Design of High-efficiency Soft-switching Converters for High-power Microwave Generation

Journal of the Korean Physical Society, Vol. 59, No. 6, December 2011, pp. 3688 3693 Design of High-efficiency Soft-switching Converters for High-power Microwave Generation Sung-Roc Jang and Suk-Ho Ahn Department of Energy Conversion Technology, University of Science and Technology, Daejeon 305-350, Korea Hong-Je Ryoo and Jong-Soo Kim Electric Propulsion Research Center, Korea Electrotechnology Research Institute (KERI), Changwon 641-120, Korea (Received 15 December 2010) In this paper, the design of three types of power supplies for 30 kw and 60 kw industrial magnetron drives, a high-voltage power supply, a filament power supply and a magnet power supply, are studied. The high voltage power supply was designed based on the series resonant converter discontinuous conduction mode (DCM) to take advantage of a clean current source that could cover the nonlinear characteristics of a magnetron, such as a zener diode. The maximum output specification of a high-voltage power supply is 42.5 kw for a 30 kw industrial magnetron drive. This can also be used for a 60 kw magnetron with a parallel operation of two power supplies. The design considerations and feasibility tests of a resonant converter for an industrial magnetron are given. Additionally, the designs of two auxiliary power supplies (a filament power supply: 15 V, 120 A and a magnet power supply: 75 V, 5 A) are also provided. These were designed based on the series resonant converter continuous conduction mode (CCM) above resonance operation for higher efficiency. Finally, the developed power supplies were tested using both a 30 kw magnetron and a 60 kw magnetron. The results prove the design procedure and the reliability of the proposed scheme. PACS numbers: 84.40.Fe, 84.30.Jc Keywords: Industrial magnetron, High-voltage power supply, Series resonant converter DOI: 10.3938/jkps.59.3688 I. INTRODUCTION Recently, research on the high power and high efficiency industrial magnetron has grown owing to its advantages for various kinds of applications, such as foodstuffs, chemicals, textiles, paper, wood, materials, communications and the environment. In order to drive the industrial magnetron, three kinds of power supplies are required, as shown in Fig. 1. The high-voltage power supply is used to generate high-power microwaves. The filament power supply is used to heat the cathode for thermo electron emission. The magnet power supply is used to control the electron trajectory. These power supplies should be highly efficient, reliable, compact, and affordable from the perspective of industrial applications. Depending upon the research, studies of high-power and high-efficiency DC-to-DC converters are increasing [1 6]. Soft switching converter topologies were adapted for industrial magnetron drives [7 9]. As a result of this research, this paper presents the design of industrial magnetron power supplies based on E-mail: scion10@keri.re.kr; Fax: +82-55-280-1490 Fig. 1. Power supply connection diagram for an industrial magnetron. a series resonant converter. An individual output voltage and current control method for each power supply is proposed to allow flexible driving conditions for the magnetron; however, an interface between the three power supply operation techniques is adapted against a fault condition to protect the magnetron. Furthermore, a filament power supply output current control algorithm, with respect to the high voltage power supply output power, is proposed due to heating from the reflected wave. The relationship between the filament power sup- -3688-

Design of High-efficiency Soft-switching Converters for High-power Microwave Generation Sung-Roc Jang et al. -3689- Table 1. Specifications of the developed high-voltage Input Voltage 3 380 V AC (+10/ 10%) Maximum Output Power 85 kw (42.5 kw 2) Maximum Output Voltage 17 kv Maximum Output Current 5 A Output for 30 kw Magnetron 14 kv, 3 A Output for 60 kw Magnetron 17 kv, 5 A Maximum Efficiency 95% Protections O.V., O.C., O.T. Size (Width Depth Height) 430 550 300 mm ( 2) Weight 42.5 kg Table 2. Summary of the design parameters for the highvoltage Resonant Capacitor 0.56 µf Resonant Inductor 13.5 µh Resonant Frequency 58 khz Characteristic Impedance 4.9 Ω Transformer Turns Ratio 10:100 Maximum Switching Frequency 24 khz Pulse Width of Switching Signal 10 µs Fig. 3. (Color online) Overall structure of a 42.5 kw power supply. Fig. 2. (Color online) Required relationship between the high-voltage power supply s output power and the filament power supply s output current. ply output current and the high-voltage power supply output power is depicted in Fig. 2. The value of the real filament current (I filament ) can be expressed, in terms of the compensation gain (K), the set current (I filament set ), and the high-voltage power supply current (I high voltage ) as I filament = I filament set K I high voltage. (1) II. DESIGN OF POWER SUPPLIES FOR AN INDUSTRIAL MAGNETRON DRIVE 1. Design of a High-voltage Power Supply In order to drive a 60kW industrial magnetron, an 85 kw (17 kv, 5 A) power supply is required because the magnetron possesses an 80 % maximum oscillation efficiency. The specifications of the developed magnetron power supply are summarized in Table 1. To generate an 85 kw output power, we proposed the parallel operation of two 42.5 kw power supplies because these can be used to drive a 30 kw magnetron for single operation. The overall structure of the 42.5 kw power supply is depicted in Fig. 3. It consists of three full-bridge series Fig. 4. (Color online) Picture of the designed high-voltage transformer. resonant converters, and each converter operates individually, except that the output part, which represents voltage-doubled rectifier outputs, is connected in series. This modular structure allows for flexible of output voltage, and current change, with the same power ratings and for ease of increasing the power by using a parallel or a series connection of the module. Moreover, if a phase delay of the switching signal for each converter is provided, the output voltage ripple can be decreased for the same filter components. A three-phase switching signal for the 30 kw industrial magnetron and a six-phase switching signal for the 60 kw industrial magnetron were applied. Therefore, detailed design procedure for the 85 kw power supply starts from the design of the 14.2 kw series resonant converter, which has a maximum output voltage and current of 5.67 kv and 3 A, respectively. Series and parallel connected resonant converter operation test, with phase shifted switching, are required. Depending on the design specifications, the parameters for the 14.2 kw converter module, including its resonant tank components and its transformer turns ratio, should be determined [9]. These results are summarized in Table 2. As a first step in the design procedure, the

-3690- Journal of the Korean Physical Society, Vol. 59, No. 6, December 2011 Table 3. supply. Specifications of the developed filament power Table 4. Summary of the design parameters for the filament Input supply voltage 3 380 V AC (+10/ 10%) Maximum load power, P load max 1.8 kw Maximum operating voltage, V out max 15 V DC Maximum operating current, I out max 120 A Switching frequency at maximum output, f s min 25 khz Ripple of output less than 10% voltage, V out rip of V out nom Ripple of output less than 5% current, I out rip of I out nom Resonant Capacitor 0.11 µf Resonant Inductor 660 µh Resonant Frequency 18.3 khz Characteristic Impedance 76 Ω Transformer Turns Ratio 42:2:2 Minimum Switching Frequency 25 khz resonant frequency should be decided, based on the definitions of the discontinuous conduction mode operation of the series resonant converter. The value of the resonant frequency is calculated from 2.5 times the maximum allowable switching frequency of the semiconductor switch, owing to the switching delay time and margin. In the second step, the transformer turns ratio is determined from the maximum voltage of the primary side. When the transformer turns ratio is fixed, the peak value of the resonant current at the primary side can be calculated based on the required average output power. From the calculated resonant current peak value and the converter DC input voltage, the characteristic impedance of the resonant tank can be determined. Finally, the values of the resonant capacitor and inductor were found from the resonant frequency and the characteristic impedance. Even though the transformer turns ratio was calculated as 1:10, the actual turn ratio should be decided by considering the saturation of the core material and the required leakage inductance, which can be used for the resonant inductance. To achieve a 13.5 µh leakage inductance without core saturation, a 10:100 turns ratio was determined with a Teflon bobbin to ensure insulation. An image of the designed high-voltage transformer is shown in Fig. 4. The pulse width of the switching signal is calculated from the resonant frequency when the switch at the diode conducting region is turned off. 2. Design of a Filament Power Supply The overall scheme and the detailed specifications of the filament power supply are given in Fig. 5 and Table 3, respectively. These have a single-phase full-bridge series resonant inverter structure with a transformer center tapped rectifier, which is proper for high output current applications owing to the reduced losses at the rectifier Fig. 5. Overall scheme of the filament Fig. 6. Overall scheme of the magnet diodes. A detailed design of the resonant tank parameters and the transformer turns ratio is performed based on the minimum component stress of the switch and the resonant tank [2,3]. The design parameters are summarized in Table 4. It should be noted that the value of the resonant inductance for above resonance zero voltage switching type converters is relatively higher than that of zero-current switching type discontinuous conduction mode converters. Therefore, an additional inductor is required, which represents lower conduction losses, due to the higher characteristic impedance of the resonant tank. 3. Design of a Magnet Power Supply Due to the low output power specification shown in Table 5, the half-bridge series resonant converter scheme is proposed for the magnet power supply from the singlephase AC input voltage. However, the power factor correction, with the boosting circuit, is used for the DC input voltage. Figure 6 shows the overall scheme and the values of the designed parameters are listed Table 6.

Design of High-efficiency Soft-switching Converters for High-power Microwave Generation Sung-Roc Jang et al. -3691- Table 5. Specifications of the developed magnet Input supplying voltage Maximum load power, Pload max Maximum operating voltage, Vout max Maximum operating current, Iout max Switching frequency at maximum output, fs min Ripple of output voltage, Vout rip Ripple of output current, Iout rip 220 VAC + G (+10/ 15%) 375 W 75 VDC 5A 20 khz less than 10% of Vout nom less than 5% of Iout nom Table 6. Summary of the design parameters for the magnet Resonant Capacitor Resonant Inductor Resonant Frequency Characteristic Impedance Transformer Turns Ratio Minimum Switching Frequency 0.2 uf 400 uh 18 Hz 45 Ω 28:20:20 20 khz Fig. 8. (Color online) Relationship between the output current and the switching frequency. Fig. 7. Experimental waveforms for 17 kw operation (resonant current (100 A/div), switching signal (10 V/div), output voltage (2 kv/div), and output current (1 A/div)). III. EXPERIMENTAL RESULTS FOR THE DEVELOPED POWER SUPPLY 1. Experimental Results of the High Voltage Power Supply Based on the designed parameters, a converter module for 14.2 kw series resonant converter was developed and tested with a resistor load. As mentioned earlier, one converter module has to generate the 3 A maximum current for the 30 kw magnetron drive, with a 4.7 kv output voltage and a 5.7 kv maximum output voltage, for the 60 kw magnetron with a 2.5 A output current. Therefore, a 17 kw operation test was performed to ensure reliable operation of the power supply for both types of magnetrons. Figure 7 shows the resonant current and the switching Fig. 9. Experimental waveforms of 13 kv operation without a voltage balancing capacitor (50 A/div, 5 kv/div). signal, with output voltage (5.7 kv) and current (3 A), for a single converter module operation. It makes clear that the switch is turned off at zero voltage and current owing to the diode conducting region and is turned on at zero current. Furthermore, a relationship between the output current and the switching frequency is observed. Because of the current source characteristics of the applied topology, the output current is almost linear in the switching frequency, independent of the output voltage, as shown in Fig. 8. In order to implement the 42.5 kw power supply, as shown in Fig. 3, the series operation of three converter modules is required with threephase switching signals. Figure 9 shows the waveforms of the resonant current for each converter with output

-3692- Journal of the Korean Physical Society, Vol. 59, No. 6, December 2011 Fig. 10. (Color online) Experimental waveforms of 14 kv operation (50 A/div, 5k V/div). Fig. 12. (Color online) Measured efficiency of the highvoltage power supply vs. output power. Fig. 13. (Color online) Output voltage and current waveforms of the filament power supply (5 V/div, 20 A/div.). Fig. 11. (Color online) Experimental waveforms of 85 kw operation for converter 1(50 A/div, 5 kv/div). voltage at 13 kv, 2.8 A operation. When it operates at a high switching frequency, which represents a high value of the output current, the problem of unbalancing was observed for the three-phase resonant current due to the series connection of the current source converter, even though each converter operated with the same switching frequency. This arises from the voltage difference between the output voltages and results in a higher loss for one converter over the others due to the higher value of the RMS current. To solve this problem, we connected a small capacitor (1 nf) to the high-voltage transformer secondary winding, which can provide the same voltage to the primary transformer, because the peak value of the resonant current depends on the voltage across the resonant tank and the characteristic impedance. The waveforms at balanced operation of the three-phase resonant converter for 42 kw shown in Fig. 10, makes clear that the unbalance problem in the resonant current was solved by applying a simple method. Additionally, it was tested in maximum voltage 17 kv operation without any unbalance problems. The last step in developing the 60 kw industrial magnetron power supply is the parallel operation of two converters, which consists of three resonant converter modules. Figure 11 shows the waveforms for the 85 kw (17 kv, 5 A) operation, including the three-phase resonant current and the output voltage. Because of the current Fig. 14. (Color online) Output voltage and current waveforms of the magnet power supply (20 V/div, 1 A/div). source characteristics of the proposed topology, a parallel operation was performed without problems. The measured efficiency, with respect to the output power (1 P.U. = 85 kw), is plotted in Fig. 12, and maximum efficiency of 96% was achieved. 2. Experimental Results for the Filament and Magnet Power supplies The developed filament and magnet power supplies were tested before combining them with the high-voltage The maximum output voltage and current were measured and are depicted in Fig. 13 and Fig. 14, respectively, which show waveforms from zero to maximum with a soft start. Furthermore, an interfacing test between the filament power supply s output current and the high-voltage power supply s output power was performed, as shown in Fig. 15. The result is very close to the required value given in Fig. 2.

Design of High-efficiency Soft-switching Converters for High-power Microwave Generation Sung-Roc Jang et al. -3693- Table 7. Summary of the experimental results with an industrial magnetron. 30 kw Microwave 60 kw Microwave Developed High-Voltage Power Supply Output Voltage: 13 kv Output Voltage: 16.5 kv Output Current: 3 A Output Current: 4.7 A Magnet Power Supply Output Voltage: 39 V Output Voltage: 53 V Output Current: 3.6 A Output Current: 4.8 A Filament Power Supply Output Voltage: 11 V Output Voltage: 10 V Output Current: 107 A Output Current: 98 A Total System Efficiency 74% 75% ACKNOWLEDGMENTS This work was supported by an Energy Efficiency & Resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government s Ministry of Knowledge Economy (No. 2008EEL02P040000). Fig. 15. (Color online) Measured relationship between the filament power supply s output current and the high voltage power supply s output power. 3. Experimental Results with an Industrial Magnetron Finally, the developed 85 kw power supply was tested with an industrial magnetron. Two kinds of test conditions for generating both 30 kw and 60 kw microwaves are summarized in Table 7, including the magnet and the filament power supply output conditions and the total system efficiency from input power to output microwave power. IV. CONCLUSION High efficiency series resonant converter topologies were applied to an industrial magnetron drive. A basic design procedure for the series resonant converter, at both the discontinuous conduction mode operation and the continuous conduction mode operation, were given. Feasibility tests of the series and the parallel operations of the converter module were done, and a solution for the unbalancing problem for each converter was provided. Finally, the developed power supply was tested with an industrial magnetron for 30 kw and 60 kw microwave generation, and the maximum efficiency was measured as 96%. These results verify that a series resonant converter can be effectively used as a power supply for an industrial magnetron. REFERENCES [1] V. Vorperian and S. Ćuk, in Proceedings of 13th annual IEEE Power Electronics Specialists Conference (Massachusetts Institute of Technology Cambridge, Mass., June 14-17, 1982), p. 85. [2] A. F. Wittulski and R. W. Erickson, IEEE Trans. Aerosp. Electron. Syst. 21, 791 (1985). [3] A. F. Witulski and R. W. Erickson, IEEE Trans. Aerosp. Electron. Syst. 22, 356 (1986). [4] R. L. Steigerwald, IEEE Trans. Power Electron. 3, 174 (1988). [5] W. C. Ho and M. H. Pong, in Proceedings of the IEEE International Conference on Industrial Technology (Guangzhou, China, December 5-9, 1994), p. 486. [6] G. H. Rim, I. W. Jeong, Y. W. Choi, H. J. Ryoo, J. S. Kim, K. H. Kim, S. P. Lee and H. K. Chang, in Proceedings of IEEE International Conference on Plasma Science (Las Vegas, USA, June 17-22, 2001), p. 342. [7] Y. J. Woo, S. K. Kim and G. H. Cho, IEEE Trans. Circuits Syst. Express Briefs 53, 1456 (2006). [8] H. Shirai, M. Nakaoka, K. Yasui, T. Kitaizumi, H. Yamashita, H. Omori, in Proceedings of The Fifth International Conference on Power Electrons and Drive Systems (Novotel Apollo Hotel, Singapore, November 17-20, 2003), Vol. 1, p. 601. [9] S. R. Jang, H. J. Ryoo, J. S. Kim and S. H. Ahn, in Proceedings of 36th Annual Conference on IEEE Industrial Electronics Society (Glendale, AZ, USA, November 7-10, 2010), p. 415.