AN3168 datasheet(13/20 Pages) STMICROELECTRONICS

13 / 20 page

AN3168

Examples of negative power supplies

Doc ID 17193 Rev 1

13/20

3.2

Capacitive power supply

Figure 15 shows the schematic diagram of a capacitive negative power supply.

Figure 15.

Capacitive power supply

To have a constant voltage across C1, the average value of the input current (IIN(av)) must be

equal to the average value of the output current (IOUT(av)). The input current is a half-wave

rectified current, whose average value (IIN(av)) is given in Equation 4 (R1 and C1

impedances and diode D2 voltage drop are neglected):

Equation 4

The standby power consumption (POFF) is set by the R1 resistor value, the average input

current and the D1 zener voltage (VZ). The resistor R1 is required to limit the inrush current

stress at power supply turn on and to avoid the overrating of the current protection of the

circuit. The standby power consumption is:

Equation 5

For a 230 V / 50 Hz mains voltage, a 60

Ω resistor R1 (typical value) and a 15 V zener diode,

the calculated average output current and standby power consumption versus different AC

capacitors given in Table 3.

The benefits of this solution are:

●

Low cost (for output current lower than 20 mA)

●

Small size

●

Easy to implement

Table 3.

Calculated output DC current and measured typical standby power

consumption

AC capacitor, C2

220 nF

470 nF

680 nF

1 µF

Average output DC current, IOUT(av)

7.1 mA

15.3 mA

22.1 mA

32.5 mA

Stand-by power consumption, POFF

0.12 W

0.3 W

0.48 W

0.8 W

AC mains

)

(

OUT

)

peak

(

mains

)

(

)

(

)

(

OFF

)

(

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