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Fast-Charge the EV Market With Silicon Carbide Power Devices

Feb 25, 2020
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Fast Charge the EV Market With Silicon Carbide Power Devices

Cars from Tesla and others have gone a long way in demonstrating the viability of electric vehicles (EVs), and the market is no more held back by the twin challenges of range and refueling time, at least in terms of technology related to larger lithium-ion batteries and fast chargers.

On the back of these solutions as well as government incentives and regulations, EV sales growth has picked up the pace. Automakers are hoping to capture an opportunity that, according to management consultant Deloitte, will reach a tipping point in 2022, when EVs will cost the same as the internal combustion engine (ICE) versions.1 This will fuel such rapid growth that battery EVs will take 10% of the total automotive market by 2024.

Charger network needed

To make EVs useful beyond the daily commute, a battery technology that offers much higher energy densities is needed. That, however, is unlikely to happen soon. While the ongoing move toward higher bus voltages will reduce cabling weight and improve range, it is critical to build easy access to recharge stations.

Automakers’ success is contingent upon the development of a network of charging stations that offer refueling times comparable to those typically spent at gasoline-filling facilities. An EV that takes hours to charge after a few hundred kilometers will not be perceived as a good ICE vehicle replacement.

A key ingredient in Tesla’s success, for instance, is its “supercharger” network of fast off-board chargers that get the vehicle battery 80% full in about half an hour.

Fast off-board charger requirements

Fast chargers in the EV ecosystem must satisfy a variety of customers — the municipalities, businesses, and EV owners. Each of these categories of customers have unique but overlapping priorities (Figure 1).

Figure 1: The design goals for fast-charging stations are driven by various customer categories with different high-priority requirements.

The design goals for fast-charging stations are driven by various customer categories with different high-priority requirements.

Fast chargers are usually built with stacked groups of power-conversion blocks that are each 15 kW to 30 kW. Such stacks ensure higher operational time — should one stack need maintenance, the charging station can stay functional at a reduced capacity until repairs can be made.

Charging stations, whether commercial or at home, may eventually be able to take advantage of future smart grids or energy storage systems with renewable sources and therefore allow bidirectionality to deliver energy from EVs to the grid, storage batteries or to your home as needed or desired.

Silicon Carbide enables fast chargers

The design goals for fast chargers call for highly efficient power electronics and very high power density that is possible with the use of wide-bandgap materials. Silicon Carbide (SiC) is such a material long known for enabling power semiconductor devices that operate at much higher voltages, frequencies, and temperatures than silicon (Si). This makes it the technology of choice for EV fast chargers.

Fast charger power blocks are made up of an AC to DC front-end followed by a DC to DC converter to provide the battery charging voltage. (Figure 2). The AC/DC section converts the power from the utility grid to a useful DC voltage, while buffering the grid from the high load swings of the charger. The DC/DC converter provides electrical isolation of the vehicle for safety purposes, while providing the necessary DC charging voltage to the car.

Figure 2: A typical fast charger design approach.
Figure 3: Wolfspeed’s E-Series MOSFETs enable simpler designs, increase system reliability and efficiency, and improve environmental ruggedness.

By replacing Si designs using either IGBTs or MOSFETs in the AC/DC block of the charger with Silicon Carbide, as shown in Figure 4, the circuit design is simplified and efficiency is increased, while power density is significantly increased, which results in reductions in size, weight, and cost. This is accomplished in a simpler circuit topology using fewer components at very high efficiency (>98% peak), switching at more than twice the frequency. The higher frequency allows smaller magnetics and capacitors which, in turn, accounts for the smaller, lighter, and less costly system.

The SiC block can also simply enable the bidirectionality necessary for allowing the vehicle battery to become a part of a smart grid. Only control software needs to be changed. With Si devices, that bidirectionality would require significantly more hardware in a much more complex circuit design.

Figure 4 shows the DC to DC block implemented in silicon and in Silicon Carbide. In a performance comparison between SiC MOSFET and Si MOSFET-based DC/DC boost converters, the Silicon Carbide MOSFET converter delivered a loss reduction of up to 50.8% when compared with the silicon converter.

Figure 4: A comparison between Si- and SiC-based implementations of the DC/DC converter shows that SiC enables not only a simpler circuit, but a lower-cost, higher-efficiency solution.

In this Wolfspeed reference design, the complex, interleaved, silicon MOSFET design has been replaced with a much simpler design topology utilizing SiC MOSFETs and diodes. The Silicon Carbide solution can be switched much faster, thereby allowing use of smaller, lighter, and less costly inductors, transformers, and capacitors. What’s more, as with the AC/DC section, the Silicon Carbide circuit can be easily modified to support bidirectional power flow. Only the diodes need to be changed to MOSFETs in the secondary stage, along with control software changes.

Managing heat is also a bigger problem with Si than it is with Silicon Carbide, and that allows SiC-based system designs to weigh less and occupy less space. Silicon Carbide components, such as Wolfspeed’s MOSFETs, Schottky diodes and power modules, further save space and cost spent on Si-based designs’ higher component count and larger magnetics and heatsinks.

An opportunity not to miss

China, which accounts for about 50% of the EV market, leads the charger infrastructure setup as well. In 2018, the country had about 167,000 EV charging stations, according to Market Insights Reports, with plans for expansion to 4.8 million stations in 2020.2

Europe, the second-largest EV market, is also poised for a rapid growth in charging infrastructure. And when Amazon announced in September 2019 that it will buy 100,000 electric delivery vehicles, it signaled an opportunity in the United States as well.

The time is ripe, therefore, to find out how you can use SiC devices to enable the EV ecosystem with fast chargers.

References

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