48 IRE TRANSACTIONS ON INDUSTRIAL ELECTRONICS May

DC to AC PowerConversion by Semiconductor Converters*

EDWARD J. DUCKETT,t SENIOR MEMBER, IRE

Summary-The rapid improvement in semicon- remaining problems that require continued engi-ductor power devices and in inverter circuits has neering effort.given rise to an accelerated introduction of newequipment for the conversion of electrical power DEVICE DEVELOPMENTin military, industrial, and commercial applicationtions. Increased power handling capability and im-proved operating characteristics have made it pos- power semiconductor devices to be widely applied.sible to perform many functions effectively which, By combining new materials techniques with im-until recently, were achieved only by nonstatic ap- proved fabrication methods the silicon diode hasparatus. The ability to convert from dc to ac effi- grown from the size appropriate to a portable radio

ci y ad rto the present-day units that have operating volt-ciently and reliably has received special emphasisae necs f50vlsadcncnrli xduring the past several years. Developmental m ages in excess of 500 volts and can control in ex-

dels of semiconductor inverters have been con- cessloc15 ps.structed, and a number of potential applications Teslcnpwrtasso,to a estructednand a number ofhpotentialyapplications veloped rapidly, largely because of a far betterhave been investigated. These new types of power

understanding of the basic physics of semiconduc-conversion apparatus should be of use in a widevariety of application areas, including emergency tors. These devices require extremely pure sili-power supplies, controlled frequency sources, and con as a basic material-less than one part infrequency converters. Predictions of future appli- impurity content is desirable. The combination ofcations and capabilities indicate that the semicon- materials research and improved fabrication tech-ductor power inverter and allied apparatus should niques has produced a new method for producingbe a significant factor in the design concepts for pure silicon, and a much better control of devicefuture electrical systems. characteristics can now be obtained.

In the past three or four years, research anddevelopment engineers have produced a new type

INTRODUCTION of semiconductor device: the silicon'controlledIn a relatively short span of time-less than 10 rectifier. Although this new semiconductor device

years-a new family of semiconductor devices and is similar to the power transistor in many ways,associated circuits have been developed and there is one basic difference. In the case of thebrought to the point where they can be employed transistor, the control element is always activeadvantageously in a large number of power appli- and the device can be turned off or on at any timecations. For the future these new types of con- by applying the appropriate voltage to the controltrolled, static power switches give promise of element. The silicon controlled rectifier, on thehaving increased importance and are expected to other hand, can be turned on by the control elementinfluence a significant part of the electrical indus- at any time; but the control is lost as soon as con-try. Commercial, industrial, consumer, and mili- duction begins. Because of this, special circuitrytary areas have need of these new power equip- is required to turn off the silicon controlled recti-ments that can offer such attributes as increased fier, i.e., the primary current must be forced to goreliability and efficiency, lower cost, less noise, to zero; and, at this point, the control elementand less occupied volume if we can extend our again becomes operative. Both transistors andpresent knowledge a littleifurther. Developmental silicon controlled rectifier devices have been usedmodels of semiconductor static inverters have in semiconductor power inverters. The relativebeen designed and constructed to illustrate some merits of each type in any specific application andof the application areas and to identify the specific the circuit that is employed will depend upon the

______________ ~~~~~~power level and other requirements.* Received September 22, 1961. Considerable study of semiconductor unitst New Products Laboratories, Westinghouse Electric shows that the internal energy losses are extreme-Corporation, Pittsburgh, Pa. ly low when the devices are at minimum or maxi-

1962 DUCKETT: DC TO AC POWER CONVERSION BY SEMICONDUCTOR CONVERTERS 49

mum conduction conditions. High losses occur from a relatively small number of watts to today'sonly under operating conditions in which the units level of 60 to 80 kvs in only two or three years.are not fully turned on or off. This makes it ex- Sustained development is expected to yield eventremely important that the circuitry permit the greater increases in the future.semiconductor unit to be operated in only twostates: either "all on" or "all off." In general, APPLICATIONSswitching time from one state to the other must bemade as short as possible for minimum dissipated One way to catalog the applications of semi-energy and highest efficiency. Since the all-on, conductor power switches is by basic function,all-off type of operation is essentially the behavior such as:of a switch, this type of operation is identified AC to DC Conversion-In this instance, controlgenerally as the "switching mode." of the conduction period is obtained by a signal ap-

All of the semiconductor power conversion plied to the control element of the semiconductorequipment described in this paper have been de- switch. These devices can perform two functionssigned with the "switching mode" operation as a in ac-to-dc conversion, i.e., rectification andbasic part of the circuitry. The semiconductor de- simultaneous control of the average voltage.vices are capable of performing the switching DC to DC-In this general area, dc regulatorsaction extremely rapidly with no deterioration at can be designed and a type of dc transformer cana high repetition rate and with a high efficiency. be fabricated to permit a change in the level of dc

Both the transistor and the controlled rectifier voltage.have been used effectively in inverters. Tran- AC to AC-In this category there are a verysistor power inverters have been constructed up large number of applications which can be con-to the 10-kw level and have produced frequencies sidered in terms of two basic operations:as high as 20 kc per second. Above the 10-kwlevel, however, the SCR device appears to be more 1) Voltage control in which the conduction anglesuitable, particularly since the technical consider- of the device is controlled to provide constantations indicate that extremely high power can be or regulated output. This is expected to findobtained in single units in the future. wide application in control circuitry at all

The basic functional element of the silicon con- power levels.trolled rectifier device is the specially prepared 2) Frequency changing. In this category a verysilicon wafer. The typical wafer shown in Fig. 1 wide expansion of present-day concepts foris smaller in diameter and thinner in thickness utilization equipment can be accomplishedthan a dime, yet is capable of controlling 100 amps since the conventional power frequenciesat 200 or more volts. Semiconductor materials need not be considered as a limiting, rigidare improving rapidly and, as a result, the power parameter if static frequency changing ishandling capabilities of SCR devices have gone utilized. Frequency changers can be designed

to provide constant output frequency or con-trolled variable output frequency. One of themethods now used frequently for increasingfrequency converts from power line frequen-cy to dc and then reconverts to the desiredhigher frequency by means of an inverter.This type of equipment will be included in

7000000lCd _ this paper as part of the total inverter appli-cation area.

DC to AC InvertersThis area includes a number of applications

ranging from emergency power supplies to specialpower sources to provide high-quality power tospecialized computers and navigational equipment.It is interesting to note that there are two types ofequipment called inverters which perform thefunction of dc-to-ac conversion. One type is

Fig. 1-Silicon controlled wafer used in 100-a device. generally employed in industrial areas in which

50 IRE TRANSACTIONS ON INDUSTRIAL ELECTRONICS May

large dc rotating machines are stopped rapidly and cuit is made capacitive and is used to store energyfrequently. In this instance the stored inertial during the initial half-cycle. This energy is thenenergy of the motor must be absorbed in the sys- discharged by use of the parallel switch and thetem. An economical and effective method is to load experiences an ac voltage. In all of theseconvert the dc power derived during the braking circuits any device which has a switching actionaction to line frequency ac so that the energy can could be employed.be fed back into the ac lines. In this case the in- Since the SCR requires circuitry which causesverter must work synchronously with the power the current to go to nearly zero after conductionline; but the frequency, voltage, and waveform of been initiated before control can be regained, thethe output power is dictated by the large power circuits employed for SCR's may differ to a de-line system components and is not included as part gree from the previous switching circuits. Gener-of the inverter apparatus. ally, as in Fig. 3, circuit elements are introduced

The second type of inverter performs the con- either across the device or in the load circuit toversion of dc to ac and also establishes the frequen- cause the current through the device to be reducedcy, voltage, and waveform as an inherent part of to a minimum and permit regaining of control be-the inverter equipment. The output of this type ofconverter can be made to be compatible with theac power line or can be selected for optimum per- +formance for a specific application. This paper isconcerned primarily with the latter type of in-verter. Il

There are a number of basic circuits that canbe conceived using the general concept of semi- LOADconductor power switches for dc-to-ac conversion.Generally an inverter requires a pair of controlledswitches: one of which supplies the positive half-cycle of the alternating current wave, and theother the negative half-cycle. In Fig. 2, simplifiedschematic diagrams are shown to illustrate thegeneral concept. It can be seen that if the units COMMUTATING BRIDGE INVERTER CAPACITOR TURN-OFF INVERTERare considered as switches and can alternately be (a) (b)made to go from a high-impedance to a low- Fig. 3-Schematic diagram of circuits used for SCR staticimpedance state, a single-phase, center-tapped or inverter applications.a three-phase bridge inverter is fundamentallyquite simple. The outputs from these basic cir-cuits are, of course, square wave voltages. 80000_A somewhat different method for utilizing these

devices is shown in Fig. 2(d) in which the load cir-

oa4 4 1 2~0 0,000@

6O~~OOO -~~SILICON

~~~~~~CONTROLLEDINPUTHALF WAVE -ITH -NERGY -TORAGE TRI RECTIFIERS

OUTPUTNPU w

INPUT OTUT OUTPUTa

L

0 >~~~~~~~~~~~~~~SINGLE PHASECENTER TAP SINGLE PHASE, BRIDGE - - - 10 9(a) (c)

Fl ass 0 ~~~~020,000-

INIPUT INPUT OUTPUT

HALF WAVE WITH ENERGY STORAGE

0 0 TRANSISTORSOUTPUT

(b) PC D ZV 0PIr -C 'LAIi Njnio (d YEA

1962 DUCKETT: DC TO AC POWER CONVERSION BY SEMICONDUCTOR CONVERTERS 51

'0000 Mw of power handling capability in the 1965 timeA/ period. If multiple units are used, much higher

MULTIPLE - power ratings might be obtained.DEVICES

ooo0 - v// -- DEVELOPMENTAL EQUIPMENTWith any new basic tool or device there is need

/z/ SttEVIES for considerable exploration and development be-,,/-/ fore suitable methods are evolved for using them

f00- ________ effectively. Development of circuits for the new100- _ _ _ I_< V >/ power switches has been active for the past sever-< W / al years, and developmental models for various

applications have been fabricated to indicate theirpotential as commercial products. Factors such

____ _ as cost, weight, and volume still need improvementC, for widespread utility; but these are rapidly di-

minishing as major problems. The equipments tobe described in this section are developmentmodels constructed to determine their electrical

l0---- _________--- - capabilities and to illustrate the broad areas of1958 1960 1962 1964 application that can be considered for the semi-

YEAR conductor power inverters.The earliest types of static semiconductor in-

Fig. 5-Power inverter development. verters utilized transistors at relatively low powerlevels. Fig. 6 illustrates an improvement overconventional techniques at low power. In this

fore forward voltage is reapplied to the SCR. Ex- instance a small flashlight battery provided dc forcept for this difference the power transistor and an inverter which produced 3000 cycles. This inthe SCR circuits are very similar. turn was used to excite an electroluminescent

The semiconductor switch devices have in- panel; the entire equipment was contained in a verycreased in power handling capability at an im- small package.pressive rate during the past several years. The need for lightweight, low volume equipmentFig. 4 shows that the silicon transistor ratings for aircraft offers an excellent opportunity for thehave increased steadily; today as much as 30 introduction of static inverters; this developmentalamps, 300 volts can be controlled by a single unit. model (Fig. 7) of an aircraft static inverter wasThis indicates a very rapid rate of rise in power designed to replace a low power rotating machine.in the few years that they have been under develop- In this instance, dc power from a battery was con-ment. verted to 400 cycles and regulated accurately both

The SCR which was developed only a few yearsago shows even a more rapid rate of rise in powerhandling capabilities and gives promise for con-tinued rise in the years ahead. You will note that F-Xin a period of only three to four years, the powerhandling capabilities have increased by a factor ofapproximately 20. It is anticipated that continueddevelopment can increase the current handlingcapabilities of these devices by as much as afactor of 3 beyond present-day capabilities; thevoltage rating should increase by a factor of 2 ormore in the not too distant future.

The semiconductor power inverter has grown ata rate comparable to the device itself. With theintroduction of the controlled rectifier the powerhandling capability has increased at a rapid rateand is expected to continue to increase for thefuture. Fig. 5 shows that the use of single devices Fig. 6-Low power, compact static inverter providingin static inverters could yield as much as 1 to 3 3000-cycle output for electroluminescent panel.

52 IRE TRANSACTIONS ON INDUSTRIAL ELECTRONICS May

one can see the conventional electronic generatorcommon in the period of 1956; this is a large pieceof apparatus with a large number of components.By 1958 the electronic generator had been reducedin complexity with a significant reduction in weightand volume and a moderate improvement in ef-ficiency. In 1960 the introduction of the SCR intothe circuitry was accomplished and again a markedreduction in weight and volume was experiencedtogether with an important improvement in ef-ficiency. Finally, in 1961, it can be seen that byuse of the newest SCR's and improved circuitry,the weight again was reduced by a factor of 2representing almost a factor of 10 reduction from

Fig. 7-Development model of low power aircraft static the original electronic generator. Volume and ef-inverter providing 400-cycle regulated output. ficiency were also improved greatly. This is an

example of how semiconductors can be effectivelyemployed for a specific application.

in frequency and voltage at about 150- to 250-va Another area in which frequency changing is ob-power rating. There are a number of circuitry tained and again by means of double conversion isproblems to be resolved in the design of equipment illustrated in Fig. 9. This 1. 5-kw unit is designedof this type since the importance of weight andvolume is acute. Usually compromises involvingthe frequency and voltage regulation and outputwaveform must be made with full consideration ofthe weight and space penalties that exist.

One of the most dramatic evidences of the im-portance of the use of silicon controlled rectifiersin power equipment is illustrated by the ultrasonicgenerators shown in Fig. 8. Although this ultra- V_0llsonic power supply performs as a frequency chang-er, since the over-all conversion is from 60 cyclesto 20,000 cycles, it is actually a double conversion,going from ac to dc and then inverting the dc to O ihigh-frequency ac. At the left side of the figure

Fig. 9-Developmental model of 1.5-kw frequency conver-ter for high-frequency lighting-1500 cycles.

Fig. 8-Evolution of 1-kw ultrasonic generator, 1956- Fig. 10-10-kw static inverter providing 400-cycle, three-1961. phase output.

1962 DUCKETT: DC TO AC POWER CONVERSION BY SEMICONDUCTOR CONVERTERS 53

to transform from 60 cycles to 1500 cycles for theexcitation of fluorescent lights. A number of ad-vantages are related to a change in frequency: thisincludes improved light output from the lightsources, reduction of size and weight in ballasts,improved efficiency of ballasts, and the opportunityto modify the ballast design to take advantage ofthe high-frequency controllable waveform that canbe generated. Ballast size and weight may be ofimportance in the internal structure of a building,and a reduction in the ballast heating can affect theair conditioning load appreciably.

This ability to change frequency is extremely 0important and suggests a number of things thatmight be done in the future. For example, an in-crease in frequency will make it possible to re-duce component size in a variety of utilizationapparatus. Consider the reduction in size of Fig. 12-100-watt static inverter powered by thermoelec-motors as frequency is increased as well as the tric generator.reduction in size of transformers, capacitors andother similar items. Power supply filters can be vides positive, reproducible rotational speeds forreduced greatly. With these advantages it would each position in the mill and can improve the use-seem extremely important that conversion of ful yield from the spinning mill equipment.frequency be considered in all future systems During the past few years a large amount ofanalyses. technical effort has been directed toward new con-

Fig. 10 shows a 10-kw inverter which has an cepts in electrical power generation. It is inter-input of 60 cycles and an output of 400 cycles, esting that a number of unconventional methodsthree phase. The size of a 400-cycle motor is for producing electrical current such as thermo-much less than the conventional 60-cycle unit and, electricity, magnetohydrodynamics, thermionics,if even higher frequencies are employed, the and fuel cells provide a dc output. As a result amotor size can be reduced by as much as a factor highly efficient and reliable method for convertingof 10 with a corresponding weight reduction. This the dc output to power line frequencies is an im-should make possible broader utilization of motors portant part of the system concept. Fig. 12 showsand other electrical products. This new degree of a developmental thermoelectric generator andfreedom should be a welcome challenge to the de- static inverter at the 100-watt level. Since thesigners of new system concepts. inverter can be designed for an output frequency

Another example of static conversion is illus- over a wide range, this type of inverter could betrated in Fig. 11. A single master oscillator can used for 60, 400, or 1000 cycles and, if desirable,control accurately a multiplicity of static power also provide a dc output at a voltage level higherinverters; these units might provide power for a than the input.number of synchronous motors in a fiber mill During the past year static power invertersdrive. The oscillator and power inverter units have been introduced into the important militaryshown here are developmental models and pro- areas. The developmental model of a 2.5-kw in-duction prototype equipments are now in test oper- verter shown in Fig. 13 was designed to receiveation at a spinning mill. This type of control pro- power from a ship's battery system and has an out-

put of 400 cycles, three phase. In this instance,the input voltage can vary by almost a factor of 2,yet the output must stay within less than one percent of a selected value. The size of this de-velopmental model is larger than anticipated forthe future, but does illustrate the present-day stateof the art for this type of inversion.

Fig. 14 shows a developmental module of a high-er power military inverter. This module is capa-ble of handling 30 kw of power. When applied

Fig. 11-Developmental model of adjustable frequency properly, the semiconductor power switch providespower supply for fiber mill drives. the opportunity to control large amounts of power

54 IRE TRANSACTIONS ON INDUSTRIAL ELECTRONICS May

300 --

M-G SETD.C TO 60 cps

2OOELECTRONICAMPLIFIER

X m V M-G SET

DC TO 400 cPs-/ INVERTER

INVERTEDC T

-' I0 100 1000POWER RATI N G-K VA

Fig. 15-Conversion equipment weight vs power rating.

POTENTIAL OF STATIC INVERTERSFor the future the semiconductor power invert-

er is expected to have a number of important ad-vantages relative to existing apparatus. In Fig. 15we have predicted the weight of future inverterequipment. The inverter weight either at 60 cycles

Fig. 13-Experimental prototype of 2.5-kw static inverter, output or 400 cycles should be significantly lower400-cycle regulated output. in weight for a given power handling capability than

the motor-generator set and the electronic ampli-fier-roughly a factor of 2.

If we predict the occupied volume Fig. 16 showsa favorable relationship for the inverter. Here theinverter is roughly a factor of 2 smaller at either60 cycles or 400 cycles than the motor-generatorset and very much smaller than the electronicamplifiers to perform the same electrical function.

Another item of importance that must bementioned when discussing the volume of apparatusis that of a major difference that exists betweenrotating machines and static equipment. Generally

10.0-

H ~~~M-G SETDLC TO 60 cps

1.0-

Fig. 14-Developmental module for high powered staticinverter system (module can control approxi- Lmately 30 kw). INVERTER

n D~~~~~~~~CTO 6Ocpslin a relatively small space and with relatively light -DC TOSET40 cpweight. INVERTE R

These are some of the areas which are now be- DC TO 400cpsling pursued because they give promise of success- -.ful application in the not too distant future, but 10 100 1000these represent only a very minute portion of the POWER RATING - KVAtotal potential for these new devices and equip-ments. Fig. 16-Conversion equipment size vs power rating.

1962 DUCKETT: DC TO AC POWER CONVERSION BY SEMICONDUCTOR CONVERTERS 55

At present the estimfiated cost of megawatt levelXD|gNCV E oTOAC a 2 semiconductor static inverters is higher than con-

90 --------_ventional systems. But, since the cost of the static

80_________ --_ ___ __ inverter will depend largely on that of the SCR de-DC-SETO60cpsvice, the inverter cost should be reduced rapidly

70-- in the years ahead. Somewhere in the 1960's theU_ DC TO 400cSE very high power semiconductor inverter should be

60_ __at the motor-generator set level, and by the 1970's

50_ . it should be comparable in cost to the ignitron in-

verter. Beyond this point the semiconductor in-40 verter should continue to drop and be substantially

0 100 1000 more economical than both motor-generator setsPOWER RATING-KVAand ignitron inverters for high power use.

Fi.17-Conversion efficiency vs power rating. For many low and medium power applications,however, the semiconductor power inverter is al-ready the most economical; for other applications,

speaking the rotating machine must be arranged in it will soon have lower costs than conventionala very specific manner. since mechanical coupling methods.must be provided between the various components.For static power converters, since they are elec-trically connected, separation of the various sub- CONCLUSIONSsystems can be accomplished easily, and packagingand deployment of the equipment can be accom- The new static inverters are now being used forplished to match more closely the contours of any a large number of applications by the military. Inassigned space. Because of this the space utiliz- missiles, submarines, and emergency power sys-ation for this apparatus is improved considerably. tems, the low volume, weight, and noise, and the

Another important item in the comparison of improved reliability under adverse conditions havethe new type of inverter with the more convention- established the value of the semiconductor invert-al unit is that of conversion efficiency. In Fig. 17 er. Many more military applications are now be-the motor-generator set at both 60 and 400 cycles ing explored and suitable units will soon be in de-is shown to have an efficiency that ranges between velopment.70 and 87 per cent, but the static inverter is ex- Commercial and industrial use of the newpected to have an efficiency of 90 per cent under inverters are just beginning to evolve; these appli-the same circumstances. Since the static inverter cations are expected to grow rapidly. One of theworks with equal efficiency at 60 or 400 cycles per more important is expected to come as an out-second, only one efficiency curve is necessary. growth of inverter engineering effort-the con-

In terms of the internal losses experienced in version of alternating current from one frequencythe conversion apparatus, it can be seen that the to another. This gives a new variable to the engi-losses are reduced by 30 to 50 per cent in the in- neer in his design of electrical equipment. Withverter compared to the motor-generator set. For frequency change, high-frequency lighting, newthe future, the inverter is expected to continue to motor designs, new ac power supplies, lower costimprove in efficiency as the voltage ratings of the ultrasonic generators, and new concepts in speeddevices are increased. It is anticipated that in the control for mill operation must be considered asnext few years the basic efficiency of the inverters possibilities.will be improved to about 95 per cent. Under A continuing objective is to increase both thethese circumstances considerably less than half power handling capability and the application areasthe internal losses that are currently experienced for semiconductor static inverters. For powerin the motor-generator set will be incurred in the from unconventional generators, for emergencystatic inverter. power, for small power systems, for power where

Of major importance to the successful appli- noise, weight, and volume must be kept to a mini-cation of the semiconductor switch is the cost mum, for low maintenance and for high reliability,picture. Costs for these new semiconductors have the semiconductor static inverter provides signifi-experienced a dramatic reduction in the past few cant advantages. Even more significant, however,years; during 1959, for example, the cost de- is the rapid rate at which static inverters are beingcreased 75 per cent. In the future rapid reduction developed. Design engineers should give carefulof cost is expected to continue, consideration to their use in any future applications.