Possibility of Power Electronics Paradigm Shift with Wide Band Gap Semiconductors

Recent researches on next-generation power devices has shown remarkable progress, especially in wide band-gap power devices such as silicon carbide and gallium nitride devices, as well as novel silicon devices, like a super junction FETs. The future direction of power electronics applications is surveyed in a term of output power density as an index of future power electronics roadmap, instead of power conversion efficiency, taking into account advanced devices and related technologies. A next-generation CPU power supply, a compact unit-inverter and an electric vehicle are selected as typical future applications. The possibility of power electronics paradaigm shift, with progress in the output power densities of more than 10 W/cm 3 to 30 W/cm 3 will be discussed in the paper. Introduction In the development of power electronics, power conversion efficiency (PCE) has been steadily increasing in the last 30 years, corresponding to the progress in power devices. PCE of small power converters such as switching power supplies was at a level of 60% in the beginning of 1970 and is now more than 90%. The efficiency of high power converters such as a motor drive system and a thyristor valve for HVDC transmition is exceeding the level of more than 98% to 99%. This means that it is gradually becoming difficult to predict future development of power electronics from PCE point of view. On the other side, power devices are still making a progress using novel device structures and wide band-gap semiconductor materials. What is a new indicator that shows a future progress of power electronics? In this paper, possibility of power electronics paradigm shift with next generation advanced power devices, focusing to a progress in out-put power density (OPD) as a new indicator of a future power electronics road map. A part of this paper was carried out by the work of an investigative research committee of the Engineering Advancement Association of Japan (ENAA), supported by the New Energy Industrial Technology Development (NEDO). A New Road Map for Power Electronics Figure of Merit of Power Converter (A) Progress in the power devices As is shown in Fig.1, power semiconductor devices have been remarkably developed with the progress in silicon technologies. The history of development can be divided into three periods. The first period from 1950 to 1960 is an age of cradle. Key technologies of semiconductor devises including the power devices, almost completed in this period. The second age from 1970 to the end of 1980 can be termed as a growth period. As results of rapid developments of major devices such as MOSFETs, IGBTs, GTOs and Light Triggered Thyristors, every need for power electronics applications were basically satisfied by the appearances of these devices. The third period from the beginning of 1990 to the present time is the age of full growth. Big efforts to realize higher performance devices are carried out. On-resistance of MOSFETs of which blocking voltage is less than several tens volts has reduced by one fifteenth. A 10MW class IGBT converter is now Materials Science Forum Online: 2004-06-15 ISSN: 1662-9752, Vols. 457-460, pp 21-28 doi:10.4028/www.scientific. net/MSF.457-460.21 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific. net. (Semanticscholar.org-11/03/20,15:23:04) 2 Title of Publication (to be inserted by the publisher) commercially available as results of marvelous progress in IGBTs with lower power consumption and higher handling power capability. A 100 MW class GTO inverter is also now in the market. (B) A new figure of merit of power electronics The PCE has been used as a major figure of merit (FOM), which indicates progress in power electronics apparatus. The PCE has been increasing, as shown in Fig.2, in the last 30 years with the development of the power devices. As shown in the figure, the progress seems to be saturated recently approaching to 100%. There are many technical issues to be improved for the power converters, instead of PCE, such as size, power consumption, low acoustic noise, low EMI/EMC, lower harmonic oscillation and remote sensing. However, none of the technical issues is suitable as a general indicator (FOM) that indicates future progress in power electronics. We need to have a general FOM and future power electronics road map. We take the OPD as a new general FOM, instead of the PCE. The OPD is defined here, as out-put power of converter per unit volume. Figure 3 shows progress of the OPD in power electronics (1) . The OPD has been increasing lineally by a factor of 2 figures in the last 30 years. The OPD is now arriving at the level of about one W/cm 3 to three W/cm 3 . An inverter with the OPD of 50 W/cm 3 is investigated by I. Takahashi (2) and is shown by the symbol □ in the figure. According to the road map, shown in Fig.3, the OPD of 20 W/cm 3 to 30W/cm 3 will appear around 2015. In the Takahashi model, almost ideal device characteristics are presumed for the study in power consumption, operation temperature and switching frequency. What is a new application, which require higher OPD in the near future? Key technical issues to realize higher OPD converter will be discussed in the following section. Key Technical Issues The higher OPD realization means size reduction of power converter. There are two main factors, which influences size of a power electronics apparatus. They are size of heat sink and passive components such as a capacitor and an inductor. For reducing the size of heat sink and passive components of converters, it is very important to realize lower power consumption, keeping higher switching frequency. Power consumption and switching frequency is mentioned in the term of the OPD. (A) Power Consumption Figure 4 shows relation between the power consumption and the OPD of switching power supplies. As is shown in the figure, the OPD increase exponentially with decease in power consumption of converter, showing that the lower power consumption, the higher OPD. It is interesting that the OPD of the above-mentioned Takahashi mode is on the same extrapolated line in the figure. Power devices consume more than 60% of total power loss of converters. So, reduction in heat dissipation of power devices is crucial problems to be solved for the size reduction of heat sink. (B) Switching Frequency Switching frequency is another important issues to realize the higher OPD. Increase in the OPD basically comes from the volume reduction of a capacitance and an inductance as well as size reduction of heat sink, because the volume of the passive components is inversely proportional to switching frequency. According to the survey results for a case of DC-DC converters, the OPD increases proportionally on a log scale with increase in switching frequency. As shown in Fig.3, the OPD of 20W/cm 3 to 30W/cm 3 will be predicted to realize after 2015.It is necessary to make switching frequency 5 to 10 times larger than that of today for the higher OPD converters of more than 20W/cm 3 . As it is necessary to manage both of increase in switching frequency and reduction of power consumption, the biggest problems to realize the higher OPD will be the development of high speed, ultra-low loss power devices. In the next section, trend of next generation advanced power devices is reviewed. Next Generation Advance Power Devices Every switching device basically has a common built-in diode structure for reverse voltage blocking by a pn-junction and forward current conducting through an active layer. Power devices are inherently 22 Silicon Carbide and Related Materials 2003