Przetwornica 12V-230V.pdf

POWER SUPPLY

Simple 12-to-230V

A mobile power outlet

Design by G. Gerards

www.aixcon.de

The absence of a mains power outlet is often keenly felt on camping sites,

with car repairs in the middle of nowhere and with picnic or party events

in the countryside. In some cases, mains power can only be brought to a

remote site by running a very long cable — which either dangerous,

impossible or not available. Whatever the occasion, planned or

unexpected, it is great to have a power inverter available capable of

changing the 12-V car battery voltage into 230 volts AC.

The idea for a simple, portable

‘power outlet’ was first suggested

by a trainee at the Aixcom company,

which is normally involved with

high-tech power inverters and spe-

cial high-current power supplies.

The trainee, called Dirk, had been

trying for quite some time to build a

power inverter for his model aircraft

club. In his enthusiastic attempts he

ran into problems obtaining the spe-

cial integrated circuit that was to

form the heart of his project. When

he was finally successful in obtain-

ing the elusive chip, albeit at horriﬁc

costs, all the circuit did was produce

a loud bang at switch-on, wrecking

a lot of components.

The company decided to continue

the design, and the result is pre-

sented here: a power inverter that

was not only successfully repro-

duced by nearly all trainees at Aix-

com, but also presented as a Christ-

mas or anniversary gift to dad, used

on a camping site and, last but not

least, deployed in a (very loud)

Elektor Electronics

2/2004

Power Inverter

POWER SUPPLY

music parade. A beefed up version of

1,000 watts was developed and

installed by Dirk at his model aircraft

club, where it has performed beauti-

fully for over a year despite rough

conditions.

lator, which is supplied by a number

of manufacturers under the compo-

nent identiﬁer xx3526, where xx is a

manufacturer-specific letter combi-

nation. The 3526 supports all known

switch-mode PSU topologies. Its

complete datasheets may be

obtained free of charge from

www.unitrode.com (part search:

UC3526 and Datasheet).

The basic operation of the power

inverter is illustrated in Figure 1 .

The SG3526 alternately switches the

current through the 12-V windings of

a mains transformer, the two central

ends of the windings having been

taken together and connected to the

positive battery terminal (+12 V). At

each switching action, the direction

of the current changes and with it

the direction of the magnetic ﬁeld in

the transformer core. The result is a

square-wave(-like) alternating volt-

age at the 230-V side of the trans-

former.

In real life, the switch consists of

two FETs in complementary

arrangement (push-pull). The source

connections of the FETs are taken to

ground by way of very low resis-

tances (compare the circuit diagram

in Figure 3).

The internal architecture of the

SG3526 is shown in Figure 2 . The

input voltage +V in may be between

7V and 35 V, and is used to create a

reference voltage V REF of 5 V. A volt-

age guard blocks the drivers stages

when the input voltage drops below

7V. The drivers are separately pow-

ered via the +V c connection. Using

resistor RT and the capacitor at CT

+12V

230V

Concept

Arguably, the circuit represents the

simplest way of creating a power

outlet for on the road. In the design

phase, the aim was a 100% bare

bones circuit, stripped of anything

that could be, well, stripped! For

example, there’s no voltage regula-

tion, and a sagging battery voltage

also causes the ac output voltage to

sag. However, because most mains

powered equipment continues to

work just fine in the face of ac volt-

age variations of ±10-15%, the

mobile power outlet does so too.

Rather than perfecting the design for

performance, Aixcom went for sim-

plicity, low component count and

utter reliability in practical use. None

the less, the 230-Vac output is short-

circuit resistant and an undervoltage

protection switches the inverter off

before the battery has been drained

to level that would no longer allow

the car to be started. The circuit is

simple enough to be reproduced suc-

cessfully by beginners, too, provided

they realise that 230 Vac is a really

dangerous voltage.

SG3526

020435 - 13

Figure 1. Block diagram of the power inverter.

In the actual circuit, the switching element

consists of two power FETs, while the resistor

to ground acts as a current sense (shunt) for the

current limiter circuit inside the SG3526.

(again compare with circuit diagram in Fig-

ure 3) the frequency is determined, which is

50 Hz in this case. The resistor at R D causes

a fixed dead time between the driver’s Out-

put A and Output B. This is done to eliminate

the risk of the two drivers (and consequently

the two power FETs) conducting at the same

time when the switch-over takes place.

The capacitor at the C SOFTSTART pin (Css,

pin 4) allows the pulse mark/space (on/off)

ratio of the outputs to be slowly raised to 48%

after the supply voltage is switched on, or

after a reset. The ‘Amp’ voltage regulator is

not used as such in our application, alterna-

tively it takes the role of an impedance con-

verter using the reference voltage as the con-

trolling quantity. In this way it is assured that

the outputs supply the full mark/space ratio

after the start-up phase.

The current limiter using shunt resistor R8

triggers a shutdown sequence when the volt-

age between +CS and –CS (in other words,

the drops across R8) exceeds 100 mV. How-

ever, the shutdown control may also be used

externally by connecting it to ground.

Because shutdown and Reset (pins 8 and 5

respectively) are interconnected in this cir-

cuit, the modulator starts again with a soft

start after an overload condition or an external

disconnect.

Pulsewidth modulation

The central part in the circuit is an

SG3526 low-cost switch-mode regu-

V REF

SYNC

+V C

+V IN

Reference

Regulator

Undervoltage

Lockout

GROUND

R D

R T

C T

To Internal

Circuitry

Oscillator

OUTPUT A

RESET

C SOFTSTART

COMPENSATION

Soft

Start

+V IN

+ ERROR

– ERROR

MEMORY

F/F

TOGGLE

F/F

Amp

More design thoughts

The transformer for the project should be a

toroidal type with a primary of 230 V and two

12-V secondary windings. Readers in coun-

tries with 110 V, 117 V or 127 V mains voltage

are, of course, advised to use a matching 200-

OUTPUT B

METERING

F/F

+ C.S.

– C.S.

SHUTDOWN

020435 - 12

Figure 2. Internal diagram of low-cost SMPSU regulator type SG3526.

2/2004

Elektor Electronics

POWER SUPPLY

watt transformer. If you are lucky to have an

old toroidal transformer lying around in a

drawer or a junk box, it should not be too dif-

ﬁcult to ‘retro-ﬁt’ two 12-V windings. Simply

wind ten turns of litze wire around the core

and connect the primary to the mains. Mea-

sure the voltage across your new winding

and then calculate how many turns you need

to get at 12 V. At an output power of

200 watts, the average current will

be about 10 A, so the cross-sectional

area (c.s.a.) of the litze wire you’re

using should be 1.5 mm 2 or greater.

It is vital that the two 12-volt

windings have exactly the same

number of turns . If there is a differ-

ence of just one turn then the trans-

former core will saturate the instant

the 12-volt battery is connected,

causing the regulator to ‘hang’ in

shutdown mode. The sense direc-

tion) of the windings is equally

important. Before installing the

transformer, connect the ends of the

two 12-volt windings in series and

apply 230 Vac to the primary. You

should measure 24 Vac across the

free ends of the secondaries.

The FETs used in the circuit can

handle up to 72 A at 55 V, and are

marked by an R D-S(ON) of just 12 mΩ .

Of course, other types may be used

provided you are sure they can han-

dle at last 40 A at 40 V, and have an

R D-S(ON) not exceeding 50 mΩ . Usu-

ally, power FETs may also be con-

nected in parallel, but please make

sure each one gets its own gate

resistor. The parallel conﬁguration is

of interest if you wish to configure

the inverter for output powers

greater than 200 watts. In that case,

the current limiter has to be

adapted, which is easiest done by

BATTERY +

D 1

1N4002

18V

1W3

R12

12V

IC1

R14

220 µ

16V

PTC

BYV27

IC1 = LM393

22k

R13

22 Ω

VIN

OUT A

IC1.A

IRFP

054

IC2

C10

+SYNC

OUT B

R15

47k

220n

+EA

–EA

VREF

COMP

IRFP

054

SG3526N

R16

+CS

IC1.B

RST

1N4148

10k

Css

–CS

R11

R10

R17

220n

220

220n

33n

63V

33n

16V

BATTERY –

020435 - 11

Figure 3. Circuit diagram of the 12V-to-230V Power Inverter. Comparator IC1 acts as a guard for temperature as well as battery voltage.

Elektor Electronics

2/2004

POWER SUPPLY

020435-1

R10

R11

R17

R16

C10

IC1

IC2

R12

020435-1

X3 X2

Figure 4. Copper track layout and component mounting plan of the PCB designed for the project. The board is single-sided and available

ready-made through Readers Services.

using a smaller value for shunt R8

and/or by modifying voltage divider

R16-R17.

Ordinary bulbs and halogen

(flood-) lights are sure sources of

trouble with most power inverters.

Both present a very low ‘cold’ resis-

tance, causing the inverter to reduce

its output voltage or even actuate

the shutdown. The result is a lock-up

with isufficient voltage to heat up

the ﬁlaments to their normal operat-

ing temperature. Fortunately, the

200-watt version of the inverter

described here should be capable of

turning on lamps of up to 150 watts

without problems. Should problems

arise, the value of capacitor C6 may

be increased — but not to any extent

because the ability of the circuit to

withstand short-circuits may well

suffer. Using C5 it is also possible to

increase the soft-start time consid-

erably, or do without it altogether.

That, in all likelihood, is the safest

solution.

Comparator IC1 monitors the bat-

tery voltage and ambient tempera-

ture and compares its measured

results with the 5-V reference volt-

age from the 3526. The two open-col-

lector outputs pull the shutdown

control input (pin 8) to ground in

case of an error. The PTC used

determines the switch-off tempera-

ture. Depending on the exact type in

your circuit, R6 may need slight red-

imensioning. Early Aixcom proto-

types of the inverter used a D901-

D60-A40 from Epcos (trip tempera-

ture 60 degrees C). However, it

should also be possible to use a tem-

perature switch of 60 to 80 degrees

C or a temperature fuse of

90 degrees C. Although the latter

component is extremely cheap at

just a few pence, you’ll need to

exchange it when it has ‘gone off’.

Provided a large enough heatsink

is used, a simple wire link may be

used instead of the PTC. The voltage

monitor switches off at about 12 volt

and this may be adapted to suit

other levels by changing R1 and R5.

On the comparators, R2 and R4

define an amount of hysteresis that

prevents the power inverter from

switching itself on again after a fault

condition. After switching on, the

reference voltage rises slowly as

determined by the charge time of C2,

hence the monitors are only acti-

vated a few seconds later.

COMPONENTS LIST

Resistors:

R1 = 15k

R2 = 22k

R3 = 2k

R4 = 10k

R5 = 12k

R6 = 4k

Ω

R8 = 0 Ω 01 (max. lead pitch 24mm)

R9 = 1k Ω (PTC, see text)

R10 = 8 Ω 2

R11 = 16k Ω

R12 = 33 Ω

R13,R15 = 22 Ω

R14 = 18 Ω

R16 = 1k Ω

R17 = 470 Ω

Car batteries supply danger-

ously high currents . To prevent the

inverter going up into flames and

causing a fire, you must protect it

with a car fuse of between 25 A and

35 A. The 230 Vac output voltage is

also very dangerous even if it is

generated by means of a battery.

F 16V radial

C3,C7,C9,C10 = 220nF

C4 = 1 µ F 63V radial

C5,C6 = 33nF

C8 = 2 µ F2 63V, 15mm lead pitch, MKS4

(Wima)

Semiconductors:

D1 = LED, red, low current

D2 = 1N4148

D3 = 1N4002

D4 = 18V 1.3W zener diode

D5,D6 = BYV27-200

IC1 = LM393N

IC2 = SG3526N

T1,T2 = IRFP054N (IRF)

Construction

The design of the printed circuit

board is shown in Figure 4 . Despite

large ground areas and wide tracks

it may be necessary to strengthen

the tracks carrying the transformer

current by tinning them. It is recom-

mended to start by mounting the

AMP (‘fast-on’) lugs (spade termi-

nals), because they require consid-

erable force to insert into the board.

After all, a mishap with the use of

pliers at this point could cause con-

Miscellaneous:

X1-X6 = AMP spade terminals, PCB mount

PCB, order code 020435-1 (see Readers

Services page)

Toroidal mains transformer, see text, e.g.,

Aixcon 230V /12-0-12 V / 200W

(www.geist-electronic.de)

2/2004

Elektor Electronics

R7 = 47k

Capacitors:

C1,C2 = 220

POWER SUPPLY

Table 1

Output voltage vs. battery voltage

(150 watt load)

Battery voltage

[Vdc]

Output voltage

[Vac]

nect it to an adjustable bench supply

and check the two guard circuits: the

voltage guard by adjusting the input

voltage, and the temperature guard

with the aid of your soldering iron, a

potentiometer or any other means

you see ﬁt. In any case, the outputs

will switch to ground and the LED

will light when the voltage at the

positive input of the comparators

drops below that at the negative

input. If the guard circuit appears to

work, you proceed by measuring the

two gate signals. If an error is pre-

sent, both will read 0 V. In the case

of an error-free circuit, an oscillo-

scope will show two clean rectangu-

lar-wave signals with 10-ms long

pulses. Using your multimeter, the

same measurement yields a readout

of about half the supply voltage.

All approved so far, you are in a

position to connect the toroidal

transformer. At this point, it makes

sense to remove IC1 from its socket,

as in that case the shutdown can

only be triggered by the current lim-

iter. If an ordinary 100-watt bulb

does not light up within a few sec-

onds, measure the voltage at the

shutdown control (pin 8 on the 3526

or the anode of D2). If you measure

less than 5 V, the current limiter or

the soft-start time has to be tweaked

as described above.

Once the bulb lights, you may

(carefully!) check if the inverter is

short-circuit resistant. If an oscillo-

scope is available, the FET current

may be measured (= the voltage

across R8) and use R16 to increase

the current limit point to about 20%

below the permissible drain current.

This is of course done with the 230-

Vac output short-circuited.

It is normal for the transformer to

make more noise under no-load con-

ditions than you would expect

when in normal use. This is caused

by the rectangular wave switching

the magnetic field hard and fast.

Core saturation under no-load con-

ditions is signalled by ugly sounds

from the transformer. Measured

with an oscilloscope the currents

will not rise in sawtooth-wise but

with peaks (overshoot). In that

case, the 12-V windings on the

transformer require just a few more

turns. If that is problematic, the

alternative is to raise the oscillator

frequency a little by using a slightly

lower value for R11. The resulting

output frequency may well be

55 Hz, but that is immaterial for

most loads and the circuit is not

suitable anyway to power an alarm

clock.

11.5

182.4

194.6

12.5

202.4

214.3

13.5

223.0

231.2

siderable damage to other components on the

board. The wire link beside the shunt resistor

R8 should not be forgotten. R8, by the way,

should be mounted a little above the board

surface to help it stay as cool as possible. If

desired a higher-wattage resistor may be

substituted (5 watts). Finally, do make sure

you mount all polarised components (transis-

tors, electrolytic capacitors, diodes and ICs)

the right way around on the board. Insulating

washers must be used when ﬁtting the tran-

sistors onto the heatsink.

Powering up

Commissioning this project only requires a

multimeter. Initially, you use the inverter

without the transformer connected . Con-

Practical results

Because a voltage regulation loop

omitted for the sake of simplicity and

cost, the output voltage is depen-

dent on the battery voltage. The out-

put voltage of the author’s prototype

loaded with a 150-watt halogen

lamp is shown in Table 1 , as a func-

tion of battery voltage.

The output voltage is dependent

on the transformer’s winding ratio

and output current. If you want to

reach the nominal output voltage of

230 Vac at 13 Vdc input, you should

consider using a transformer with

two 11-volt windings. On the proto-

type, a maximum efficiency of 94%

was measured and the circuit was

found to be Dirk-proof.

(020435-1)

Elektor Electronics

2/2004

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