Control of high-frequency AC link electronic transformer

Transcription

1 Control of high-frequency AC link electronic trnsformer H. Krishnswmi nd V. Rmnrynn Astrct: An isolted high-frequency link AC/AC converter is termed n electronic trnsformer. The electronic trnsformer hs size nd cost dvntges over conventionl trnsformer ecuse of high-frequency opertion of the mgnetic core. Of the vrious topologies of electronic trnsformer, the high-frequency AC link electronic trnsformer chieves high-frequency AC power trnsformtion without DC link. The circuit uses the stndrd H-ridge, one on either side of the high-frequency trnsformer. A novel PWM scheme is proposed, which symmetriclly delys nd dvnces the phse of the left nd right legs of the front-side converter with respect to the outputside converter. The proposed scheme introduces freewheeling su-periods, which results in zero voltge switching in the output-side converter. The electronic trnsformer s n AC utomtic voltge regultor (AVR) offers distinct dvntges over conventionl servo voltge stiliser in terms of size nd speed of response. The AVR ppliction is discussed nd experimentl results of 5 VA AVR re presented. A four-qudrnt switch presents difficulties in turning off inductive lod current ecuse of the sence of freewheeling pth in the switch. A ising circuit is proposed to convert the potentilly lossy switching trnsition into lossless trnsition. Simultion nd experimentl results with the ising circuit re presented. Introduction Line frequency trnsformers (5 nd 6 Hz) re hevy nd ulky items in power conversion nd distriution systems. Trnsformer size is inversely proportionl to the frequency of opertion nd sturtion flux density. Hence reduction in volume nd weight cn e otined y high-frequency opertion of the mgnetic core. Introduction of highfrequency link to relise smll-size electronic trnsformer hs een widely discussed in the literture [ 4]. Low cost nd esy vilility of ferrite core mteril hs helped in the implementtion of high-frequency link power trnsformtion. The electronic trnsformer utilises power electronic converters long with high-frequency trnsformer to otin overll size nd cost dvntges over conventionl trnsformer. Severl topologies of electronic trnsformer hve een discussed in [4]. One of the topologies of the electronic trnsformer is the high-frequency AC link AC/ AC converter first proposed in []. It hs two cycloconverters with high-frequency AC link in etween, s shown in Fig.. H-ridge nd push pull circuits hve een proposed in [] nd [3], respectively. It my e seen tht the literture hs extended the ppliction of high-frequency link AC trnsformtion not only for low-level electronic power Pper first received 9th My nd in finl form 9th August 4. Originlly pulished online: 8th April 5 H. Krishnswmi ws with the Deprtment of Electricl Engineering, Indin Institute of Science, Indi, nd is now with the GE Helthcre Technologies, 584 E-Block, AECS Lyout, Kundlhlli, Bnglore, Krntk 5637, Indi V. Rmnrynn is with the Deprtment of Electricl Engineering, Indin Institute of Science, Bnglore, Indi Fig. AC in conversion ut lso for medium-level distriution power trnsformers [, 4]. At such high powers the uthors hve proposed only voltge trnsformtion nd isoltion. The mjor pyoff expected is size reduction. The dvntges extend to more innovtive control possiilities s well. Principle of opertion The principle of opertion of the high-frequency AC link electronic trnsformer is sed on mplitude modultion. Consider low-frequency sine wve (5 Hz) v in ðtþ eing modulted y high-frequency squre wve. These two re defined s: v in ðtþ ¼ sinðw i tþ H M ðtþ ¼ t = ¼ p ðþ = t w s The resultnt voltge fter modultion cn e otined y multiplying v in ðtþ nd H M (t), the Fourier series of which is given y, v HF ðtþ¼ p cycloconverter/cycloconverter X n¼ HF link Electronic trnsformer using HF AC link ðn Þ ½cosððn Þw s w i tþ cosððn Þw s þ w i tþš AC out ðþ

2 voltge voltge time, s Evluting the Fourier series of the output voltge, the fundmentl output voltge V fund s function of is otined s: V fund ¼ 4 when ¼ ; V fund ¼ ¼ =; V fund ¼ ¼ =; V fund ¼ ð4þ This pper presents the stndrd H-ridge dpted to AC/ AC power conversion. The circuit is illustrted in Fig. 3. When the ove-mentioned control is pplied i.e., H M (t) controlling converter nd H D (t) controlling converter, the unfiltered output voltge otined is s shown in Fig. 4 long with the frequency spectrum. In Fig. 4, it is oserved tht with the ove scheme the output voltge wveform is ipolr. Also since the trnsformer voltge does not hve ny zero voltge instnts, i.e. no freewheeling su-periods, some of the switching trnsitions in the converters re hrd switching trnsitions. In this pper, n lterntive control method is proposed wherein some of the switches chieve soft trnsitions time, s converter converter S highfrequency 5 V pri Vo voltge trnsformer 5 S time, s c Fig. 3 Topology of high-frequency AC link electronic trnsformer mgntitude of hrmonic voltge frequency in Hz Fig. Wveforms illustrting derivtion of v HF ðtþ v in (t) H M (t) cv HF (t) d Fourier spectrum of v HF (t) Eqution () proves tht the lowest hrmonic strts t w s w i. Hence this voltge cn e trnsformed with high-frequency trnsformer. The modulted wveform is showninfig.. To control the AC output voltge [] hs proposed the demodulting function H D (t) s delyed signl of H M (t), which is defined s: H D ðtþ ¼ t þ T s þ t T ð3þ s þ d 3 Proposed symmetric modultion scheme In this pper, symmetric modultion scheme is proposed wherein H M (t) ndh D (t) re defined s: 8 9 < t = = H M ðtþ¼ = þ t T ð5þ : ; elsewhere t T H D ðtþ¼ s = = t ð6þ H M (t) controls the switches in converter in Fig. 3. H D (t) controls the switches in converter. Evluting the Fourier series of the unfiltered output voltge, the fundmentl voltge V fund s function of is otined s: V fund ¼ when ¼ ; V fund ¼ ¼ =; V fund ¼ ¼ ; V fund ¼ ð7þ The nture of the function H M (t) implies tht converter is phse modulted. The control pulses for switches derived from H M (t) ndh D (t) reshowninfig.5.itis cler from (6) tht, when is vried from to, the output voltge vries from to through zero. Figure 6 shows the unfiltered output voltge wveforms for the

3 . V t, s 6. mg (V) f ( 3 ) Fig. 4 Unfiltered output voltge wveform for existing control scheme S α Fig time, s ( 3 ) c Control signls for switches derived from H M (t) nd H D (t) proposed control scheme. Note the difference in wveform in Figs. 4 nd 6. At t ¼ / in Fig. 5, switches in converter nd in converter chnge from to simultneously. If in Fig. 5 is symmetriclly distriuted on either side of /4 within the hlf cycle / then, t ny point of time only one trnsition tkes plce, i.e. either in converter or in converter. Erlier S ws phse shifted with respect to. After this modifiction, the phse vritions of nd S with respect to re positive nd negtive, respectively. Hence it is given the nme symmetric modultion. The resultnt pulses for the switches re shown in Fig. 7. This symmetric nture of the scheme proves dvntgeous during the switching process of converter. AC/AC converters employing four qudrnt switches hve the prolem of high-current nd high-voltge surges during commuttion. This decreses the efficiency of the converter s well s the reliility of the system. These fcts re gret rrier to mking this converter prcticle. In this proposed control scheme the trnsformer voltge wveform hs zero voltge sttes. Owing to the symmetric nture of the scheme ll the switches in converter re turned on t this instnt. Hence zero voltge switching (ZVS) is chieved in converter. A method to reduce switching losses in converter is discussed in Section 5 of this pper. Also the control scheme nturlly leds to unipolr output voltge, which hs its dvntges known in the literture. Two methods of relistion of this PWM scheme re proposed in this pper. The first method uses tringulr wve nd cn give only n in-phse output voltge, i.e. cn vry from to / only. The second method uses

4 . V t, s 6. mg (V) f ( 3 ) Fig. 6 Unfiltered output wveform for proposed control scheme α/ S α/ Fig time, s ( 3 ) c Control signls for switches to illustrte symmetric modultion doule rmp crrier wve to give oth in-phse nd out-ofphse output voltge. Figs. 8 nd illustrte the two methods. In Fig. 8 V R is the crrier voltge nd V C is the control voltge. V R hs two identicl rmps shifted y 8. This method does not introduce discontinuity t the zero output voltge point, i.e. when ¼ /, hence it is useful in closed-loop control. The second method finds its ppliction in control of the utomtic voltge regultor (AVR), which is explined in the following Section. 4 Appliction s n utomtic voltge regultor Power-line disturnces such s undervoltge, overvoltge, voltge sgs nd swells in sensitive equipment such s computers, communiction services etc., cn often led to loss of vlule dt nd interruption of communiction services. Trditionlly servo voltge stilisers fill this need. They use vrile utotrnsformer nd n isoltion trnsformer to inject compensting voltge in series with the utility power. Slow dynmic response nd ulky line frequency trnsformer re the min disdvntges of such system. Vrious methods of providing vrile AC hve een discussed in [5, 6]. But the line frequency mgnetics still form prt of the system. An electronic trnsformer cn replce the line frequency utotrnsformer nd isoltion trnsformer y single high-frequency trnsformer. The size of the system reduces drsticlly s result. Such n AVR is shown in Fig. 9. The output voltge of the AVR cn e derived s: V o ¼ð n þ DnÞ where D ¼ ð8þ

5 V T V c V c, S, S α/ α/ trnsitions re lredy ZVS, s explined in Section 3. Consider the opertion of single leg in converter, s shown in Fig.. A cpcitive snuer is dded cross ech switch to prevent the interruption of lod current. This cpcitnce my e the device cpcitnce or n externl cpcitnce dded cross the switch. To prevent the source getting short-circuited, ded time is given etween two switches in one leg. Figure explins the working of the single-leg circuit during the ded time with the trnsition from to tken for exmple. Both positive nd negtive input voltges re considered with the current in thesmedirection. V R C C V c V c, I I α/ S, S C C Fig. 8 Control scheme relistions Tringulr crrier Doule rmp crrier V Fig. Switching trnsition from to during dely time showing direction of current nd chrging of cpcitors C nd C Lossy trnsition Lossless trnsition 7V 7V Fig. 9 S where V o is the regulted output voltge of the AVR nd n is the trnsformer turns rtio. Thus when 5 Bising circuit converter converter S highfrequency V pri trnsformer D ¼ ; V o ¼ð nþ ðuckþ D ¼ :5; V o ¼ D ¼ ; V o ¼ð þ nþ ðoostþ ð9þ In this Section ising circuit is proposed to reduce the switching losses of converter. Converter switching L C filter V o 3V AVR using high-frequency AC link electronic trnsformer Consider initilly tht is on nd the lod current is flowing through, s shown in Fig.. When is turned off, the inductive lod current splits etween the two cpcitors, C nd C. The lod current is not in the proper direction to id the chrge trnsfer from C nd C. Insted, the cpcitnce cross switch gets chrged continuously to vlue of higher thn the us voltge. Both C nd C re chrged in the opposite direction, s in Fig.. When is now turned on, the stored energy gets dissipted in the device. This results in high current spikes. This is lossy trnsition. Consider the opertion in Fig.. When is turned off, the reflected lod current ids in the energy trnsfer etween the output cpcitnce of the two complementry switches, nd. When C is dischrged completely to zero, cn e turned on. This is lossless trnsition. A method to prevent overvoltges hs een proposed in [8], which controls ech two-qudrnt switch in four-qudrnt switch element. In this pper novel ising circuit is proposed to convert the potentilly lossy trnsitions into lossless trnsitions. Turn-on nd turn-off losses re the two min constituents of switching losses. In ZVS converters, the device voltge is rought to zero just prior to turn-on. During turn-off therteofvoltgeriseislimited,sothtthedevicecurrent flls to zero efore the voltge rises sustntilly. In the electronic trnsformer circuit, the turn-off losses re negligile s cpcitive snuer is dded cross the switches. But during turn-on there exists voltge cross

6 the MOSFET nd the turn-on losses re determined y the energy stored in the cpcitnce cross the MOSFETS (intrinsic nd extrinsic). An nlysis of converter in Fig. ws performed to identify the lossy nd lossless switching trnsitions (turn-on) in mnner similr to tht shown in Fig.. Tle presents the results of the qudrnt-yqudrnt nlysis of switching trnsitions in converter. The qudrnts re defined sed on the fundmentl lod voltge nd lod current. Out of the 6 switching trnsitions in converter eight re lossy nd eight re lossless. It is oserved tht lossy nd lossless trnsition depends on the qudrnt of opertion. Reference [9] hs suggested n externl ising circuit to reduce the lod dependence of the ZVS chrcteristics in phse-modulted DC/DC converter. This technique of ising pproprite legs with suitle currents to ensure ZVS is pplied to the electronic trnsformer in this pper. Tle : Summry of switching trnsient nlysis Switch Qudrnt Qudrnt Qudrnt 3 Qudrnt 4, lossy lossless lossy lossless S, S lossless lossy lossless lossy The sic principle of opertion of the ising circuit is tht t the instnt of switching trnsition the current is mde lgging. The concept is illustrted in Fig.. An inductor is used cross the midpoint of the split cpcitor V I inj I o C C I inj L I I off I L C C on C3 o V Fig. Bising circuit Opertion of ising circuit explined with turn-on I inj I C4 L I I S off S S C S C S on (point o ) nd the centre point of one leg. This ising circuit injects high-frequency current into the system to overcome the effect of lod current during trnsitions. The injected current is designed to e twice the vlue of pek of the reflected lod current. The opertion of the ising circuit is explined considering the switching trnsitions from to. Before is turned off, it is crrying the sum of the reflected lod current (I L ) nd the injected current (I inj ) nd the resultnt switch current (I ) is positive (direction of I shown in Fig. 3). Note tht the sme current I is negtive in lossy trnsition. During the ded time, the injected current divides eqully etween the two cpcitors C nd C.SinceI is positive the cpcitor C gets dischrged nd the switch is turned on when the voltge cross it is zero. In this wy ZVS is chieved in in qudrnt. A similr nlysis is crried out for ll switches in converter nd it is found tht the ising circuit ensures ZVS for switches nd S in ll four qudrnts. 6 Experimentl results A 5 VA prototype hs een fricted to demonstrte the suitility of the electronic trnsformer s n AVR nd to demonstrte the dvntges of the proposed control scheme. The input voltge rnge is 7 7 V. The output voltge is regulted to 3 V nd with 5 W lod the lod current is.7 A. The switching frequency of the power devices is fixed t khz. The voltge is dded or sutrcted in series with the mins (series voltge regultor) nd hence the high-frequency trnsformer hndles only certin frction of the output power. The mximum voltge dded is 6 V nd hence the high-frequency trnsformer VA rting is 3 V.7 A, which is equl to 3 VA. In the developed AVR the trnsformer is designed to hndle mximum power of 75 VA. The core re of the trnsformer ws 35 mm, which is lesser thn conventionl line frequency trnsformer hndling the sme mount of power. The power MOSFETS used were IRF84, which hs n on-stte resistnce of.85 O. A mjor prt in the control circuit is the genertion of doule rmp. Digitl circuitry with counters nd comprtors hs een used to generte the rmps. The MOSFETS were driven through pulse trnsformers to provide isoltion etween the drive nd the power. A PI controller is used in the feedck circuit to regulte the output voltge. The output voltge styed firly constnt in the entire lod ( 5 W) nd line (7 7 V) rnge. The overll efficiency of the system with 7 put with output regulted to 3 V is 95% t full lod. In Section 4, ZVS in converter ws discussed. Figure shows the turn-on nd turn-off chrcteristics of switch in converter. It is cler from the Figures tht ZVS occurs in converter. In Fig. the voltge tht swings etween positive nd negtive is the gte voltge. The conduction losses in converter, which mount to 4%, re minly due to the high R DS(on) of the MOSFETS used. Switching losses re zero in the output side converter. In converter since resistive lod is connected t the output, the qudrnt of opertion is lwys nd 4. Hence the ising circuit needs to e connected only for the right leg of converter. Simultion results of switch in qudrnt re shown in Fig. 3 with nd without ising circuit nd the Figure shows cler reduction of voltge spikes in the wveforms. Figure 4 shows the experimentl results of S turn-on. It is seen from the Figure tht the drin voltge decreses to lower vlue efore turn-on ut does not rech zero voltge. The switching losses re thus

7 V/ 35.V/.s.s/ Stop 44V gte voltge drin voltge Fig. Oserved gte nd drin voltge wveform of switch in converter (note ZVS).. voltge cross switch (F s = khz) reduced nd the voltge cnnot e mde zero owing to the sence of freewheeling diode cross the four-qudrnt switch. This is still dvntgeous ecuse in lossy trnsition the voltge cross the switch goes higher thn the us voltge. The conduction nd switching losses re clculted for n input voltge of 7 V nd n output voltge of 3 V, with the AVR delivering 5 W output. The estimted conduction loss in converter ws W, forming the mjor loss component, nd the switching losses were negligile. The totl mesured losses for oth the converters were 5 W. The experimentl wveforms of the AVR for the operting point ¼ 7 V, V o ¼ 3 V nd P o ¼ 48 W re shown in Figs. 5 nd 6. Figure 5 shows the oserved primry voltge nd current wveform. The primry voltge envelope is not shown for the ske of clrity. Figure 6 shows the oserved lod voltge nd lod current. The wveforms re recorded when the sine voltge is t its mximum nd they re shown only for two switching cycles. All the wveforms will hve 5 Hz envelope, which is not shown in the grphs. 7 Conclusions V t, s voltge cross switch with ising circuit (F s = khz) The electronic trnsformer hs een introduced nd its principle of opertion explined. A symmetric modultion scheme for control of electronic trnsformer is proposed. The dvntges of this scheme, such s unipolr output voltge wveform nd ZVS in the output-side converter, re presented. An ppliction of the electronic trnsformer s n AVR is explined in detil. PWM genertion methods for the control scheme re presented. Experimentl results of 5 VA AVR re discussed. This pper explins only one ppliction ut the electronic trnsformer cn lso find its use in uninterruptile power supplies wherein the isoltion trnsformer t the output side cn e replced y high-frequency trnsformer. Finlly novel ising. V/.s. u s/ Stop 8V V. T. Fig. 3 circuit.5.5 t, s Simultion results of turn-on with nd without ising 4 m v/ u.s. s/ Stop m v V/ 35.V/ 6. u s n s/ Stop 8V T T drin voltge gte voltge 4 Fig. 4 turn-on Oserved gte nd drin voltge wveform of switch S Fig. 5 Oserved trnsformer primry voltge wveform Oserved trnsformer primry current wveform

8 V/.s. m s/ Stop 5.V circuit is proposed to convert ll lossy trnsitions in converter to lossless trnsitions nd the results re lso presented. 8 References T 4 T 4 m v/ m.s. s / Stop 4.8 m v McMurry, W.: Power converter circuits hving high-frequency ink, US Ptent 3573, 3 June 97 Kng, M., Enjeti, P.N., nd Pitel, I.J.: Anlysis nd design of electronic trnsformers for electric power distriution system, IEEE Trns. Power Electron., 999, 4, (6), pp Hrd, K., Skmoto, H., nd Shoym, M.: Phse-controlled DC- AC converter with high-frequency switching, IEEE Trns. Power Electron., 988, 3, pp Hienemnn, L., nd Muthe, G.: The universl power electronics sed distriution trnsformer n unified pproch. IEEE PESC, Conf. Rec. 5 Kwon, B.H., Youm, J.H., nd Choi, J.H.: Automtic voltge regultor with fst dynmic speed, IEE Proc. Electr. Power Appl., 999, 46,(), pp. 7 6 Hietps, S.M., nd Nden, M.: Automtic voltge regultor using n AC voltge-voltge converter, IEEE Trns. Ind. Appl.,, 36, pp Hrihrn, K.: High frequency AC link electronic trnsformer. MSc Thesis, Deprtment of Electricl Engineering, Indin Institute of Science, April 8 Enjeti, P.N., nd Choi, S.: An pproch to relize higher power AC controller. Proc. IEEE APEC Conf., Mrch 993, pp Rjpndin, A., nd Rmnrynn, V.: A Constnt Frequency Resonnt Trnsition Converter, J. Indin Inst. Sci., 996, pp Fig. 6 Oserved output voltge of AVR V ¼ 3 V Oserved lod current of AVR I L ¼.7 A Scle: A/div