09-02-2012, 11:40 AM
A Scalable SIC Device for DC/DC Converters in Future Hybrid Electric Vehicles
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2)INTRODUCTION
This work is motivated by an industry wide interest to integrate SiC power devices in future automotive applications. The need to mount electric motor drives and other power converter electronics under the hood and as close to the motor as possible demands that the power electronics installed must either be capable of enduring the high under-hood temperatures or incorporate an extra cooling system. The use of SiC power device in under hood power electronics would allow for more relaxed cooling requirements, elimination of a second cooling system, and integration with the current motor coolant system. The ability to reduce the necessary cooling mechanisms and overall package design is what will balance out the higher cost of using SiC devices over silicon in future automotive electronics
3)CONVERTER DESIGN AND RESULTS
A classic boost converter was designed to deliver roughly1 kW of continuous power, operating at 100 kHz, and using SiC devices as the switching pair. This prototype boosts an input voltage of 200 V simulating minimum battery bank voltage to an output voltage of 500 V, the regulated DC bus voltage within the Prius II [1]. Even though the JFET is generally thought of as a normally on device, SiC JFETs can be fabricated and practical gate drivers designed to support JFETs with less negative, or even positive, threshold voltage. Such devices can exhibit an ideal combination of enhancement mode functionality, similar to conventional normally off silicon devices, with the advantage of bias-enhanced blocking voltage unique to the JFET that permits higher blocking voltage and lower on resistance [2]. In a conventional boost converter, as well as many types of resonant converters, enhancement mode functionality and bias-enhanced blocking greatly simplifies the gate driver and start up circuit design such that SiC JFET converter has essentially the same safety as a converter switched with a purely normally off silicon switch. This is because many converters, including the boost converter considered here, stress the switch at two different characteristic voltage levels: a static or dc voltage level, and a higher dynamic voltage level. If the SiC JFET is rated to block the lower dc value at VGS = 0 V, and the gate driver is designed to supply the small negative gate-source bias voltage (say VGS ≤ -3 V) necessary for the JFET to block the higher dynamic voltage, an inherently safe design results because the higher dynamic voltage can only exist if the converter, including gate-drive circuitry, is working properly. Typically, the enhancement-mode JFET can block 50% of rated voltage (∼300 V) at VGS = 0 V, and 100% of rated voltage (600 V) with VGS ≤ -3 V. Fig. 2 provides evidence of the enhancement mode blocking capabilities in
a typical BVDS vs VGS curve.
Fig. 1 provides a block diagram of the high voltage system used in the current Toyoto Prius HEV. Within the Prius a 200V battery bank is established by connecting several individual batteries in series. In the first generation vehicles, original Prius, this 200V battery voltage was used to directly drive the electric motor. In the second generation vehicles, Pruis II, a high voltage power converter was included to boost this voltage to create a 500-V bus voltage for the motor/generator. While the power electronics in a modern “hard” HEV power train, such as the system in the Toyota Prius II, is typically rated for 50kW, a scaled prototype was designed and fabricated to investigation gate driver design to provide initial results that show the feasibility of SiC in HEV power electronics.
