Silicon carbide shows potential for improved DC-DC technology and single multirole converters

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  • Image: Prodrive11-12Peter Tibbles.jpg
Image: Prodrive11-12Flexible power converter 3.jpg

The Prodrive-led consortium's new flexible converter can handle multiple voltages simultaneously on input and output sides.

The development of silicon carbide for use with multivoltage DC-DC technology—with one multirole converter doing the work of several—may prove crucial to the successful long-term application of EVs, believes automotive engineering consultancy, Prodrive. Work on the technology is at the center of a business consortium’s R&D program headed by the company, which sees it simplifying powertrain electrification, bringing meaningful weight and cost savings.

Significant challenges facing powertrain electrification concern the high power and currents required as a result of replacing traditional mechanical systems with their electric equivalent. Today’s 12-V architectures struggle to meet the requirements of high-power systems including stop-start, EPS (electric power steering), air-conditioning compressors, and other comfort priorities, plus the inexorably increasing demands of safety systems.

There is now renewed interest in 48-V systems and paralleling this is a growing problem emerging in EV and HEVs that concerns the need for differing DC bus voltages to interface with batteries, motors, fuel cells, and super capacitors—plus existing 12-V systems.

“What we need is the the ability to provide each system with the voltage at which it can work most efficiently while continuing to deliver 12-V for legacy technologies,” said Prodrive’s Research Manager, Pete Tibbles. “The availability of very high voltages from hybrid vehicle traction batteries gives us the opportunity to do this and more—if we can solve the challenges of stepping voltages up and down efficiently, without additional weight and complexity.”

The solution, believes Tibbles, is a new approach to the design of DC-DC converters, an established technology used for interfacing between the voltage levels. “At present, every time a new voltage is needed, another converter is added. That’s additional weight, cost, and packaging volume. We believe the solution is to have one compact, lightweight converter that provides multiple voltages.”

Prodrive is leading a British consortium now well into a development program that recently won additional U.K. government funding to take this concept forward to be demonstrated on a vehicle toward the end of 2013. The key innovation is a move from a silicon-based power device, which limits switching speeds to around 25 kHz, to one based on silicon carbide.

He adds that silicon carbide-based components permit switching speeds up to 150 kHz, allowing the use of substantially smaller inductors and capacitors: “The very high efficiency of the new technology also reduces the need for heavy and complex cooling systems. We have been able to reduce the size and weight by around two-thirds—from around that of a flight bag to more like a shoe box.”

As well as leading the consortium, Prodrive is also responsible for the vehicle integration, converter control system, and low-level embedded software. The converter is based on a topology developed by the University of Manchester working in conjunction with Raytheon U.K., which has developed the silicon-carbide devices, and with IST Power Products, which has developed the magnetic components. The consortium’s software specialist SCISYS has developed the high-level vehicle interface software. The work scope includes the development of safety-critical software and establishing a potential supply chain to serve low-volume programs.

The first application is expected to be hybrid powertrains, where there is an existing need to interface multiple systems at different voltages. Tibbles stated that as well as reducing the complexity of installation and maintenance, this will also reduce the cost and weight of the complete, integrated system as well as providing designers with greater flexibility. “Standards for any of these technologies are still emerging, so the ability to offer an ‘open architecture’ for the power circuit is a big step forward.”

Tibbles also sees an important role for the technology in conventional IC powertrains, where higher voltages can enable a new generation of active systems with smaller and lighter componentry: “I’m thinking in particular of chassis systems, electric pressure chargers, electric steering, HVAC, and active valvetrains.

“The multiport DC-DC converter allows manufacturers to integrate a variety of electrical systems without having to redesign their electrical architecture to account for varying system voltages, providing a low-risk, low-cost, controlled progression to where we all know vehicle power electrical systems need to go.”

Now, Prodrive is considering the next research step: downsizing or removing the liquid cooling system. Silicon-based power electronics assemblies usually require a dedicated cooling system because they must operate at a lower temperature than the standard vehicle cooling system. However, silicon carbide allows a much higher junction temperature, offering the potential of using the standard vehicle cooling system and allowing potentially significant cost and weight savings.

At such high temperatures, conventional power device packaging methods and materials may not be suitable, claims the company. New concepts will be required to manage heat transfer and differential thermal expansion between the different materials in the power module. Although it is possible to demonstrate its efficacy under laboratory conditions, the challenge is to ensure its operation and reliability in a car over a typical 200,000-km (124,000-mi) service life.

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