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Novel comB.-H. Kw on6.-D.MinJ.-H.Kim

Indexing terms:AC-AC conver

Abstract:The AC-Pconverter is very 5powerful controllabhave been few practpower electronics fieis the commutationa novel commutatisimple to implemenallows the dead tirrnonideal switches ana current path ofvoltage spikes. Expe5kW prototype are

1 IntroductionSince the matrix conve[l], it has receivedbecause of the followin1. A large capacity an(be designed because thilink circuit and, as a rstorage component sucsmoothing capacitor.2. The system has higlof devices connected inin the conventional rec3 . Four-quadrant openby controlling switchiioutput voltages and inThus the matrix conveprovide direct ACIACage elements except fielimination of switchirbeen few practical arelectronics fields. Onemutation problem. Foiquadrant switches, a IThas been given in [ 5 ] . 1complex to implementof the load current diiload current directionQ IEE, 1998ZEEProceedings online no. 195Paper firs t received 26th AuguiThe authors are with the IEngineering, Pohang UniversitSan 31 Hyoja Dong, Pohang, 'IE E Proc-Electr. Power Appl , , V

nutation technique of AC-AC converters

Ma trix converter, Commutation problem

converter called the matrix.ple in structure and hasy. However, to date there1 applications, especially in;. One of the major reasons2blem. The paper proposestechnique which is veryThis commutation schemeto avoid current spikes ofit the same time establishes; inductive load to avoidiental results obtained on acussed.

r was originally introduced ininsiderable attention [2-131,tdvantages::ompact converter system canystem does not have any DC-ilt, does not need any energy3s a smoothing inductor or afficiency, because the numberries is less in this system thaner-inverter system.In is very easy. What is more,patterns appropriately, botht currents become sinusoidal.c has its major potentiality tonversion without energy stor-a small AC input filter forripples. However, there havecations, especially in powerthe major reasons is the com-afe commutation of the four-tistepped switching procedurewever, the switching policy isi also requires exact detectiontion. Inexact detection of theiblishes a short circuit during$69nd in revised form 12th December 1997artment of Electronic and Electrical. cience and Technology (POSTECH),,784 Republic of Korea145, No . 4, uly 1998

the commutation process [5]. Since the available cur-rent sensors have hysteresis characteristic of the core,the exact detection of the load current is very difficult.This paper proposes a novel commutation techniquewhich is very simple to implement. The commutationscheme allows the dead time to avoid current spikes ofswitches and at the same time establishes a currentpath of the inductive load to avoid voltage spikes. Theswitching policy is described for AC choppers andunlimited frequency changers, respectively. The pro-posed AC choppers use regenerative DC snubbers

attached directly to power semiconductor modules toabsorb energy stored in line stray inductance. TheseDC snubbers enhancing the conversion efficiency fea-ture a very simple structure consisting of a capacitoronly with no need for discharge resistance or for acomplicated regenerative circuit for snubber energy.Experimental results obtained on a 5kW prototype arediscussed.2problemSwitching policy solving the commutation

2. I Commutation problemThe switches of the matrix converter shown in Fig. 1are four-quadrant switches. Four-quadrant switches areable to block voltage of both polarities and may con-duct current of both polarities. As four-quadrantswitches are not available to date, they can be imple-mented by connection of two diodes and two switches.Four-quadrant switches have the commutation prob-lem. To understand this problem, a single-phase AC-AC converter called a single-phase AC chopper withtwo four-quadrant switches shown in Fig. 2 is consid-ered. Suppose that a four-quadrant switch SI is turnedon and conducts the load current iL. After a time, wewish to commutate the current iL to the switch S,.%a-+

inductiveload

LdFig.1 Three-phase to three-phase converter with an AC inputJilter295

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Fig.2 Single-phas e A C chopper with an inductive loadUnfortunately it is impossible to define the timingwhich will lead to safe commutation of these switches.Theoretically the switching must be instantaneous andsimultaneous. For practical realisations we have to takeinto account finite switching times and delays in thedrive circuits and switches. So, if the switch S2 isturned on before S1 s turned off, a short-circuit path isestablished through e,-SI-S2. Current spikes generatedthis way will destroy the switches. Similarly, if SI isturned off before S2 is turned on, a dual situation willoccur: there is no path for the conduction of the cur-rent of the inductive load. Voltage spikes are inducedon the opened switches and will destroy the switches.

The basic elements for construction of four-quadrantswitches are two-quadrant switches which are commer-cially available. To construct a fully four-quadrantswitch, two-quadrant switches are connected anti-parallel or antiseries as shown in Figs. 3a and c. Volt-age two-quadrant switches are able to block voltage ofboth polarities and may conduct current only in thegiven reference direction of i,, depending on the gatecontrol signal. On the other hand, current two-quad-rant switches are able to conduct current of both polar-ities and may block voltage only in one direction.Four-quadrant switches can be constructed connectingtwo voltage two-quadrant switches antiparallel or twocurrent two-quadrant switches antiseries and then areable to block voltage of both polarities and conductcurrent of both polarities. Therefore, an equivalentsymbol of a four-quadrant switch can be expressed asshown in Figs. 36 and d.

Gf

Gra b

C dFig.a Using two voltage two-quadrant switchb Equivalent symbolc Using two cuirent two-quadrant switchd Equivalent symbol

Constructions and symbol of four-quadrant switch

2.2 Single-phase AC chopperThe switching policy solving the commutation problemfor a single-phase AC-AC chopper with an inductiveload will be introduced using Fig. 4a. In this system,voltage control of the output is possible but frequency296

control is not possible. Two four-quadrant switches areconnected alternatively to the inductive load accordingto the duty ratio D determined by the control strategy.Sl y denotes the forward directional switch in the cur-rent direction side from the input voltage to the outputand SI, the reverse directional switch. Suppose thattwo-quadrant switches Slyand S1,are turned on. Theload current flows through one of these switches,depending on its direction. If we attempt to turn off SI(or S l f and SI,)and turn on S2 (or S2sand S,,) simulta-neously, the commutation problem described willoccur. The dead time is requisite to avoid currentspikes of practical nonideal switches and at the sametime a current path of the inductive load has to be pro-vided to avoid voltage spikes. For this, a switching pol-icy solving the commutation problem is based on thepolarity of the switch-to-switch voltage e, across twobidirectional switches; two unidirectional switches S1,and S2f are additionally turned on during the positiveperiod of e, and the switches Slf and S2, during thenegative period of e, without respect to the controlstrategy determining the duty ratio. Then the inductiveload is bypassed through the input side or output sidedepending on its direction during the dead time.

a

e,>O

b c

e, 0 and zL > 0c For e, 9 0 and zL c 0dFor e, < 0 and zL > 0e Fo r e, c: 0 and zL < 0

Current paths during dead tune or single-phase AC chopper

During the powering mode connecting the input andoutput, the switch S,, is turned off during the positiveperiod of e, but S2,during the negative period of e,. Allthe other switches are turned on. As a result, the inputvoltage is connected to the inductive load, providingenergy. The freewheeling mode giving zero output volt-age is complementary to the powering mode. Duringthis mode, the switch Slf is turned off during the posi-tive period of e, but SI, during the negative period ofe,. All the other switches are turned on so that the out-put current may be freewheeled. Figs. 46-e show possi-ble current paths during the dead time. Thus, a currentpath of the inductive load always exists every currentdirection and the two switch pairs (Sly, 2f)and (SI+S2,) are turned on alternately every half-period of e,. Itis noted that these switch pairs also have the switchingat zero voltage of e,.

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2.3 Modified sing/( AC chopper

line strayinductance-

Therefore, the snubtierswitches must be anabsorb a bidirectionalstray inductance. Further,switch is turned on orfor regenerating thevided for enhancing t1.eequipment. However,regenerating the snubberadditional regenerativeIEE Proc -Electr Power A p p l , Vcl

" I

circuit of four-quadrantAC snubber circuit that canturn-off spike energy due to linesince each four-quadrantoff at high frequency, a circuitsnubber energy needs to bo pro-conversion efficiency of thethe circuit shown in Fig. 5c isenergy without using anycircuit. In the configuration of145, No 4, July 1998

--a[bower semiconductor module

inductiveload

bers

Fig. 5c , the commutation policy is that two unidirec-tional switches SI, and S2r in one leg are additionallyturned on during the positive period of e, and theswitches Slf and S,, in another leg during the negativeperiod of e , without respect to the control strategydetermining the duty ratio. The other switches exceptswitches for safe commutation are modulated accord-ing to the duty ratio.2.4 Three-phase to single-phase AC-ACconverterThe commutation policy can be extended to three-phase to single-phase AC choppers using regenerativeDC snubbers. Let us define emlnand emaxas follows:

emaz = ma+,, e,, e,]emzn = min[e,, e, , e,]

Thus emax selects the highest phase voltage and eminselects the lowest phase voltage among the input volt-ages U, and w.

Lia

A three-phase to single-phase converter with aninductive load is shown in Fig. 6u . In this system, bothvoltage and frequency of the output can be controlledsimultaneously. Three four-quadrant switches are con-necting alternatively the input voltage sources e,, e, ande , to the inductive load. The load current flowsthrough one of these three bidirectional switches,turn off S, and turn on S , simultaneously, the commu-tation problem described will occur. The dead time isrequisite to avoid current spikes of nonideal switchesand at the same time a current path of the inductive

depending on its direction. Similarly, if we attempt to

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load has to be provided. For this, a switching policy isbased on the magnitude of the switch-to-switch volt-ages euv,ev wand ewu across two bidirectional switches;two unidirectional switches of the reverse switch corre-sponding to the highest voltage emax and a forwardswitch corresponding to the lowest voltage eminamongthe switch-to-switch voltages are additionally turned onfor safe commutation without respect to the controlstrategy, as shown in Fig. 6b. This concept can also beextended to multiphase to multiphase unlimited fre-quency changers. Turn-on and turn-off of bidirectionalswitches are made according to the control signal.Therefore during the dead time the unidirectionalswitches for commutation, i.e. the reverse switch corre-sponding to the highest voltage emax and forwardswitch corresponding to the lowest voltage emin,remainto be turned on. Then the inductor current is bypassedthrough the input side or output side depending on itsdirection during the dead time. Figs. 7 a and b showpossible current paths during the dead time for the timeinterval I. Thus, a current path of the inductive loadalways exists every current direction during the deadtime. Additional turn-on of the unidirectional switchesfor commutation of the load current does not give anyhazard such as current spikes.

e"-@ + tgia

bThree-phase to single-phase converter with an inductive loadig .7

a Current path for iL < 0 during time interval Ib Current path for iL > 0 during time interval I

2.5 Three-phase to three-phase matrixconverterA three-phase to three-phase matrix converter basicallyconsists of nine bidirectional switches arranged in amatrix so that any input phase can be connected to anyoutput phase at any time as shown in Fig. 8. The previ-ous commutation concept is applied to the three-phaseto three-phase matrix converter. For safe commutation298

of three inductive load currents, three reverse switchescorresponding to the highest voltage emaxand three for-ward switches corresponding to the lowest voltage eminamong the switch-to-switch voltages across two bidirec-tional switches as shown in Fig. 9 are additionallyturned on without respect to the control strategy.According to this switching policy solving the commu-tation problem, the six unidirectional switches withbold arrows in Fig. 8 are shown to be turned on duringthe time interval I .

inductionmotor

Fip .8 , Power circuit of three-phase to three-phase converter with sym-bo I C signals

j Sua, j Svar j Swar ji Sbr %br i Swbrj su,, j svcr j swcr j

SVCf ; Swcf j Sucf ; Svcfr - - - - - - - - - 7 - - - - - _ _ -I-- - - -- - - -_-_ . -___

Svaf I Swaf I Suaf I %af'vbf j 'wbf 'ubf 'vbfFig.9phase converterTurn-on switches for safe commutation for three-phase to three-

3 Experimental resultsTo show the validity of the proposed scheme, the 5kWAC chopper shown in Fig. 5c is implemented with thefollowing parameters:

Load inductance L = 0.2mHLoad resistance R = 2 QRegenerative DC snubber CL = 1CLFLine frequency f = 60HzInput peak voltage V,,= 170VOutput peak voltage V,, = 170VDuty ratio D = 0.8.

A test model of the converter is built using 600V, 1OOAinsulated gate bipolar transistors (IGBTs) with the car-rier frequency 15.4kHz. To obtain the information ofthe input voltage, only one operational amplifier isadditionally used in control circuits. So the implemen-tation of the switching policy is possible with a smallIEE Proc -Elect? Power A p p l , Vo l 145, N o 4, July 1998

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Fig.10

a

b

C

d

e

a

Picture o whole set-&

t

Voltare and current have orms o f switches at switchinaig. 11a Gate-source voltage of SI ( 2 .51b Drain-source voltage of dz, 50'Time scale: lOp &c i, (50Ndiv)d iz (50Aidiv)e Inductor current iL (50Aidiv)

I l l I

I I t I

I I t I

Fig.12 Gate-source voltages of switchesa Gate-source voltage of S , (SVi,1v)Time scale: 2 msidiv

increase of system costs. Fig. 10 shows a picture of thewhole set-up. Fig. 11 shows the case when the switch-ing policy is implemented and the dead time of 2 p sused. As expected, not only the voltage but also thecurrent waveform is without spikes. The current com-mutation from switch SIf o S2, occurs during the deadtime. Fig. 12 shows gate-source voltages of the switchesduring one period of the input voltage. Each switch isadditionally turned on during a half-period of the linevoltage for safe commutation. Negative gate-sourcevoltage (-5V) is provided for faster turn-off of theIGBTs. Fig. 13 shows the drain-source voltage of Slfioutput voltage and output current. Owing to propercommutation, the voltage stress of the switch is limitedto the peak line voltage.

I + I

I." I I

I IFia. 13 Waveforms of switch and outautTiGe scale: 2msidiv "a Drain-source voltage of S,, (SOVidiv)b Output voltage (SOVidiv)c Output current (5OAidiv)4 ConclusionThis paper has proposed a novel commutation tech-nique which is very simple to implement. The switchingpolicy is based on the magnitude of the switch-to-switch voltages across two bidirectional switchesrelated to one inductive load: two unidirectionalswitches of the reverse switch corresponding to thehighest voltage and a forward switch corresponding tothe lowest voltage among the switch-to-switch voltagesare additionally turned on for safe commutation with-out respect to the control strategy. This commutationscheme also allows the dead time to avoid currentspikes of nonideal switches and at the same time estab-lishes a current path of the inductive load to avoidvoltage spikes. The switching policy has been describedfor AC choppers and unlimited frequency changers,respectively. The proposed AC choppers use regenera-tive DC snubbers attached directly to power semicon-ductor modules to absorb energy stored in line strayinductance. These DC snubbers enhancing the conver-sion efficiency feature a very simple structure consistingof a capacitor only with no need for discharge resist-ance or for a complicated regenerative circuit for snub-ber energy. Experimental results obtained on a 5kWprototype have been shown.

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3 ZIOGAS, P.D., KHAN, S.I., and RASHID, M.H.: Analysis anddesign of forced commutated cycloconverter structures withimproved transfer characteristics, ZEEE Trans. Znd. Electron.,4 ALESINA, A.,, and VENTURINI? M.: Analysis and design ofoptimum-amplitude nine-switch direct ac-ac converters, ZEEETrans. Power Electron., 1989, 4, (l ), pp. 101-1125 BURANY, N.: Safe control of four-quadrant switches. Conf,Rec. IEEE IAS, 1989, pp. 1190-11946 OYAMA, J., HIGUCHI. T., YAMADA, E., KOGA, T., andLIPO, T.A.: New control strategy for matrix converter. Conf.Rec. IEEE PESC, 1989, pp. 360-3677 KIM, Y. , and EHSANI, M.: Contro l of forced-commutateddirect frequency changers. Conf. Rec. IEEE IAS, 1990, pp. 1163-

11708 ROY, G., and APRIL, G.E.: Direct frequency changer operatingunder a new scalar control algorithm, IEEE Trans. Power Elec-tron., 1991, 6, (l), pp. 100-107

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9 HOIMES, D.G., and LIPO, T.A.: Implementation of a control-led rectifier using ac-ac matrix converter theory, IEEE Trans.Power Electron., 1992, 7, (l),pp. 240-24910 NEFT, C.L., and SCHAUDE, C.D.: Theory and design of a 30-HP matrix converter, ZEEE Trans. Ind. Appl., 1992, 28, (3), pp .11 TENTI, P., MALESANI, L., and ROSSETTO, L.: Optimumcontrol of N-input K-output matrix converters, IEEE Trans.Power Electron., 1992, 7, (4), pp . 707-71312 KAZERANI, M., and 001, B.T.: Feasibility of both vector con-trol and displacement factor by voltage source type ac-ac matrixconverter, IEEE Trans. Ind. Electron., 1995,42, (5), pp. 524-53013 HUBE, L. , and BOROJEVIC, D.: Space vector modulatedthree-phase to three-phase matrix converter with input power fac-tor correction, ZEEE Trans. Znd. Appl., 1995, 31, (6), pp. 1234-1246

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