Comparison Of SiC-Based DC-DC Modular Converters For EV Fast DC Chargers

III.CHARGER STRUCTURE AND TOPOLOGIESDue to the high power demand absorbed from the grid andin order to cope with the challenge of fast charging multipleEVs simultaneously, one solution is to connect the chargingstation system to a medium voltage (MV) grid connection [5],[18].The classical approach for the conversion form AC-DC (orDC-AC e.g. bidirectional power flow) is to employ an LFtransformer and a rectifier/inverter as the first conversion stage,as shown in Fig. 2a. As the input voltage of the charging systemis in several kVs, the AC/DC stage should withstand the highinput voltage with low power losses. Multilevel converterswould be the best candidate to interface with the grid due totheir capabilities of withstanding high input voltage [19], [20].This AC-DC conversion stage guarantees power factorcorrection and produces a constant high output DC bus voltagefor the second conversion stage. The second conversion stagein this configuration can be a non-isolated converter type. Thealternative architecture of a fast DC charging station is shownin Fig. 2b. An isolated converter is employed as the secondconversion stage to perform the isolation requirements throughan integrated HF transformer whereas the first AC-DC stageremains the same as the first architecture. However, a higherrated AC/DC converter is required to withstand the high inputvoltage [2], [21]. The first conversion stage (AC-DC) is out ofthe scope of this paper and will not be discussed any further.the nominal value, respectively [2], [11], [24]. Anotherrequirement is the ability to deliver high power to reduce thecharging time to less than 15 minutes. These requirementswould require a higher number of semiconductor devices toshare the high current and voltage.The DAB and IBC topologies were found the most suitableconverters that are capable of meeting the above requirements[8], [11]. These two converters are compared, and the besttopology is chosen based on several aspects: efficiency,converter volume, the number of components, current ripple,and control complexity. A resistive load has been considered,as the main purpose of the paper is to compare the performanceof the two converters in terms of operation, efficiency, etc.Hence, the model of the battery is not required at this stage.The switching frequency will determine the size of thepassive elements and losses in the semiconductor devices,while the right components can help in reducing the losses. Aswitching frequencyof 50 kHz is considered for the twoconverters in this paper, which is reasonable when high powerSiC devices are utilised. A commercially available B6C(CAS300M17BM2) SiC module is considered to be used forboth converters in this study [25].The specifications of the fast DC charger module aresummarised in Table 2. The dc charger module compromises ofparallel DC-DC converters to meet the desired power level of350 kW. Hence, 5 parallel cells are required, each rated at 70kW. Connecting the converter output in parallel enables highcharging current and hence faster charging. The maximumoutput current of each cell is 350 A. This output current is underminimum output voltage i.e. 200 V. For voltage levels higherthan 200 V, the maximum output current is reduced to keep theoutput power around 70 kW for each cell. For instance,maximum output current in the maximum output voltage, 920V, is limited to 76 A. The main reason for this current reductionis to fulfil the maximum operating condition of the lineconnected devices like cables, switches and power electronicdevices. In this work, the output nominal voltage is chosen tobe 500 V.Table 2 Specifications of the fast DC chargerFigure 2 Level 3 fast charging: a) LF transformer; b) HF transformer [2]IV. THE FAST DC CHARGER SPECIFICATIONSIn addition to the general aforementioned comparisonquantities, some other important requirements and factors mustbe considered when designing a fast DC charger. The firstessential requirement of a charger is the ability to provide awide dc output voltage to interface different types of EVsoccupied with different battery technologies as well as theability to regulate the output voltage according to the voltage ofthe EV battery pack. The voltage range of future 350 kW fastDC chargers is 200-920 V [3], [22]. The second requirement isthe electrical quantities (voltage and current) ripples as theoutput of the converter which is set to a very narrow value toguarantee secure operation of the converter and prevent anydamage to the connected battery pack [23]. In fact, themaximum current and voltage ripples are set at 5% and 1% ofParametersCharger unitInternal DC-link Voltage1200 VCharging Current Ich(Each cell)350 kW / 5 parallel convertersof 70 kW200 - 920 Vnominal 500 V 140 AMax 350 ASwitching frequency (Fs)50 kHzEffective Rated Power Pch,effBattery Pack Voltage VbatBattery Voltage Ripple(peak-peak)Inductor current ripple(peak-peak) 1% 5%V. DESIGN OF THE DC-DC CONVERTERS BASED ON THE DABAND IBC TOPOLOGIESA. Design of DAB Converter:

The Dual Active Bridge (DAB) is one of the mostpromising DC-DC circuit topologies for high powerapplications [26]. The series and/or parallel connections ofmultiple DABs enable to increase the current and powermaking this topology feasible for high power chargingapplications (e.g. fast DC chargers). One of the mainadvantages of the DAB converter is the galvanic isolationwhich is provided within the converter by means of an HFtransformer. Hence, no bulky line frequency transformer isneeded in the front-end of the charging system.Fig. 3 shows the schematic diagram of the bidirectionalDAB converter with SiC MOSFET as switching devices. Theconverter consists of two active full bridges linked with aninductive element where the first side operates as an inverterand the second side operates as a rectifier. The energy transferleakage inductor L1 is used as the main energy storage andtransfer element between the two bridges [27], [28]. For abattery charging application, a DAB converter can operate as abuck converter to transform the bus voltage to charge thebattery and on the contrary, operate as a boost convertertransforming the battery voltage to the bus voltage whendischarging [29]. The amount of power being transferred PDand the direction of the current is determined by the phase shiftd between the primary V1 and secondary V2 voltages. Inaddition, other parameters that are affecting the power transferare the transformer turn ratio N, the leakage inductor L1 and thef switching frequency [27]. . .21[W](3)where L is the sum of the transformer leakage inductance L1and the auxiliary inductances of the transformer.An output capacitor Cout is employed to smooth out theoutput voltage, which can be calculated as follows: 50 .(5)[F]Three main modulation techniques can be applied to DABconverters, namely, single-phase-shift (SPS), extended-phaseshift (EPS) and dual-phase-shift (DPS) control. However, themost commonly used control technique for DAB is the SPScontrol due to its advantages of simple control, high dynamicand small inertia [27]. Each bridge of the DAB converter isdriven by a 50% duty cycle square voltage waveform. Theresulted output V1 and V2, which are the equivalent ac outputvoltages of full-bridges H1 and H2, respectively, and theinductor current iL of the DAB converter are shown in Fig. 4.V1tV2tiLtFigure 4 Typical voltage and current waveforms of the DAB converterThe maximum allowable leakage inductance value of theDAB converter can be calculated from (4), which is set as at80% of Lmax to ensure sufficient bandwidth of the duty cycle[28]. Considering a switching frequency of 50 kHz, this yieldsthe final design value of 41.14. At this value ofleakage inductor, the phase shift is calculated from (3) to beas 0.28.The HF transformer is the main component in the design ofan isolated converter. A transformer with Amorphous materialwas selected due to its high saturation flux density Bsatpresenting the best balance between cost losses per unit volume,operating temperature, and availability when compared withother magnetic materials such as ferrite and Nanocrystalline[31]. All the data needed to design the HF transformer and thecalculated parameters of the transformer are summarised inTable 3.Table 3 HF Transformer parametersFigure 3 Topology of DAB converter [30]From the previous equation, it can be derived that themaximum amount of transferring power occurs when 0.5.However, operating the converter at the maximum phase-shifte.g. 0.5 (90 ) results in high reactive currents flowingthrough the converter and consequently reducing the efficiencyof the converter due to power losses of power devices [30]. .8.[W](4)Magnetic coreAmorphousSaturation flux density(Bmax)1.56 [ T ]Primary winding (N1)38 Turns, 63 strands, AWG 23Secondary winding (N2)16Turns, 150 strands, AWG 23Current density (J)4.5 [ A / mm2 ]Primary resistance7.93 [mΩ]Secondary resistance1.4 [mΩ]

The DAB converter and the parameters of the HFtransformer were simulated and calculated off-line usingMATLAB. The gate signals of the two bridges operating at50% duty ratio, implemented with SPS modulation are shownin Fig. 5, while the AC voltage and current waveforms areshown in Fig. 6. The secondary voltage is shifted by 0.28 49.7 ) from the primary voltage in order to avoidany resulted excessive reactive power when operating theconverter at the maximum phase shift of 0.5 (90 ) asmentioned earlier. The primary and secondary RMS currents atthe high-voltage and low-voltage sides are 1 72.8 A and 2 174.7 A, respectively. The output voltage and output currentof the converters are shown in Fig. 7 and Fig. 8, respectively.The output voltage at the full load is at around 498 V with apeak to peak ripple voltage of 2.8 mV. The transformer lossesare calculated according to [32] and are summarised in Table 4.Figure 8 Output current of DAB converterTable 4 HF Transformer lossesParameterValueCore losses (pfe)84.36 WCopper losses (pcu)1.54 kWTotal losses (ptotal)1.63 kWTransformer efficiency (ηt)97.73%B. Design of IBC Converter:Figure 5 Gate signals of the DAB converterFigure 6 Operational waveforms of DAB converterFigure 7 Output voltage of DAB converterA different approach to an off-board charger at the inputstage of the system is through using an LF transformer whereinsuch case, non-isolated converters can be employed as a DCcharger. Typically, at low power application, the buckconverter can be used to step down the input voltage, which ishowever not feasible for high power applications. Instead ofusing an over-sized single-phase converter, multiphaseinterleaved buck converters (IBC) can be employed for highcurrent applications due to its advantage of sharing the outputcurrent between the number of phases N resulting in lower ratedinductors and power switching devices. Moreover, theefficiency of the converter is typically high due to the fact thatfundamental frequency is multiplied by N resulting in a highersystem frequency, improved transient response, smooth outputcurrent with low ripple and low EMI and output filters size [33],[34]. However, as the number of phase’s increases, the numberof active switches, cost, weight, complexity, and size alsoincreases.Figure 9 Topology of three-phase IBC [6]

Hereafter, a three-phase buck converter is considered in thispaper as shown in Fig. 9. The upper and lower active switchesof each phase operate in a complementary manner and eachphase is shifted by 360 /N from one another. In this case, theswitches have a phase shift of 120 between them with 50 kHzswitching frequency.The passive components of the three-phase IBC converterare designed and calculated according to the specifications ofthe fast charger shown in Table II. The converter is assumed tobe operating under continuous conduction mode (CCM) as theoperation in discontinuous conduction mode (DCM) yields tovital limitations in high current applications especially on activecomponents as well as high core losses in inductors [35]. Theoutput inductor value per phase can be obtained from (6) asfollows: . 1. Δ[H]Figure 11 Output current of IBC(6)where f is the switching frequency, D is the duty cycle ofthe converter andis the current ripple of each phase.Considering the advantage of current cancelation in the IBC,which results in a low output current, a current ripple of 25 was selected for each phase to minimise the size of theinductor [36]–[38]. This leads to an inductance value of 233.33per phase.The output capacitor at the charger output furthermoresmoothes the output charging current and satisfy the requiredvoltage ripple of 1%. The minimum output capacitor iscalculated using (7), whereis the ripple current of a singlephase of the converter. 8.Δ. Δ.[F](7)Figure 12 Output voltage of IBCThe total output current due to the cancellation effect ofsumming up all the three-individual phase inductor current isshown in Fig. 11 resulted in reducing the output inductor rippleto 6.4 A. This ripple is further reduced to 0.1145 A by the outputcapacitor at the load side. Fig. 12 shows that the output voltageat full load, which is around 500 V with a peak to peak ripplevoltage of 1 mV, which is way less than the value specified of10 mV due to the interleaving technique.VI. COMPARISON AND DISCUSSIONThe DAB and IBC converters are compared considering theperformance indices aforementioned utilising the same ratedsemiconductor devices and switching frequency of 50 kHzproviding the same output power of 70 kW. Table 5 shows aquantitative comparison between the two converters, in whichsome features of each converter are presented.Table 5 Comparison of DAB and IBC convertersCriteriaFigure 10 3-phase inductor currents of IBCThe inductor currents of each phase of the converter, shiftedby 120 from each other as shown in Fig. 10. It can be seen thatthe current is equally shared between each phase with anaverage value of 46.6 A, which corresponds to the total outputcurrent of 140 A divided by N (i.e. N 3 in this paper), and apeak-to-peak ripple value of 25 A.Dual activebridge(DAB)Interleavedbuck converter(IBC)Output capacitor280 mF16 µFInductor41.14 µH3 233.33 µHTransformer Active switches86Internal isolationBidirectional powerflowControl complexity SimpleSimpleVolume and sizeHeavyLightEfficiency97.7%99.2%

The IBC required merely a 16 µF output capacitor in orderto regulate the output voltage and filter out the ripple. This isdue to the current ripple cancellation, resulted from theinterleaving technique, as well as the output inductors sincethey are directly connected to the load. On the other hand, abulky capacitor of 280 mF is required in the DAB converter.Ideally, to ensure the battery is efficiently charged and itslifetime is not affected, the output current ripple and the outputvoltage ripple of the converters should be minimum. These twofactors can be easily achieved and were significantly lower inthe simulation results of the IBC converter.Due to the current sharing technique in the three-phaseIBC, three output filter inductors of 233.33 µH minimally ratedat 47 A are required, while the leakage inductor of the DABconverter is 41.14 µH and rated at a minimum full outputcurrent of 140 A. The magnetic elements such as thetransformer and inductors have the most effect on the volumeand weight of a converter. Hence, even though, IBC converterrequires 3 inductors, the overall volume and size of thetransformer considerably outweigh the volume and weight ofthe DAB converter.Moreover, with the aforementioned advantages of SiCdevices, IBC converters can relatively operate at very highswitching frequency (e.g. 300 kHz) resulting in furtherreduction the output filter components (equation (6)), whereasthe nominal frequency of the HF transformer within the DABconverter is usually limited to 120 kHz, especially for highpower converters, due to the electromagnetic compatibility(EMC) and harmonic emissions [39].In terms of the number of active switches, the DABconverter has more switches than the three-phase IBCconverter. The active switches of the DAB converters must berated and withstand the highest current in the converter in caseof bidirectional power flow, whereas the switches in the IBCconverters is usually minimally rated at / , where N is thenumber of phases. As some immediate consequences, the costand the conduction losses are significantly lower in IBC whencompared to DAB due to the current sharing between thephases.Moreover, in addition to the conduction and the switchinglosses, DAB converter suffers from the HF transformer lossesin high current applications resulting in reducing the overallefficiency of the converter. Contrarily, the efficiency of the IBCis only affected by the conduction and the switching losseswhere in which is the conduction losses is lower due to thecurrent sharing feature as highlighted earlier.The DAB converter as mentioned provides the advantageof galvanic isolation by means of HF transform in the DC-DCconverter stage, which results in eliminating the need for abulky LF transform in the front end of the charging system.However, deviation from the nominal conditions (e.g. outputvoltage) in which the HF transformer was initially designed for,would result in reducing the system efficiency. Hence, for wideinput/output voltage variation applications, the use of DABconverter is limited due to this disadvantage [40], [41].Moreover, the semiconductor devices of the AC/DC converterstage must be highly rated due to the connection of the MV gridwithout the use of up-front LF transformer.Hence, since an LF transformer seems necessary in thefront-end of a high-power charging system, a non-isolatedconverter such as the discussed IBC is preferable to serve as adc charger due to its compactness, reliability, efficiency andrelatively simple control when compared to other candidates ofhigh-power topologies.In addition, for a modular configuration of the dc chargersin order to obtain high charging output current andconsequently reduce the charging time, the rated power of asingle cell DAB converter is limited due to the power stress onthe semiconductor devices and the conduction losses. On theother hand, a higher power can be achieved with less numberand a lower power rated components by means of a single cellof an IBC with multiple phases as the output current isseparately shared over the different converter phases. Thisconfiguration would result in a better efficiency and lessnumber of components when compared with a modular DABconfiguration. In addition, the output power can be furtherenhanced by connecting multiple cells of an IBC in a modularoutput parallel configuration.VII. CONCLUSIONFor a fast DC charger, considerable several requirementsmust be taken into accounts such as high power, isolation, lowcurrent and voltage ripples etc. In this paper, the two mostsuitable high-power converter, DAB, and IBC are introduced,analysed, simulated and compared in terms of multiple criteriaby using state-of are switching devices e.g. SiC modules. Inaddition, the strengths and weaknesses of both fast chargingarchitecture approaches have been analysed and discussed.Based on the discussion, the IBC, when compared to theDAB, is found to be the optimum candidate combining therequirement of a DC charger, in particular, simplicity, highefficiency, very low current ripple and a low number ofcomponents.ACKNOWLEDGEMENTThe authors would like to thank SP Energy Networks fortheir partial financial support of this work through the NetworkInnovation Allowance Project No. RES/0560/7466/002.REFERENCES[1] M. Yilmaz and P. T. Krein, “Review of Battery ChargerTopologies, Charging Power Levels, and Infrastructure for Plug-InElectric and Hybrid Vehicles,” IEEE Transactions on PowerElectronics, vol. 28, no. 5, pp. 2151–2169, May 2013.[2] L. Rubino, C. Capasso, and O. Veneri, “R

Comparison of SiC-Based DC-DC Modular Converters for EV Fast DC Chargers Mohammed Alharbi 1, Mohamed Dahidah 1, Volker Pickert 1 and 2 James Yu 1 School of Engineering, Newcastle University, Newcastle upon Tyne, United Kingdom 2 Scottish Power Energy Networks, Blantyre,United Kingdom m.a.alharbi2@newcastle.ac.uk mohamed.dahidah@newcastle.ac.uk volker.pickert@newcastle.ac.uk James.Yu .