Electric cars driving new innovations

Prodrive is leading a consortium of companies to develop a new technology for electric and electric hybrid vehicles that will reduce the weight and space taken by the electrical power system. The 3 year project to develop a flexible DC-DC converter is supported by investment from the government-backed Technology Strategy Board. Prodrive is joined by the University of Manchester, Raytheon Systems Ltd, SciSys, International Transformers and Tata Motors European Technical centre. The first year of the project will be devoted to fundamental research, followed by two years of application and development, leading to a drivable demonstration car by the end of year three; market introduction is said to be 5-7 years away.

Current practice is to integrate DC-DC converters into the power management system to step down or step up the battery voltage to meet the needs of different devices such as traction motors, cabin electrical systems, fuel cell stacks or supercapacitors. In complex architectures this requires several converters. A single, flexible converter will save cost, weight and package space, enabling vehicle manufacturers to move more easily to the next generation of sophisticated plug-in and range-extended hybrids.

“Existing hybrids, such as the Toyota Prius, need one DC-DC converter for the traction motor and another for the vehicle’s 12 volt system," explains Pete James, Prodrive’s technical specialist. “In future, there will be further voltage steps for supercapacitors or fuel cells; it isn’t viable to keep adding extra converters for every additional voltage. Having worked hard to reduce the cost, weight and size of battery packs and motors on hybrid vehicles, manufacturers are clearly unwilling to see those gains swallowed up by growth in the power management hardware."

Solving this problem will require the development of fundamentally new technology; The flexible converter will have to be capable of handling multiple voltages simultaneously on both the input and output sides, while achieving conversion efficiencies equal to the best single-range converters currently available.

Prodrive is building a reputation as a specialist in the development and integration of electric and flywheel hybrid systems and future powertrains. James concludes, “We concentrate on the control systems, architecture, strategy and techniques for efficiency enhancement. This whole-vehicle perspective means that we work well with specialist technology suppliers as we see the overall picture. That’s the only way to get real value from innovation."

What is a DC-DC converter?

A DC to DC converter is an electronic circuit which converts a source of direct current (DC) from one voltage level to another. This is particularly important in hybrid and electric vehicles because the battery cell voltage varies with state of charge which would cause vehicle performance to vary with battery state of charge without a DC-DC converter to maintain the voltage level. Conversely, generator charging voltage varies with speed and would, without a converter, present a variable charging voltage to the battery that would affect battery life and limit the practical speed range in which regenerative charging was possible.

Switched DC to DC converters convert one DC voltage level to another, by storing the input energy temporarily in inductors and capacitors and then releasing that energy to the output at a different voltage. Physically small inductors and capacitors can be used by operating at high switching frequencies, and high efficiencies are possible when using high power, high frequency devices such as insulated gate bipolar transistors (IGBTs). A switched DC to DC converter regulates the output voltage, presenting a constant voltage to the output device (e.g. a traction motor drive).

While most DC-DC converters work in one direction only, hybrid and electric vehicles require bi-directional control to recover energy from regenerative braking. Bi-directional DC-DC conversion provides a constant supply voltage to the traction system, stepping up the battery voltage during motoring operation, and providing a controlled charging current to the battery during regenerative braking.