AN3168

Examples of negative power supplies

Doc ID 17193 Rev 1

14/20

The drawbacks of this solution are:

●

Low average output current. The maximum average output current is about 30 mA. For

higher output current, the size and the cost of the C2 capacitor becomes prohibitive.

Moreover, the inrush current stress at power supply turn on increases and will need

higher resistor R1 power.

●

High standby power consumption. For example, the standby power consumption

equals about 0.8 W for a 230 V power supply with a capacitor C2 of 1 µF, a 15 V zener

diode and a 60

Ω R1.

3.3

Resistive power supply

Figure 16 show the schematic diagram of a negative resistive power supply.

Figure 16.

Resistive power supply

rectified current, whose average value is given by the following equation (C1 impedance and

diode D1 voltage drop neglected):

Equation 6

Equation 7

For a 230 V mains voltage and a 15 V zener voltage, the calculated average output current

and standby power consumption versus different resistors are:

Table 4.

Calculated average output DC current and standby power consumption

Resistor R1

25 k

Ω

18 k

Ω

12 k

Ω

8 k

Ω

3.9 mA

5.5 mA

8.2 mA

12.3 mA

1 W1.3 W2 W

3 W

C1

AC mains

V

DD

D2

R1

D1

V

SS

)

av

(

OUT

z

)

peak

(

mains

)

av

(

IN

I

1

R

V

V

I

=

π

×

−

=

1

R

4

)

V

V

(

P

2

z

)

peak

(

mains

OFF

×

−

=