NCP1203_15 Datasheet, PDF (12/15 Pages) ON Semiconductor – PWM Current-Mode Controller for Universal Off-Line Supplies Featuring Standby and Short Circuit Protection

NCP1203
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. Also, the
power dissipation of this resistor is extremely small since it
only heats up during the startup sequence.
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.
+
1
Cbulk 2
3
4
Rbulk
> 4.7 k
8
7
6
+
5
CV_CC
+
1
Cbulk 2
3
4
8
D3
7
1N4007
6
5
+
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
www.onsemi.com
12