NCP1200
Figure 21. A negative spike takes place on the Bulk capacitor at the switch−off sequence
Simple and inexpensive cures exist to prevent from
internal parasitic SCR activation. One of them consists in
inserting a resistor in series with the high−voltage pin to
keep the negative current to the lowest when the bulk
becomes negative (Figure 22). Please note that the negative
spike is clamped to –2 x Vf due to the diode bridge. Please
refer to AND8069/D for power dissipation calculations.
Another option (Figure 23) consists in wiring a diode from
to the bulk capacitor to force V to reach UVLOlow
sooner and thus stops the switching activity before the bulk
capacitor gets deeply discharged. For security reasons, two
diodes can be connected in series.
V
CC
CC
3
Rbulk
> 4.7 k
2
3
1
2
3
4
8
7
6
5
1
2
3
4
8
7
6
5
D3
1N4007
+
+
Cbulk
Cbulk
1
1
+
CV
+
CV
CC
CC
Figure 22. A simple resistor in series avoids any
latchup in the controller
Figure 23. or a diode forces VCC to reach
UVLOlow sooner
A Typical Application
inherent short−circuit protection of the NCP1200, you only
need a bunch of components around the IC, keeping the final
cost at an extremely low level. The transformer is available
from different suppliers as detailed on the following page.
Figure 24 depicts a low−cost 3.5 W AC−DC 6.5 V wall
adapter. This is a typical application where the wall−pack
must deliver a raw DC level to a given internally regulated
apparatus: toys, calculators, CD players etc. Due to the
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