NCP1200A
http://onsemi.com
13
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 V
CC
capacitor to deplete at the same rate as the bulk
capacitor does. Figure 24 shows the solution.
Figure 24. A Diode Forces the V
CC
Capacitor to
Quickly Discharge at Power−off
CV
CC
8
7
6
5
1
2
3
4
+
Cbulk
+
3
1N4007
NCP1200A
or
1N4007
1N4007
When the bulk naturally depletes at power−off, the diode brings the V
CC
down as soon as Vbulk drops below V
CC
. This
ensures a clean turn−off and the above problems go away.
V
CC
VFB
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 V
CC
capacitor at turn−off. To summarize:
1. Wire a diode between V
CC
and the bulk capacitor as illustrated by Figure 24.
2. Select a MOSFET affected by a standard V
T
, minimum of 2 V, maximum of 4 V.
3. Check that final waveforms match Figure 25 signals
