Comparison of SiC and Si Power Semiconductor Devices to Be Used in 2.5 kw DC/DC Converter

Comparison of SiC and Si Power Semiconductor Devices to Be Used in 2.5 kw DC/DC Converter M. G. Hosseini Aghdam Division of Electric Power Engineering Department of Energy and Environment Chalmers University of Technology Gothenburg, Sweden ghasem.aghdam@chalmers.se T. Thiringer Division of Electric Power Engineering Department of Energy and Environment Chalmers University of Technology Gothenburg, Sweden torbjorn.thiringer@chalmers.se Abstract With the fast development of silicon carbide (SiC) technology, SiC-based power semiconductor devices have started to complete Si components in transportation applications. In this paper, two dc/dc converters for hybrid electric vehicles (HEs) application are designed and analyzed. The losses, efficiency, junction temperature, and the volume and weight of heat sinks of two converters are calculated for a Si and SiC solution for a 2.5kW dc/dc converter. A performance comparison of the parameters mentioned above gives that SiC-based technology shows better performances than Si-based power semiconductor devices in the investigated dc/dc converter system. Finally, an economical evaluation shows that the SiC components can cost almost 2.5 times more in order to have the same total cost as for a Si solution for a 15 years operation. Keywords-Si and SiC power semicondcutors; dc/dc converter; hybrid electric vehicle I. INTRODUCTION Currently, almost all power electronics converters use Sibased power semiconductor devices. However, in recent years, a large effort has been devoted on the development of SiCbased power semiconductor switches [1-2]. This development is caused by the fact that SiC-based power semiconductor devices have higher breakdown voltages because of their higher electric breakdown filed. Since SiC is a wide-bandgap semiconductor, SiC allows higher concentration of doping and consequently a lower specific on-state resistance. SiC power semiconductor devices can as a consequently of this fact operate at high temperature. SiC-based power semiconductor devices also have excellent reverse recovery characteristics [3]. With less reverse recovery current, the switching loss is reduced. The high- frequency switching capability of a semiconductor is directly proportional to its drift velocity. The drift velocity of SiC is more than twice the drift velocity of Si. Therefore it is expected that SiC-based power semiconductor devices could be switched at higher frequencies than their Si counterparts. Many devices have been proposed for SiC, but only SiC Schottky diodes are commercially available so far. Single device rating is up to 1200 / 50 A [4]. Semi South, SiCED and Rock- well have also developed some prototypes. Recently, SiC MOSFETs (from Cree) are also available for research purpose. Moreover, other high power modules have been fabricated and tested, such as the 1200 /300 A Si IGBT/SiC Schottky diode single phase module (Cree) [5] and 55 kw Si IGBT/SiC Schottky diode inverter [6]. At the end of 2006, Cree announced the first Si IGBT/SiC Schottky diode co-package products (CID150660) [7]. Power electronics circuits play an important role in HEs. The hurried demand for HEs enhanced the significance of the power electronics circuits in these vehicles. The switches used for HEs application are typically MOSFETs or IGBTs, although other types of switches may be used [8]. But, MOSFETs are widely preferred power devices in low power applications because they can be operated at high frequencies with relatively low loss switching behavior. The using of MOSFET in power electronics circuits of HEs shows quite good results [9]. The purpose of this contribution is to present the comparison of SiC and Si based power semiconductor devices to be used in 2.5 kw insulated full-bridge dc/dc converter for HEs application. In order to make a performance comparison of SiC-based devices, two full-bridge dc/dc converter systems are designed and analyzed based on conventional Si MOSFETs/Si diodes and SiC MOSFETs/SiC Schottky diodes. The main objective is to determine and analyze of losses in two cases. a) With the same heat sink for two systems, comparing the system temperature. b) With the same junction temperature; comparing the needed volume and weight of heat sink. The final aim is an economical comparison of Si and SiC based power semiconductor devices to be used in a dc/dc converters. II. DC/DC CONERTER SYSTEM In HEs, a dc/dc power supply is needed for the 12 supply. There are various power electronics converters available for dc/dc power supplies. In this paper, an insulated full-bridge dc/dc converter is selected. A full-bridge, transformer isolated, dc/dc converter circuit is depicted in Fig. 1. This topology uses a high frequency transformer between the source and the load to provide galvanic isolation. The full-bridge dc/dc converter consists of four parts: a high frequency inverter, a high frequency transformer, a rectifier, and an LC filter. The high frequency inverter consists 1035

of four switching modules (Q 1 Q 4 ). According to four-switch combination, three voltage levels, + dc, - dc, and 0, can be synthesized for the output voltage. When switches Q 1 and Q 4 are turned on simultaneously, the input voltage + dc appears at the output. If switches Q 2 and Q 3 are turned on at the same time, the output voltage is reversed to - dc. When all the switches are off, the primary side of transformer sees no voltage. Diodes, D 1 and D 2, rectify the voltage fed to them from the secondary side of the transformer. This rectified voltage then passes through an LC filter to feed the dc load. The operation waveforms of the converter are shown in Fig. 2. The pure current of MOSFETs and diodes can be observed in Fig. 3 and Fig. 4, respectively. Also the average and rms values of the currents, which flow through the MOSFET and diode, can be calculated as I Q,ave =4.51 A, I Q,rms =2.44 A, I D,ave =112.83 A, and I D,rms =61.10 A. TABLE I. SYSTEM PARAMETERS Power rating 2.5 kw Input voltage 300-450 Output voltage 12 Switching frequency 100 khz oltage ripple 1% Current ripple 5% TABLE II. POWER DEICES USED IN THE CONERTERS Item oltage rating Current rating Company SiC MOSFETs 800 10 A*2 Cree Si MOSFETs 600 20 A*1 IXYS SiC Schottky Diodes 300 10 A*25 Infineon AG Si Diodes 300 10 A*25 Infineon AG Fig. 1. The full bridge, transformer-isolated, dc/dc converter. TABLE III. DEICE CHARACTERISTICS AT ROOM TEMPERATURE Characteristics Si SiC MOSFET on-state resistance 350 mω 125 mω (0.25Ω/2) MOSFET output capacitance 420 pf 42 pf (21pF*2) Diode series resistance 8.61 mω 3.36 mω Diode reverse recovery parameters α=3.5424*10-8 α=2.1670*10-8 β=1.2698*10-8 β=2.3300*10-8 Fig. 2. Operation waveforms of the insulated full-bridge dc/dc converter. The dc/dc converter investigated here is designed to supply a 2.5 kw load with a regulated output voltage of 12 and the input voltage is assumed to be fluctuating between 300 and 450. The system parameters are summarized in Table I. In the Si-based converter, four 600 and 20 A singleswitch MOSFET modules from IXYS [10] are used as the main switches with a switching frequency 100 khz. In the SiCbased converter system, the Si MOSFETs and Si diodes are replaced with SiC MOSFETs and SiC Schottky diodes, respectively. In this system, an 800 /10 A SiC MOSFET from Cree is used as the main power switch. Since commercial SiC MOSFET module data are currently not available, their parameters are calculated by 2D numerical simulations and theoretical analysis [11]. SiC Schottky diodes and Si diodes used in this study are rated 300 and 10 A from Infineon AG which their parameters obtained from laboratory results [1]. Based on the above discussion and also considering the availability of devices, the selected devices and their parameters at room temperature are listed in Tables II and III. Fig. 3. MOSFET current waveform. Fig. 4. Diode current waveform. III. CONERTER POWER LOSSES CALCULATION In order to investigate the performance of a SiC-based converter system, power losses analysis is performed based on 1036

the parameters in Table III. The converter has two kinds of power losses, conduction losses and switching losses. A. Conduction Losses The expression for the conduction loss of a diode is given by [12]: 2 P D, cond I D, rms RD + I D, ave = (1) where I D,rms and I D,ave are the rms and average value of the current value through the diode, respectively. The conduction loss of a MOSFET depends only on the onstate resistance of the MOSFET; therefore, the conduction loss expression is as simple as D 2 Q, cond = IQ, rms Ron. (2) P R on depends on the specific on resistance of the material (R D ). R D is proportional to a power of the temperature. This is γ γ because RD = 1/ μn and μn 1/ T. Thus, R D T where γ is a constant and is 2.42 for Si and 1.3 for 6H-SiC at 300 o K [13]. B. Switching Losses Although the reverse recovery current is much smaller for Schottky diodes than that of pn diodes, the reverse recovery loss dominates its switching losses [1] and is accordingly of interest. So in this model, the other losses are neglected and only reverse recovery loss is considered. Assuming that the diode sees a constant reverse voltage when it is off and it is switched at constant frequency [1], then PD, = f i dt. (3) SW s R d The reverse recovery time-integral current can be approximated linearly as a function of the forward current [1]: i dt = α I + β (4) d F where ε r is the permittivity of semiconductor, E c is the electric breakdown field, b is the breakdown voltage, E on and E off are the losses during the charging and discharging of two device capacitances: drain-source and drain-gate. These capacitances are charged and discharged by effective currents of (K 1-1) J and (K 2 +1) J, respectively. ( ) g m GH th where K1 = and K J g = ( ) m th GL 2. where g m is the transconductance, J is the current density, GH is highest gate voltage applied, GL is lowest gate voltage applied, and th is the threshold voltage [14]. The energy loss for a turn-on and a turn-off of a MOSFET is the sum of (7) and (8). 1 1 1 Etot = Eon + Eoff = ε r Ec + (7) 3 b K 1 1 K 2 + 1 If a MOSFET is switched at a frequency of f s, then its switching losses can be represented as 1 1 1 PQ, sw = Etot fs = fs ε r Ec +. (8) 3 b K1 1 K2 + 1 The conduction, switching and total loss profiles of a diode and a MOSFET for the different power level, calculated using the methods introduced above, are shown in Fig. 5 to Fig. 10. The total power losses of the converter (four MOSFET modules and two diodes) are plotted in Fig. 11. As it can be seen in these figures, the power losses of the SiC-based power semiconductor devices are lower than those of the Si-based power semiconductor devices. Consequently, the efficiency of the SiC- based converter system is higher than the one of the Si-based system. Note that the parameters of the power semiconductor devices are considered at room temperature. J For a Si diode, α and β are temperature dependent and their values are given in Table III at room temperature. But, for a SiC Schottky diode, α and β are temperature independent and their values are given in Table III [1]. The switching losses of a MOSFET can be calculated using piece-wise linear turn-on and turn-off waveforms. This is an approximation, which does not consider the physics behind the switching. The turn-on and turn-off energy loss equations are derived in [14] as E on 1 = ε E (5) 3 r c ( K1 1) b E off 1 = ε E (6) 3 r c ( K2 + 1) b Fig. 5. The conduction loss profile of a MOSFET versus load. 1037

Fig. 6. The switching loss profile of a MOSFET versus load. Fig. 9. The switching loss profile of a diode versus load. Fig. 7. The total loss profile of a MOSFET versus load. Fig. 10. The total loss profile of a diode versus load. Fig. 8. The conduction loss profile of a diode versus load. Fig. 11. The total loss profile of the converter versus load. 1038

I. JUNCTION TEMPERATURE AND HEATSINK In order to investigate the performance of a SiC-based converter system, power losses analysis is performed based on the parameters in Table III. The converter has two kinds of power losses, conduction losses and switching losses. A. With the same heat sinks per device for the two converters The expression for the conduction loss of a diode is given by [12]: In this case, the same heat sinks per devices are used for the two converters. The parameters of heat sinks are as follows: Heat sink of diodes: olume: 1284.90 cm 3 Weight: 675.86 grams Thermal resistance: 0.28 o C/W Heat sinks of MOSFETs: olume: 196.64 cm 3 Weight: 121.56 grams Thermal resistance: 1.30 o C/W With the same ambient temperature 40 o C, the junction temperature of the SiC MOSFET and the Si MOSFET are 49.93 o C and 131.42 o C, respectively. It is noted that unlike the Si MOSFET, the on-state resistance of the SiC MOSFET decreases as temperature increases. The reason for this is that the relative large channel resistance in the SiC MOSFET. Also, the junction temperature of the SiC Schottky diode and Si diode are 93.94 o C and 110.66 o C, respectively. Therefore, with the same heat sink for two converters, the SiC system has a much lower junction temperature. B. With the same junction temperature and different heat sinks With the same ambient temperature 40 o C and the same junction temperature for the SiC and the Si-based power semiconductor devices, it is possible to select different heat sinks for Si and SiC devices. The parameters of required heat sinks are as follows: Heat sink of Si-based diodes: olume: 1284.90 cm 3 Weight: 675.86 grams Thermal resistance: 0.28 o C/W Heat sink of SiC-based Schottky diodes: olume: 700.84 cm 3 Weight: 353.81 grams Thermal resistance: 0.40 o C/W Heat sinks of Si-based MOSFETs: olume: 196.64 cm 3 Weight: 121.56 grams Thermal resistance: 1.30 o C/W Heat sinks of SiC-based MOSFETs: olume: 55.00 cm 3 Weight: 34.00 grams Thermal resistance: 4.7 o C/W As it can be seen, the required heat sink size of the SiCbased power semiconductor devices is only a fraction of the heat sink size needed for the Si devices if the device junction temperatures are kept the same. As a result, the SiC-based converter system is smaller in size using the same thermal limit.. ECONOMICAL EALUATION The total cost for a power semiconductor solution consists of two parts: loss cost, and investment cost. Assume that the dc/dc converter works 2 hours per day for a HE application, 50% in full-load and 50% in one-fourth of full-load. It also is assumed that it will be used for 15 years and the cost for loss is 0.1 /kwh. The investment cost of the Sibased power semiconductor devices is 74.27. The total energy loss for Si-based components will be: 429.57 1 365 15 + 60.39 1 365 15 = 2682.53 kwh (9) Finally, the loss cost for Si-based components is give by (10): Loss Cost of Si = 2682.53 kwh 0.1 /kwh = 268.25 (10) The total cost for the Si solution is the sum of loss cost and investment cost, i.e.: The total cost for the Si solution = 74.27 + 268.25 = 342.52 (11) The total energy loss for SiC-based power semiconductor devices will be: 259.45 1 365 15 + 46.77 1 365 15 = 1676.60 kwh (12) Finally, the loss cost for Si-based power semiconductor devices is give by (13): Loss Cost of SiC = 1676.60 kwh 0.1 /kwh = 167.66 (13) Therefore, the SiC components can be 174.86 more expensive in order to have the same total cost as for the Si solution. I. CONCLUSION In this paper, a SiC-based power semiconductor devices full- bridge insulated-transformer dc/dc converter system is designed and compared with a Si-based devices converter system. The simulation results show the power loss of the SiC converter system is 60% of the Si-based system in full-load condition. If the device junction temperatures are kept the same, the heat sink size and weight of the SiC converter is 46% 1039

of the Si system. Finally, an economical evaluation shows that the SiC-based power semiconductor devices can almost 2.5 times more in order to have the same total cost as for a Si solution for a 15 years operation. REFERENCES [1] B. Ozpineci, and L. M. Tolbert, Characterization of SiC Schottky Diodes at Different Temperatures, IEEE Power Electronics Letters, ol. 1, No. 2, pp. 54-57, 2003. [2] T. F. Zhao, J. Wang, A. Q. Huang, and A. Agarwal, Comparisons of SiC MOSFET and Si IGBT Based Motor Drive Systems, Proceeding of the 42 nd Annual Meeting IEEE Industry Application Conference, pp. 331-335, 2007. [3] A. Elasser, M. Kheraluwala, M. Ghezzo, R. Steigerwald, N. Krishnamurthy, J. Kretchmer, and T. P. Chow, A comparative Evaluation of New Silicon Carbide Diodes and State-of-the-Art Silicon Diodes for Power Electronic Applications, IEEE Transactions on Industry Applications, ol. 39, No. 4, pp. 915-921, 2003. [4] Cree Inc., Kansai Electric and Cree Demonstrate a 100 ka Silicon Carbide Three Phase Inverter, 2006. [5] H. Zhang, L. M. Tolbert, and B. Ozpineci, System Modeling and Characterization of SiC Schottky Power Diodes, Proceedings of the IEEE Workshops on Computers in Power Electronics, pp. 199-204, 2006. [6] B. Ozpineci, M. S. Chinthavali, L. M. Tolbert, A. Kashyap, and H. A. Mantooth, A 55 kw Three-Phase Inverter with Si IGBTs and SiC Schottky Diodes, Proceeding of the 21 st Annual IEEE Applied Power Electronics Conference and Exposition, pp. 448-454, 2006. [7] Cree Inc., Cree Announces First Power Switch and Diode Co-Pack, 2006. [8] B. Welchko, J. M. Nagashima, and K. M. Rahman, Inverter for Electric and Hybrid Powered ehicles and Associated System and Method, United States Patent, No. 7057371, 2006. [9] A. ezzin, and K. Reichert, Power Electronics Layout in a Hybrid Electric or Electric ehicle Drive System, Proceeding of the IEEE Workshop on Power Electronics in Transportation, 1996. [10] IXYS Semiconductors Website, http://www.ixys.com/. [11] L. Reddy, L. M. Tolbert, H. Zhang, and T. Cheek, Performance of Ultra-High Efficient Electronic Ballast for HID Lamps Using SiC Devices, Proceeding of the 42 nd Annual Meeting IEEE Industry Application Conference, pp. 471-477, 2007. [12] B. J. Baliga, Modern Power Devices, John Wiley & Sons, Ltd, New York, 1987. [13] M. Bhatnagar, and B. J. Baliga, Comparison of 6H-SiC, 3C-SiC, and Si for Power Devices, IEEE Transactions on Electron Devices, ol. 40, No. 3, pp. 645-655, 1993. [14] Q. Huang, and B. Zhang, Comparing SiC Switching Power Devices: MOSFET, NPN Transistor, and GTO Transistor, Solid State Electronics, ol. 44, No. 2, pp. 325-340, 2000. 1040