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EETE MAY 2014

POWER design SiC power devices gain traction among electric vehicles manufacturers By Dr. Thomas Barbieri As the market demand for electric vehicles continues to increase – driven in part by government regulations on fuel efficiency, escalating fuel costs and an overall trend toward “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 subsystems 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 Fig. 1: EV charging systems benefit from the improved efficiency and thermal characteristics of Cree’s SiC Schottky diodes. 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 vehicle’s 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 percent (for a maximum observed conversion efficiency of 96.4 percent), compared to the system employing all silicon devices. Cree’s 600V and 1200V SiC Schottky diodes have already been implemented in several EV charger designs, and the diode portfolio has recently been expanded to include packaged devices and bare die in voltage ratings ranging up to 1700V. Cree SiC Schottky diodes feature a proprietary internal design with a Merged PiN Structure (MPS), providing extreme surge resistance against the most intense fault events. As seen in figure 4, a 10A, 1200V Schottky diode with the MPS structure exhibits surge resistance greater than 700A at 25˚C under a Dr. Thomas Barbieri is product marketing engineer at Cree, Inc – www.cree.com Fig. 2: SiC Schottky diodes show zero reverse recovery charge (Qrr), which is a result of silicon carbide’s ability to support high voltage with unipolar devices. 24 Electronic Engineering Times Europe May 2014 www.electronics-eetimes.com


EETE MAY 2014
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