better place 50kw charger

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 60, NO. 12, DECEMBER 2013 5391

Battery Charger for Electric VehicleTraction Battery Switch Station

A. Kuperman, Member, IEEE, U. Levy, J. Goren, A. Zafransky, and A. Savernin

Abstract—This paper presents the functionality of a commer-cialized fast charger for a lithium-ion electric vehicle propulsionbattery. The device is intended to operate in a battery switchstation, allowing an up-to 1-h recharge of a 25-kWh depletedbattery, removed from a vehicle. The charger is designed as adual-stage-controlled ac/dc converter. The input stage consists ofa three-phase full-bridge diode rectifier combined with a reducedrating shunt active power filter. The input stage creates an un-controlled pulsating dc bus while complying with the grid codesby regulating the total harmonic distortion and power factoraccording to the predetermined permissible limits. The outputstage is formed by six interleaved groups of two parallel dc–dcconverters, fed by the uncontrolled dc bus and performing thebattery charging process. The charger is capable of operatingin any of the three typical charging modes: constant current,constant voltage, and constant power. Extended simulation andexperimental results are shown to demonstrate the functionalityof the device.

Index Terms—Battery charger, electric vehicle (EV), powerconverters, power quality.

I. INTRODUCTION

THE traction battery is undoubtedly the most critical com-ponent of an electric vehicle (EV), since the cost and

weight as well as the reliability and driving range of the vehicleare strongly influenced by the battery characteristics [1]. Mod-ern rechargeable lithium batteries, which are, by far, the mostpower or energy dense among modern batteries, are commonlyused in traction applications [2]. The high energy/power contentrequires appropriate battery management to ensure safety andoptimal performance. In particular, proper recharging is essen-tial in order to utilize the full capacity of the battery pack andpreserve its nominal lifetime [3]–[6].

There are two common types of vehicle battery chargers. Theonboard (often referred to as slow or low power) charger islocated on board. The propulsion battery is recharged via theslow charger, plugged into a charging spot, while the vehicleis at parking lot [7]–[14]. The offboard (so-called fast or highpower) charger is located at the battery switch station (BSS).The battery must be removed from the vehicle to be recharged

Manuscript received March 12, 2012; revised June 21, 2012 andNovember 3, 2012; accepted November 24, 2012. Date of publicationDecember 12, 2012; date of current version June 21, 2013.

A. Kuperman is with the Hybrid Energy Sources R&D Laboratory,Department of Electrical Engineering and Electronics, Ariel University Centerof Samaria, Ariel 40700, Israel (e-mail: [email protected]).

U. Levy, J. Goren, A. Zafransky, and A. Savernin are with GamatronicElectronic Industries Ltd., Jerusalem 97774, Israel (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIE.2012.2233695

via the fast charger (FC) [15], [16]. The slow charger usuallyoperates at 0.15–0.25-C rates, while the FC rate may typicallyreach 2-C rates, i.e., while charging a 25-kWh battery; the slowcharger supplies 3–4 kW, while the FC peak power is typically30–50 kW.

The typical concept of EV includes urban driving only, wherethe full battery charge is sufficient for medium-range routes of50–100 miles. Recharging is accomplished by plugging the carinto charge spots placed at different city locations throughoutthe day and at driver’s home during the night. Recently, aparadigm shift toward closing the gap between EV and con-ventional vehicles has occurred, forcing the infrastructure tosupport EV intercity driving as well. The following conceptof BSS was developed: When out of charge, the EV batterycan be replaced at a BSS, allowing nearly uninterrupted long-range driving. The replacement process takes 2–4 min, similarto the duration of conventional refueling process [17]. The near-empty battery, removed from a vehicle at the BSS, is rechargedby an FC to be available as quickly as possible for the nextcustomer. The charging time is obviously crucial, affecting thebattery stock. For example, assuming 4-min battery replace-ment time, 15 vehicles per hour may be processed by eachservice lane. If battery charging time is 1 h, the minimumstock of 15 batteries per lane should be present at the station.Reducing the charging time obviously reduces the stock as well.It is worth noting that, since there is no human involvementin the fast charging process, galvanic isolation is usually notrequired to be present in an FC.

The FC is basically a controlled ac/dc power supply, drawingthe power from the three-phase ac utility grid, converting it todc and injecting it into the traction battery [18]. In order tocreate a feasible solution, the FC must both satisfy the gridcode in terms of power factor (PF) and harmonic content fromthe utility side and support lithium-ion charging modes fromthe battery side. Since the BSS usually contains multiple FCs,its impact on the distribution grid is very significant, as shownby previous research [19]–[25]. Therefore, the input stage ofthe FC usually performs PF correction (PFC) according to theregulation requirements in addition to rectification. It can beaccomplished by employing either an active rectifier [26]–[28],or a diode rectifier combined with a PFC circuit [29]. The well-known single-phase PFC approach, utilizing an uncontrolledrectifier followed by a full-rating boost dc–dc converter [30],is unsuitable for the three-phase diode rectifier case. However,it can be modified by splitting the three-phase rectifier intoeither two single-phase legs followed by two independent PFCconverters [31] or three single-phase Δ- or Y-connected stages[32], [33]. Alternatively, a more elegant approach employs a

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5392 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 60, NO. 12, DECEMBER 2013

shunt-connected active power filter (APF) at the uncontrolledrectifier input, supplying the reactive current to the diode rec-tifier, thus achieving both near-unity PF and near-zero totalharmonic distortion (THD) by letting the utility to supply theactive current only, which is in phase with the utility voltageand of the same shape. The use of either one three-phase[34]–[37] or three single-phase [38]–[40] APF configurationsis potentially feasible for implementing a three-phase PFCstage. The additional advantage of the approach is the fact that,because of the shunt connection, the APF rating is less than one-third of the bridge rectifier rating, since the APF supplies thereactive and harmonic power only, while the series-connectedPFC converter rating is equal to the load kilovoltampere rating.The resulting loss reduction is an extremely desirable featurefor an FC since the dissipated heat must be removed from theBSS by means of a cumbersome ventilation system, whosecomplexity is proportional to the amount of the heat to beremoved.

From the battery side, a conventional lithium-ion batterycharging is characterized by two main phases: constant current(CC) and constant voltage (CV). Recently, constant power (CP)charging became popular in large vehicle battery packs [41].Hence, the charger output stage (typically consisting of dc–dcconverters) must be capable of operating as either a current orvoltage source. Alternatively, it can be operated as a voltagesupply with dynamic current limitation. CP mode is usuallyachieved by operating as a current source, constantly varyingaccording to the power profile. Moreover, the charger outputcurrent ripple should be kept as low as possible in order toprevent undesired influence on the battery chemistry. The well-known solution, allowing splitting the load power betweenmultiple modules in order to reduce both the conduction lossesand current ripple, is interleaving [42]–[45]. Interleaving em-ploys parallel operation of converters, whose output current isequally shifted with respect to others such that, when summed,the current ripples partially cancel each other, creating a lowripple total output current. In addition, interleaving also reducesthe implementation challenge of designing a single full-ratingconverter by using several lower rating converters instead at theexpense of somewhat increased hardware cost, volume/weight,and more complex control circuitry.

This paper describes the development of a 50-kW commer-cial FC, employed in the first generation of BSS in Israel.Rather than presenting a novel topology, the main goal ofthis paper is to present a successful industrial application ofwell-known power electronics concepts. The charger employsa three-phase diode rectifier combined with three single-phaseAPFs as the input stage and twelve buck dc–dc converters,separated into six interleaved groups as the output stage. Thepower stage of the charger operates as a programmable volt-age supply with controllable dynamic current limitation. Thecharger draws power from the three-phase utility grid and isable to charge lithium-ion batteries within the voltage rangeof 230–430 V by supplying currents up to 125 A. The controlstage of the FC supports the required communication protocolsboth of the battery and the Control and Management Server(CMS) via controller area network (CAN) bus and Ethernet,respectively.

TABLE IMAJOR SPECIFICATIONS OF THE TRACTION BATTERY

Fig. 1. FC output performance envelope.

The rest of this paper is arranged as follows. Section IIcontains an overview of a typical vehicle traction battery.Section III presents the FC design requirements, relevant to thispaper. The topology of the charger power stage and the appro-priate control circuitry and simulation results are described inSection IV. Experimental waveforms as well as some of thequalification test results are presented in Section V. This paperis concluded in Section VI.

II. FC REQUIREMENTS

The FC, described in this paper, was designed to charge355-V 70-A traction batteries formed by 96S2P connection of3.7-V 35-A · h lithium manganese spinel cells. The main elec-trical specifications of the battery are summarized in Table I.

A. Output stage

In order to charge such a battery, the FC must be able tooperate in the full range of the possible battery voltages. Inaddition, power cable voltage drop should be taken into ac-count. Hence, the maximum output voltage design requirementwas set to 430 V. The maximum charging current was limitedby the battery manufacturer to 125 A for safety reasons, lead-ing to the charger output performance envelope requirement,shown in Fig. 1.

B. Input Stage

The charger was designed to draw the power from 380- to415-Vrms 50-Hz three-phase grids with neutral and protec-tive earth connections. Since typical grid operators providethe ac power with 10% accuracy, the charger must be capa-ble of functioning in the range of 342–457-Vrms input linevoltages.

KUPERMAN et al.: BATTERY CHARGER FOR ELECTRIC VEHICLE TRACTION BATTERY SWITCH STATION 5393

Fig. 2. FC system-level block diagram.

The minimum consumer PF allowed by the Israel ElectricCompany in Israel is 0.92. Note that devices with active powercorrection usually operate with near-unity displacement factor(DF); therefore, the PF is affected mostly by the THD accordingto the following well-known relation:

PF =DF√

1 + THD2. (1)

In case the DF is kept near unity, the theoretical maximumallowed value of THD is 42.6%. However, since the rated inputcurrent exceeds 16 A, the charger emissions must comply withthe IEC61000-3-4 standard, leading to the THD upper limit of15%.

C. Additional Requirements

The efficiency requirements are dictated mostly by thecooling ability of the BSS, as mentioned earlier. The air-conditioning system of the BSS under study was designed toremove up to 2.5 kW of heat from each FC. Therefore, whensupplying full-rated power, the chargers are allowed to operatewith minimum efficiency of 95%. For derated operation, theefficiency requirement may be reduced down to 92% at 10 kW.

The physical size requirements are typically derived fromBSS space limitations. The target weight of the charger was70 kg, and the upper limit of the dimensions was set to 500×650× 600 mm in order to support the BSS infrastructure. Thesedemands led to the minimum gravimetric and volumetric powerdensities of 720 W · kg−1 and 0.26 W · cm−3.

III. SYSTEM OVERVIEW

The system-level block diagram of the proposed solution ispresented in Fig. 2. On the signal level, the FC communicateswith traction battery management system (BMS) and CMSterminal via CAN and Ethernet buses, respectively. The chargersupports both master and slave mode charging. In the mastermode, the charger performs either CC–CV of CP charging

according to the CMS commands while monitoring the batterycondition. In the slave mode, the battery manages the charg-ing process by sending current/voltage/power requests to thecharger. The CMS is the highest level management layer of aBSS, performing administration and billing tasks in addition tothe ability of limiting or completely shutting down the FC fleetpower in case of a safety issue or according to the utility gridoperator request. In the charger under study, both communica-tion protocols are realized by Zilog eZ80F91 microcontrollerwith the assistance of Gridconnect RS232-CAN adaptor sinceeZ80F91 does not support the CAN bus directly. Zilog micro-controller is a bidirectional gateway between the CMS, batteryBMS, and the Atmel Atmega 128 microcontroller, which isin charge for the power management and monitoring of thecharger power circuitry. The APF and buck control boards areindependent and based on fully analog controllers.

On the power level, there are low- and high-voltage linksbetween the charger and the battery. The high-voltage linktransfers the charging power, while the low-voltage (12 V)low-power link supplies power to the battery contactors. Notethat the 12-V power supply, located inside the FC, is usedto power both charger and battery contactors. The batterycontactors are used to isolate the high-voltage power pack fromthe environment when the battery is not located in the vehicle orcharging.

Additional feature of the battery contactors is precharging ofthe vehicle inverter dc link capacitor upon battery connection.The charger contactors serve similar purposes since the chargerdc–dc stage contains output capacitors which must be carefullyprecharged prior to the battery connection in order to preventexcessive inrush currents.

When a charging process is terminated, the charger outputvoltage is usually higher than 400 V. After the battery isdisconnected, the output capacitors should be discharged to alow voltage because of safety reasons. This is accomplishedby a bleeder circuit, discharging the output capacitors quicklybelow 50 V. The bleeder and the 12-V contactor power supplyare both operated by the Atmega 128 microcontroller.


Fig. 3. Buck cell structure.

Fig. 4. Simulation results. (Upper) Individual buck cell inductor currents.(Lower) FC output current before and after capacitor filter.

IV. OUTPUT STAGE

The dc–dc stage of the charger consists of six interleavedcells of two parallel 4.5-kVA buck converters [47], [48]. Asingle cell structure is shown in Fig. 3. The converters are ofbasic buck topology with an enhanced switch, consisting of aparallel connection of insulated gate bipolar transistor (IGBT)and MOSFET. A small delay between the turn-on and turn-off instants of the transistors allows relatively high switchingfrequency operation of 50 kHz and significant loss reduction.Detailed operation of the combined switch is out of the currentpaper scope and will be reported in a separate paper.

The pulsewidth-modulation (PWM) signals of the two con-verters in the same cell are synchronized while the PWMsignals of the two adjacent cells are 60◦ shifted in order toimplement time-based interleaving, as shown by PSIM softwaresimulation results in Fig. 4. While the ripple of cell outputcurrent iOn is around 6 A (n = 1, . . . , 6), the FC output currentripple is reduced to 0.7 A as a result of interleaving. In addition,the ripple frequency is multiplied by six, and as a result, itsinfluence on the battery current further reduces because of thelow-pass characteristics of the battery internal impedance [49].The dc–dc stage input current ripple is much improved as well,as shown in Fig. 5, leading to the electromagnetic emissionreduction as follows. Since basic buck topology is used, theinput cell current is highly discontinuous, dropping to zeroeach time the switch is open. However, due to interleaving, theinput current ripple is significantly reduced, and its frequency is

Fig. 5. Simulation results. (Upper) Individual buck cell input currents.(Lower) DC–DC stage input current.

Fig. 6. Simulation results. Rectified voltage at the dc–dc stage input.

multiplied by six as well. The input current of the dc–dc stageloads the diode bridge; hence, the current ripple is polluting themains since the APF stage is unable to suppress high-frequencyharmonics. Both ripple reduction and frequency increase leadto THD reduction and electromagnetic compatibility filter re-quirements loosening.

The voltage at the input of the dc–dc stage is the rectifiedinput voltage, shown in Fig. 6 (for 400-Vrms grid). Since bucktopology is employed at the output stage, the minimum valueof the rectified voltage must be higher than the maximumoutput dc voltage in order to ensure undistorted operation. Theminimum value of the rectified voltage is given by

VINmin =

√6 · Vrms

2(2)

hence, the global minimum of the rectified voltage (neglectingthe voltage drop of the diode bridge) is VINmin = 419 V forVrms = 342 V, and the requirement of maximum charge outputvoltage of 430 V cannot be met. As a result, the modificationof the maximum output voltage requirement shown in Fig. 7was proposed and accepted by the customer. It is worth notingthat 410-V output is sufficient to charge the mentioned batteryin most cases if the power cable is of a reasonable lengthand cross-sectional area since, during CV and CP charging


Fig. 7. Modified maximum output voltage envelope.

Fig. 8. DC–DC stage control circuitry diagram.

stages, the current diminishes and cable voltage drop reducesproportionally with the current.

The architecture of the dc–dc stage control circuitry is shownin Fig. 8. The analog control board receives from the Atmega128 microcontroller reference voltage command in case of CVoperation or reference current command in case of CC or CPoperation, senses the real values of the appropriate voltage/current, and creates PWM commands to switch drivers. Thecontrol algorithm has a conventional dual-loop structure, wherethe slow outer loop controls the charger output parameter(voltage or current, according to the operation mode) and theinner current mode control (CMC) loop controls the inductorcurrents of the individual converters to follow the outer loopgenerated reference. The interleaving is achieved by shifting theclocks used by CMC. The PWM signal, created by the currentloop, is split into two time-delayed signals to the drivers of thecombined switch transistors.

Actual currents, voltages, and cell temperatures are continu-ously monitored by the Atmega 128 microcontroller, which canshut cells down in case of malfunction and perform correspond-ing output power derating. Control loop design and componentselection issues are omitted for the sake of brevity.

V. PFC STAGE

The FC input stage consists of a three-phase diode rectifierand three single-phase APFs [50]. The reason of using threesingle-phase APFs instead of a single three-phase APF is the

Fig. 9. Input stage R-phase diagram.

Fig. 10. Simulation results. Worst case bridge rectifier output current.

fact that a single-phase APF module was developed earlier bythe company for another application and was found suitable forthe first version of the FC. A three-phase APF employment iscurrently being developed for the future versions of the device.

The diagram of the input stage phase R is shown in Fig. 9.The diode bridge is represented by a nonlinear current sourceiRD, which is supplied by both the mains and the APF. Themain idea is forcing the APF to supply the reactive and har-monic content of the nonlinear current, leaving the mains tosupply the fundamental harmonic only. The APF consists of acontrolled full bridge with a dc bus capacitor (since it does notprovide real power to the diode bridge), connected to the ac busvia an inductor. Although the diode bridge is represented by acurrent source, its actual behavior resembles a CP load since itdrives a battery through dc–dc converter stage. Hence, the worstcase bridge rectifier output current, occurring when the mainsvoltage is minimal, is shown in Fig. 10.

In order to understandably describe the CP load effect, theoutput stage of the FC was represented in the simulation bya 50-kW CP load, and the high-frequency harmonics (Fig. 5)were neglected to prevent confusion. For the R phase, the mainsvoltage and diode bridge current are

vRM (t) =√2Vrms sin(ωt) (3)

iRD(t) =

∞∑n=1

In sin(nωt+ φn) (4)


Fig. 11. Simulation results. (Blue; dashed) Input stage R-phase mains voltageand (green; solid) diode bridge current.

respectively. In order to force the mains to supply the funda-mental harmonic only

iRM (t) = I1 cosφ1 sin(ωt) (5)

the APF must supply the difference, given by

iRF (t) = I1 sinφ1 cos(ωt) +

∞∑n=2

In sin(nωt+ φn). (6)

Note that, according to Fig. 11, where the phase R voltageand input current of the bridge rectifier are shown, the DF of thediode bridge current is actually in unity (φn = 0); hence, theAPF must supply harmonic content only, given by the secondterm of the right-hand side of (6), to the diode bridge. Thedesired APF current is created by impressing the followingvoltage at the filter terminals:

vRF (t) =

∞∑n=2

nωLRF In sin(nωt+

π

2

). (7)

In order to realize (7), the dc link voltage of the APF shouldbe kept above the absolute maximum of the mains voltage be-cause of the buck structure of the circuit. In addition, since thereare internal switching and conduction losses in the APF, someamount of active current should be drawn by the filter from themains to maintain the dc link voltage nearly constant. More-over, according to (7), there is a following tradeoff between thedc link voltage level and filter inductor: In order to prevent theswitching frequency leakage, the inductor value should be ashigh as possible; however, a high inductor leads to the high-frequency harmonics compensating ability deterioration sincethe dc link voltage increase is limited by the capacitor voltagerating. The tradeoff values are eventually determined by PF andTHD design requirements. If no solution is available satisfyingall the constraints, an LC filter instead of a single inductor maybe considered. However, in this case study, a single inductorsolution turned out to be sufficient.

Time and frequency domain simulation results of the inputstage performance are shown in Figs. 12 and 13, respectively.The frequency domain results clearly demonstrate that the

Fig. 12. Simulation results. Input stage performance (phase R): (Upper)(Blue; dashed) Mains voltage and (green; solid) current; (lower) (blue; dashed)diode bridge and (green; solid) APF currents.

Fig. 13. Simulation results. Input stage current spectra.

Fig. 14. Input stage control circuitry diagram.

mains current consists of the fundamental harmonic only andthe rest of the bridge current is supplied by the APF.

The bridge-per-phase current rating determined for the worstcase of mains voltage is around 88.9 Arms (assuming losslessconversion), while the APF current rating is 26.5 Arms, i.e., theshunt-connected APF rating is about 30% of the load rating, asexpected.

The APF control circuitry architecture is presented in Fig. 14.The dual-loop analog control structure consists of the outerslow voltage loop, keeping the dc link voltage at the prede-termined level, set by the Atmega 128 microcontroller. Thevoltage controller generates mains current amplitude referencecommand, which is multiplied by the mains voltage sinusoidal


Fig. 15. FC pictured. (a) Front view. (b) Rear view.

template to create the main current reference. The fast innerloop forces the mains current to follow the reference by gener-ating the appropriate duty cycle command to the PWM logiccircuitry. The APF operates at 17-kHz switching frequencyemploying unipolar PWM technique. APF currents, voltages,and temperatures are monitored by the Atmega 128 microcon-troller for safety reasons. Control loop design and componentselection issues are omitted for the sake of brevity.

VI. EXPERIMENTAL RESULTS

The photography of the designed FC is shown in Fig. 15.The weight and the physical dimensions of the device are

Fig. 16. Experimental results, 230-Vrms mains, 35-kW charging. (Upper)Mains voltage (phase R). (Lower) Mains current.

Fig. 17. Experimental results. (Upper) Output voltage. (Middle) Output cur-rent. (Lower) Output power.

67 kg and 495× 650× 598 mm, respectively, complying withthe design requirements. The charger was tested under both 35-and 50-kW charging powers. Sample input and output wave-forms of a 35-kW CP charging mode are presented in Figs. 16and 17, respectively. It can be concluded that the input currentis nearly sinusoidal (the THD content is observable in theshape; moreover, note that the switching frequency componentswere filtered out to improve readability) and in phase withthe mains voltage. The waveform was obtained by measuringthe phase current without an electromagnetic interference filterconnected and filtering it in software by a filter with a similar(theoretically) frequency response. The nonperiodic spikes arejust noise traces, while the periodic ones are the distortions atswitching instants. As to the output stage, the output currentand voltage (and, thus, the power) are stable and possess lowripple. The PF, THD, and efficiency for two different outputpowers and upper and lower limits of the input voltages arepresented in Figs. 18 and 19. According to the results, the PFis kept near unity, and the THD is within permissible limitsof 15%. The efficiency is well above the lower limit of 95%.It can be concluded that the FC complies with all the designrequirements.


Fig. 18. Experimental results. PF, THD, and efficiency at 35-kW charging.(Solid) Vrms = 342 V. (Dashed) Vrms = 457 V.

Fig. 19. Experimental results. PF, THD, and efficiency at 50-kW charging.(Solid) Vrms = 342 V. (Dashed) Vrms = 457 V.

VII. CONCLUSION

A 50-kW FC design for a lithium-ion EV traction batteryhas been presented in this paper. The device is capable ofsupplying up to 50-kW charging power to any battery, operatingin 240–430-V voltage range in either CC, voltage, or powermode. The charger topology may be referred to as a two-stage controlled rectifier. The input stage consists of a three-phase full-bridge rectifier combined with a reduced rating APF(three single-stage power filters are actually employed). Theinput stage creates an uncontrolled dc bus while complyingwith the grid codes by keeping the THD and PF within thepermissible limits. The output stage is formed by six interleavedgroups of two dc–dc converters, reducing the input and outputcurrent ripples. Two independent control boards are employed:active filters control circuitry and the dc–dc control circuitry.The former is operated according to the predetermined gridinterfacing behavior, while the operation of the latter is dictatedeither by preprogrammed charging sequence or by the requestsfrom the BMS. The designed device performance is shown tocomply with main design requirements, and extended simula-tion/experimental results are presented.

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A. Kuperman (M’08), photograph and biography not available at the time ofpublication.

U. Levy, photograph and biography not available at the time of publication.

J. Goren, photograph and biography not available at the time of publication.

A. Zafransky, photograph and biography not available at the time ofpublication.

A. Savernin, photograph and biography not available at the time of publication.

better place 50kw charger

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