Partial Power Processing Based Converter for Electric Vehicle Fast Charging Stations

electronics

Article Partial Power Processing Based Converter for Electric Vehicle Fast Charging Stations

Jon Anzola * , Iosu Aizpuru and Asier Arruti

Department of Electronics and Computer Science, Mondragon Unibertsitatea, 20120 Hernani, Spain; [email protected] (I.A.); [email protected] (A.A.) * Correspondence: [email protected]

Abstract: This paper focuses on the design of a charging unit for an electric vehicle fast charging station. With this purpose, in first place, different solutions that exist for fast charging stations are described through a brief introduction. Then, partial power processing architectures are introduced and proposed as attractive strategies to improve the performance of this type of applications. Further- more, through a series of simulations, it is observed that partial power processing based converters obtain reduced processed power ratio and efficiency results compared to conventional full power converters. So, with the aim of verifying the conclusions obtained through the simulations, two downscaled prototypes are assembled and tested. Finally, it is concluded that, in case galvanic isolation is not required for the charging unit converter, partial power converters are smaller and more efficient alternatives than conventional full power converters.

Keywords: electric vehicle; fast charging stations; partial power processing; partial power converters; series connected converter; dual active bridge

1. Introduction Citation: Anzola, J.; Aizpuru, I.; Through the last years, the automotive market is showing a big interest on the in- Arruti, A. Partial Power Processing clusion of the electric vehicle (EV). Indeed, due to its efficient performance and reduced Based Converter for Electric Vehicle greenhouse emissions, the EV is turning into a real alternative to conventional combustion Fast Charging Stations. Electronics based vehicles. This can be confirmed by analyzing its shelling data, which increases year 2021 10 , , 260. https://doi.org/ by year [1]. However, one of the main disadvantages of EVs is their low autonomy, which 10.3390/electronics10030260 can reach up to 550 km [2]. In order to solve this problem, there exist 2 main solutions: invest on technologies that can extend the capacity of the energy storage system (ESS) Received: 10 December 2020 or build an extensive and solid EV charging stations grid. The first solution is usually Accepted: 19 January 2021 Published: 22 January 2021 high time consuming, since experimental tests of different battery technologies require long periods. However, the second solution is much faster and if the charging stations are

Publisher’s Note: MDPI stays neutral correctly located, the driving rage anxiety of the EV user can be reduced [3]. with regard to jurisdictional claims in According to ref. [4], there exist different concepts of EV charging stations: exchanging published maps and institutional afﬁl- vehicle’s battery [5], contactless inductive coupled chargers [6] and conductive coupled iations. chargers. Regarding conductive coupled chargers, this type of chargers are the most popular solution for EV charging and currently, inside Europe there exists a great number of charging points based on this technology [7]. When it comes to the different types of conductive EV charging stations, they can be divided by their power level. On the one hand, up to 10 kW AC charging stations can be found. Usually, this type of charging Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. structures are located where people spend great part of the day, for example: at home or at This article is an open access article work. On the other hand, DC wise, two main groups exist: fast charging stations (between distributed under the terms and 20 kW and 120 kW) and extreme fast charging stations (higher than 120 kW). Due to the conditions of the Creative Commons high peak power values, the charging times can be reduced up to 15 min [8], which makes Attribution (CC BY) license (https:// the EV more attractive to the customer. creativecommons.org/licenses/by/ Concerning conductive EV fast charging stations, Figure1 shows different imple- 4.0/). mented structures. As it can be observed, in all the solutions, fast charging stations are

Electronics 2021, 10, 260. https://doi.org/10.3390/electronics10030260 https://www.mdpi.com/journal/electronics Electronics 2021, 10, x FOR PEER REVIEW 2 of 17

Electronics 2021, 10, 260 2 of 16 Concerning conductive EV fast charging stations, Figure 1 shows different implemented structures. As it can be observed, in all the solutions, fast charging stations are connectedconnected to to a a medium medium voltage AC gridgrid and,and, then,then, the the voltage voltage level level is is reduced reduced and and rectified. rectified.Finally, Finally, a DC-DC a DC-DC power power converter converter is implemented is implemented to charge to charge each each EV at EV the at station. the station. How- However,ever, the locationthe location of galvanic of galvanic isolation isolation and and the waythe way to step to step down down the the voltage voltage are are managed man- agedin different in different ways. ways. For example,For example, in Figure in Figu1a,re the 1a, voltage the voltage is reduced is reduced by aline by a frequency line fre- quencytransformer transformer and then and rectified then rectified by an AC-DCby an AC-DC converter. converter. This way, This away, common a common DC bus DC is busobtained is obtained from from which which the rest the of rest the of DC-DC the DC-DC individual individual charging charging units units are connected. are connected. In the Incase the of case Figure of 1Figureb,c, the 1b,c, line the frequency line freque transformerncy transformer is avoided is byavoided implementing by implementing solid-state solid-statetransformers transformers on the AC-DC on the rectifier AC-DC or rectifier on the DC-DCor on the converter. DC-DC converter.

(a)

(b)

(c)

FigureFigure 1. 1. SimplifiedSimpliﬁed single-wire single-wire diagram diagram of different of different electric electric vehicle vehicle (EV) fast (EV) charging fast charging station station solutions.solutions. (a) Galvanic (a) Galvanic isolation isolation is provided is provided by a by line a line frequency frequency transformer; transformer; (b ()b Galvanic) Galvanic isolation isolation is is providedprovided by by the the AC-DC AC-DC rectifier; rectiﬁer; (c (c) )Galvanic Galvanic isolation isolation is is provided provided by by the the DC-DC DC-DC charging charging unit. unit.

ThisThis paper paper will will focus focus on on the the design design of the of theDC-DC DC-DC converter, converter, which which is connected is connected betweenbetween a common a common DC bus DC and bus the and ESS the of ESSthe EV. of theSince EV. the Since concerned the concerned converter converteris required is torequired process togreat process power great values, power its size, values, cost itsand size, performance cost and performanceare key factors are that key must factors be optimizedthat must through be optimized its design. through Due itsto this, design. recent Due literature to this, recentaround literature EV fast charging around EVappli- fast cationscharging presents applications advance presents architectures advance based architectures on partial power based onprocessing partial power (PPP). processing This type of(PPP). architectures This type aim of to architectures reduce the power aim to processed reduce the by power the converter, processed achieving by the converter,reduced sizeachieving and more reduced efficient size converters. and more efficient Due to converters.the mentioned Due benefits, to the mentioned their applications benefits, theirare diverse.applications For example, are diverse. authors For example, from [9–11] authors propose from PPP [9–11 based] propose architectures PPP based for architectures photovol- taicfor (PV) photovoltaic systems, (PV) whereas systems, [12,13] whereas suggest [12 PPP,13] suggestsolutions PPP for solutionsESS applications. for ESS applications. There, the concernedThere, the converter concerned is converter designed is to designed control the to control power theflow power between flow the between source the and source the load.and Moreover, the load. Moreover, through [14,15], through authors [14,15 pres], authorsent DC presentwindfarms DC based windfarms on PPP based technology. on PPP Aparttechnology. from that, Apart with from the that, aim with of theachieving aim of achieving current balancing current balancing of series of seriesconnected connected elements,elements, refs. refs. [16–19] [16–19 pr]esent present PPP PPP architectures architectures for for applications applications where where there there exist exist series series connectedconnected elements. elements. Last Last but but not not least, least, when when it it comes comes to to EV EV fast fast charging charging applications applications authorsauthors from from [8,20,21] [8,20,21 ]propose propose PPP PPP based based charging charging units. units. There, There, the the authors authors implement implement a a unidirectional topology such as the phase shifted full bridge and they show that the PPP solution achieves converter rating reduction and efficiency improvements compared to its

Electronics 2021, 10, x FOR PEER REVIEW 3 of 17

Electronics 2021, 10, 260 unidirectional topology such as the phase shifted full bridge and they show that the3 PPP of 16 solution achieves converter rating reduction and efficiency improvements compared to its full power processing (FPP) version. Therefore, it is concluded that PPP based converters are very suitable for the concerned application. full powerFinally, processing in relation (FPP) to the version. normative Therefore, around itEV is charging concluded systems, that PPP at present, based converters the great majorityare very suitableof countries for the oblige concerned to implement application. isolated DC-DC converters. This fact affects negativelyFinally, to in PPP relation architectures, to the normative which can around never EV ensure charging galvanic systems, isolation at present, (unless the an greatextra transformermajority of countriesis added). oblige Nevertheless, to implement through isolated the last DC-DC years, converters. the normative This around fact affects EV chargingnegatively stations to PPP is architectures,suffering considerable which can changes. never ensureFor example, galvanic the isolationrequirement (unless of gal- an vanicextra transformerisolation is on is discussion added). Nevertheless, and, in some through cases, the the application last years, of the non-isolated normative aroundsupply equipmentEV charging is allowed stations for is suffering EV charging considerable [22]. To be changes. more precise, For example,the only countries the requirement that are includedof galvanic at the isolation corrigendum is on discussion of the normative and, in someare the cases, United the States application and Canada. of non-isolated Although theresupply are equipment only two countries is allowed that for permit EV charging non-is [olated22]. To solutions, be more precise,they can the be onlyconsidered countries as that are included at the corrigendum of the normative are the United States and Canada. worldwide references that can encourage others to follow the same route. Although there are only two countries that permit non-isolated solutions, they can be Bearing all this mind, the present paper aims to design a PPP based DC-DC charging considered as worldwide references that can encourage others to follow the same route. unit for an EV fast charging station and compare its performance against a conventional Bearing all this mind, the present paper aims to design a PPP based DC-DC charging full power converter (FPC). Through the concerned comparison, special interest will be unit for an EV fast charging station and compare its performance against a conventional shown on the temperature evolution of the PPC prototype. Indeed, these results help to full power converter (FPC). Through the concerned comparison, special interest will be understand the real benefit of processing less power and to have a bigger picture when shown on the temperature evolution of the PPC prototype. Indeed, these results help to carrying out the design of a partial power converter. To achieve this, in first place, Section understand the real beneﬁt of processing less power and to have a bigger picture when 2 introduces and describes the concept of PPP. Then, Section 3 presents the case studio to carrying out the design of a partial power converter. To achieve this, in ﬁrst place, Section2 simulate and the followed design procedure. Next, Section 4 and Section 5 present the introduces and describes the concept of PPP. Then, Section3 presents the case studio to obtained results from the simulations and the experimental tests, respectively. Finally, simulate and the followed design procedure. Next, Sections4 and5 present the obtained Sectionresults from6 and the Section simulations 7 detail and the theobtained experimental conclusions tests, through respectively. the paper Finally, and Sections the pro-6 posedand7 detail future the lines. obtained conclusions through the paper and the proposed future lines.

2. Basis of Partial Power Processing As its name indicates, a power converter based on the PPP concept only processes a reduced percentage of the total power that goes from the source to the load. As example, Figure 22 shows the the power power flow flow of of a aconverter converter based based on onFPP FPP and and a converter a converter based based on PPP. on OnPPP. the On one the hand, one hand, as it ascan it canbe observed be observed in Figure in Figure 2a,2 thea, the FPP FPP converter converter is isdesigned designed to processto process the the 100% 100% of ofthe the power power consumed consumed by by the the load, load, generating generating a a given quantity of losses. On On the the other other hand, Figure 22bb showsshows thethe PPPPPP concept,concept, whichwhich isis basedbased onon achievingachieving a reduction of the power processed by the converter.converter. This way, the losses generated by the power converter are reduced, as wellwell asas itsits size.size. Furthermore,Furthermore, maintainingmaintaining thethe samesame efficiencyefficiency for the power converter, thethe globalglobal efficiencyefficiency ofof thethe systemsystem increases.increases.

(a)

(b)

Figure 2. Power flow ﬂow diagram. ( a)) FPP; FPP; ( b) PPP.

The literature around PPP presents different strategies that achieve a reduction of the total power processed by the converter [23]. On one side, differential power converters (DPC) can be found, whose main objective is to correct current unbalances between series Electronics 2021, 10, 260 connected elements. On the other side, there are the partial power converters (PPC), 4which of 16 control the power flow between a source and a load with different voltage and current values. Concerning PPCs, different types of architectures can be found in the literature. Figure 3 presents two examples for step-down applications [12,21,24,25]. The architecture from Fig- values. Concerning PPCs, different types of architectures can be found in the literature. Figureure 3a3 ispresents referred two as examples“Input-Series-Output-Para for step-down applicationsllel” (ISOP) [ and12,21 the,24 ,on25e]. from The architectureFigure 3b is fromdefined Figure as “Input-Parallel-Output-Series”3a is referred as “Input-Series-Output-Parallel” (IPOS). In order to (ISOP) compare and the the architectures one from 𝐾 Figureshown3 bin is Figure defined 3, asthe “Input-Parallel-Output-Series” first step is to calculate the processed (IPOS). active In order power to compare ratio ( the) of architecturesthe converter shown at each in one. Figure For3 that, the purpose, first step th ise toarchitecture calculate thefrom processed Figure 3a active is taken power as an example. Firstly, Kirchhoff’s laws are applied on the architecture, obtaining expressions ratio (Kpr) of the converter at each one. For that purpose, the architecture from Figure3a is takenEquations as an (1) example. and (2). Firstly,In addition, Kirchhoff’s the efficiency laws are of appliedthe system on is the defined architecture, as shown obtaining in Equa- expressionstion (3). Equations (1) and (2). In addition, the efficiency of the system is defined as shown in Equation (3). 𝑉 −𝑉 =𝑉 (1) Vsource − Vin = Vload (1) 𝐼 +𝐼 =𝐼 (2) Isource + Ipc = Iload (2) 𝑉 ·𝐼 𝜂 =Vload·Iload (3) ηsystem = 𝑉 ·𝐼 (3) Vsource·Isource

(a)

(b)

FigureFigure 3. 3.Step-down Step-down PPC PPC architectures. architectures. ( a(a)) ISOP; ISOP; ( b(b)) IPOS. IPOS.

Then,Then, the theK pr𝐾is deﬁnedis defined as theas the division division between between the processedthe processed power power by the by converter the con- andverter the and source’s the source’s power Equationpower Equation (4). (4). 𝑃 𝑉 ·𝐼 Pconv Vout·Ipc Kpr𝐾= = == (4) Psource𝑃 Vsource𝑉·Isource·𝐼

Applying Equations (1)–(4), it is possible to obtain the 𝐾 curve in function of the Applying Equations (1)–(4), it is possible to obtain the Kpr curve in function of the static voltage gain 𝐺 = Vload , see Equation (5). static voltage gain G = , see Equation (5). V Vsource

𝐾 =𝜂 −𝐺 (5) Kpr = ηsystem − GV (5)

Following the same procedure with the IPOS, Figure4 is obtained, where the Kpr curve of each converter from Figure3 is compared to a FPC. In ﬁrst place, it is evident that the FPC always processes the 100% of the power that ﬂows from the source to the load. However, PPC wise, it is observed that the ISOP and IPOS architectures achieve lower Kpr values when GV gets closer to 1. This means that the closer Vsource and Vload are between them, the lower power is processed by the converter. Comparing the curves achieved by Electronics 2021, 10, x FOR PEER REVIEW 5 of 17

Following the same procedure with the IPOS, Figure 4 is obtained, where the 𝐾 curve of each converter from Figure 3 is compared to a FPC. In first place, it is evident that

Electronics 2021, 10, 260 the FPC always processes the 100% of the power that flows from the source to the5 ofload. 16 However, PPC wise, it is observed that the ISOP and IPOS architectures achieve lower 𝐾 values when 𝐺 gets closer to 1. This means that the closer 𝑉 and 𝑉 are between them, the lower power is processed by the converter. Comparing the curves Electronics 2021, 10, x FOR PEER REVIEWtheachieved ISOP and by the ISOP IPOS and architectures, the IPOS architecture it is concludeds, it is that concluded the ISOP that obtains the ISOP the lowest obtains5K ofprthe 16

curve.lowest Indeed, 𝐾 curve. when Indeed,GV is lowerwhen than𝐺 is 0.5,lower the than IPOS 0.5, obtains the IPOS absolute obtainsK prabsolutevalues 𝐾 higher val- thanues 1.higher This meansthan 1. that This the means IPOS that converter the IPOS is no converter longer working is no longer on the working PPP range on andthe PPP all itsrange beneﬁts and areall lost.its benefits are lost.

Figure 4. Processed power ratio of each converter from Figure 3. FigureFigure3. Design 4. 4.Processed Processed of the powerPPP power Based ratio ratio ofDC of each each-DC converter converter Charging from from Unit Figure Figure 3. 3.

3.3. Design DesignAs shownof of the the PPP inPPP Figure Based Based DC-DC 1, DC-DC the concerned Charging Charging UnitDC Unit-DC converter is connected between a common DC bus and the ESS of the EV. This means that the application consists of a AsAs shown shown in in Figure Figure1, the1, the concerned concerned DC-DC DC-DC converter converter is connected is connected between between a common a com- constant voltage source (DC bus) and a variable load (EV’s ESS). When it comes to the DC DCmon bus DC and bus the and ESS the of theESS EV. of the This EV. means This that means the applicationthat the application consists ofconsists a constant of a voltageconstant bus, according to the SAE J1772 standard, its voltage value can vary from 200 V to 400 V sourcevoltage (DC source bus) and(DC abus) variable and a load variable (EV’s load ESS). (EV’s When ESS). it comes When to it the comes DC bus, to the according DC bus, in applications with power levels lower than 90 kW [4]. For this case, authors decided to toaccording the SAE J1772to the standard, SAE J1772 its standard, voltage value its voltage can vary value from can 200 vary V to from 400 V200 in applicationsV to 400 V in stablish it at its maximum value, 400 V. On the other hand, ESS voltage wise, its value withapplications power levels with lower power than levels 90 kWlower [4]. than For this90 kW case, [4]. authors For this decided case, authors to stablish decided it at its to changes from one manufacturer to another, but, in most cases does not exceed 380 V [4]. maximumstablish it value,at its 400maximum V. On thevalue, other 400 hand, V. On ESS the voltage other wise,hand, its ESS value voltage changes wise, from its value one Considering this, Figure 5 shows the stablished voltage and power curves for a manufacturerchanges from to one another, manufacturer but, in most to another, cases does but, notin most exceed cases 380 does V [4 not]. Considering exceed 380 this,V [4]. hypothetical EV fast charging application. As it can be observed, it consists of an electric FigureConsidering5 shows this, the stablishedFigure 5 shows voltage the and stablished power curves voltage for and a hypothetical power curves EV for fast a charging hypothet- battery charging: constant current, up to 60% of state of charge (SOC), and constant application.ical EV fast Ascharging it can application. be observed, As it it consists can be ofobserved, an electric it consists battery of charging: an electric constant battery current,voltage, up up to to 60% an of80% state of SOC. of charge (SOC), and constant voltage, up to an 80% of SOC. charging: constant current, up to 60% of state of charge (SOC), and constant voltage, up to an 80% of SOC.

Figure 5. Voltage and power profile of the modeled ESS. Figure 5. Voltage and power proﬁle of the modeled ESS.

BearingBearing in in mindmind that it it consists consists of of a voltage a voltage step step-down-down application, application, according according to Figure to Figure4, the4 appropriate, the appropriate PPC architecture PPC architecture is the is ISOP the ISOP(Figure (Figure 3a). In3a). addition, In addition, topology topology wise, a dual active bridge (DAB) is implemented for its simplicity and its bidirectional power flow (in case the application requires vehicle to grid functions). Figure 6 shows a high level schematic of the two solutions to compare: a DAB-FPP and a DAB-PPP.

Electronics 2021, 10, x FOR PEER REVIEW 6 of 17

400 90

80 350

70 300 60

250 50

200 40 0 20406080 SOC (%) Figure 5. Voltage and power profile of the modeled ESS. Electronics 2021, 10, 260 6 of 16 Bearing in mind that it consists of a voltage step-down application, according to Figure 4, the appropriate PPC architecture is the ISOP (Figure 3a). In addition, topology wise,wise, a a dual dual active active bridge bridge (DAB) (DAB) is is implemented implemented for for its its simplicity simplicity and and its its bidirectional bidirectional powerpower ﬂow flow (in (in case case the the application application requires requires vehicle vehicle to to grid grid functions). functions). Figure Figure6 shows6 shows a a highhigh level level schematic schematic of of the the two two solutions solutions to to compare: compare: a DAB-FPPa DAB-FPP and and a DAB-PPP.a DAB-PPP.

(a)

(b)

FigureFigure 6. 6.High High level level schematic schematic of of the the selected selected solutions solutions for for the the DC-DC DC-DC charging charging unit. unit. (a ()a DAB-FPP;) DAB-FPP; (b()b DAB-PPP.) DAB-PPP.

TakingTaking into into account account the the voltage voltage and and power power curves curves from from Figure Figure5, 5, Table Table1 speciﬁes 1 specifies thethe values values of of different different electrical electrical parameters parameters related related to to the the converter converter design. design. As As it it can can be be observed,observed, voltage voltage wise, wise, both both converters converters have have the the same same output output voltage. voltage. However, However, in in case case ofof the the DAB-FPP DAB-FPP the the input input voltage voltage is is constant, constant whereas,, whereas, in in the the DAB-PPP DAB-PPP consist consist of of the the differencedifference between between the the source source and and the the load. load. Then, Then, when when it it comes comes to to the theK pr𝐾, the, the DAB-PPP DAB-PPP isis expected expected to to process process just just the the 40% 40% of theof the total total power. power. Furthermore, Furthermore, this this value value is reduced is reduced to ato 5% a through 5% through the charging the charging process. process. Next, sinceNext,each since converter each converter has a different has a different input-output input- voltage relation, the required transformation ratio (n) and inductor (L) result in a different value.

Table 1. Main electrical parameters of the DAB-FPP and DAB-PPP.

Value Parameter DAB-FPP DAB-PPP

Vin (V) 400 173 ÷ 20 Vout (V) 227 ÷ 380 227 ÷ 380 Kpr (p.u.) 1 0.413 ÷ 0.05 fsw (kHz) 10 10 P[conv (kW) 90 23.25 n 1.0919 0.0919 L (µH) 23.05 1.94

Finally, before going into the simulation results, it is important to deﬁne the comparison parameters that will be used through the next subsections. In ﬁrst place, there is the processed active power by the converter Equation (6), which has been already described.

Pconv Kpr = (6) Psys Electronics 2021, 10, 260 7 of 16 Electronics 2021, 10, x FOR PEER REVIEW 7 of 16

energyIn second processed place, there by storage is the non-active components power (capacitors processed and by the inductors) converter. and According that is to not thetransferred definition givenfrom bythe IEEE source [26 ],to during the load the is steady considered state of as a non DC-DC-active converter, power theflow energy (푁), see processedEquations by storage(7) and components(8). In addition, (capacitors due to the and internally inductors) processed and thatis power not transferred by the converter, from theit source is worth to theconsidering load is considered the non-active as non-active power flowing power flow through (N), seethe Equationsinput source (7) andterminals (8). Inof addition, the converter due to Equation the internally (9). Then, processed the total power non by-active the converter, power processed it is worth by considering the converter theis non-active calculated power by summing flowing the through results the obtained input source from terminalsEquations of (7) the–(9), converter see Equation Equation (10). (9). Then, the total non-active power processed by the converter is calculated by summing 퐷·푇푠| ( ) | the results obtained from Equations2 (7)–(9),· Δ퐸퐿 see2 · Equation∫0 푣퐿 푡 (10).· 푖퐿(푡) 푑푡 푁퐿 = = (7) 푇푠 · 푇푆 · 2· R D Ts |v (t)·i (t)|dt 2 ∆EL 0 퐷·푇L L NL = 2 · Δ=퐸 2 · ∫ 푠|푣 (푡) · 푖 (푡)|푑푡 (7) Ts 퐶 0 TS 퐶 퐶 푁퐶 = = (8) 푇푠 푇푆 R D·Ts 2·∆EC 2· 0 |vC(t)·iC(t)|dt NC = = 2 2 (8) Ts 푁푖푛 = √푆푖푛 T−S 푃푖푛 (9) q 2 2 Nin푁=푡표푡 =S푁in퐿 −+ P푁in퐶 + 푁푖푛 (9)(10)

Last but not least, the efficiencyNtot = ofN theL + converterNC + Nin and the efficiency of the system(10) are calculated. When implementing FPP strategies, since the power processed by the Last but not least, the efficiency of the converter and the efficiency of the system are converter is the same as the one that flows from the source to the load, 휂 and 휂 are calculated. When implementing FPP strategies, since the power processed by푐표푛푣 the converter푠푦푠 the same Equation (11). However, PPP wise, the values of 휂푐표푛푣 and 휂푠푦푠 are related by is the same as the one that flows from the source to the load, ηconv and ηsys are the same the 퐾푝푟, see Equation (12). Equation (11). However, PPP wise, the values of ηconv and ηsys are related by the Kpr, see Equation (12). 푉푙표푎푑 푉표푢푡 휂 = = 휂 = (11) 푠푦푠퐹푃푃V 푐표푛푣Vout η = load푉푠표푢푟푐푒= η = 푉푖푛 (11) sysFPP V conv V source ( in ) 휂푠푦푠푃푃푃 = 1 − 퐾푝푟 · 1 − 휂푐표푛푣 (12) ηsysPPP = 1 − Kpr·(1 − ηconv) (12)

4. Simulation4. Simulation Results Results OnceOnce the the case case studio studio has has been been described, described, the the next next step step is tois simulateto simulate it andit and observe observe thethe obtained obtained results. results. On On the the one one hand, hand, Figure Figure7 shows 7 shows the the processed processed active active power power ratio ratio of of eacheach converter. converter. As expected,As expected, the DAB-FPPthe DAB-FPP always always processes processes a 100% a of100 the% totalof the power total thatpower ﬂowsthat from flows the from source the to source the load, to nothe matter load, no the matter working the point working of the point application. of the However,application. theHowever, DAB-PPP the achieves DAB-PPP a reduction achieves ofa reduction the power of processed the power by processed the converter, by the at converter, least, up at toleast, a 40% up of to the a total40% of power. the total Furthermore, power. Furthermore, this value decreasesthis value asdecreases the ESS as inside the ESS the inside EV charges,the EV achieving charges, achieving a minimum a minimum value of 5%. value This of is 5%. due This to the is factdue thatto the as fact the ESSthat charges,as the ESS itscharges, voltage valueits voltage increases, value provoking increases, GprovokingV to get closer 퐺푉 to to get 1. closer to 1.

Figure 7. Processed power ratio comparison between the DAB-FPP and the DAB-PPP. Figure 7. Processed power ratio comparison between the DAB-FPP and the DAB-PPP.

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OnOnOn the the the other other other hand, hand, hand, therethere there isis is the the non-activenon non-active-active power. power.power. In InIn this thisthis case, case, based based on on on Equations Equations Equations ( 7)(7) (7)–––(9),(9), (9),FigureFigure Figure 8 8 presents8 presents presents the the the different different different non non non-active-active-active power power power curves curves curves that that that exist exist exist inside inside inside the the the converter. converter. converter. As As Asitit canit can can be be beobserved, observed, observed, the the themost most most critical critical critical value value value in in both both in bothsolutions solutions solutions is is the the isinput input the inputnon non-active-active non-active power, power, power,followedfollowed followed by by the the by output output the output capacitor. capacitor. capacitor. This This is Thisis mainly mainly is mainly due due to dueto the the to high thehighhigh RMS RMS RMS currents currents currents that that thatexist exist existinin a a inDAB DAB a DAB topology. topology. topology.

(a(a) ) (b(b) ) FigureFigure 8. 8. Processed Processed non non-active-active power power by by the the components. components. ( a(a) )DAB DAB-FPP-FPP; ;( b(b) )DAB DAB-PPP.-PPP. Figure 8. Processed non-active power by the components. (a) DAB-FPP; (b) DAB-PPP.

InInIn order order order to t haveoto have have a bettera a better better view view view of theof of non-activethe the non non-active-active power power power results, results, results, Figure Figure Figure9 shows 9 9 shows theshows total the the non-activetotaltotal non non- curvesactive-active curves ofcurves each of of solution. each each solution. solution. Again, Again, theAgain, DAB-PPP the the DAB DAB offers-PPP-PPP betteroffers offers resultsbetter better results throughresults through through great partgreatgreat of part thepart chargingof of the the charging charging process, process, process, except except forexcept the for ﬁrstfor the the charging first first charging charging periods periods periods (SOC (SOC < (SOC 5%), < < where5%), 5%), where where the DAB-PPPthethe DAB DAB- processesPPP-PPP processes processes higher higher higher amounts amounts amounts of non-active of of non non-active-active power. power. power. However, However, However, considering considering considering that thethat that greatthethe great majoritygreat majority majority of the of EVsof the the will EVs EVs reach will will reachthe reach charging the the charging charging station station station with SOC with with values SOC SOC values highervalues higher thanhigher a 5%,than than thisaa 5%, 5%, period this this canperiod period be dismissed.can can be be dismissed. dismissed.

FigureFigure 9. 9. Processed Processed non non-active-active power power compari comparisonson between between the the DAB DAB-FPP-FPP and and the the DAB DAB-PPP.-PPP. Figure 9. Processed non-active power comparison between the DAB-FPP and the DAB-PPP.

Finally,Finally,Finally, efﬁciency efficiency efficiency wise, wise, wise, Figure Figure Figure 10 10 10shows shows shows that that that the the the DAB-PPP DAB DAB-PPP-PPP is is ais a morea more more efﬁcient efficient efficient solution solution solution thanthanthan the the the DAB-FPP. DAB DAB-FPP.-FPP. Similarly Similarly Similarly to to to Figure Figure Figure9, 9, at9, at lowat low low SOC SOC SOC values, values, values, the the the DAB-PPP DAB DAB-PPP-PPP does does does not not not offer offer offer a good performance but, this quickly changes as the ESS of the EV charges and the 퐾퐾푝푟 a gooda good performance performance but, but, this this quickly quickly changes changes as as the the ESS ESS of of the the EV EV charges charges and and the theK pr 푝푟 decreases.decreases.decreases.

ElectronicsElectronicsElectronics2021 2021 2021, 10, ,10 ,10 260, ,x x FOR FOR PEER PEER REVIEW REVIEW 99 of9 of of 16 16 16

FigureFigure 10. 10. Efficiency Efficiency comparison comparison between between the the DAB DAB--FPPFPP and and the the DAB DAB--PPP.PPP. Figure 10. Efﬁciency comparison between the DAB-FPP and the DAB-PPP.

5.5.5. Experimental ExperimentalExperimental Platform PlatformPlatform WithWithWith the thethe aim aimaim of ofof conﬁrming confirmingconfirming the thethe beneﬁts benefitsbenefits of ofof implementing implementingimplementing PPCs PPCsPPCs on onon EV EVEV fast fastfast charging chargingcharging applications,applications,applications, the the the present present present section section section details details details the the the experimental experimental experimental results resultsresults obtained obtained obtained through through through the the the comparisoncomparisoncomparison between betweenbetween a aa DAB-FPP DABDAB--FPPFPP and andand a a DAB-PPP.a DABDAB--PPP.PPP.

5.1.5.1.5.1. Prototype PrototypePrototype Design DesignDesign TheTheThe experimental experimental experimental platform platform platform built built built for for for the the the concerned concerned concerned tests tests tests consists consists consists of of of a a adownscaled downscaled downscaled prototypeprototypeprototype whose whose whose voltage voltage voltage and and and power power power curves curves curves are are are shown shown shown in in in Figure Figure Figure 11 11. 11.. As As As it it it can can can be be be observed, o observed,bserved, ititit models modelsmodels the thethe behavior behaviorbehavior of ofof the thethe charging chargingcharging process processprocess of ofof an anan electric electricelectric battery. batterybattery..

FigureFigure 11. 11. Modelled Modelled voltage voltage and and power power profile profile of of the the load load for for the the design design of of the the prototype. prototype. Figure 11. Modelled voltage and power proﬁle of the load for the design of the prototype.

BasedBasedBased on on on the the the curves curves curves from from from Figure Figure Figure 11, Table 11, 11, 2TableTable shows 2 2 the showsshows main the the electrical main main designelectrical electrical parame- design design tersparameparame of thetersters experimental ofof thethe experimentalexperimental platform. platform.platform. There, it canThere,There, be observeditit cancan bebe observed thatobserved it consists thatthat itit of consistsconsists a constant ofof aa voltageconstantconstant source voltagevoltage and sourcesource a variable andand a load.a variablevariable When load.load. it comes WhenWhen to itit the comescomes voltage toto thethe values, voltagevoltage 220 values,values, V has been220220 VV decidedhashas beenbeen to decided deﬁnedecided as toto initial definedefine charging asas initialinitial point. chargingcharging This point. way,point. it ThisThis is assumed way,way, itit thatisis assumedassumed the EV willthatthat arriveththee EVEV withwillwill at arrivearrive least withwith a SOC atat ofleastleast 1.5%. aa SOCSOC ofof 1.5%.1.5%.

TableTable 2. 2. Main Main electrical electrical parameters parameters of of the the experimental experimental platform. platform.

ParameterParameter ValueValue

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Table 2. Main electrical parameters of the experimental platform.

Parameter Value

P\source (kW) 3 Vsource (V) 320 Vload (V) 220 ÷ 300

Based on Figure 11 and bearing in mind the electrical parameters presented in Table2, the main electrical parameters of the DAB-FPP and the DAB-PPP prototypes are obtained, see Table3.

Table 3. Main electrical parameters of the DAB-FPP and DAB-PPP prototypes.

Value Parameter DAB-FPP DAB-PPP

Vin (V) 320 100 ÷ 20 Vout (V) 220 ÷ 300 220 ÷ 300 Kpr (p.u.) 1 0.31 ÷ 0.06 fsw (kHz) 50 50 P[conv (kW) 3 0.69

Then, it is time to calculate the adequate design values for the components that exist inside the converter. In ﬁrst place, there are the active components, which consist of 8 semiconductors: half of them located at the primary side of the transformer (Q1–4) and the other half located at the secondary side of the transformer (Q5–8). Bearing in mind Table3 , it can be concluded that switches Q5–8 must withstand at least 300V at both architectures. However, switches Q1–4 will observe different voltage values at the DAB-FPP and at the DAB-PPP, 320 V and 100 V respectively. Applying a security margin, it is decided to implement switches rated for the double of the voltage, see Table4. As it can be observed, the same component is implemented for all the switches except for Q1–4 at the DAB-PPP. In consequence, the on resistance of the devices at its primary side is three times lower.

Table 4. Active and passive components of the DAB-FPP and DAB-PPP.

Value Component DAB-FPP DAB-PPP 1 2 Q1–4 IPT65R033G7 IPT111N20NFD Q5–8 IPT65R033G7 IPT65R033G7 C (µF) MKP1848C 3 MKP1848C n 1.31 0.261 L (µH) 101.5 305.34 4 1 2 3 VDS = 650 V, ID = 69 A, RDS = 33 mΩ. VDS = 250 V, ID = 96 A, RDS = 11.1 mΩ. VISHAY 500 V 100 µF (2 times paralleled). 4 The inductance is located at the secondary side of the transformer.

On the other hand, when it comes to the transformation ratio (taking as example the DAB-FPP), its value has been designed for an intermediate value between two extreme working conditions: starting the battery charge Equation (13) and ending the battery charge Equation (14). The obtained value is shown in Equation (15).

Vin 100 n1 = = = 0.45b (13) Vout 220

Vin 20 n2 = = = 0.06ˆ (14) Vout 300 0.45b + 0.06ˆ n = = 0.261 (15) DAB,PPP 2 Electronics 2021, 10, 260 11 of 16

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Once the transformation ratio is deﬁned, inductor’s value must be designed for the most critical point. In this case, the moment in which the converter processes more power is is at the start of the charge. Equation (16) shows the obtained inductance refereed to at the start of the charge. Equation (16) shows the obtained inductance refereed to primary primary side. side. 0 n·Vin·Vout 0.261 × 100 × 220 ′≤ 푛 · 푉푖푛 · 푉표푢푡= 0.261 × 100 × 220= µ L퐿 ≤ = 3 20.8= 20.8H µH (16) 8· fsw·Pconv 8 × 50 × 10 ×3690 (16) 8 · 푓푠푤 · 푃푐표푛푣 8 × 50 × 10 × 690 InIn orderorder toto reducereduce thethe currentcurrent throughthrough thethe inductor,inductor, itit isis decideddecided toto locatelocate itit atat thethe secondarysecondary sideside ofof thethe transformertransformer EquationEquation (17).(17). L0퐿′ 20.820.×8 ×1010−6−6 L =퐿 = == ==305.33305.33µ HµH (17)(17) n2푛2 0.2610.2612 2 Finally,Finally, thethe twotwo prototypesprototypes toto comparecompare areare shownshown inin FigureFigure 1212.. AsAs itit can be observed, thethe mostmost apparentapparent differencedifference betweenbetween themthem areare thethe magnetics.magnetics. To bebe moremore precise,precise, thethe nucleusnucleus usedused forfor thethe DAB-FPP DAB-FPP is is a a PM8770 PM8770 type, type, whereas, whereas, the the one one used used for for the the DAB-PPP DAB-PPP is aisPM6249. a PM6249. This This supposes supposes a 65%a 65% reduction reduction in in the the volume volume of of the the magnetics. magnetics.In In relationrelation toto thethe transformertransformer andand thethe inductor,inductor, allall ofof themthem areare assembledassembledat at thethe laboratory.laboratory.

(a)

(b) Figure 12. Experimental prototype. (a) DAB-FPP; (b) DAB-PPP. Figure 12. Experimental prototype. (a) DAB-FPP; (b) DAB-PPP.

5.2.5.2. Test ConditionsConditions OnceOnce thethe entireentire prototypeprototype hashas beenbeen described,described, thethe nextnext stepstep isis toto testtest it.it. For this purpose, thethe powerpower converterconverter isis forcedforced toto workwork atat fourfour differentdifferent points,points, seesee TableTable5 .5. These These points points modelmodel the charging charging process process of of the the battery battery and and they they have have been been extracted extracted from from FigureFigure 11. 11As. Asit can it can be be observed, observed, the the source source voltage voltage always always remains remains constant constant and and the load load voltage voltage increasesincreases togethertogether withwith thethe power.power. TheThe ﬁnalfinal pointpoint correspondscorresponds toto thethe constantconstant voltagevoltage period,period, where,where, thethe powerpower consumedconsumed byby thethe loadload decreases.decreases. InIn thisthis case,case, thethe selectedselected valuevalue isis aroundaround aa 50%50% ofof thethe nominalnominal power.power.

Table 5. Testing conditions extracted from Figure 11.

퐒퐎퐂 (%) 푽풔풐풖풓풄풆 (V) 푽풍풐풂풅 (V) 푷풍풐풂풅 (kW) 1.5 320 220 2.2 5.7 320 260 2.6

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(b) Figure 12. Experimental prototype. (a) DAB-FPP; (b) DAB-PPP.

5.2. Test Conditions Once the entire prototype has been described, the next step is to test it. For this purpose, the power converter is forced to work at four different points, see Table 5. These points model the charging process of the battery and they have been extracted from Figure 11. As it can be observed, the source voltage always remains constant and the load voltage in- Electronics 2021, 10, 260 creases together with the power. The final point corresponds to the constant voltage12 of 16pe- riod, where, the power consumed by the load decreases. In this case, the selected value is around a 50% of the nominal power.

TableTable 5. 5.Testing Testing conditions conditions extracted extracted from from Figure Figure 11 11.. SOC (%) V (V) V (V) P (kW) 𝐒𝐎𝐂 (%) sourse𝑽𝒔𝒐𝒖𝒓𝒄𝒆 (V) load𝑽𝒍𝒐𝒂𝒅 (V) 𝑷load𝒍𝒐𝒂𝒅 (kW) 1.51.5 320320 220220 2.22.2 5.75.7 320320 260260 2.62.6 6060 320320 300300 3 3 8080 320320 300300 1.51.5

OnOn the the other other hand,hand, when it it comes comes to to the the ex experimentalperimental set set up, up, Figure Figure 13a,b 13 a,bshow show a sim- a simpliﬁedplified electric electric diagram diagram of the of the implement implementeded set-ups set-ups for forthe theDAB-FPP DAB-FPP and and the theDAB-PPP, DAB- PPP,respectively. respectively. As it Ascan it be can observed, be observed, in the ca inse the of casethe DAB-FPP, of the DAB-FPP, 4 parameters 4 parameters are measured are 𝐼 𝐼 𝑉 𝑉 measured( , (Isource, , Iload, andVsource and). VHowever,load). However, DAB-PPP DAB-PPP wise, 6 wise, different 6 different parameters parameters are re- arequired required (𝐼 (Isource, 𝐼, Iload, 𝐼, Iout, 𝑉,Vsource, 𝑉,V inandand 𝑉Vout).).

(a)

(b)

FigureFigure 13. 13.Simpliﬁed Simplified electric electric circuit circuit of of the the experimental experimental set-up. set-up. (a ()a DAB-FPP;) DAB-FPP; (b ()b DAB-PPP.) DAB-PPP.

Electronics 2021, 10, x FOR PEER REVIEWThen, Then, with with the the aim aim of of securing securing that that the the semiconductors semiconductors do notdo exceednot exceed the temperaturethe tempera-13 of 17 limit,ture alimit, thermocouple a thermocouple is stuck is tostuck the to PCB the and PCB to and one to of one the of semiconductors the semiconductors of each of full-each bridge. Finally, Figure 14 shows a real image of the set-up used for the experimental tests. More detailed information about the equipment used through the test is given in Appendixfull-bridge.A. Finally, Figure 14 shows a real image of the set-up used for the experimental tests.

Power meter

Power conversion circuit Source

Temperature measurement Load

Figure 14. The built set-up for the experimental tests. Figure 14. The built set-up for the experimental tests.

5.3.5.3. Experimental Experimental Results Results OnceOnce the the test test conditions conditions have have been been correctly correctly deﬁned, defined, the the next next step step is is to to observe observe the the obtainedobtained results.results. Following Following the the testing testing points points defined deﬁned in inTable Table 5,5 Figure, Figure 15 15shows shows the the tem- temperatureperature evolution evolution of the of the semiconductors semiconductors and and the thePCBs. PCBs. It is It worth is worth mentioning mentioning that that each eachdark dark dot dotmarks marks the thebeginning beginning of a of new a new testing testing point point and and that, that, since since the the power power block block has hasbeen been oversized, oversized, the the obtained obtained temperature temperature values values are are not not disturbing. disturbing. On the one hand, Figure 15a shows the temperature results obtained by the DAB- FPP. There, it can be observed that the most critical component is the secondary side semiconductor, which achieves a maximum temperature of 32.4 °C when working at maximum power point (𝑉 = 300 V & SOC = 60%). On the other hand, Figure 15b shows the results obtained by the DAB-PPP. In this case, the most critical component is the primary side semiconductor, which achieves a maximum temperature of 29 °C when working at the initial charging point (𝑉 = 220 V). Then, as the load voltage increases, the temperature of the concerned component decreases. This thermal behavior is completely different to the one obtained by the DAB-FPP but, it can be very interesting to take it into account when designing a PPC. Indeed, assuming that the initial charging conditions vary in voltage rapidly, see Figure 11, the engineers can optimize the designing criteria by expecting a lower thermal stress.

38 30 Primary-side PCB Primary-side PCB 36 Primary-side Q Primary-side Q Secondary-side PCB 28 Secondary-side PCB 34 Secondary-side Q Secondary-side Q 32 26 30

28 24

26 22 24

22 20 0 500 1000 1500 0 500 1000 1500 2000 t (s) t (s)

(a) (b)

Figure 15. Temperature evolution. (a) DAB-FPP; (b) DAB-PPP.

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working at the initial charging point (푉푙표푎푑 = 220 V). Then, as the load voltage increases, the temperature of the concerned component decreases. This thermal behavior is completely different to the one obtained by the DAB-FPP but, it can be very interesting to take it into account when designing a PPC. Indeed, assuming that the initial charging Electronics 2021, 10, 260 13 of 16 conditions vary in voltage rapidly, see Figure 11, the engineers can optimize the designing criteria by expecting a lower thermal stress.

(a) (b)

Figure 15. Temperature evolution. (a) DAB-FPP; (b) DAB-PPP. Figure 15. Temperature evolution. (a) DAB-FPP; (b) DAB-PPP.

OnThe the fact one that hand, the temperature Figure 15a shows of the thecomponent temperature decreases results as obtained the battery by charges the DAB- is 퐾 FPP.directly There, related it can to bethe observed 푝푟. This that is proved the most in criticalTable 6 component, where the issystem the secondary and converter side semiconductor,powers are shown which for each achieves testing a maximumpoint. Analyzing temperature the DAB of- 32.4PPP column,◦C when it workingis observed at that 푃푠표푢푟푐푒 increases through the first three working points (constant current mode) and, maximum power point (Vload = 300 V & SOC = 60%). On the other hand, Figure 15b showsin the last the one, results it decreases obtained (constant by the DAB-PPP. voltage mode). In this However, case, the most when critical it comes component to the input is thepower primary (푃푖푛) sideof the semiconductor, converter, it always which decreases achieves a in maximum value. This temperature is due to the of 29fact◦C that, when as the battery charges, 푉 is getting closer to 푉 , causing a reduction in 퐾 . At the working at the initial charging푙표푎푑 point (Vload = 220 V푠표푢).푟 Then,푐푒 as the load voltage increases,푝푟 the temperaturelast point, the of 퐾 the푝푟 concernedis not reduced component because decreases. 푉푙표푎푑 is maintained This thermal at 300 behavior V. is completely different to the one obtained by the DAB-FPP but, it can be very interesting to take it intoTable account 6. Processed when power designing by the a DAB PPC.-FPP Indeed, and DAB assuming-PPP. that the initial charging conditions vary in voltage rapidly, see Figure 11, the engineers can optimize theValue designing criteria by expectingTest Condition a lower thermal stress.Parameter DAB-FPP DAB-PPP The fact that the temperature of the component decreases as the battery charges is 푃푠표푢푟푐푒 (kW) 2.24 directly related to the Kpr. This is proved in Table6, where2.21 the system and converter 푉 = 220 푃 (kW) 0.699 powers푙표푎푑 are shown (V) for each testing푖푛 point. Analyzing the DAB-PPP column, it is observed 퐾 (p.u.) 1 0.312 that Psource increases through the푝푟 ﬁrst three working points (constant current mode) and, in 푃 (kW) 2.53 the last one, it decreases (constant푠표푢푟푐푒 voltage mode). However,2.58 when it comes to the input power푉푙표푎푑 (=Pin260) of the(V) converter, it푃푖푛 always (kW) decreases in value. This is due to the0 fact.474 that, as the battery charges, Vload is getting퐾푝푟 (p.u. closer) to Vsource, causing1 a reduction in Kpr0..187 At the last point, the Kpr is not reduced푃 because푠표푢푟푐푒 (kWVload) is maintained at 300 V. 2.85 푉푙표푎푑 = 300 (V) 3.02 푃푖푛 (kW) 0.176 TableSOC 6. Processed= 60% power by the DAB-FPP and DAB-PPP. 퐾푝푟 (p.u.) 1 0.062 푃푠표푢푟푐푒 (kW) Value 1.38 푉푙표푎푑Test= Condition300 (V) Parameter 1.54 푃푖푛 (kW) DAB-FPP DAB-PPP0.086 SOC = 80% 퐾 (p.u.) 1 0.062 Psourse푝푟 (kW) 2.24 2.21 Vload = 220 (V) Pin (kW) 0.699 Kpr (p.u.) 1 0.312

Psourse (kW) 2.53 2.58 Vload = 260 (V) Pin (kW) 0.474 Kpr (p.u.) 1 0.187

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Table 6. Cont.

Value Test Condition Parameter DAB-FPP DAB-PPP

Psourse (kW) 2.85 V = 300 (V) 3.02 load P (kW) 0.176 SOC = 60% in Kpr (p.u.) 1 0.062

Psourse (kW) 1.38 Vload = 300 (V) 1.54 Electronics 2021, 10, x FOR PEER REVIEW Pin (kW) 0.086 14 of 16 SOC = 60% Kpr (p.u.) 1 0.062

EfficiencyEfficiency wise, wise, Figure Figure 16 16shows shows the the obtained obtained results. results. As As it canit can be be observed, observed, although although thethe efficiency efficiency of of the the DAB-PPP DAB-PPP converter converter is notis not extraordinary extraordinary (most (most of of the the cases cases bellow bellow the the DAB-FPP),DAB-FPP), the the efficiency efficiency of of the the DAB-PPP DAB-PPP system system achieves achieves the the best best results, results, with with a peaka peak valuevalue of of 99.62%. 99.62%. Compared Compared to to the the maximum maximum efficiency efficiency obtained obtained by by the the DAB-FPP DAB-FPP (94.7%), (94.7%), it canit can be be concluded concluded that that the the DAB-PPP DAB-PPP offers offers a mucha much efficient efficient performance. performance.

FigureFigure 16. 16.Efﬁciency Efficiency obtained obtained by by the the prototypes. prototypes.

6.6. Conclusions Conclusions TheThe present present paper paper compares, compares, in in terms terms of of processed processed power power by by the the converter converter and and efficiency,efficiency, the the performance performance between between a DAB-FPPa DAB-FPP and and a DAB-PPPa DAB-PPP for for an an EV EV fast fast charging charging application.application. Through Through the the simulations, simulations, it isit is observed observed that that the the DAB-PPP DAB-PPP reduces reduces the the active active andand non-active non-active power power to to process process by by the the converter converter and and that that it improvesit improves the the efficiency. efficiency. This This isis confirmed confirmed by by testing testing two two downscaled downscaled prototypes. prototypes. On On the the one one hand, hand, regarding regarding the the temperaturetemperature obtained obtained by by the the most most critical critical component, component, it isit concludedis concluded that that the the DAB-PPP DAB-PPP achievesachieves lower lower temperature temperature stress. stress. On the On other the hand, other processed hand, processed power wise, power it is observed wise, it is thatobserved the DAB-PPP that the processesDAB-PPP betweenprocesses a between 30 and a a 6%30 and of the a 6% total of the power total that power flows that from flows the source to the load. In consequence, the achieved efficiencies always improve the ones from the source to the load. In consequence, the achieved efficiencies always improve the obtained by the DAB-FPP. Apart from that, when it comes to the size of the magnetics ones obtained by the DAB-FPP. Apart from that, when it comes to the size of the magnetics (inductor and transformer), it is observed that a volume reduction of 65% is achieved (inductor and transformer), it is observed that a volume reduction of 65% is achieved with with the DAB-PPP. To sum up, in case the regulation of the country does not oblige to the DAB-PPP. To sum up, in case the regulation of the country does not oblige to implement galvanic isolation at the charging unit, PPCs turn to be more efficient and lower implement galvanic isolation at the charging unit, PPCs turn to be more efficient and size solutions than conventional FPP converters. lower size solutions than conventional FPP converters. 7. Future Lines 7. Future Lines With the aim of improving the present paper, the authors propose the next future lines are: With the aim of improving the present paper, the authors propose the next future lines are:  Include a processed non-active power analysis on the prototypes that verifies the reduction of energy processed by storage elements.  As observed in Figures 9 and 10, the DAB-PPP presents higher processed non-active power and lower efficiency at initial charging periods. This is due to the fact that power converters inside PPC architectures are required to work at a wider operation range than FPCs [20]. In consequence, lower performance is achieved at extreme working points. In order to improve this, extended analysis on advanced modulation methods is proposed.  Although the main objective of the paper is to compare the behavior of a given topology (in this case a DAB) when it is implemented on a FPP architecture and on a PPP architecture, this comparison may not be considered completely fair, since one

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• Include a processed non-active power analysis on the prototypes that veriﬁes the reduction of energy processed by storage elements. • As observed in Figures9 and 10, the DAB-PPP presents higher processed non-active power and lower efﬁciency at initial charging periods. This is due to the fact that power converters inside PPC architectures are required to work at a wider operation range than FPCs [20]. In consequence, lower performance is achieved at extreme working points. In order to improve this, extended analysis on advanced modulation methods is proposed. • Although the main objective of the paper is to compare the behavior of a given topology (in this case a DAB) when it is implemented on a FPP architecture and on a PPP architecture, this comparison may not be considered completely fair, since one of the solutions provides galvanic isolation and the other one does not. Therefore, it is proposed to extend the comparison by adding the AC-DC stage. Indeed, as shown in Figure1, every EV fast charging station consists of an AC-DC and a DC-DC stage and, when it comes to the galvanic isolation, it can be located at one stage or the other. This way, the galvanic isolation can be provided by both solutions: non-isolated AC-DC stage with an isolated DC-DC stage (DAB-FPP) and an isolated AC-DC stage with a non-isolated DC-DC stage (DAB-PPP).

Author Contributions: Conceptualization, J.A. and I.A.; validation, J.A., A.A.; investigation, J.A.; data curation, J.A.; writing—original draft preparation, J.A.; writing—review and editing, J.A., I.A. and A.A.; supervision, I.A.; All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Appendix A The objective of the present appendix is to list the equipment that has been used for the correct measuring of the experimental tests, see Table A1.

Table A1. Equipment used for the measuring of the experimental tests.

Description Reference Source ITECH IT6012C-800-40 Load EA-ELR 9750-22 Power meter YOKOGAWA WT500 Temperature measurement Pico TC-08

References 1. Irle, R. EV-Volumes—The Electric Vehicle World Sales Database. 2018. Available online: http://www.ev-volumes.com/ (accessed on 6 March 2019). 2. Gorzelany, J. The Longest Range Electric Cars For. 2019. Available online: https://insideevs.com/features/342424/the-longest- range-electric-cars-for-2019/ (accessed on 6 March 2019). 3. Dickerman, L.; Harrison, J. A new car, a new grid: The electric car is back (with Help from New Batteries, a Smarter Grid, and Uncle Sam). IEEE Power Energy Mag. 2010, 8, 55–61. [CrossRef] 4. Christen, D. Analysis and Performance Evaluation of Converter Systems for EV-Ultra-Fast Charging Stations with Integrated Grid Storage. Doctoral Dissertation, ETH Zurich, Zürich, Switzerland, 2017; p. 195. 5. Etter Place: What Went Wrong for the Electric car Startup? | Environment | The Guardian. Available online: https://www. theguardian.com/environment/2013/mar/05/better-place-wrong-electric-car-startup (accessed on 23 September 2020). 6. Bosshard, R.; Badstubner, U.; Kolar, J.W.; Stevanovic, I. Comparative evaluation of control methods for Inductive Power Transfer. In Proceedings of the 2012 International Conference on Renewable Energy Research and Applications (ICRERA), Nagasaki, Japan, 11–14 November 2012. 7. Chargemap—Electric Cars’ Charging Station Map. Available online: https://chargemap.com/map (accessed on 23 September 2020). Electronics 2021, 10, 260 16 of 16

8. Iyer, V.M.; Gulur, S.; Gohil, G.; Bhattacharya, S. Extreme fast charging station architecture for electric vehicles with partial power processing. In Proceedings of the 2018 IEEE Applied Power Electronics Conference and Exposition (APEC), San Antonio, TX, USA, 4–8 March 2018; pp. 659–665. 9. Zapata, J.W.; Kouro, S.; Carrasco, G.; Meynard, T.A. Step-down partial power DC-DC converters for two-stage photovoltaic string inverters. Electronics 2019, 8, 87. [CrossRef] 10. Zapata, J.W.; Kouro, S.; Carrasco, G.; Renaudineau, H. Step-up partial power DC-DC converters for two-stage PV systems with interleaved current performance. Energies 2018, 11, 357. [CrossRef] 11. Zientarski, J.R.R.; Martins, M.L.d.; Pinheiro, J.R.; Hey, H.L. Series-Connected Partial-Power Converters Applied to PV Systems: A Design Approach Based on Step-Up/Down Voltage Regulation Range. IEEE Trans. Power Electron. 2017, 33, 7622–7633. [CrossRef] 12. Mira, M.C.; Zhang, Z.; Jorgensen, K.L.; Andersen, M.A.E. Fractional Charging Converter with High Efficiency and Low Cost for Electrochemical Energy Storage Devices. IEEE Trans. Ind. Appl. 2019, 55, 7461–7470. [CrossRef] 13. Mishra, S.; Tamballa, S.; Pallantala, M.; Raju, S.; Mohan, N. Cascaded Dual-Active Bridge Cell Based Partial Power Converter for Battery Emulation. In Proceedings of the 2019 20th Workshop on Control and Modeling for Power Electronics (COMPEL), Toronto, ON, Canada, 17–20 June 2019; pp. 1–7. 14. Pape, M.; Kazerani, M. Turbine Startup and Shutdown in Wind Farms Featuring Partial Power Processing Converters. IEEE Open Access J. Power Energy 2020, 7, 254–264. [CrossRef] 15. Pape, M.; Kazerani, M. An Offshore Wind Farm with DC Collection System Featuring Differential Power Processing. IEEE Trans. Energy Convers. 2019, 35, 222–236. [CrossRef] 16. Qin, S.; Barth, C.B.; Pilawa-Podgurski, R.C.N. Enhancing Microinverter Energy Capture with Submodule Differential Power Processing. IEEE Trans. Power Electron. 2014, 31, 3575–3585. [CrossRef] 17. Kasper, M.; Bortis, D.; Kolar, J.W. Unified power flow analysis of string current diverters. Electr. Eng. 2018, 100, 2085–2094. [CrossRef] 18. Iyer, V.M.; Gulur, S.; Bhattacharya, S.; Ramabhadran, R. A Partial Power Converter Interface for Battery Energy Storage Integration with a DC Microgrid. In Proceedings of the 2019 IEEE Energy Conversion Congress and Exposition (ECCE), Baltimore, MD, USA, 29 September–3 October 2019; pp. 5783–5790. 19. Candan, E.; Shenoy, P.S.; Pilawa-Podgurski, R.C.N. A Series-Stacked Power Delivery Architecture with Isolated Converters For Energy Efficient Data Centers. IEEE Trans. Power Electron. 2014, 31, 3690–3703. [CrossRef] 20. Iyer, V.M.; Gulur, S.; Gohil, G.; Bhattacharya, S. An Approach towards Extreme Fast Charging Station Power Delivery for Electric Vehicles with Partial Power Processing. IEEE Trans. Ind. Electron. 2019, 67, 8076–8808. [CrossRef] 21. Rojas, J.; Renaudineau, H.; Kouro, S.; Rivera, S. Partial power DC-DC converter for electric vehicle fast charging stations. In Proceedings of the IECON 2017-43rd Annual Conference of the IEEE Industrial Electronics Society, Beijing, China, 29 October–1 November 2017; Volume 2017, pp. 5274–5279. 22. Buck, J. International electrotechnical commission. Handb. Transnatl. Econ. Gov. Regimes 2016, 573–584. [CrossRef] 23. Anzola, J.; Aizpuru, I.; Romero, A.A.; Loiti, A.A.; Lopez-Erauskin, R.; Artal-Sevil, J.S.; Bernal, C. Review of Architectures Based on Partial Power Processing for DC-DC Applications. IEEE Access 2020, 8, 103405–103418. [CrossRef] 24. Zhao, J.; Yeates, K.; Han, Y. Analysis of High Efficiency DC/DC Converter Processing Partial Input/Output Power. In Proceedings of the 2013 IEEE 14th Workshop on Control and Modeling for Power Electronics (COMPEL), Salt Lake City, UT, USA, 23–26 June 2013; pp. 1–8. 25. Chen, L.; Wu, H.; Xu, P.; Hu, H.; Wan, C. A high step-down non-isolated bus converter with partial power conversion based on synchronous LLC resonant converter. In Proceedings of the 2015 IEEE Applied Power Electronics Conference and Exposition (APEC), Charlotte, NC, USA, 15–19 March 2015; Volume 2015, pp. 1950–1955. 26. IEEE. IEEE Standard Definitions for the Measurements of Electric Power Quantities Under Sinusoidal, Nonsinusoidal, Balanced or Unbalanced Conditions. Available online: https://pdfs.semanticscholar.org/29a9/36f02016a71ca26ea5d4fa05a2a974984b14. pdf?_ga=2.41965094.2034374545.1611191539-1641229895.1606892986 (accessed on 8 April 2019).