NCP1200AP60G Просмотр технического описания (PDF) - ON Semiconductor

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NCP1200AP60G

PWM Current-Mode Controller for Universal Off-Line Supplies Featuring Low Standby Power

ON Semiconductor 

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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