NCP1217 ONSEMI [ON Semiconductor], NCP1217 Datasheet - Page 14

• Manufacturer Part Number: NCP1217
• Description: Enhanced PWM Current−Mode Controller for High−Power Universal Off−Line Supplies
• Manufacturer: ONSEMI [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 28). Please note that the negative
spike is clamped to (−2*Vf) thanks 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 28. A simple resistor in series avoids any
Figure 27. A Negative Spike Takes Place on the Bulk Capacitor at the Switch−Off Sequence
+
Cbulk
latch−up in the controller . . .
1
2
3
4
Vcc 5 V/DIV
Vlatch 1 V/DIV
Time 10 ms/DIV
8
7
6
5
2
NCP1217, NCP1217A
3
1
+
Rbulk
u4.7 k
CV
http://onsemi.com
CC
14
from V
VCC
the bulk capacitor gets deeply discharged. For security
reasons, two diodes can be connected in series.
Another option (Figure 29) consists in wiring a diode
Figure 29. . . . or one diode forces V
ON
CC
sooner and thus stops the switching activity before
to the bulk capacitor to force V
+
Cbulk
VCC
1
2
3
4
ON
sooner.
8
7
6
5
3
CC
to reach
CC
1
+
D3
1N4007
CV
to reach
CC