NCP1200A
A simple and inexpensive solution helps circumventing both problems, negative biasing, and gate high transient. It consists
in a solution using one 1N4007 (or two in a series for safety) forcing the VCC capacitor to deplete at the same rate as the bulk
capacitor does. Figure 24 shows the solution.
+
Cbulk
NCP1200A
1
8
2
7
3
6
4
5
3
1N4007
or
1N4007
1N4007
+
CVCC
Figure 24. A Diode Forces the VCC Capacitor to
Quickly Discharge at Power−off
When the bulk naturally depletes at power−off, the diode brings the VCC down as soon as Vbulk drops below VCC. This
ensures a clean turn−off and the above problems go away.
VFB
VCC
Vbulk
Vgs
Figure 25. The Diode Addition Forces a Clean Turn−off Sequence both Negative Biasing
and Gate High State Troubles
Once implemented, please make sure that your operating waveforms match those of Figure 25. That is to say, a bulk level
depleting the VCC capacitor at turn−off. To summarize:
1. Wire a diode between VCC and the bulk capacitor as illustrated by Figure 24.
2. Select a MOSFET affected by a standard VT, minimum of 2 V, maximum of 4 V.
3. Check that final waveforms match Figure 25 signals
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