Comparisons of 6.5kV 25A Si IGBT and 10-kV SiC MOSFET in Solid-State Transformer Application Gangyao Wang Xing Huang Student Member Student Member Tiefu Zhao Jun Wang Student Member Subhashish Bhattacharya Student Member Member Alex Q. Huang Fellow FREEDM Systems Center, Department of Electrical and Computer Engineering North Carolina State University, Raleigh, NC [email protected] Abstract --A 6.5kV 25A dual IGBT module is customized and packaged specially for high voltage low current application like solid state transformer and its characteristics and losses have been tested under the low current operation and compared with 10kV SiC MOSFET. Based on the test results, the switching losses under different frequencies in a 20kVA Solid-State Transformer (SST) has been calculated for both devices. The result shows 10kV SiC MOSFET has 7-10 times higher switching frequency capability than 6.5kV Si IGBT in the SST application. operation is given; in the section III, the theoretical switching frequency and break down voltage for Si and SiC device is compared. In section IV, the tested switching loss for 6.5kV Si IGBT is given and compared with switching loss of 10kV SiC MOSFET under the same test condition. The switching loss model of 10kV SiC MOSFET has been reported in . In section V, the switching frequency capability of 6.5kV Si IGBT and 10kV SiC MOSFET is compared for SST application when considering the same losses. The conclusions are presented in Section VI. Index Terms-- IGBT, Solid state transformer, SiC MOSFET I. II. INTRODUCTION 1 Solid-state transformer (SST) is an AC-AC converter which is used to replace traditional 60Hz transformer. It has many advantages compared with traditional transformer like unity power factor, instantaneous voltage regulation, DC output etc. as reported in . It converts 7.2kV ac to 120/240Vac for utility applications or vice verse for feeding back energy to the grid. In order to reduce the size of the SST, the switching frequency must be very high, unfortunately the highest voltage for currently available silicon IGBT is 6.5kV with a minimum current rating 200A . The switching frequency capability of this kind of IGBT is typical less than 1 kHz and not suitable for low current application. For this reason, a 6.5kV 25A silicon IGBT has been specially packaged and used as switching device of a seven level rectifier, which is being considered as the rectifier stage of SST. On the other side, SiC power semiconductor device has much higher break down voltage, lower switching loss, and higher operation temperature, so it can operate under much higher frequency and becomes a good power device candidate for SST application. In the section II of this paper, the packaging of 6.5kV 25A IGBT is introduced, its switching characteristics and losses under very low current Fig.1, 6.5kV 25A IGBT dual module layout The final prototype is shown in Fig. 2, for this package design, the minimum clearance distance in air is 19mm and creepage distance is 78mm which complies with IEC-600771 standard. This work was supported by ERC Program of the National Science Foundation under Award Number EEC-0812121. 978-1-4244-5287-3/10/$26.00 ©2010 IEEE INTRODUCTION OF 6.5KV 25A IGBT DUAL MODULE Because of absence of high voltage, low current power device on the market, a 6.5kV IGBT dual module with antiparallel diode has been packaged. Fig 1 shows the chips layout on DCB, it contains two 6.5kV 25A IGBT chips and two 6.5kV 50A diode chips. 100 Vg I Vc inte(Vce*Ic Vce*I Fig.2, 6.5kV 25A IGBT dual module prototype Fig 3 shows an optical signal triggered gate driver installed on the IGBT, plated slots are specially designed in order to keep enough creepage distance between high voltage terminals. Fig.5 IGBT Turn Off under 3000V 10A Fig.4 shows the 10A turn off waveform with 3kV bus under 125degree. The Turn off loss is 52.23mJ. III. THEORETICAL COMPARISON OF SILICON AND SILICON CARBIDE DEVICES The limitations of power devices based on silicon and silicon carbide can be evaluated from Baliga’s Figure of Merit, the relationship of specific resistance of the ideal drift region and the breakdown voltage : Ron −ideal = Fig.3 6.5kV IGBT Gate Driver Prototype The packaged 6.5kV 25A IGBT is tested with an 18.5mH clamped inductive load, the DC bus voltage is 3000V, Rg-on is 100ohm while the Rg-off is 10ohm, the test is conducted with 125 ºC case temperature. The Fig.4 gives the 10A turn on waveform; the turn on loss is 66.26mJ. 4VB 2 ε S μ n EC3 Where, Ron-ideal is the specific on resistance, VB is breakdown voltage. εS , μn and EC is dielectric constant, carrier mobility and critical electric field respectively. Fig.6 shows the limitation of two different materials. With the same 10kV break down voltage, the Ron_sp of Si power device is several hundred times of SiC power device, so silicon power device needs multiple modules in parallel to achieve acceptable low on resistance. Ron-BV 1.00E+03 I 1.00E+02 Vce*I R on,sp (oh m *cm 2) Vc inte(Vce*Ic 1.00E+01 1.00E+00 Silicon 1.00E-01 Silicon Carbide 1.00E-02 1.00E-03 1.00E-04 1000 10000 BV (V) Fig.4 IGBT Turn On under 3000V 10A Fig.6 Theoretical limit for silicon and silicon carbide power devices for trade-off between Ron and BV Fig.4 shows the 10A turn on waveform with 3kV bus under 125degree. The Turn on loss is 66.26mJ. 101 Another factor that limits silicon devices’ application in SST is the frequency limitation. From Fig.7, one can see that at the same breakdown level, silicon carbide device can operate at a much higher switching frequency . Fig.8 compares the forward I-V curves of these two devices. It shows that SiC MOSFET has higher forward voltage drop than Si IGBT at 125 ºC. B. Comparison of turn-on loss Fig.9 shows a comparison of the measured turn-on energy losses of the 10kV SiC MOSFET and 6.5kV IGBT with an inductive load and a DC-link voltage of 3kV. The inductive load parallel diode is 6.5kV 50A ABB silicon diode and 10kV SiC JBS separately for different tests. The test condition for IGBT is 125 ºC, since the IGBT switching performance is highly dependent on the operation temperature, and the switching loss is higher at higher operation temperature, so 125 ºC can be considered as worse case operation condition; while for MOSFET, the operation temperature has very little impact on switching loss, so the test here is only conducted under 25ºC. The result shows the 6.5kV IGBT has 20 times larger losses compared to 10kV SiC MOSFET. The above two formulas predict the maximum switching frequency for semiconductor devices with minimized power loss. Fig.6 gives us a comprehensive view about the difference between silicon and silicon carbide devices’ switching capability. Since device’s high frequency and high voltage operation is preferred for SST application, silicon carbide shows far leading characteristics of trade-off limit. At a frequency of 10 kHz, silicon carbide device can support two times higher voltage than silicon one. Fig.9 Turn-on loss of 6.5kV IGBT and 10kV SiC MOSFET Fig.7 Limitation of switching frequency and breakdown voltage IV. LOSS COMPARISON OF 6.5KV SILICON IGBT AND 10-KV SILICON CARBIDE MOSFET A. Comparison of forward I-V characteristics The forward I-V characteristics of a 6.5 kV IGBT has been measured. The forward I-V characteristics of a 10-kV SiC MOSFET were measured and reported . C. Comparison of turn-off loss Fig.10 shows a comparison of the measured turn-off energy losses of the 10kV SiC MOSFET and 6.5kV IGBT in the same test circuit and the same test condition as turn-on test. The result shows the 6.5kV IGBT has 20 times larger losses compared to 10kV SiC MOSFET at 1A current, and about 80 times at 5A current. Fig.8 Forward I-V curves of 6.5kV IGBT and 10-kV SiC MOSFET at 125 ºC Fig.10 Turn-off loss of 6.5kV IGBT and 10kV SiC MOSFET 102 the calculation method described above. The results are plotted in Fig.13. V. SWITCHING LOSS AND FREQUENCY CAPABILITY OF 6.5KV SILICON IGBT AND 10-KV SILICON CARBIDE MOSFET FOR SST The basic configuration of a proposed 20 kVA SST interfaced to the grid is shown in Fig.11. It contains three stages, AC/DC rectifier, DC/DC dual active bridge (DAB), and DC/AC inverter. The input voltage is 7.2kV, 60Hz ac. For 6.5kV Si IGBT based SST, there will be three cascaded H-bridges for rectifier as shown in Fig.11, each high voltage DC bus is 3.8kV. But for 10kV SiC MOSFET based SST, only two cascaded H-bridge are required, in this case, the dc bus voltage will be 6kV. Fig. 12 Loss simulation circuit for 10kV MOSFET While for 10kV MOSFET, a Pspice model has been reported in  and verified by test data, the switching characteristics of 10kV SiC MOSFET with 6kV dc bus can be simulated through the circuit shown in Fig.12 and the fig.13 gives the simulated turn on and turn off waveforms. Vd Vds(V) Fig. 11 Topology of Solid State Transformer Id*100 For the AC/DC rectifier, the switching device is turned on and off under the hard switching condition. The total power loss for one IGBT in the SPWM modulated rectifier can be calculated by using the following equations: tn = f T n , N sw = sw , n = 1,2,..., ( N sw − 1) × f grid 2 N sw Time(us) (1) I n = 2 I rms ⋅ sin(2π × f grid × t n ) (2) 1 Dn = (1 + m ⋅ sin(2π × f grid × tn )) 2 N sw −1 V × I × Dn Ptotal = ∑ ( on ,n on ,n + Eon , n + Eoff , n ) × f grid f sw n =1 (3) Vgs*10 Fig. 13 Switching Characteristics of 10kV MOSFET with 6kV dc bus So the switching loss can be simulated and the results for both turn on loss and turn off loss under different current are plotted in Fig.14. (4) For the DC/DC dual active bridge converter, the switch can achieve ZVS for turn-on, so only conduction loss and turn off loss is calculated and given by equation (5), π − φ Von (φ ) I on (φ ) φ / 2 + × × + Eoff × f sw (5) 2π 2 2 2π where, ϕ is the phase shift angle between DAB primary and secondary voltage under the rated power output. Since the 6.5kV IGBT loss model has been obtained based on test data, the IGBT losses under different switching frequencies for both rectifier and DAB can be calculated via Ploss = Von (φ ) × I on (φ ) × 103 Fig. 14 Switching Loss of 10kV MOSFET with 6kV dc bus Similarly by using the same calculation method, the SiC MOSFET losses under different switching frequencies can be obtained for both rectifier and DAB and plotted in Fig.15. It can be found SiC MOSFET can operate at much higher frequency than Si IGBT if considering the same power loss. Take 50W loss for example, 6.5kV IGBT can operate under 1750Hz for rectifier and 2650Hz for DAB, while 10kV MOSFET can operate under 12.5 kHz for rectifier and 18.2 kHz for DAB.    Fig.15 Power loss of 6.5kV IGBT and 10-kV SiC MOSFET for rectifier and DAB under different switching frequency VI. CONCLUSIONS In this paper, the switching characteristics of 6.5kV 25A IGBT have been tested, and switching loss behavior under very low current operation has been investigated for high voltage silicon device. Loss characteristics has been compared between 6.5kV IGBT and 10kV MOSFET. Moreover, the switch loss for both devices under different switching frequencies in the 20kVA SST application has been calculated, the results illustrate that the 10kV MOSFET has much higher frequency switching capability. ACKNOWLEDGMENT The authors gratefully acknowledge the Powerex Inc. for packaging 6.5kV IGBT dual modules. REFERENCES   S. Bhattacharya, T. Zhao, G. Wang, S. Dutta, S. Baek, Y. Du, B. Parkhideh, X. Zhou, A.Q. Huang, “Design and Development of Generation-I Silicon based Solid State Transformer”, Applied Power Electronics Conference and Exposition, APEC 2010, palm Springs, CA, February 21-25, 2010, pp. 1666-1673 Kopta, A., M. Rahimo, U. Schlabach, D. Schneider, E. Carroll, and S. 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