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Electric Vehicle Fast Charging ChallengesWith the pressure on governments to reduce carbon emissions continuing, the interest in battery electricvehicles (BEV) continues to grow as part of the solution to this challenge. The BEV market continues tooffer ever more choice at increasingly attractive price points. However, range angst remains a key concernamong consumers. This issue is compounded by the need to re-think refuelling. Parking the vehicle while atwork could be the perfect opportunity to recharge, but a lack of infrastructure means that many BEV ownersfeel bound to recharge at home. For longer journeys, such as vacations, consumers expect that rechargingcan be undertaken with a rapidity that matches, or comes close to, refuelling an internal combustion engine(ICE) vehicle. Pradip Chatterjee and Markus Hermwille, Infineon Technologies, Germany

In order to facilitate charging at home,most vehicles provide support for chargingvia a household single-phase alternativecurrent (AC) supply. This allows them to becharged overnight. Solutions range fromsimple provision of a cable to connect thevehicle to a power outlet, to in-cable controland protection devices (IC-CPD), and wall-box chargers that may include furthercomplexity, such as communicationbetween the vehicle and power unit, withgrounding and protection inside.The batteries themselves of course

require a direct current (DC) supply forcharging, with the conversion from AC toDC occurring in charging electronics builtinto the vehicle. This approach requires that

every vehicle be fitted with a chargingsolution that must be designed according toall the normal constraints of cooling,efficiency and weight, factors that ultimatelylimit charging power and, therefore,charging speed. The obvious way forward isto develop universal off-board DC chargers.

Approaches to fast DC chargingA typical 22 kW AC charger providesenough charge in 120 minutes to providean additional 200 km of vehicle range,more than enough for topping up while atwork. However, in order to reduce the 200km range charge time to 16 minutesrequires recourse to a 150 kW DC chargingstation. At 350 kW, the charging time for this

range can be reduced to around just 7minutes, somewhere approaching an ICEvehicle refuelling visit. These figures, ofcourse, additionally assume that the targetbattery can support such charging rates.And, just like refuelling at the pump,consumers expect the industry to provide astandardized fuelling experience regardlessof where they recharge.In Europe, the organization CharIN e. V.

focusses on developing and promoting theCombined Charging System (CCS). Theirspecifications define the charging plug,charging sequence and even datacommunication. Other regions, such asJapan and China, have similar organizationssuch as CHAdeMO and GB/T respectively,

Figure 1: Block diagrams for two potential high-power DC charger approaches

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while Tesla has its own proprietary system.The CharIN specifications define support

for both AC and DC charging via their plugand socket implementation. They alsoenvisage a maximum constant currentoutput of 500 A at 700 VDC, with supportup to 920 VDC. System efficiency is also setat 95 %, although this will rise to 98 % inthe future. It should be noted that a 1 %loss in efficiency for a 150 kW charger isequivalent to 1.5 kW. Thus, reducing lossesto an absolute minimum is a priority in fastDC charger designs.

Fast DC charger architecturesHigh-power DC charger design typicallyfollows one of two basic approaches. Thefirst converts a 3-phase AC supply into avariable DC output that feeds a DC/DCconverter. The exact DC voltage is definedafter communication with the vehicle beingcharged. The alternative approach is toconvert the incoming AC to a fixed DCvoltage, whereupon a DC/DC converteradjusts the output voltage to match theneeds of the vehicle’s battery (Figure 1).With neither approach considered to have aclear advantage or disadvantage incomparison with the other, it is systemchallenges that determine the optimalapproach. Such high-power solutions willnot use a monolithic approach; instead thedesired power output will be achieved bycombining multiple charging subunits, eachcontributing somewhere between 15 and60 kW. Thus, the key design goals are theminimization of cooling effort, delivery ofhigh-power density, and the reduction ofoverall system size.Design efficiency starts at the front end

with the AC/DC conversion stage. Theimplementation of this power factorcorrection stage is usually implemented byVienna rectifier topology. The possibility ofusing 600 V active devices helps it toachieve the right balance of cost andperformance. With the availability of high-voltage SiC devices, a normal two- levelPWM type AC/DC conversion stage is alsobecoming popular in the power range of 50

kW or higher. With both the approaches, acontrolled output voltage, sinusoidal inputcurrent can be achieved with a power factorabove 0.95, a THD of below 5 %, andefficiencies of 97 % or better. Withapplications where grid-side isolation by amedium-voltage transformer is a possibility,multi-pulse rectifier topologies with a diodeor thyristor at the front end are becomingpopular due to their simplicity and reliabilityalong with higher efficiency.In the DC/DC conversion stage, resonant

topologies are often preferred due to theirefficiency and galvanic isolation. This designfulfils demand for higher power density andsmaller volume and the zero-voltageswitching (ZVS) reduces switching lossesand contributes to overall higher systemefficiency. The phase-shifted full-bridgetopology with the SiC devices alsoconstitutes an alternative solution in theisolated categories. For grid-isolatedarchitectures, multi-interleaved buckconverters are the DC/DC topology ofchoice. This has the advantage of loadsharing across phases, reduced ripple andfilter size, but at the cost of a larger numberof components.In the 15 to 30 kW power range,

subunits are most optimally implementedusing discrete components (Figure 2).Implementing the Vienna rectifier usingTRENCHSTOP™ 5 IGBTs together withCoolSiC™ Schottky diodes is a goodcombination for more cost-sensitiveapplications. Slight efficiency improvementsare gained by replacing the IGBTs withCoolMOS™ P7 SJ MOSFETs. For the DC/DCconverter, a resonant converter usingCoolMOS CFD7 MOSFETs achieves arespectable efficiency, while selection ofMOSFETs from the CoolSiC portfolio isrecommended when targeting highestefficiency.When developing subunits that can be

combined or upgraded to provide fast DCcharging, or chargers at the top end of thespectrum, a solution based upon powermodules is recommended. At this powerlevel, liquid cooling is preferred, although

air-cooling remains a possibility. The Viennarectifier is implemented using CoolSiC Easy2B modules with a switching frequency of40 kHz. The DC/DC section leverages aninterleaved 3-phase or multi-phase buckconverter, switching at up to a few hundredkHz. Here a combination of CoolSiC Easy1B modules combined with discreteCoolSiC diodes make for a highly efficientcombination.The CoolSiC family offers the

F3L15MR12WM1_B69, a Vienna rectifiertopology device in its Easy 2B package. Witha RDS(ON) of 15 mΩ, the devices provide highpower density in a package that simplifiesdesign implementation. The isolation gel-filled ceramic devices have a lowcapacitance, and their switching losses areindependent of temperature. Half-bridgetopologies are available in both the Easy 2Band smaller Easy 1B packages, featuringRDS(ON) values as low as 6 mΩ (Figure 3).

Control, communication and securityControl of the power stages is typicallyimplemented using a microcontroller.Devices such as the XMC4000 seriesprovide flexible analogue-to-digitalconverters (ADC) along with highlyconfigurable timers and pulse-width-modulation (PWM) peripherals toimplement the control loop. CAN

Figure 2: Typical topology for a charger made of discrete devices

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connectivity ensures that subunits cancommunicate with one another andrespond to the varying needs of differentbattery types. Service billing andauthentication of software updates orhardware changes can be handled by theHardware Security Module (HSM) of theAURIX™ family of microcontrollers, a familythat is well-known for safety-relevantapplications in the automotive space.

The authentication of replacementsubunits can be ensured using devices suchas the OPTIGA™ Trust B anti-counterfeitsecurity chip, while more demanding

integrity protection is offered by the OPTIGATPM trusted platform module.

SummaryThe roll-out of fast DC charginginfrastructure is an essential part of thestrategy to increase the numbers of BEVs.Without the availability of reasonablecharging opportunities offering anacceptable charging time, BEVs willinevitably remain restricted to those with adisposition toward green transport solutionsand consumers with limited-distance dailyjourneys. The preparatory work, in specifying

the chargers and connectors, has beendone. In addition, the necessary offerings ofinnovate semiconductor solutions areavailable. These range from traditionalSilicon power devices to Silicon carbidesolutions that offer higher-frequencyswitching and more efficient powerconversion, ensuring that chargers areefficient and reliable. When coupled withmicrocontroller devices, and cleverauthentication and security solutions, it isclear that multi-subunit approaches todelivering the DC charging infrastructure areready to power the future of transport.

Figure 3: Typical solutions for a charger made of module devices

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