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SiC power devices boost EV performance, efficiency

Posted: 13 Jun 2014     Print Version  Bookmark and Share

Keywords:electric vehicles  HEVs  PHEVs  BEVs  silicon carbide 

As the market demand for electric vehicles continues to rise driven in part by government regulations on fuel efficiency, escalating fuel costs and an overall trend towards greener transportation options, a growing number of automotive manufacturers are incorporating the latest power electronic technology in their designs to improve overall performance, increase efficiency, and reduce cost, weight and complexity.

Hybrid electric vehicles (HEVs), plug-in hybrid electrical vehicles (PHEVs) and battery electrical vehicles (BEVs) all contain several critical systems that stand to benefit from wide bandgap power devices; these devices have the potential to enhance both the energy efficiency and performance of electric vehicles, which could enable early adopters to achieve a significant market advantage over their competitors.

As one of the leading wide bandgap semiconductor materials, silicon carbide (SiC) offers a number of proven performance advantages over conventional silicon technology, including higher voltage blocking capability, faster switching speed, lower on-state and switching losses, higher thermal conductivity, and higher surge resistance. These characteristics provide the platform for advanced power electronics sub-systems that are at the heart of electric vehicle drivetrains, power converters and charging systems.

In a typical electric drivetrain vehicle, sophisticated power electronics are employed to manage the flow of energy between energy storage devices (batteries) and motor drive inverters. Improving the efficiency of these power electronics systems, which currently depend on conventional silicon power devices with limited voltage and power ratings, is critical for improving overall electric vehicle efficiency and reliability. By using the performance advantages of SiC power devices, electric drivetrains can achieve increased efficiency, higher power levels and power density, and reduced cooling system requirements. These system-level benefits yield increased vehicle performance, driving range per charge and decreased energy and/or fuel cost.

The significant performance enhancement that SiC can provide in an electric vehicle application can be shown by replacing the conventional silicon PiN diodes with SiC Schottky diodes in both the high voltage DC/DC boost converter circuit of the traction drive system and also in the onboard battery charging system. Note that these applications require high voltage devices (> 300V) with ultrafast switching speed. Conventionally, silicon PiN diodes are used since high voltage silicon Schottky diodes are not available. However, these bipolar silicon PiN diodes have poor reverse recovery characteristics, which reduce achievable switching frequency and efficiency. In comparison, the zero reverse recovery characteristic of the unipolar SiC Schottky devices virtually eliminates diode switching losses and permits increased switching frequencies, making the overall power management system much more efficient.

Another critical area for enhancing EV performance is also in the design of the vehicles charging system. Plug-in vehicle owners want rapid charging from readily accessible electrical outlets and hybrid owners desire reliable and long-lasting battery charging systems. The key to both of these performance enhancements is the design of power electronics systems that feature high efficiency power conversion, high operating temperature capability and high charging current and power.

A significant increase in system efficiency can be achieved by replacing the silicon PiN diode with a SiC Schottky diode in the buck-boost converter of a 6.6kW charging system. In a recent study by Global Power Electronics, this drop-in replacement of SiC diodes for silicon diodes in an IGBT-switched power module increased the system efficiency by approximately 2 per cent (for a maximum observed conversion efficiency of 96.4 per cent), compared to the system employing all silicon devices.

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