Drive those TTLcircuits withthis 5 volt10 amp (max)supply.

WHILST the introduction of CMOShas lowered the power requirements ofdigital equipment using it, many large‘scale systems, because of cost andavailability, are still designed aroundTTL logic. For such systems a five-voltsupply having a capability of up to 10amps is often required.

The choice of power supply for asystem depends very much on theoutput requirements. In very lowpower applications a shunt regulatorconsisting of a series resistor and azener may be entirely adequate. Formedium power systems however aseries-pass transistor regulator isnormally used.

Whilst the series pass regulator is verygood with regards to ripple andregulation the specification of the transformer is critical if the supplyefficiency is to be above 50%. In alarger system this can be a veryimportant factor.

With a switching regulator therequirements on the transformer aregreatly relaxed and an efficiency of70% or more can readily be obtainedwith mains-input variations of from160 to 260 volts.

A fourth type is the switch-modesupply where the mains voltage is firstrectified and filtered. The rectifiedmains then drives a high-frequency

54

inverter which employs a ferritetransformer. Regulation is obtained bycontrolling the inverter and by thismeans very high efficiencies may beobtained. Nearly all the components insuch a system work at mains voltageand hence for safety reasons thisapproach was not used in our project.

CONSTRUCTIONAll components, with the exception

of the transformer and the choke arebest mounted on a printed-circuitboard such as the one specified. Thechoke should be wound as detailed in

Table 2 with four layers close woundof 16 swg wire. Due to the dccurrent in the choke an air gap isnecessary to avoid saturation. Theeasiest method of adjusting this gapfor best performance is to run thesupply at the maximum currentrequired and adjust the gap byinserting that thickness of insulationbetween the cores which givesminimum ripple voltage. We foundthat a 3 mm gap was required at 10amps for a ripple of 50 mVpeak-to-peak.

The prototype was mounted in a b

a

e

dn

eatd

die-cast box which acted as theheatsink as well as a shield to preventthe radiation of RFI generated by theswitching action of the supply. Ifanother form of box is used a heatsinkmust be added to the transistor-diodebracket for cooling.

An external LC filter will reduce theripple even further if required. Forexample a series choke of 20 turns of1.6 mm wire on a 10 mm ferrite rodand a parallel combination of 1000µF electrolytic and 0.47 polyestercapacitors external to the box willprovide considerable extra rippleattenuation.

SWITCHING REGULATOR IFig. 3, Component overlay.

* w _. we 11 *,*s, 1* *_, *, c ,I I -Ve OUT

I

+Ve OUT

SUPPLY

-TOCHOKE d

HOW IT WORKS - ETI 119I n a conventional series regulatorpower supply the resistance of aseries transistor is controlled in orderto maintain the correct outputvoltage. The series transistordissipates considerable power andtherefore at very high load currentsseries regulators are quite inefficient.In the switching regulator a Seriestransistor is still used but does notoperate in its linear range. Instead itswitches ON and OFF at high speedsuch that the load is alternatelyconnected and disconnected to asupply voltage that is higher thanthat required across the load. Bycontrolling the ratio of ON to OFFtime we effectively control theaverage voltage as seen by the load.For example if it is on for 25% of thetime the average output voltage willbe 25% of the input. Thus bycontrolling the ON/OFF ratio theoutput voltage may be stabilizedwhilst dissipation in the seriestransistor is very greatly reduced.

However since most loads do notlike their supply to be in the form ofa square wave an LC tilter is usedbefore the load to pass only the dccomponent.

Referring to the main circuitdiagram we see that transistors Q5and Q6 are used as the series switch.Ll and C7 form the output filter.Due to the inductance of the choke aflywheel diode is required, not onlyto protect the transistor, but toprovide proper operation. When theswitch is on, the load current flowsthrough the transistor, the choke,and into the capacitor and the load(Fig. A). When the switch is openedthe load current must continue toflow through the choke and this is

Fig. B). The current through thechoke will thus rise during the onperiod and fall during ‘the off period.The current never falls to zero exceptat very low load currents and theaverage is the same as the loadcurrent.

The operating frequency is set bythe UJT Q1 which runs about20 kHz; the higher the operatingfrequency the lower the ripplevoltage on the output. However asthe operating frequency goes up soalso do switching losses in bothtransistor Q6 and diode DS. The20 kHz was chosen as a compromise.it is high enough not to be audiblebut low enough to keep these lossesto a minimum.When the UJT fires the pulse

generated is coupled into the base ofQ4 by C4 turning Ql on. This, in turn,turns on Q2 and the switch Q5/6.When Q2 turns on Q4 also turns onand both latch on. If the currentthrough Q6 rises above about 12 to14 amps Q3 will turn on robbingcurren t from the base of Q2 allowingboth it and Q4 to turn off. This alsoturns off the output switch Q5/6.This is the current protectioncircuitry.

A voltage- proportional to theoutput is provided by RVl to Q7 forcomparison to the voltage of ZDl. IfQ7 is turned on sufficiently it willalso turn on Q3 thus unlatching Q2/4and turning off the output switch.Once the supply has stabilised this.action will control the on time of theswitch in each cycle of the 20 kHz,such that the output voltage ismaintained at a voltage as set by RVlin a smooth and even manner.

We used a 240 V to 30 V 2 Atransformer, which is adequate for,

however any transformer having anoutput of 20 to 30 volts and a powerrating of 60 VA would do. If up to10 amps output is required then atransformer with a rating of 75 to 80VA would be required.

It is also possible to supply theregulator from a dc supply of 10 to40 volts. If the voltage available isless than 20 volts R2 should bereplaced by a link to ensure that theUJT operates correctly.

LOAD

Fig. A. Current paths with switchingtransistor on.

LOAD

Fig B. Current paths with switchingtransistor off.

56 ELECTRONICS TODAY INTERNATIONAL-APRIL 1976

TABLE 2 Choke winding details.CORE Philips E core 4322-020-

34720 two requiredFORMER Philips 4322-021-

31830 or 4312-021- --23622 one required

Four layers close wound of 1.6mm wire core gap 3 mm (seetext).

CHOKE COMPONENTSWe have, as yet, been unable to find a source of supply to the amateur of the chokecore and former. However the value of this component is not critical and is, in anycase, the subject of experimentation in the adjustment of the airgap. We would there-fore suggest that, although we haven’t tried this, the laminations and former of a6.3V ac heater transformer may be of suitable dimensions. It may be, in fact, thatthe secondary of a heavyduty heater transformer may serve without modification,although we recommend that a lk resistor be connected across the primary toprevent the effects of a build-up of induced voltage. Please note that this is a matterfor experimentation.