External Circuit Design

AN2317 - Application

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1.5

Capacitive Power Supply Circuit

The capacitive power supply circuit is shown in

Figure 10 and includes:

●

a varistor,

●

the capacitive power supply, and

●

the Electromagnetic Compatibility (EMC) filter.

Figure 11.

Capacitive power supply (with EMC Filter)

1.5.1

Varistor

The varistor is a surge protection device that is connected directly across the AC input.

When a power surge or voltage spike exceeding a specified voltage (varistor voltage) is

sensed, the varistor's resistance rapidly decreases, creating an instant shunt path for the

overvoltage, thereby saving the sensitive control panel components. The varistor and the

line fuse are subject to damage or weakened because the shunt path creates a short circuit.

An essential point of varistor selection is that the varistor can handle the peak pulse current,

which is 110% of the maximum current at which the varistor voltage does not change. If the

peak pulse current rating is insufficient, then the varistor may be damaged. The main

K10*300V varistor is chosen for this application.

VDD

Transient

Protection

Current

Limiter

Voltage

Divider

Filter 1

Filter 2

GND

C1

1n

D3

5.1V

RV1

510V

D2

DIF60

D1

DIF60

LINE

NEUTRAL

AI12909

1

2

R1

82R

L1

220m

L2

220m

C3

1000m

C2

470n