor (C
connected to the high voltage line (V
must be chosen so that the charging current is greater than
the IC bias current during startup. The maximum value for
the startup resistor is calculated using Equation 1.
where V
bias current during startup (20 mA maximum).
EA(MAX)
CC
CC
The NCP1927 is a combination power factor correction
This device includes frequency jittering, a shutdown
The flyback controller of the NCP1927 is enabled once
Once the flyback controller detects regulation on the
As the PFC stage approaches regulation on the output, the
When power is initially applied to the application, the V
MANAGEMENT
reaches V
in
, (typically 20 mA) to linearly charge the PControl
is the rectified dc input voltage and I
VCC
CC(on)
PControl
) begins charging through a resistor
, provided it is not in thermal shutdown
R
). Soft−start is achieved as C
start
SSTART
+
IENABLE(high)
I
V
CC5
in
EA
(typically 4.0 ms). This
, gradually reduces to
in
). The resistor value
DETAILED OPERATING DESCRIPTION
EA
(typically 5 V).
reaches 0 mA,
CC5
PControl
is the IC
PControl
(eq. 1)
http://onsemi.com
CC
is
10
On Reset occurs. This resets all logic states on the device. As
V
until V
reaches V
bias current increases to I
the total I
load at the flyback drive output (FDRV). Once the flyback
is in regulation, the PFC controller can be enabled. When the
PFC is enabled, the I
gate charge load at the PFC drive output (PDRV). The
increase in I
where, f
charge of the external MOSFET X.
V
supply voltage increases during startup. If C
V
the auxiliary winding powers up the controller. The total I
current after the flyback controller is enabled (I
I
often useful to connect a small V
to the V
the V
This allows minimum startup time while providing enough
V
This implementation is shown in Figure 3 and the startup
sequence is shown in Figure 4.
CC(FDRV)
CC
CC(off)
CC
CC
When V
C
VCC
falls below V
continues to rise, the IC bias current remains at I
CC
capacitance to operate during light load conditions.
CC
CC
OSC
pin through a diode and charged by the aux winding.
must be sized such that a V
CC
(9 V typical) is maintained while the auxiliary
CC(on)
) must be considered to correctly size C
CC
pin, while a larger capacitor (C2) is connected to
reaches V
CC
current is greater than this due to the gate charge
is the switching frequency and Q
reaches V
per MOSFET is calculated using Equation 2.
, the flyback controller is enabled and the IC
CC(off)
CC
I
CC(x)
CC(on)
CC(reset)
current increases further due to the
and the controller turns off before
CC3
+ f
(typically 17 V). Once V
OSC
(1.5 mA typical). However,
(typically 6.5 V), a Power
CC
@ Q
CC
capacitor (C1) directly
G(x)
voltage greater than
VCC
G(X)
is too small,
is the gate
VCC
CC3
(eq. 2)
. It is
plus
CC5
CC
CC