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Silicon Carbide Drives Cost, Performance Benefits in EV Fast Chargers

Feb 18, 2020
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Silicon Carbide Drives Cost, Performance Benefits in EV Fast Chargers

In 2018, battery electric vehicle (BEV) and plug-in hybrid EV (PHEV) sales grew year on year by 68% and 84%, respectively, according to Yole Développement.1 This year, the market is expected to be awash with new fully electric models from the likes of Audi, Porsche, Volvo, Polestar, Mercedes-Benz, M.G., and BMW. For instance, the upcoming fully electric XC40 Recharge SUV from Volvo will have a 78-kWh battery to keep it going more than 322 km.2 Mercedes-Benz claims that its EQC 400 electric SUV’s 80-kWh battery delivers a range of about 445–471 km.3

The growth in the EV market has brought an opportunity to create a supporting infrastructure of charging stations. Market analysis firm Market Insights Reports estimates that the EV charging station market will record a CAGR of over 38% during 2019–2024.

Fast-charging opportunity

Fast-charging station customers require that chargers be small, rugged, reliable, and highly efficient while also delivering less-than-30-minute charge times. The typically 3- to 7-kW on-board chargers take four hours or more to charge today’s EVs. To reduce charging time, fast off-board chargers must deliver much higher power — anywhere between 80 kW and 350 kW.

Such fast chargers are modular in design with 15-kW to 25-kW blocks. This allows charging stations to continue operating, although at a reduced capacity, during repair and maintenance.

EV chargers comprise two main blocks: an AC/DC converter and a DC/DC converter. The AC/DC converter gets the three-phase power from the grid to DC intermediate voltages, which are then converted by the DC/DC block to the voltage required for fast charging the EV batteries.

AC/DC Conversion

There are several common silicon (Si) implementations of the AC/DC converters using MOSFETs as well as IGBTs. The issue with Si MOSFETs is that it is not possible for blocking voltages to go much beyond 650 V while keeping the die size small and getting high switching efficiencies. Although a two-level topology with Si IGBTs can deliver up to 1.2 kV, size and efficiency remain a challenge.

Silicon Carbide (SiC) based AC/DC converters, such as the one shown in Figure 1, can address all those issues with Si. The reference design shown uses 1-kV blocking devices at 65 mΩ, has a high switching frequency of 48 kHz, and outputs 800 V to the DC/DC section of the fast charger. Experimental results demonstrate that this circuit offers over 98.5% peak efficiency at 480-V input.

The Silicon Carbide circuit in the figure also shows significant savings in terms of component count — six active devices (C3M0065100K) in the SiC-based design versus 12 active devices in the Vienna topology for the Si MOSFET-based design and 18 active devices in the two-level IGBT design. In terms of size, the Silicon Carbide based circuit consumes 25% to 30% less area.

Figure 1: The two-level topology of Wolfspeed’s SiC MOSFET-based design is simpler, smaller, and more efficient than the silicon AC/DC converters.

DC/DC conversion

The DC/DC section of the fast charger may be required to convert close to 800 VDC down to 570 VDC maximum. Figure 2 compares the Si design with the Silicon Carbide solution.

Figure 2: The SiC advantages of higher efficiency, smaller size, lower component count, and lower costs are obvious in the DC/DC section as well.

In the DC/DC converter as well, the Si implementation uses more active devices than the Silicon Carbide solution. The SiC-based design uses two 65-mΩ MOSFETs with 1-kV blocking capability in parallel per switch position, for a total of eight in the circuit. The rectifier part of this circuit uses eight 20-A, 650-V Silicon Carbide Schottky diodes.

This design is implemented in Wolfspeed’s CRD-20DD09P-2 20-kW, full-bridge resonant LLC converter. It is rated for an input range of 650 V to 750 V, output range of 300 V to 550 V, and an output current of 35 A maximum. The reference design reaches a peak efficiency of 98.3% with an input of 750 V and output of 570 V. Compared with the 15-kW Si-based design shown in Figure 2, the Silicon Carbide solution delivers 33% higher power from approximately 25% smaller space.

The Silicon Carbide based implementation of a 20-kW block using the AC/DC and DC/DC sections together results in over 96% of delivered efficiency.

The future of charging stations

Fast-charging stations will be connected to a smart grid and will therefore likely be bidirectional, something that can be achieved much more simply with Silicon Carbide devices versus silicon. The fast chargers are likely to offer additional features, such as entertainment and e-commerce or electronic retailing (e-tailing) services, while simultaneously fast charging up to four EVs.

To deliver on the efficiency, power density, reliability, and ruggedness demanded by various consumers (see Fast Charging the EV Market), Silicon Carbide devices and modules will be required. That reality is partly reflected in Yole Développement’s forecast of Silicon Carbide power module sales growing at 48% CAGR during 2018–2024 — more than twice the CAGR for IGBT modules during the same period.

And it is reflected in Wolfspeed, continuing to develop Silicon Carbide reference designs for fast chargers that deliver increasingly higher power and high-frequency operation with off-the-shelf passives and magnetics.

Wolfspeed, which has more than 6 trillion global device service hours and counting, is uniquely positioned to speed up fast-charger design with reference designs, including downloadable schematics, board layout, bill of materials (BoM), and all the information needed to realize your own power blocks.

References

1. Yole Développement, EV/HEV is driving power electronics innovations
(http://www.yole.fr/PowerElectronics_EV-HEV_IndustryOverview.aspx)

2. Volvo Cars, The XC40 Recharge
(https://www.volvocars.com/us/cars/xc40-pure-electric)

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