Electric & Hybrid Vehicle Technology - July 2021
Charging point
Richard Gooding 2021-07-20 03:50:58
TAKING CHARGE
Higher charging power reduces electric vehicle battery top-up times but creates other more complicated issues for onboard charging technology. However, charger design is changing to deal with, and future-proof against, these increasingly complex demands
Electric passenger cars typically use 400V charging architecture, more expensive 800V technology only fitted to trucks or buses with higher power requirements. The Porsche Taycan was the first production electric car to employ 800V, and the Hyundai Motor Group’s new electric E-GMP platform uses patented technology to support both charging architectures, controlling the motor and inverter to boost 400V to 800V. Audi’s PPE architecture uses 800V tech to off er charging power of up to 270kW, up from the 150kW more commonly found.
“800V systems off er the benefit of doubling power delivered at the same current, cutting DC fast-charging times,” states Gianfranco Di Marco, wide bandgap technical marketing engineer, power transistor macro-division, automotive and discrete group, ST Microelectronics. “It also reduces resistive losses, and increasing the voltage, instead of current, has very beneficial effects in using smaller cables and reducing charger cooling requirements.”
These benefits do mean that 800V architectures ask more from onboard charger (OBC) design. An OBC manages electricity flow from the grid to the battery, needing DC current to charge it. AC charging requires a conversion between the vehicle charging socket and the battery.
“New development topologies of OBCs, such as totem-pole Power Factor Correction are helping to increase charging efficiency” Elijah Bunin, product manager, VisIC Technologies
“800V onboard charging is simpler than charging at single phase,” says Richie Frost, CEO, Sprint Power. “With single phase, you need an AC-DC converter on board, which is another vehicle component and cost. With faster DC charging, all you need is extra power contactors in the battery system and additional software in the battery management system to talk to the onboard charger – all the fast charging control electronics are off-board. With 800V you’re just connecting the battery to the onboard charger. It’s talking to that through a standardized communication protocol, like the Combined Charging System (CCS).”
“OBCs are usually lower powered than the inverter and high-voltage DC-DC converter, typically up to a maximum of 43kW,” Di Marco reports. “However, most vehicles only include OBCs for single-phase power up to 7kW or three-phase up to 22kW. For economic reasons, faster charging rates are usually achieved using an external DC charger instead.”
Efficiency gains
As mass can affect vehicle range, an OBC’s weight and dimensions are important. “Designers need to increase the switching frequencies to reduce OBC volume and weight,” says Frank Vondenhoff, product development manager at Bel Power Solutions. “For 450V applications, we use silicon carbide (SiC) or gallium nitride (GaN) metaloxide semiconductor field-effect transistors (MOSFET). For 850V batteries, SiC MOSFETs are used.” “Historically, 800V architectures required 1200V power devices which were less powerful than 650V devices, and frequencies had to be kept low. Efficiency was also lower than 400V architectures,” says Elijah Bunin, VisIC product manager. “The current limitations of 800V architectures are related to the use of insulated-gate bipolar transistors (IGBT) or SiC. SiC performs better than IGBT, so systems using 1200V devices can achieve improved performance metrics. GaN devices rated at 900V will provide better switching energies and losses than SiC devices.”
“While SiC currently represents the most suitable choice as OBC power devices, we believe GaN semiconductors will gain prominence due to lower power and voltage requirements, with potential overall efficiency gains,” agrees Di Marco. “SiC use will remain high through technological development, but GaN may become the preferred design choice in the longer term.”
Higher OBC efficiency means fewer requirements for the thermal management, but it comes at a price. “Higher efficiency results in a higher cost,” states Vonderhoff. “Currently our OBC products reach an efficiency between 95-96% at nominal output power. An increase to 97%-98% means higher complexity and cost.”
“New development topologies of OBCs, such as totempole Power Factor Correction (PFC – a single-face PFC), are helping to increase charging efficiency,” Bunin says.
“For bi-directional charging, you need bi-directional DC-DC converters, following the PFC section. For DC-DC converters, we have a new C-LLC topology, or dual active bridge topologies. There are several solutions for helping with thermal management, such as SMT power devices with reduced thermal resistance. You do not need to parallel multiple devices, or have a large heat sink to cool the devices effectively.”
High-power charging infrastructure is now more common, with networks such as Ionity installing 350kW pumps. Keeping up with these roll-outs results in more standards and testing, says Vonderhoff.
“Customers require full certification of our products according to IATF 16949 and ISO 26262. Most development time is spent testing OBCs to the current standards.”
“At high-powered DC charging stations, the chargers require conversion from the grid AC supply, including PFC circuits,” Di Marco adds. “Global power supplies include phase-to-phase voltages of up to 440V and require conversion to 400V for vehicle batteries. As we adopt 800V systems, we’ll need to convert to this, too. Chargers may also be connected directly to higher-voltage distribution networks. SiC will be used in fast chargers of 50kW and higher to support the increased voltages and power requirements.”
“Onboard AC-DC converters will phase out because when there is more rapid charging infrastructure, you won’t need to charge via AC supply” Richie Frost, CEO, Sprint Power
Stacked-block approach
“We are expecting rapid chargers to be installed at a rate of approximately one charger per 50 BEVs sold in the near-term,” Di Marco continues. “Towards 2030, the number of BEVs per charger will likely grow as charging power increases and average charging time decreases. We also believe there will be a trend to build chargers designed around a stacked-block approach with the same circuit architecture as OBCs.”
Future topologies are hard to predict, but Vonderhoff says Bel’s current OBC started with the introduction of a 15kW single phase split phase onboard charger with integrated DC-DC converters in one box for the US market followed by the arrival of a 25kW liquid-cooled onboard charger with export functionality. “With a DC output range from 250V to 800V, we cover all battery packages with a single product. We are currently in discussion with fuel cell manufacturers to develop next generation DC-DC converters.”
As the power demand to charge EVs increases, Vehicleto- Grid (V2G) technology becomes essential.
“V2G requires bi-directional charging,” Bunin says. “VisIC’s bi-directional topologies mean our devices don’t need to be paralleled for 7.2kW. We are also weakening switching loss frequencies to above 200kHz without paralleling. For bi-directional DC-DC, the same devices can be used.”
Hyundai’s new Ioniq 5 and Kia’s EV6 models also offer Vehicle-to-Load charging. “This has the advantage that you can create your own grid in export mode where you can power your three-phase equipment from the batteries,” says Vondenhoff. “But in charge mode you are even able to charge the batteries of another vehicle [V2V].”
Will wireless charging have a bigger role to play in the future? “It’s an infrastructure thing,” confirms Frost. “It’s not that the technologies don’t exist today. We’re looking at creating wireless taxis, which is a clear application case. It is impractical for drivers to keep getting out to plug in when they need to move forward in a vehicle queue. Longer term, wireless is a feature because every time your vehicle stops it could be charged: at traffic lights, in a shopping mall, on a driveway, or at work. Wireless static charging is also a stepping stone to dynamic wireless charging.”
Frost believes that AC charging systems will disappear. “Onboard AC-DC converters will phase out because when there is more rapid charging infrastructure, you won’t need to charge via AC supply. It’s better for the vehicle – you just need high current cables from the inlet to the battery. With traditional AC charging, you need a converter, which needs cooling by liquid or air, and the connecting systems around it. It’s also another mass on the vehicle.”
A WIRELESS NETWORK
Wireless charging is supported by OBC technology, but it does still need a converter.
“Whatever output comes from the coils on the vehicle would have to be converted into DC and controlled into the battery. There is a receptacle on the vehicle to transfer the power from the coil in the road to a coil on the vehicle. That will be done almost like an AC waveform. There will be still be a need for onboard for wireless to be an AC to DC piece of power electronics to match the state-of-charge voltages in the battery, whether that’s 400V or 800V,” says Sprint Power’s Frost.
“Sprint Power is supporting Innovate UK programs on wireless charging and we are asking how wireless chargers communicate with a vehicle – do we need to change the vehicle itself or the wireless system? One concept is a ‘magic box’ sitting between the two replicating a CCS or CHAdeMO comms protocol. Any wireless system can be plugged in and retrofitted to any vehicle. The vehicle thinks it is plugged into an off-board charger, it communicates in the same way, and it will charge itself. But it’s actually being charged onboard wirelessly.”
FINE-TUNING STANDARDS
More electric vehicles from more manufacturers across the globe mean additional sets of charging standards. Harmonization is crucial. “In the past, requirements for OBCs differed from country to country,” Vonderhoff reports. “But now they are becoming harmonized and this is the same for certification. It makes life for the design engineer easier.”
“It has been historically a problem,” says Frost. I think now everyone’s honed into CCS as being a universal charging standard, which makes things easier. When you are dealing with more than one system, you cater for some redundancy on vehicles that don’t have that particular standard.”
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Charging point
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