e991040.pdf

POWER SUPPLIES

step-up converter

no-iron converter for mobile

charging of low-power battery packs

The voltage step-up converter described in

this article is a transformerless design

based on just one integrated circuit

and a handful of passive parts. Effi-

ciency is excellent given the sim-

plicity of the circuit, which requires

no modifications for any input volt-

age between 6 V and 12 V, for output

voltages of about 10 V and 22 V

respectively.

One of the most frequently used appli-

cations of voltage step-up converters is

that of a battery charger using the 12-

V vehicle battery as its input power

source. After all, to charge a battery,

you need a voltage which is always

greater than the maximum voltage

supplied by the battery when fully

charged. So, charging a 12-V NiCd bat-

tery pack as used in, say, a portable

mobile radio or a laptop computer

from the car battery calls for a circuit

that increases (‘steps up’) the 12-V

input voltage to, say, 20 V or so which

may be applied to the charger circuit.

Not so long ago, it was practically

impossible to design DC-DC step-up

converters without recourse to special

transformer techniques using the

inverter principle: use the input volt-

age to power an oscillator which drives

a step-up transformer; next, rectify the

high voltage at the secondary. Such cir-

cuits are typically bulky and not terri-

bly efficient, although there are notice-

able exceptions.

Today, most step-up converters are

tailor-made switch-mode power sup-

plies (SMPSUs) based on purpose-

designed ICs. The design presented

here is an exception in that it employs

a low-cost audio power amplifier IC,

the TDA2822M.

Design by W. Zeiller

S TEP UP THE VOLUME

Looking at the circuit diagram in Fig-

ure 1 you will not fail to note the sim-

plicity of the circuit. Basically, the

inputs and outputs of the two ampli-

fiers in the TDA2822M are cross-cou-

pled by capacitors C2 and C7 to cause

a (controlled) amount of oscillation. In

fact, you are looking at a double AMV

(astable multivibrator) acting as a push-

pull oscillator/charge pump driving a

classic diode-based voltage multiplier.

Elektor Electronics

1/99

DC-DC

Simple as it may be, the circuit acts as

a reasonably efficient voltage doubler

(theoretically, that is).

Through their output capacitors (C4

and C9) and associated diode pairs

(D1-D2 and D3-D4), amplifiers IC1a

and IC1b alternately contribute to the

energy (charge) built up in output

capacitor C10. This energy is available

for use by the load connected to the

converter output terminals.

Theoretically, the input voltage is

doubled, but there are derating factors.

Firstly, the output transistors of the

TDA2822M are not ideal devices and

cause a small voltage loss. Add to that

the voltage drop across the diodes and

you will appreciate that an input volt-

age of 12 V produces an output voltage

of just 22 V instead of the theoretically

expected 24 V. Unfortunately, the out-

put voltage drops a little more when

the converter is actually loaded, but

that will not be a problem in most bat-

tery chargers thanks to

their internal regulator

circuits (for constant cur-

rent or constant voltage).

The oscillator operates

at a frequency of about

2 kHz. This value

depends to some extent

on the actual supply volt-

age and the load current.

The Boucherot networks at the ampli-

fier outputs, R3-C3 and R4-C8, may

come as a surprise here because they

typically occur in audio amplifiers

were they serve to ‘straighten’ loud-

speaker impedances. Here, the main

purpose of the networks is to stabilize

the converter when the diodes are

switching.

12V

C12

C11

100n

1000 µ

16V

SB130

33k

25V

22n

IC1

12V

47n

TDA2822M

100n

22n

SB130

22n

33k

470 µ

25V

C10

100n

1000

16V

980073 - 11

Figure 1. This transformerless DC-DC

step-up converter is based on a stereo

power amplifier IC, the TDA2822M. Here,

the two opamps are wired as a cross-

coupled double AMV driving a traditional

diode-capacitor voltage doubler. Switch-

ing frequency is about 2 kHz.

Figure 2. Copper track

layout and component

overlay of the printed cir-

cuit board designed for

the converter (board not

available ready-made).

C ONSTRUCTION

The circuit is best built on a printed cir-

cuit board of which the copper track

layout and component-mounting plan

are given in Figure 2 . Construction

should be a piece of cake, the board

being single-sided, and only common-

or-garden components are used. Do

make sure, however, that the following

parts are mounted the right way

around on the board:

- electrolytic capacitors C4, C9, C10,

C11;

- diodes D1, D2, D3 and D4;

- integrated circuit IC1.

Having finished the solder work you

should subject the board to a thorough

visual inspection, and correct any obvi-

ous errors before you power up for the

first time.

If difficult to obtain locally, the type

SB130 diodes may be replaced by

almost any other medium-power

Schottky diode capable of passing at

least 1 A. In the prototype, the well-

known BYW29 was tried and found to

give good results, too.

Finally, be sure to use the TDA2822 M

only. Because of its 8-pin DIL enclosure,

it is the only version that can be used on

this printed circuit board.

COMPONENTS LIST

Resistors:

R1,R2 = 22k Ω

R3,R4 = 4

P ERFORMANCE

The maximum continuous output cur-

rent that can be supplied will be about

300 mA. The no-load current con-

sumption of the converter is between

6 and 8 mA. A prototype of the con-

verter was put through its paces in our

design laboratory, with the following

results:

Ω

Capacitors:

C1,C2,C7,C7 = 22nF

C3,C8,C12 = 100nF

C4,C9 = 470

F 25V radial

C5 = 47nF

C10 = 470

F 40V radial

C11 = 1000

F 16V radial

U in I in U out I out Efficiency

6 V 0.22 A 10 V 0.1 A 80%

12 V 0.44 A 21.3 V 0.21A 85%

Semiconductors:

D1-D4 = SB130, BYR745 or BYW29

IC1 = TDA2822M (SGS Thomson)

Not spectacular, but not bad either for

such a simple design!

(980073-1)

Elektor Electronics

1/99

470

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