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024

Symmetrical Switching Power Supply

This circuit shows one way to generate both

positive and negative output voltages using a

step-down switching regulator. The Micrel

( www.micrel.com ) MIC4680 is used here.

Transformer winding T1a, diode D1, voltage

divider R1/R2 and the output capacitors

together form the well-known output-side cir-

cuit of a step-down regulator. The secondary

winding L1b and capacitor C2 are used to

generate the negative output voltage. The

pulses from the switching output SW that

have been inverted in this manner are recti-

fied by diode D2, which provides a negative

U IN

U OUT

Tr1a

2

3

2

1

IN

SW

R1

IC1

*

MIC4680

1

4

FB

SHDN

R2

D1

GND

C1

C3

C5

*

5

678

47

µ

35V

100

µ

16V

100n

SS26

Tr1b

4

Tr1 = CTX68-4P

C2

C4

C6

* siehe Text

*

* voir texte

*

see text

zie tekst

220

µ

25V

100

µ

16V

100n

3

U OUT

D2

SS14

014077 - 11

12/2001

Elektor Electronics

47

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SMALL CIRCUITS COLLECTION

voltage of the same magnitude to capacitors C4 and C6.

The SMD transformer is available ready-made from Coil-

tronics ( www.coiltronics.com ) and has a winding ratio of

1:1 (268 µH).

Sample component values for use with the MIC4680 shown

here are listed in the table. As you can see, the input poten-

tial must lie a few volts above the desired output potential.

The upper limit for the input potential is determined only

by the specifications of the switching regulator IC used.

(014077-1)

U out

I out

U in

R1

R2

±5 V

0.5 A max.

+7–34 V

3.01 k

1.01 k

±12 V

0.5 A max.

+ 16–34 V

26 k Ω

3.01 k Ω

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