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.
3
Rbulk
> 4.7 k
2
+
Cbulk
1
2
3
4
8
7
6
5
1 +
CV
CC
Another option (Figure 23) consists in wiring a diode from
V
CC
to the bulk capacitor to force V
CC
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.
3
+
Cbulk
1
2
3
4
8
7
6
5
D3
1N4007
1 +
CV
CC
Figure 22. A simple resistor in series avoids any
latchup in the controller
Figure 23. or a diode forces V
CC
to reach
UVLOlow sooner
A Typical Application
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
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.
http://onsemi.com
11
ONSEMI [ ON SEMICONDUCTOR ]
