Possibility of Power Electronics Paradigm Shift with Wide Band Gap Semiconductors

Transcription

1 Materials Science Forum Online: ISSN: , Vols , pp 2-28 doi:.4028/ Citation & Copyright 2004 Trans (to be Tech inserted Publications, by the publisher Switzerland ) Possibility of Power Electronics Paradigm Shift with Wide Band Gap Semiconductors H. Ohashi Tokyo Institute of Technology, South 3 rd Building 5F- 509, Ookayama Campus 2-2- Ookayama, Meguro-ku, Tokyo, Keywords: Power Electronics, Output power Density, Wide band gap semiconductor Abstract. 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 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 970 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., 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 950 to 960 is an age of cradle. Key technologies of semiconductor devises including the power devices, almost completed in this period. The second age from 970 to the end of 980 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 990 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 MW class IGBT converter is now 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, (ID: , Pennsylvania State University, University Park, USA-05/03/6,3:37:55)

2 22 Silicon Carbide and Related Materials 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 0 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 0%. 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 (). 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 205. 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 205.It is necessary to make switching frequency 5 to 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 s 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

3 Materials Science Forum Vols Journal Title and Volume Number (to be inserted by the publisher) 3 limited by the built-in diode, that is, a width of active layer and a doping density. It is becoming obvious that conventional power devices gradually approaching material limitation determined by the diode. As previously explained, most important key technologies for the high OPD power electronics system will be high speed, ultra low-loss power devices and related technologies for their practical use. Two technical trends are under way for breaking through the device limitation. They are a novel structure and a wide band gap semiconductor device. In this section, next generation advanced power devices will be discussed conjunction with high OPD converter possibility, focusing on the wide band semiconductor devices as well as novel structure devices. Novel Structure s Novel structure devices such as a super junction (SJ) and a floating junction (FJ) device are developed. The design concept of the devices is lowering of maximum electric field around pn-junction by introducing three-dimensional structure in the active layer. In the case of the SJ MOSFET, vertical pn-stripe structure is installed three dimensionally in the active layer of the common diode. On-resistance can be reduced proportionally with reduction in the stripe width keeping blocking voltage. Now a 600 V SJ MOSFET (Cool MOS) of which on-resistance is less than that of /3 (about 40 mω cm 2 ) of a conventional MOSFET is commercially available. A 600V- SJ MOSFET with on-resistance of less 26 mω cm 2 is reported. On resistance of less than mωcm 2 will be commercially available for the 600 V-SJ MOSFET, in the near future. Concerning to the FJ device, Fabrication of a Schottky Barrier Diode (SDB) is reported showing breakthrough of on-resistance limitation of conventional SDB in Silicon. Application possibility of FJ MOSFET to SiC material is also studied theoretically looking at expansion of SiC device possibility. Wide Band Gap Semiconductor s Introducing wide band-gap semiconductors has carried out breakthrough of silicon device limitation. As maximum breakdown electric field of the materials determines maximum blocking voltage, trade-off relations between the maximum blocking voltage and on-resistance can be improved by introducing the wide band-gap semiconductor materials such as Silicon Carbide (SiC), Gallium Nitride (GaN) and Diamond. Many research results have been reported for SiC devices and GaN devices. On-resistance of the MOS gated devices is expressed roughly by summation of MOS gate channel resistance and active layer resistance. Because of poor channel mobility of SiC, Static Induction Transistor (SIT) is dominant in SiC devices. A GaN-HEMT structure shows possibility to have excellent characteristics as a power device. Overview of Next Geration dvices Recent research activities on the advanced power devices are shown in Fig.5. As is shown in the figure, the wide band-gap semiconductor devices such as SiC-SIT, SiC-SDB and GaN-HEMT is making a big progress in the reduction of on-resistance. In the case of 00V class devices, the on-resistance is now around 3mΩ cm 2, showing almost one several th reduction in comparison with Si-MOSFET, and about one th in comparison with Si- IGBT. Concerning to SiC-MOSFET progress, big efforts to realize increase in channel mobility have been carried out. Recently SiC-MOSFET with on-resistance of less than several tens mω cm2 have reported. The uni-polar device on-resistance will become the value of less than mω cm 2 to several mω cm 2 in the voltage blocking rage from several hundreds to several thousands volts. This means that there will be possibility to replace Si-IGBT and Si-MOSFET with wide band gap uni-polar devices such as MOSFET, SIT (JFET) and HEMT. As the built-in voltage of SiC pn-junction (about 2.5V) is higher than that of Si (0.7-08V), SiC-Bipolar devices such as IGBT and pin diode have not advantages for the reduction of conduction loss in the range of several thousand volts, but have advantage of the

4 24 Silicon Carbide and Related Materials Title of Publication (to be inserted by the publisher) improvement for increase in switching speed due to thinner active layer width in comparison with Si devices. As will be discussed in the next section, it will be necessary to evaluate total power consumption of the devices in term of on-resistance and capacitance product (3) From this point of view, Investigation of a new candidate device for the lower voltage range applications is necessary. For an accretion of research and development of the devices, motif applications are necessary as a driving force of the new device developments. High Output Power Density Applications Power Electronics New Road Map The survey committee of ENAA investigated future requirements for high output power density applications, form 2000 to 2002 (4). The possibility of power electronics innovation with higher OPD converter was discussed in the committee. The result is shown in Fig.6, summarizing survey activities as a road map of power electronics, focusing to OPD as a new indicator. A power supply for next generation CPU, a compact unit-inverter and an electric vehicle are selected as typical future applications, which will be necessary to realize OPD of W/cm 3 to 30 W/cm 3 in the future. In the following section, details will be shown. All of the applications will open a new age of power electronics. Pursuit of high OPD power electronics changes paradigm from power conversion efficiency to the age of component, enabling lower watt cost due to mass-production of standardized unit converter. New Motif Applications (A) A Power Supply for Future Generation CPU According to the road map of CPU progress (4), it is predicted that in 206, output voltage and power consumption of a power supply are 0.4V, and 290W for a high performance CPU and 60W for a general purpose CPU respectively. This also means that current consumed in the CPU will be 700A and 400A respectively. Clock frequency will be 90GHz for the high performance case and 45GHz for the general-purpose case. Switching frequency of a CPU power supply has been increased with ratio of /00 to the clock frequency. For single power supply setup, switching frequencies of 45MHz and 90MHz for each CPU will be necessary respectively. In the case of the general-purpose CPU power supply in the future, it will be required to develop technical issues as follows, output voltage of 0.4V with output current of 400A, switching frequency of 45MHz and current through-rate of 400A/μs. For the actualization of the target issues, it will be necessary to realize a synchronous rectifier of which on-resistance and capacitance product is 0.65Ω pf. Moreover, advanced assembly technologies for making decoupling condensers and the power supply as possible as closer to the CPU will become important to make the large current and current through-rate come true. (B) A Compact Unit-inverter In advanced countries, including Japan, around 40% of total input energy is consumed by electrical energy. Although, 50% to 60% of total electrical energy is used for motor driving. Ratio of inverter driven industrial motors is, for example, still less than %. A big energy saving effect can be expected by increasing inverter driven motors. Biggest roadblock to popularize inverter driven motors is the cost reduction. According to the survey (4) by the hearing from a solution business company for energy saving, if the cost reduction exceeds a certain threshold value, drastic increase in the inverter driven motor can be expected. Cost reduction will be possible by the mass-production of standardized compact unit-inerter, introducing high OPD converters. A possibility of 200V-3kW-class unit-inverter with 30W/cm 3 OPD was discussed. It is confirmed that actualization of the 30W/cm 3 OPD are possible putting the following issues into the practice, 600V power devices of which power consumption is less than 5W and related technologies such as a low loss rectification circuit, a high speed gaiting circuit, technologies for AC filter size reduction and for power electronics system integration. A 3kW-0cm 3 unit-inverter has a big possibility to enables

5 Materials Science Forum Vols Journal Title and Volume Number (to be inserted by the publisher) 5 expansion of energy saving market, consumer business, distributed power supply market and energy management system. (C) Electric Vehicle Power Control Unit (PCU) of electrically controlled vehicles, such as a Electric Vehicle (EV), Hybrid Electric Vehicle (HEV) and a Fuel Cell Vehicle (FCV) are likely to be designed, keeping a constant volume (about litters) with no relation to power capacity of motor. In the case of a FCV with 0kW motor, OPD of PCU will be about W/cm 3. According to the road map shown I Fig.5, W/cm 3 OPD will be realize in 20. A new EV structure concept is proposed recently, looking at the age of FCV. The new concept tries to install all driving system under a floor of car body. In the case of new the new EV structure, more size reduction of PCU will be required. In the discussions, 3 litters PCU is forecasted. In the age 3 litters PCU with OPD of 30W/cm 3, the 0KW PCU will be by completely fabricated by technologies of hybrid power integrated circuits, with an aid of design concept of power electronics system iteration. In addition to development of the high speed, ultra low loss device, it will be required to make new reserach sechema for the power electronics system integuration. With out wide cooperation among the people who are involved in the reseachs of materials, devices, passive compornet, circuit, assembly technoloies and so on. Conclusion The possibility of power electronics innovation is discussed focusing to the progress in the next generation advanced power devices. The future direction of power electronics applications is surveyed in the term of output power density as an index of future power electronics roadmap, instead of power conversion efficiency. According to the new road map, the output power densities of more than W/cm 3 to 30 W/cm 3 will be appear from 20 to A next-generation CPU power supply, a compact unit-inverter and an electric vehicle will be the typical candidates of the future applications.all of these applications will open a new age of power electronics. Acknowledgment Author would like to thank ENAA investigative committee members, especially Drs. I. Oomura (Toshiba), S. Matsumoto (NTT), Y. Sato (Chiba Univ.) and H. Tadano (Toyota CRD) for their kind help in preparing the manuscript. Reference [] H. Ohashi, Journal of IEEJ, Vol.22, No.3, 2002, p68 [2] I. Takahashi, IEEJ 200 Annual Meeting [3] K. Adachi, JSAP 64th Autumn Annual Meeting, 2003, Ext. Abs., No., p360 [4] 2002 NEDO Report, ENAA Investigative Committee, Investigation on practical use of next generation power devices

6 26 Silicon Carbide and Related Materials Title of Publication (to be inserted by the publisher) 0 SIT Uni-polar MOSFET Si MOSFET IGBT IGBT Super Junction MOS 950 pn-diode Bi-polar (trench) IGTT Bipolar Transistor GTO IEGT Darlington Thyristor MCT/EST SiC 990 GCT 2000 Diamond GaN Fig. History of progress in power semiconductor devices PCE (%)90 80 (%)(%)(%)70 60 Loss Low Ruggedness :Switching Regulator :AC Drive :Thyristor Valve Year Low power gating Fig.2: Progress of conversion efficiency in power electronics OPD(W/cm3) 0 Board PS Thyristor Valve + サイリスタバルブ + + General purpose Inverter Unit PS 0. Inverter for air conditioner PKG-PS Year HEV/EV What is a new application? Fig.3: Progress of out put power density in power electronics apparatus. The power density increases linearly on a log scale Unit PS Board PS PKG PS 30 PS for A.M. 20 Takahashi model Power consumption / Input Power(%) Fig.4: Relations between the OPD and the power loss of converter OPD(W/cm 3 ) Ron(mΩ cm 2 ) Si-MOSFET Theoretical limitation Super Junction SiC SBD Novel Si-MOSFET front line Blocking Voltage:V B (V) SiC SIT GaN-HEMT Si-IGBT Fig.5: Recent research state of the advanced power devices OPD(W/cm 3 ) 0 HVDC + Air conditioner G.P. INV PS PS EV/HEV Year Fig.6: A road map of power electronics, focusing to the OPD as indicator. The map shows appearance of 20-30W/cm 3 converters FCEV-0kW Inverter(30 30W/CC) FCEV-5kW Inverter(5-20W/CC) 20kW Converter 50W/CC,4MHz 3kW-Inverter 30W/CC 500kHz Next Gene. 0W-CPU PS. 5W/CC 5MHz

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