ncp1927 ON Semiconductor, ncp1927 Datasheet - Page 13

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

Short−Winding Protection
output diode short−circuit, the primary current increases
above V
dangerously high current from flowing, an additional
comparator senses when V
comparator toggles, the controller immediately latches off.
mode, however, the V
input power is removed and C
below V
NCP1927 operates according to the initial power−on
sequence. The V
protection is shown in Figure 9.
Feedback
threshold, K
limit threshold. This means that the feedback voltage when
the current limit threshold equals V
a resistor divider. To ease system design, the FFB pin is
represented by a Thevenin equivalent circuit containing a
voltage source and series resistor, V
and R
Soft−Start
soft−start circuit. Every time the controller starts (i.e. the
controller was off and starts, or restarts due to a fault), a
Under some conditions, like a transformer winding or
The effect of latching off the IC is identical to shutdown
The ratio from the feedback voltage to the current limit
The FFB pin is connected to the internal V
The NCP1927 flyback controller features an internal
V
FCS Pin
CS(stop)
Figure 9. V
V
V
FFB
CC(on)
CC(off)
DRV
I
I
I
V
CC2
CC4
CC6
I
CC
CC
ILIM
CC(reset)
(typically 20 kW).
FFB
before the LEB timer expires. To prevent
Detected
Winding
Short
CC
(typically 5), determines the peak current
. When input power is reapplied, the
CC
Behavior During Short Winding
CC
Protection
behavior during short winding
cycle repeats indefinitely until the
FCS
VCC
reaches V
FFB(open)
is allowed to discharge
ILIM
is 3.5 V (typical).
CS(stop)
DD
(typically 5 V)
rail through
. Once this
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time
time
time
time
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soft−start is applied when V
limit threshold is linearly increased from 0 until it reaches
V
setpoint lower than the one imposed by the soft−start (the 2
comparator outputs are OR’ed together). Figure 10 shows a
typical startup sequence.
Ramp Compensation
subharmonic oscillations. These oscillations take place at
half the switching frequency and occur only during
continuous conduction mode (CCM) when the duty ratio is
greater than 50%. To prevent these oscillations, one
typically lowers the current loop gain by injecting between
50% and 75% of the inductor downslope. This is done by
inserting a resistor (R
current sense resistor. Figure 11 shows an example of this.
The ramp signal is disconnected from the FCS pin during the
off time.
ILIM
Ramp compensation is a known method for preventing
FDRV
Reset
from FFB Setpoint
(in 4.0 ms), or until the feedback loop imposes a
Soft−start ramp
+
−
Figure 11. Inserting a Resistor
CS Setpoint
V
V
ILIM
ILIM
V
ON
FB
Figure 10. Soft−Start Timing
I
0 mA
ramp(MAX)
L.E.B.
SCOMP
over soft−start
V
FB
I
ramp
takes
CC
) between the FCS pin and the
reaches V
FCS
t
SSTART
CC(on)
R
SCOMP
Time
Time
Time
R
. The current
sense