1. macro-trends power devices

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Macro-Trend and A Future Expectation ofInnovations in Power Electronics and PowerDevices

H. Shigekane*, T. Fujihira*, K. Sasagawa**, Y. Seki*, Y. Takahashi%, and A. Takai**Fu~jilectric Device Technology Co., Ltd., Tokyo, apanFuji Electric Advanced Technology Co., Ltd., Tokyo, apan

Abstract-The improvement of power conversion efficiencyis important for preventing global warming and forprotecting global environment. Expecting the direction offuture innovations in this field is also important andeffective. Macro-trend of innovations in power electronicsand power devices are reviewed to derive two laws ofinnovations in this field. According to these laws, anexpectation to future innovation of power electronics, PDMtechnology using soft switching of SiC or GaN devices athigh frequency, is presented.

1. INTRODUCTIONAs 30 to 40% of the world primary energy is consumedfor electricity generation and as this share is forecasted tocontinue increasing, power electronics and power devicesare key technologies for reducing CO2 emission to preventglobal warming and to protect global environment. Yearby year, the power conversion efficiency and the cost ofpower electronic systems have been being improved.However, innovative improvements, or innovations, of

power electronic systems have not been introduced sooften. Therefore, to expect the direction of future

innovations of this field is very important and effective. Inthe present paper, macro-trend of innovations in powerelectronics and power devices are reviewed to derive twolaws of innovations in this field. According to the twolaws, an expectation to future innovation of powerelectronics and power devices are presented.11. MACRO-TREND OF INNOVATIONS

There have been a number of innovations introduced inpower electronics. For example, the industrial use of PA M(Power Amplitude Modulation) and PWM (Pulse WidthModulation) have introduced great advancements of themotor drive and of the motion control technologies, and,then, have contributed to the progress of the modemindustrialized society. For example, the industrial use ofresonant circuits or matrix-converters have greatlyimproved the power conversion efficiency in inductionheating and in power supply or in elevator control.Reviewing these innovations in power electronics, theauthors have find two guiding laws that can be used tostudy future expectations of power electronics.

TABLE 1. THE TREND OF POWER DEVICES AND POWER ELECTRONICS EQUIPMIENTSYear 1960 1970 1980 1990 2000

A 961 A1984Thyristor GTO

A 975 A 90 A198, A 99 4 A2002PoerTrnsstr raflsistor IGTM ule IGBT Module IGBT ModulePoevie Module (I en. (3Gen.) (5Gen.)A2003

A 1986 Al8 RB-IGBTMOSFETM-Po er

Low,Medium Transistor MtiMotor Pwr Thyristor PAM Inverter, IBDrive High Load Cretsuc T \ PMIvre

PowerMeiunDvd ertier Conve nvererPowe Trnsisor esonnt ConvererteThyri storT

PwrConverter Converter

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A. The FirstLaw of Innovations in PowerElectronicsTh e first law of innovations in power electronics wewould like to present is that the innovation of powerelectronics occurs together with the innovation of powerdevices.TABLE I shows the trend of power devices and power

electronic equipments [1]. Since the advent of thyristorsas power devices in the 1960s, power electronics hadmade rapid progress. Thyristors can be turned on at anytime, bu t cannot be turned off by gate control. Thyristors,therefore, applied to rectifiers, DC motor drives, and soon. The circuit topology shown in Fig. I had applied for apart of a variable speed drive system for AC motors. TheDC-link voltage, Ed, can be controlled by the thyristorsand an output voltage having a rectangular shape isobtained by the inverter. Therefore, PAM can beachieved. Since commutation circuit (CC) is required forturning off the thyristors, the circuit configuration iscomplicated.Emerging of self turn-off device, which, can meet therequirement of the advanced control applications, forexample, variable speed control drives of AC motors anduninterrupted power systems, was expected. Transistors,which were mainly used for the small signal amplifier,were focused. Through the research of semi-conductorphysics and the innovation of the manufacturingtechnology, the transistor was changed to the powertransistor for the switching use.Power electronics have been rapidly developing sincepower transistors, which have been commercialized in the1970's, were used. Fig. 2 shows the typical circuittopology [2]. A constant DC-link voltage is obtained bythe diode bridge rectifier and an AC power is output fromthe voltage source inverter. Th e inverter controls theamplitude and the frequency of the output voltage byPW M control. PW M control is widely employed inpower electronics equipments and controllability fo r

voltage, current, an d frequency is greatly improved.Micro-controllers and modern control theories are appliedin the control unit and use of digital control technologiesare dramatically increased. On the other hand, isolated

modules for power transistors have been developed andpackaging technology have been rapidly changed. Thisbrings that main circuits are simplified and the volume isreduced.The switching frequency of such power transistors canbe limited within a few kHz, resulting in occurring

undesirable magnetic sounds. To overcome such an issue,a new type of power devices, IGBT (Insulated-GateBipolar Transistor), has been developed. The device hasspecific characteristics of high-speed switching andtremendous low drive power due to having combinationstructure of a MOSFET (Metal-Oxide-SemiconductorField Effect Transistor) and transistor. The IGBT hasbeen improved rapidly year after year, and some haveachieved to have considerably high voltage and highcurrent rating as almost same as those of GTOs (GateTurn-Off Thyristor) for an alternative device to thyristors.Hence, the IGBT has been playing an important role forvarious kinds of power electronics equipments rather thanthe others these days.Until now, the trade-off relationship between the on-state voltage and the switching loss of the IGBTs has

been improved. However, it is said that such performanceimprovement is close to the theoretical limit. Therefore,technological development will advance to the functionimprovement than the performance improvement in thefuture. One of the function improvements is that an IGBTpossesses reverse-blocking voltage capability. This typeof the IGBT is called RB-IGBT (Reverse-BlockingIGBT). Now, RB-IGBTs, with 600V blocking voltagehave been commercialized, and by using these IGBTsseveral type of matrix converters which is able to directlyconvert to AC voltage and frequency without a DC-linkvoltage (cf. Fig. 3), have been developed [3]. In the future,the blocking voltage and the current rating of RB-IGBTsshould be improved. These RB-IGBTs enable bi-directional switching, and they are approaching to anideal switch. This progress of the power devicetechnology will expand the possibility of variousconverters, such as AC-AC direct converters and currentsource converters.

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(a)Circuit ConfigrationFflUl-[I[ EdIJJLII0U

(b)WavefornisFig. 2. Transistor PWM Inverter

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NO,=si qJ(EcI W0) =2sEC2qV8

LL 6On-state Resistancek~si)R.,ic2;R.t 1300U @same b~tocking voltageDrift Layer Thicktness

Fig. 5.Features of SiC(GaN) material compared with Sithe PFC (Power Factor Control Circuit) of blade servers indata centers. SiC MOSFET and GaN SBD have alreadybeen announced very close to enter the market. However,there is almost no announcement for full SiC or GaNpower devices to be applied to real market use. Thereasons why these devices seem not to be applied to realmarket use are very high cost and high current andthermal densities. Especially because of very high cost,there will be almost no approval in the market if MOSFETor IGBT will be simply replaced by SiC or GaN devices inthe existing power electronic equipments.

The key for SiC or GaN devices to be accepted in themarket will be the change or the innovation of powerelectronics to achieve certain advantage that compensatevery high cost of these devices. Very high carrierfrequency to be realized by these devices can be the originof the innovation.B. An Expectation of Innovation

Th e social and market requirements to power electronicequipments and systems are low loss, high preciseness,'low price, low emission, and low weight and volume.State-of-the-art AC-AC power conversion topology isDC-link sinusoidal PWM using hard switching of IGBTor MOSFET. In this type of systems, switching noise isvery large and, then, carrier frequency is limited.Especially in medium to high power systems using IGHT,the carrier frequency is limited to as low as several toseveral ten's of kHz. Because of the limited carrierfrequency, output filters of this type of systems are largeand high cost. Because of very large switching noise,switching speed of the device is suppressed resulted inhigh switching loss. Because of very large switching noise,input filters and noise suppressing components are largeand high cost.

The authors' expectation of innovation in near futurepower electronics is in this type of systems. What willhappen if PD M (Pulse Density Modulation) technologyusing soft switching, for example resonant zero-voltage-switching, of SiC or GaN devices with 10 to 100 timeshigher carrier frequency is applied to DC-link AC-ACpower conversions? The weight, volume, and cost ofoutput and input filters, and of noise suppressioncomponents will be greatly reduced. It may compensatethe very high cost of SiC or GaN power devices. The lossof the converter will be greatly reduced because theconduction loss of SiC or GaN devices is very low and

because the switching loss of SiC or GaN devices is alsovery low in soft switching applications.Th e PDM technologies using soft switching were triedpreviously, for example in ref. [51, but they were no taccepted in the market at that moment. The reason wh ythey were not accepted was because Si devices, for

example IGBT, were used for switching devices, so thatthe carrier frequency could not be increased enough andalso that the switching loss was large. In the near future,these problems will be resolved by using SiC or GaNdevices.Th e expected innovation, introduction of PDM,technology using soft switching, seems to be feasibleaccording to the first law of innovations because theinnovation of the device, the introduction of SiC or GaNdevices, will support the innovation of power electronics.Th e total advantages of the new device, low conductionloss and high carrier frequency, will be fully converted tothe system advantages, low loss and low noise. Accordingto the second law of innovations in power electronics, italso seems feasible. Resonant soft switching technologiesare already widely used in low power applications and we

have enough experiences to extend them to higher powerapplications. PDM technology may look like new.However, PD M has already been being used intelecommunication area as signal transmitting technologyup to very high carrier frequency.C. The Key to The Expected Innovation

The authors have pointed out several disadvantages ofSiC and GaN devices in the section III A. The high costissue of the devices may be compensated by the reductionof passive components cost. However the remainingissues, high current and high thermal densities, must beresolved at the device side.In SiC or GaN dies, the current density will be severaltimes higher than that of IGBT at the top side electrodeand the thermal density also several times higher at the

bottom side. To maintain the reliability and the lifetime ofthe device at elevated temperature, Aluminum wirebonding technology cannot be used for full SiC or GaNpower modules and the thermal resistance from the die tothe cooling fin must be reduced several times. Keytechnology to resolve top side issue will be lead flameinterconnection [6] as shown in Fig. 6. By using leadflame interconnection, current density at the top sideelectrode of the die can be maintained as the same level astoday's IGBT. Adding to this, the conduction of heatthrough the lead flame reduces the heating up of the diesurface as shown in Fig. 7. Key technologies to resolvebottom side issue are DC B (Direct Copper Bonding)substrate and die arrangement. For DCB, insulationmaterials with higher thermal conductivity and thickerCopper foil should be introduced. Die arrangement onDC B will be more important. The thermal interventionsbetween neighboring dies must be avoided an d the heatspreading from the bottom of dies should be designedlarger. For maintaining reliability at elevated temperature,improvement of the solder between die and DC B andbetween die and lead flame will be required [7]. Anexample is shown in Fig. 8, where the tensile strength ofthe solder at elevated temperature has been greatlyimproved by changing the additive components to Tinbased solders.

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hAgt~flasFig. 6.Comparison of he cross section between wire bonding and lead-frame inter-connections

Wi

LJF -qPPinterconnectionFig. 7.Simulation results of temperature distnibutions

i.iSnf-Sbolde

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601C x24hr 1501C X 100hr 175VGx 1OO0hrFig. 8.Experimental results of change rate of tensile strength under thethermal aging test

IV. CONCLUSIONSMacro-trend of innovations in power electronics andpower devices has been reviewed to derive two laws ofinnovation of this field. The first law derived is that theinnovation of power electronics occurs together with theinnovation of power devices. The second law derived isthat the innovation of power electronics occurs in innovation,

and based on the recognition that next generation powerdevices will be SiC or GaN, an expectation of cominginnovation in power electronics has relatively low powerapplications and that it is extended to high powerapplications. According to the two laws of been presented.PD M technology using resonant soft switching of SiC orGaN devices is expected to be introduced to DC-link AC-AC converters. By increasing carrier frequency 10 to 100times higher than today's PW M using hard switching ofIGBT or MOSFET, PDM using soft switching of SiC orGaN devices will be feasible to compensate high devicecost by reduced passive components cost with reducedloss and noise. The key to realize this expected innovationwill be advancement of packaging technologies, forexample lead flame interconnection, DCB, diearrangement, solder, and so on.

ACKNOWLEDGMENTThe data and drawings of this paper are provided by themembers of Electron Device Lab, Fuji Electric DeviceTechnology Co., Ltd. and by the members of ElectronicsTechnology Lab, Fuji Electric Advanced Technology Co.,

Ltd.REFERENCES

[11 H. Shigekane, et al., "Developments in modem high power semi-conductor devices," Procof Int. Symp. on Power Semicon.Devices and /Cs, 1993, pp. 16-21.[21 T. Tsukahara et al., "FRENIC 5000 series power transistor PWMtype VVVF inverter," Fuji Electric Journal,vol. 55. No.10, 1982,pp. 15-21.[31 1. ato et al., "Technologies for practical motor drive system withmatrix converter," Procoqfint. Symp. on Power Semicon. Devices'and i10, 2008.[4] H. Ota et al. ,"Soft-switching type multiple-chip power device(M-Power) ," Proc. of ni.Symp. on Power Semicon Devices and10~, 2001, pp. 373-376.[5] D. M. Divan, G. Venkataramanan, an d R. W. De Doncker,"Design Methodologies for Soft Switched Inverters," IEEE-lAS

Annual Conference Records, 1988, pp. 758-766.[6] M.Honio, et al, "Investigations of High Temperature IGBTModule Package Structure," Proceedings ofPCIM Europe, 2007.[7] M.Otsuki, et al, "Advanced thin wafer ICRTs with ne w thermalmanagement solution," Proc. of Int. Symp. Power Semicon.Devices and I1s, 2003, pp. 144-147,

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1. macro-trends power devices

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