System Design Considerations for Optimizing the Benefit by Unipolar SiC Power Devices.
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System Design Considerations for Optimizing the Benefit by Unipolar SiC Power Devices. Roland Rupp and Ilia Zverev Infineon Technologies AG, Power Management & Supply Balanstrasse 73, D-81609 Munich, Germany ABSTRACT
Focussing on unipolar SiC power devices a variety of applications are described, where cost reduction can be a achieved on system level even for SiC device costs being several times higher than the costs of the competing Si devices. Based on the specific properties of SiC devices like Schottky diodes and JFETs it is explained with the help of these examples how this is attainable. INTRODUCTION
Intensive research and development aiming at silicon carbide power devices during the last decade resulted in an outstanding performance of demonstrators with respect to achievable blocking voltages and reduced static and dynamic losses [1,2]. This development was accompanied by a very encouraging progress in SiC base material quality, cost and wafer size leading to commercial available 3 inch 4H SiC wafers with a micropipe defect density below 20 cm-2. Triggered by these positive trends a new situation arose for SiC power devices during the last two years. They are now beyond the point of technological demonstration and have entered the market for commercial applications. Specifically, SiC Schottky Barrier Diodes (SBDs) act as a precursor being introduced in Spring 2001 by Infineon [3] and also by Cree/Microsemi [4]. With this step a new focus becomes important for the SiC researchers: it’s no longer most important to show basic capabilities and new world record data of the power devices, but to consider which properties and trade off make such devices most valuable for the system designers. One always has to be aware, that SiC devices are and will be significantly more expensive than the Si parts they shall replace. Of course, just looking on the physical properties it is a clear deal for SiC. But unfortunately in many cases these advantageous properties can be more than compensated by just larger silicon chip area. In other cases innovative device structures like compensation devices (e. g. CoolMOS [5]) have expanded the predicted physical limit of Si power switches by a factor of 3-4 towards lower RDS,on. On the other hand SiC allows unipolar devices for much higher blocking voltages than it is reasonable with Si. So the major competition today happens between unipolar SiC and bipolar Si devices. No doubt, that such Si devices are always much cheaper for a given current rating. Furthermore bipolar devices typically show a (slightly) negative temperature coefficient in on-resistance, making them much more stable under surge current conditions with respect to unipolar SiC devices. It is the major justification for a SiC device in this case, that switching losses are dominant in the specific application. So as a precondition for a broad market acceptance it has to be clear, that the system engineer gets a clear benefit by applying a comparatively high switching frequency. Another important barrier to b
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