NCP1377, NCP1377B
www.onsemi.com
10
An internal timer prevents any restart within 8.0 ms
further to the driver going−low transition for NCP1377,
and 3.0 ms for NCP1377B. This prevents the switching
frequency to exceed (1.0/T
ON
+ T
blank
) but also avoid false
leakage inductance tripping at turn−off. In some cases, the
leakage inductance kick is so energetic, that a slight
filtering is necessary.
The NCP1377 demagnetization detection pad features a
specific component arrangement as detailed by Figure 22.
In this picture, the zener diodes network protect the IC
against any potential ESD discharge that could appear on
the pins. The first ESD diode connected to the pad, exhibits
a parasitic capacitance. When this parasitic capacitance
(10 pF typically) is combined with Rdem, a restart delay
is created and the possibility to switch right in the
drain−source wave exists. This guarantees QR operation
with all the associated benefits (low EMI, no turn−on
losses etc.). Rdem should be calculated to limit the
maximum current flowing through pin 1 to less than
+3.0 mA/−2.0 mA: If during turn−on, the auxiliary
winding delivers 30 V (at the highest line level), then the
minimum Rdem value is defined by: 30 + 0.7/3.0 mA =
10.2 kW. This value will be further increased, e.g. to
introduce a restart delay and also a slight filtering in case
of high leakage energy.
Figure 23 portrays a typical Vds shot at nominal output
power.
Figure 23. The NCP1377 Operates in
Borderline/Critical Operation
400
300
200
100
0
Overvoltage Protection
The overvoltage protection works by sampling the
plateau voltage after the turn−off sequence. A 4.5 ms delay
for NCP1377 and 1.5 ms for NCP1377B guarantees a clean
plateau, providing that the leakage inductance ringing has
been fully damped. If this would not be the case, the
designer should install a small RC damper across the
transformer primary inductance connections. Figure 24
shows where the sampling occurs on the auxiliary winding.
Figure 24. A Voltage Sample is Taken 4.5 ms
After the Turn−Off Sequence
8.0
6.0
4.0
2.0
0
SAMPLING HERE
4.5 ms
When an OVP condition has been detected, the
NCP1377 enters a latchoff phase and stops all switching
operations. The controller stays fully latched in this
position and the startup source being still active, it keeps
the Vcc going up and down between 12.5 V and 5.6 V. This
state lasts until the Vcc is cycled down to 4.0 V, e.g. when
the user unplugs the power supply from the mains outlet.
By default, the OVP comparator is biased to a 5.0 V
reference level and pin1 is routed via a divide by a 1.44
network. As a result, when Vpin1 reaches 7.2 V, the OVP
comparator is triggered. The threshold can thus be adjusted
by either modifying the power winding to auxiliary
winding turn ratios to match this 7.2 V level or insert a
resistor from pin1 to ground to cope with your design
requirement.
Latching Off the NCP1377
In certain cases, it can be very convenient to externally
shut down permanently the NCP1377 via a dedicated
signal, e.g. coming from a temperature sensor (Figure 25).
The reset occurs when the user unplugs the power supply
from the mains outlet. To trigger the latchoff by an external
signal, a simple PNP transistor can do the work, as
Figure 26 shows.
