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High Voltage, Step-Down Converter Design using 20-kV Silicon Carbide IGBTs Miguel Hinojosa and Aderinto Ogunniyi Army Research Laboratory, Adelphi, MD 20783, USA Abstract β Single-die, silicon carbide (SiC) Insulated-Gate Bipolar Transistors (IGBTs) fabricated on wide epitaxial drift regions have been developed reaching avalanche breakdown voltages greater than 24 kV and rated currents up to 20 A. This work utilizes similar research-grade, SiC IGBT devices to investigate their continuous switching capabilities, by integrating them in a high voltage, non-isolated, step-down DC-DC converter circuit. The details on the design and construction of the buck converter test-bed and its components are covered in this document, as well as initial results on the device capabilities. The goal of our work was to explore the device capabilities beyond previously-reported levels with a focus on frequency and operating voltage. The SiC-based step-down converter was successfully operated at 10 kHz and 10 kV for a test period of 60 min. While the results found here are not conclusive, the demonstration of the system shows that SiC technology for power conversion systems is very promising. Keywordsβ UHV IGBT, Buck Converter, Silicon Carbide (SiC), High voltage solid state switch, 20kV 4H-SiC, insulated-gate bipolar transistor, pulsed testing. capabilities beyond previously-reported levels with a focus on frequency, operating voltage, and duty cycles. II. BUCK TOPOLOGY The step-down (buck) converter is one of the basic building blocks in power electronics and it is commonly used in switchmode DC-DC converter designs. This topology is easily scalable to high voltages, as presented in Fig.1. The buck converter's transfer function, which can be written in its simples form as πππ’π‘ π‘ππ = =π· πππ π‘ππππππ (1) depends on the on-time, ton, of the switch (U1) and its period (tperiod), or as a function of duty cycle (D), when the converter is operating in continuous-conduction mode (CCM) [5]. In CCM operation, the current through the inductor L1 flows continuously and it does not fall down to zero. I. INTRODUCTION Silicon carbide (SiC) Insulated-Gate Bipolar Transistors (IGBTs) fabricated on wide epitaxial drift regions are desirable for pulsed-power and continuous power conversion systems due to their high avalanche breakdown voltages, as well as their ability to switch currents at fast repetition rates. For mobile platforms, the use of SiC-based electronics is advantageous as it allows for a significant reduction of space in comparison to silicon and gas switches, in addition to faster switching speeds, continuous/burst mode of operation, and advanced triggering capabilities. This work reports on the design and construction of a stepdown converter test-bed utilized to evaluate the continuous high-frequency operation of newly-fabricated, research-grade, 20-kV 4H-SiC IGBTs. These devices, manufactured by Wolfspeed Inc. under the U.S. Army High Voltage Power Technology Program (HVPT), have wide epitaxial drift regions greater than 200 µm, reach avalanche breakdown voltages greater than 24 kV, and have continuous current ratings up to 20 A. This paper expands on previous work reported for similar devices [1-2], where resistive and inductive switching were performed, and from [3] where they were used in a burst-mode solid-state Marx generator. In [4], the previous generation of 4H-SiC IGBTs with epitaxial regions of 140 µm to 160 µm were switched in a buck-boost converter at 8 kV with an operating frequency of 5 kHz. The goal of our work is to explore the device Isolated gate driv er R3 Rsupply Dsupply 1 L1 2 2 U1 HVcharger 20kVIGBT D2 Rload C_bank Cout 15kVJBS 1 0 Fig. 1. Non-isolated step-down buck converter circuit. The second mode of operation of the buck converter is discontinuous-conduction mode (DCM), where the inductor current flattens upon reaching zero. In this case, the output becomes a function of duty cycle, inductance (L1), the load resistance (Rload), and the switching period (Tsw). The transfer function for this mode was defined by Pressman [6] as πππ’π‘ = πππ 2 β π· π· + βπ·2 + 8 β πΏ1 π ππππ β ππ π€ (2) ο ο III. CONVERTER COMPONENTS A. Power source group The buck converter circuit shown in Fig. 1 was implemented in the laboratory utilizing an adjustable 15-kV, 8-kJ/s, 8-kW TDK-Lambda capacitor charger (HVcharger) that draws its power from the 480 VAC 3-phase power outlet. In addition to the manufacturer's internal protection circuits, the power supply is safeguarded from reflections and unexpected events with two series-connected, 10-kV silicon puck diodes (Dsupply) and a low-inductance ceramic resistor (Rsupply). A capacitor bank (C_bank) follows the protection components and it is comprised of four, series-connected 256-µF, 10-kV film capacitors with respective balancing resistors. The bank's equivalent capacitance is 64 µF, which maintains the high voltage rail constant during operation. B. Silicon Carbide IGBTs and JBS diodes The devices-under-test are the 20-kV SiC IGBTs labeled U1 in Fig. 1. The 4H-SiC IGBTs utilized in this experiment were vertical power devices with a P-type, 300-µm substrate and an epitaxially-grown N-type drift region with a thickness of 230 µm. The devices have a double-diffused MOS structure, an asymmetrical punch-through design, and a field-stop buffer of approximately 2 µm. Their nominal breakdown voltage was 2224 kV at a leakage current of 10-14 µA. Their active area was 0.28 cm2 and a chip area of 0.81 cm2. Fig. 2 shows the forwardconduction and forward-blocking characteristics of one IGBT. (Additional device fabrication details can be found in [1]). Fig. 3(a) shows the 9-mm by 9-mm chip die, where the backside region serves as the die attach to a gold-plated, copper collector terminal. The topside interface consists of four emitter fingers and one gate pad, and these pads are connected to their respective terminals using two 10-mil wire-bonds per pad. Fig. 3(b) shows the SiC IGBT on a high-temperature polyethertherketone (PEEK) package and it is mounted on an actively cooled aluminum heat sink. The heat sink circulates Castrol 399 gas turbine oil, which is pumped by a Mydax 1H100W water-cooled heat exchanger, at a flow rate of 1.6 gpm. (a) (b) Fig. 3. (a) IGBT die and dimensions. (b) IGBT package mounted on an oilfilled, actively-cooled aluminum heat sink. Silicon carbide Junction Barrier Schottky (JBS) diodes were utilized in the step-down converter to protect the IGBT from the inductor's stored energy. These SiC rectifiers, which have forward-conduction and blocking characteristics shown in Fig. 4, have a chip area of 1 cm2 and are rated for 15 kV and 17 A. In the 10-mm by 10-mm JBS diode die, the cathode terminal is located on the backside of the chip and it attaches to a goldplated, copper plate via SnPb solder. The top side of the die attaches to the anode terminal using multiple strands of 10-mil bond wires. Similar to the SiC IGBTs, the JBS diodes are packaged in PEEK and cooled with an aluminum heat sink and dielectric fluid. (a) (b) Fig. 4. (a) JBS diode conduction I-V plots. (b) JBS diode blocking I-V plot. (a) (b) Fig. 2. (a) IGBT Forward-conduction I-V plots. (b) Blocking I-V plot. C. Floating gate driver The SiC IGBT in the buck converter is conFig.d as a high side switch, which requires a floating gate driver and an isolated power supply. The gate driver board seen in Fig. 2(b), is based on the IXYS IXDD430 gate driver chip, it is capable driving the gate voltage up to 30 V, and it is optically triggered. The gate board is powered by a custom multi-stage, isolated power supply that employs an open-loop forward converter and two regulated buck converters. The gate board was conFig.d to operate at a voltage range from +20 V to -6 V, based on the IGBTs forward-conduction I-V curves. Fig.5 shows the block diagram for the isolated power supply that feeds the gate driver board. TABLE 1. +20 V (Adjustable) Buck Stage -6 V (Adjustable) Fig. 5. Isolated power supply used for gate driver board. D. Inductor Designing the inductor was a challenging task due to the switching times (frequency up 10 kHz, but pulse widths as low as 2-10 µs), the intended high operating voltage (~10 kV), and the initial, low operating current levels (1-10 A range). The design parameters included a target inductance which had to be tens to hundreds of mH, an acceptable saturation current, no winding-to-winding and winding-to-core dielectric breakdown, and acceptable thermal management. The inductor was designed using AMCC1000 cores due to their high saturation flux density, high permeability, window size, and availability. These cores are manufactured with iron-based METGLASS 2605 SA1 amorphous alloys. The current inductor, seen in the CAD drawing in Fig. 6(a) uses four, C-shaped cores held together with steel banding, a 3D-printed polycarbonate bobbin, and 90 turns of 16 AWG magnet wire enclosed in a 15kV sleeve. DC Hi-pot tests indicated isolation between the core and the windings to be greater than 30 kV. Dynamic tests were also conducted, such as the one illustrated in Fig. 6(b) during a 5-kV, 10-kHz test, to verify the transient electrical characteristics and ensure the core received proper ventilation and would not overheat. (a) C-Cores Gap Sleeved Magnet wire windings (b) Gap Steel banding INDUCTOR PROPERTIES WITH 20 MIL AND 40 MIL GAPS Properties n turns gap Ls @ 10 kHz Rs @ 10 kHz Ls @ 20 kHz Rs @ 20 kHz I_sat Com Buck Stage Forward Stage Input 36 V 40-kV Isolation Design 1 90 20 mil 46.1 mH 944 β¦ 45.3 mH 289 β¦ 14 A Design2 90 40 mil 28.4 mH 37.8 β¦ 28.4 mH 392 β¦ 25 A E. Output capacitor and load The output capacitor is part of the LC filter needed to obtain a constant voltage level in the output. The output capacitor used was a 20-kV film capacitor with a measured capacitance of 7.5 µF and an equivalent series resistance of 18 mβ¦. This value was selected based on initial calculations to obtain an output ripple lower than 100 V, and because its availability in the laboratory. A reconfigurable resistive load was designed to test the buck converter using non-inductive bulk ceramic resistors. Each resistive element is 250 β¦ and rated for 22 kV, with peak energy of 17.5 kJ and a maximum operating temperature of 350 °C. The test fixture contains 10 slots, in which the units can be connected in series or parallel configurations. Fig.7 (a) shows an image of the resistive load prior to its installation. A circulating fan with an air flow of 550 CFM was installed on top of the unit to pull air out of the mid-section (not shown in Fig. 7a). Fig.7 (b) shows a thermal image of the load while operating the buck converter at 10 kV and 10% duty cycle for 1.5 hr. (a) (b) 13 in. 25 in. Bobbin Fig. 6. (a) Inductor design using 4 AMCC1000 C-cores. (b) Inductor temperature during 5-kV tests with 50% duty cycle. The inductor saturation current (I_sat) was determined using a pulsed clamped inductive circuit, where the voltage and pulse width were varied to find the current level where the current began to increase exponentially. During these experiments, sheets of 10-mil Mylar were inserted between the gaps to increase the saturation current, while decreasing the inductance. Table 1 shows a summary of the results of these tests. Fig. 7. (a) Reconfigurable resistive load with 10 elements in place (2.5 kβ¦). (b) Load temperature during 10-kV test at 10% duty cycle. IV. DEVICE EVALUATION Isolated supply JBS diode Load Inductor SiC IGBT Capacitor Bank Fig. 8. Image of high-voltage, step-down converter system. A small number of SiC IGBTs were evaluated in the laboratory with the buck converter test bed shown in Fig. 8. As mentioned previously, the goal of this exercise was to demonstrate the SiC IGBT's performance while switching continuously at high voltage and high frequency. The test procedure consisted of an iterative sequence of steps where the lowest operating voltage and switching frequency were selected and the operating current and duty cycle were slowly increased. These tests were then repeated with a higher voltage, and then eventually with higher switching frequencies. Due to the limited number of devices, the tests would vary in time from 10 min and up to 1-1.5 hr as the extreme cases. The targeted maximum voltage was 10 kV and the maximum operating frequency was 10 kHz. The limits were based on previous simulations and experimental double-pulse clamped-inductive tests. The power stage of the DC-DC converter was successfully operated in an open-loop configuration as seen in the snippet of the input and output voltage waveforms versus time in Fig. 9. The output voltage follows the transfer function given in equation (2), for discontinuous-conduction mode. For an input of 10.5 kV, for example, the measured output was 1.98 kV, at an operating frequency of 10 kHz, 10% duty cycle, an inductor value of 28.4 mH, and a load resistance of 2500 β¦. In this case, the output power was 1.5 kW with a voltage ripple of approximately 30 V using the 7.5 µF output capacitor. Fig.10 illustrates the collector-to-emitter voltage (Vce) and the collector current (Ic) as they were driven with a gate-toemitter voltage swing from -6 V to +20 V. The voltage drop across the IGBT was calculated by measuring at two locations with PVM-12 Northstar high voltage probes, and then finding the difference between the two measurements. The collector current was measured using an insulated Pearson current transformer. Fig. 10 shows a decaying voltage as the IGBT cycles on and off. The early drop in Vce is attributed to the DCM nature of the converter and it begins to fall when the inductor current approaches zero. This behavior is expected for DCM because the output capacitor must support the load for that short period of time. Fig. 10 also shows the current amplitude of Ic as a function of time when the IGBT cycles in the converter. The current waveforms indicate that the capacitor is charging steadily at about 3.5 A when the IGBT is on. The instantaneous peak current of 22.5 A is caused by the Miller capacitance of the IGBT, and it can be dampened by utilizing different resistors for turn-on and turn-off. This feature will be implemented in future test evaluations. For this 10-kHz case with a supply voltage of 10.5 kV, the turn-on losses in the IGBT were 1.47 mJ and the turn-off losses were 10.6 mJ. Fig. 11 shows the inductor current (I_L) and its respective power losses as a function of time. These waveforms confirm the DCM operation as the inductor current falls below zero. Fig. 10. Collector-to-emitter voltage (Vce) and collector current (Ic) transient characteristics. The supply voltage was 10 kV at an operating frequency of 10 kHz and 10% duty cycle. Fig. 11. Inductor current (I_L) and inductor loss as a function of time. Fig. 9. Snippet of the input and output voltage waveforms as a function of time. The supply voltage was 10 kV at an operating frequency of 10 kHz and 10% duty cycle. Table 2 highlights the salient results of the buck converter with a high operating frequency of 10 kHz. The input voltages were increased from 5 kV up to 10 kV with testing times ranging from 10 min up to 1.5 hrs. At the lower voltages, the duty cycles were increased up to 50% and at the higher voltages the duty cycles were kept at a conservative value of 10%. successfully at high voltages with low duty cycles, which is very encouraging for devices with thick epitaxial regions greater than 200 µm. In order to proceed further with higher TABLE 2. RESULTS SUMMARY AT 10 KHZ voltage, duty cycle, and current levels, a series of tools need to be incorporated to prevent device destruction. First, thermal Vin Test Time Dty Vout P_load I_LMAX E_turn-on E_turn-off E_cond (kV) (min) (%) (kV) (kW) (A) (mJ) (mJ) (mJ) models and simulations need to be included to predict structure heating, packaging, and heatsink interaction. Continuing work 5.1 90 50 3.1 3.84 1.03 0.12 2.5 includes the development of Silvaco, ANSYS, and PSPICE 6.1 10 25 2.4 2.30 2.81 0.14 5.3 models to better understand the heating and cooling 8.5 10 10 1.58 1.00 1.7 0.83 5.3 mechanisms. Second, a correlation must be made between the junction temperature and the saturation on-state voltage (Vsat), 9.6 15 10 1.78 1.27 1.93 0.91 7.32 to determine when the device reaches its peak operating 10.5 60 10 1.96 1.54 2.13 1.74 10.6 temperature. Future evaluations will include feedback from the Vsat probe to the switching controller, which can stop the gate signal in case of device overheating and degradation. In Operation in discontinuous-conduction mode (DCM) was not addition to developing thermal simulations and adding a limitation in these tests because the output current levels were feedback to the IGBT controller, future evaluations will also relatively low. At the moment, the 7.5 µF output capacitor require the design and fabrication of liquid-cooled testing loads collects enough charge to support the 2500 β¦ load while the and high power inductors. inductor is off, but this might become an issue in future tests requiring higher output power. As more current is drawn by the load, the capacitor discharges at a higher rate, which means the ACKNOWLEDGMENTS output capacitance will have to be increased or the output The authors would like to thank Dr. Sei-Hyung Ryu and Dr. voltage ripple will increase. The operation of the buck converter Edward Van Brunt of Wolfspeed Inc., a Cree Company, for in continuous-conduction mode (CCM) is possible, for example fabricating the devices and for their technical support. Special in the 10-kHz, 10-kV operation case, if the load resistance is thanks to Mr. Oladimeji Ibitayo of ARL for his help with the reduced to 250 β¦. At this point, both the SiC IGBT and the load inductor development. would be operating at a significant higher power levels which could be detrimental to the components. These conditions will be considered in future tests once thermal model and REFERENCES simulations are incorporated with the device evaluations. E.V. Brunt, L. Cheng, M. J. OβLoughlin, J. Richmond, V. Pala, J. Palmour, C. Tipton, and C. Scozzie, β 27 kV, 20 A 4H-SiC n-IGBTsβ. Silicon Carbide and Related Materials. Materials Science Forum. 2014 [2] M. Hinojosa, A. Ogunniyi, S. Bayne, and C. Scozzie. βEvaluation of High Voltage, High Power 4H-SIC Insulated-Gate Bipolar Transistorsβ, IEEE International Power Modulator and High Voltage Conference. Santa Fe, NM. 2014. [3] M. Hinojosa, H. O'Brien, E. Van Brunt, A. Ogunniyi and C. Scozzie, "Solid-state Marx generator with 24 KV 4H-SIC IGBTs," 2015 IEEE Pulsed Power Conference (PPC), Austin, TX, 2015, pp. 1-5. [4] A. Kadavelugu and S. Bhattacharya, "Design considerations and development of gate driver for 15 kV SiC IGBT," 2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014, Fort Worth, TX, 2014, pp. 1494-1501. [5] N. Mohan, T.M. Undeland, & W.P. Robbins, Power electronics-Converters, Applications and Design (Hoboken, New Jersey: John Wiley & Sons Inc., 2003). [6] A. Pressman, Switching Power Supply Design, McGraw-Hill, Inc., New York, NY, 1997. [1] Gate Driver Board SiC IGBT Package Gate Collector Fig. 12. Thermal image of the SiC IGBT during operation with a supply voltage of 10 kV and an operating frequency of 10 kHz and 10% duty cycle. V. CONCLUSION AND FUTURE WORK A step-down buck converter was designed and constructed to evaluate state-of-the-art SiC IGBTs at high voltage, high frequency and low duty cycles. The converter's continuous switching performance was demonstrated up to 10 kV and 10 kHz, which goes beyond the highest limit published in literature. Results show that the SiC IGBTs operated