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Dec 3rd, 2012
UK silicon carbide dc-dc converters for cars
A British consortium is working on a flexible converter for electric and hybrid cars using UK-made silicon carbide power transistors.
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"The success of a research project to develop a multi-voltage DC-DC converter has led to funding from the UK government’s Technology Strategy Board to develop the technology for manufacture," said Oxfordshire-based Prodrive, which is leading the project.

In the multiport design, power can flow in or out of any port or any combination of ports.

"The demonstration converter is 97% efficient and has 58kW peak flow under acceleration or braking between the traction motor and the batteries, plus a 25kW interface for supercaps, plus a 1kW interface for 12V ancillaries," said Peter Tibbles, research manager at Prodrive.

According to Tibbles, the operating voltages of the various parts in electric and hybrid vehicles are in no way settled.

"Early hybrid vehicles matched battery and motor, but keeping the battery voltage low reduces its cost and size. Operating the motor at high voltage can reduce the size of current-carrying components. The Prius has a 220V battery boosted to 650V for the motor," he said. "What you are tending to see with the industry in this early state is that different motors and batteries at different voltages are being considered. There are no standard solutions. We see our technology as a bridge between these different solutions."

With an efficient customisable multi-port converter, vehicle integrators have complete freedom to tie together the components they need, without being limited by their voltage needs.

"As hybrid and electric vehicles become more complex, conventional technology would require an additional converter for each voltage step, increasing cost, weight and package requirements," said Prodrive. "The multiport device accommodates all the required voltages from a single, compact unit, allowing each system to be optimised for maximum efficiency."

In a design example (photo), the firm is proposing a 300V battery, 600V motor, 100V supercapacitor bank, 12V for legacy systems like lights, and 150V for electric power steering and air-conditioning. It would allow, for example, regenerative braking energy to be divided between the battery and supercapacitors.

In an echo of 42V systems that never were, 48V is another rail on the cards,

"German motor manufacturers are challenged their suppliers to explore 48V as another rail for electric power steering and air conditioning," added Tibbles.

SiC components can run far hotter than their silicon counterparts and so can be squeezed into a smaller space. They also switch faster - 150kHz as opposed to 25kHz - said the firm, allowing smaller inductors and capacitors to be used. And they can run at higher voltages.

“The system is around a third the size of a silicon-based device,” said Tibbles. “The size comes down from that of a flight bag to more like a shoe box, with a corresponding reduction in weight.”

Multi-port bi-directional converters have a reputation for needing a lot of semiconductors.

This topology, which is still under wraps, has been developed by the University of Manchester.

Its it transistor hungry?

"This is not a technology that uses excessive number of semiconductors, the innovation is in the use of magnetics," said Tibbles.

Along with Prodrive and the University, consortium members are Raytheon Systems which is making bipolar SiC transistors in Glenrothes, International Transformers (IST Power Products) for the magnetics, SciSys for vehicle interface software, and Tatra which is supplying the development vehicle.

"Work-scope includes the development of safety-critical software and establishing a potential supply chain to serve low-volume programmes," said Prodrive. "Applications are expected to include cars, heavy-duty trucks, aerospace and defence."

The firm is seeking tier 1 automotive suppliers interested in taking the system into production.


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