NCP1203 ON Semiconductor, NCP1203 Datasheet - Page 12

• Manufacturer Part Number: NCP1203
• Description: Current Mode Controller Fixed Frequency Operation
• Manufacturer: ON Semiconductor

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. Also, the
power dissipation of this resistor is extremely small since it
only heats up during the startup sequence.
Simple and inexpensive cures exist to prevent from
Figure 22. A simple resistor in series avoids any
Figure 21. A negative spike takes place on the Bulk capacitor at the switch-off sequence
+
Cbulk
latchup in the controller
1
2
3
4
8
7
6
5
+
Rbulk
> 4.7 k
CV
CC
http://onsemi.com
NCP1203
12
V
sooner and thus stops the switching activity before the bulk
capacitor gets deeply discharged. For security reasons, two
diodes can be connected in series.
CC
Another option (Figure 23) consists in wiring a diode from
Figure 23. or a diode forces V
to the bulk capacitor to force V
+
Cbulk
UVLOlow sooner
1
2
3
4
8
7
6
5
CC
CC
to reach
to reach UVLOlow
+
CV
D3
1N4007
CC