AUTOMOTIVE POWERDesign Considerations for fast DC Chargers Targeting 350 Kilowatt

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Design Considerations for fast DC Chargers Targeting350 KilowattWorkings groups within standards organisations have, around the world, defined everything from theoperational envelope and charging sequence, to the communication and connectors of High PowerChargers (HPC). In Europe and the US interested parties have coalesced around CharIN and the CombinedCharging System (CCS). Elsewhere other alternatives have developed, such as CHAdeMO in Japan andGB/T in China. Some vehicle manufacturers have also placed value on developing proprietary chargingsolutions. For manufacturers looking to address this market it quickly becomes clear that a modularapproach is required. The article shows how to proceed in this way. Pradip Chatterjee, ApplicationOwner of EV Charging, Infineon Technologies, Warstein, Germany

The car owner has, unknowingly, beenspoiled for many decades by a seamlessnetwork of refuelling stops. The thought ofhaving to plan a journey optimised aroundthe location of gas and service stations isutterly unfamiliar. However, this is probablyone of the initial thoughts going throughthe minds of those considering purchasingor leasing a battery electric vehicle (BEV).Although the automobile is used by manyprimarily for short journeys well within therange of a BEV, it is the exceptions, such asa weekend away or the annual vacation,that cause concern.

When parked at home our BEV can becharged, slowly, overnight. Many of ourlarger cities and towns have also started to

provide municipal charging piles, enablingus to top-up our vehicle’s charge whileshopping. The reality is, for longer journeysat least, the charging time has to comesomewhere close to that required forrefuelling an internal combustion enginevehicle. A 22 kW home AC charger candeliver charge equivalent to around 200km of range in a time frame of 120minutes. Reducing this to seven minuteswould require a fast DC charger supplying350 kW.

For manufacturers looking to addressthis market it quickly becomes clear that amodular approach is required. This allowsreuse of some aspects of the end solution,such as a common housing and cooling

concept, while connectors, cables and thepower electronics can be selected tomatch the specifications of the targetmarket.

Approaches to the fast DC chargerpower electronics designFast charging HPC refuelling points willrequire dedicated electrical low or mediumvoltage (LV/MV) infrastructure as theirsupply. It is expected that this will beinstalled primarily in locations such asmotorway service stations along key routesbetween cities. The incoming AC supplyfeeds into an isolating transformer whosesecondary will be converted to DC.Transformers with a double secondary

Figure 1: An Active Front End can be easily implemented using a single 1200 V CoolSiC™ MOSFET module

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� and Y winding are a popular solution.These phase-shifting transformers are

then combined with multi-pulse rectifiersoperating in series or parallel that reduceharmonic content at the input. In suchdesigns the transformer is mandatory evenif isolation can be provided through thechosen topology for the DC/DC stage,mainly due to the harmonic contentimprovement its presence provides. Thefirst design decision to be made here iswhether to take a common AC or commonDC bus approach.

In the common AC bus approach, thesecondary side of the transformer feedsmultiple AC/DC rectifying stages that feedtheir own DC/DC stages. This has thebenefit of simplifying the overall designconcept for the charger. However, it doesrequire a replication of the AC/DC stagethat results in higher total cost due to theneed for several sets of filters, controlstages and sensing. Currently, support forinjecting energy back into the grid, such asVehicle-to-Grid (V2G) and Vehicle-to-Building (V2B), is not mandatory. However,should this change, this approach wouldresult in further cost and complexity.

The common DC bus approach uses asingle AC/DC stage to create a DC outputthat supplies all the DC/DC stages. Inmany cases, this proves more optimal as itreduces device count and cost, andimproves overall efficiency. Should V2Gand V2B become mandatory it would alsobe simpler to retrofit. A DC bus is alsoeasier to integrate with other energysystems (local battery storage,photovoltaic) that may be implemented.Finally, current DC charger standardssupport the idea of a centralized chargingstation operating as an active front end forseveral battery chargers. The main

downside surrounds dimensioning such ahigh power-rated active front end.

Charging parks supporting 2 to 3 MW ofpower prefer the common DC bus, using itto supply between six and eight high-power DC/DC charging stages.

Focussing on the AC/DC rectificationThanks to modern power transistortechnology, coupled with highperformance microcontrollers (MCU) anddigital signal processing (DSP), highlyefficient AC/DC rectification circuits can beimplemented. These ensure a sinusoidalcurrent draw from the grid, low harmonicdistortion (THDi ≤ 5 %), and independentcontrol of active and reactive power flowwhile ensuring high dynamic control.Operation at unit power factor ensuresthere is no reactive power consumptionfrom the grid. Finally, if the chosentopology supports it, bi-directional powerflow between the AC and DC sides isrelatively straightforward.

One of the most widely used topologiesis the Two-Level Voltage Source Converter(2L-VSC). This consists of an array of sixswitching devices, typically IGBTs or SiCMOSFETs, together with a capacitor as aDC link, generating an output voltagehigher than the input phase voltages. Thisactive front end also supports bi-directionalenergy flow and provides a fully adjustablepower factor. The switching approach canmake use of either pulse-width modulation(PWM) or space vector modulation (SVM).

This can be easily implemented usingthe single-package 1200V CoolSiC™MOSFET Module FS45MR12W1M1_B11(Figure 1). This contains six switchingdevices in the EasyPACK™ 1B package thatfeatures a low inductive design andcontains an integrated NTC temperature

sensor. Half-bridge solutions, such as theFF11MR12W1M1_B11 in the EasyDUAL™1B package, could also be considered.Designs based on these componentscould support 60 to 100 kW at switchingfrequencies of 25 to 45 kHz.

If bi-directional current flow is deemedunnecessary, the three-phase, three-levelVienna rectifier is becoming the popularchoice. It requires only three activeswitches and provides dual boost powerfactor correction (PFC). In the event of amalfunction in the control circuit it isprotected against a short-circuit of theoutput or front end, and can even operatewith the loss of one input phase. Assemblyeffort for such designs can be high usingdiscrete components, but in such high-power applications, integrated powermodules are more commonly used.

A Symmetric Boost PFC Vienna rectifiercan be implemented using SiC modulessuch as the F3L15MR12W2M1_B69,offered in an Easy 2B package (Figure 2).Each module contains two 1200V fastrectifying diodes, two 1600 V slow rectifierdiodes, and two 1200 V, 15 mΩ SiCMOSFETs. Three such Easy 2B packagescan easily be combined to create acompact high current, low loss design(Figure 3).

Delivering the variable DC chargingvoltageThe CharIN specification for DC chargersdefines that the supported output voltagemust lie between 200 V and 920 V, supplya maximum of 500 A, and operate withina power envelope of 350 kW. There is arange of DC/DC topologies, both isolatedand non-isolated, that can be used totackle this challenge.

Regardless of the topology chosen,

Figure 2: Half-bridge modules integrated into Easy 2B packages, such as the F3L15MR12W2M1_B69, are ideal for a Vienna rectifier

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there are several key requirements thatneed to be fulfilled. Physical size andoverall cost are focus areas, whileelectromagnetic interference (EMI)regulations must also be observed. Zerovoltage or zero current switching(ZVS/ZCS), highest efficiency andsupport for the high power required arealso on the list. Finally, low ripple of bothvoltage and current at the output areessential to avoid battery heating.

Topologies utilizing a high-frequency(HF) isolation transformer, such as a fullbridge LLC resonant converter, are knownfor their high efficiency at their resonantfrequency. They are also inherentlycompact thanks to their ZVS primary sideswitches and ZCS secondary side diodes.Unfortunately, supporting the desired

wide output voltage range makes chargerdevelopment exceptionally challengingwith this approach.

Above 100 kW power outputs, andsince the galvanic isolation is guaranteedby the grid transformer, a non-isolatedBuck/Boost converter can be used. In amulti-phase configuration, it can provideefficiencies of up to 98.5 percent. Thisapproach also significantly reducescurrent pulsation due to the shiftedvoltage pulses. Its modular design allowsits dimensions and operationalparameters to be easily adapted tochanges, both in output andperformance or physical shape.

Managing heat dissipationDespite the incredible efficiencies thatpower converters can achieve today, amere 1 percent drop in efficiency isequivalent to 3.5 kW of power dissipation,emitted as heat, when a fast DC charger isoperating at full power. The cable alonecan add an additional loss of 100 W permetre length. HPCs require more than aforced air-cooling approach to heatdissipation. Additionally, it is not only thepower electronics but also the connectorand cable that requires manufactures tomove to liquid cooling.

The challenge here is that many liquidcoolants have issues with flammability,

degradation, corrosion, and toxicity. Todaya water-glycol mix has shown itself as apopular coolant for both the cable andconnector. Dielectric coolants have alsobeen developed, such as the 3M™Novec™, with successful deployment inthe ITT Cannon HPC. The cooling systemis then coupled with a separate or centrallylocated heat exchanger, depending on theconfiguration of the charging park.

SummaryThe uptake of BEVs is, to some degree,dependent on the available charginginfrastructure. Some worries could bealleviated through better promotion of theexisting network of charging points,although investment in fast DC chargingHPCs, specifically to diminish range-angstfor those worried about their longerjourneys, is also required. Liquid coolingwill be an essential part of the heatdissipation strategy, requiring that theselected electrical topologies andcomponents are both highly efficient andprovide easy integration with themechanics of the heat extraction approach.SiC devices, including diodes and switches,will form an essential part of the designchoices made, starting at the rectificationstages and moving through to the DC/DCtopologies chosen to deliver the batterycharging output.

Figure 3: Highefficiency 60 kWdesign utilising Easy2B Vienna rectifierphase leg modules

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