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1646 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 16, NO. 2, JUNE 2006 Voltage Rating Reduction of Magnet Power Supplies Using a Magnetic Energy Recovery Switch Takanori Isobe, Taku Takaku, Takeshi Munakata, Hiroaki Tsutsui, Shunji Tsuji-Iio, and Ryuichi Shimada Abstract—A new concept of magnet power supplies that can reduce voltage ratings of the power supplies is proposed. Circuit diagram and operation principles of magnetic energy recovery switch (MERS) are described. MERS consists of a capacitor and four semi-conductor devices such as MOSFETs and IGBTs. It is connected in series to a power supply and a coil. MERS is a switch module and it has no power supply in itself. Because MERS generates a voltage required for the inductance of the coil, the power supply only has to supply a voltage required for the resistance of the coil. Therefore, using MERS can reduce voltage rating and capacity of the power supply. Two types of power supply using MERS and voltage rating reduction are discussed. Comparatively small power supplies for high-repetition pulsed magnets and alternating magnetic field coils can be designed. Some experiments were carried out and confirmed that MERS can reduce voltage ratings of power supplies. Index Terms—Magnet power supply, pulsed magnet. I. I NTRODUCTION P OWER supplies for magnet coils often use power elec- tronics devices, such as MOSFETs and IGBTs. In general, a high voltage is required to control high currents at high rep- etition rates because of an inductance. Consequently the power supply becomes comparatively large scale because the capacity of the power supply becomes large. Therefore, some types of rapid-cycling synchrotrons are op- erated by power supplies using LC resonance to reduce their voltage ratings [1], [2]. However, these types of power supply can not change output current frequency. In addition, aged de- terioration of capacitor may cause some problems. The authors proposed some types of power supply using Mag- netic Energy Recovery Switch (MERS) [3]–[5]. In this paper, power supplies for magnet coils using MERS are proposed. MERS is a switch module with a capacitor. From other point of view, it is a capacitor controlled by semi-conductor devices. The power supplies using MERS can reduce their voltage rat- ings regardless of resonant frequencies by using forced LC res- onance. These power supplies can be applied to high frequency coil excitations such as magnetic levitations, liner motors, and induction heating. II. M AGNETIC E NERGY R ECOVERY S WITCH (MERS) A. Circuit Diagram Fig. 1(a) shows bi-directional type MERS. Four MOSFETs are connected in two parallel arms. Each arm consists of two Manuscript received September 20, 2005; revised November 24, 2005. The authors are with the Research Laboratory for Nuclear Reactors, Tokyo In- stitute of Technology, Meguro-ku, Tokyo 152-8550, Japan (e-mail: tisobe@nr. titech.ac.jp). Digital Object Identifier 10.1109/TASC.2006.870491 Fig. 1. Magnetic Energy Recovery Switch (MERS). (a) Bi-directional type of MERS consists of four MOSFETs and a capacitor. The pair of MOSFETs 1 and 2 is used to control upward current and the other pair of MOSFETs is used to control downward current. (b) Mono-directional type of MERS consists of two MOSFETs and two diodes and a capacitor. MOSFETs connected in series. Four MOSFETs are connected in reverse direction each other in both of series and parallel con- nection. The middle points of series are connected to a capacitor. Semi-conductor devices which can turn off current are needed for MERS. Therefore, MOSFETs or IGBTs can be used because they can turn on and off in any time by gate control. In the case of upward current control in Fig. 1(a), MOSFETs 1 and 2 are controlled, 3 and 4 are left turned off and used as diodes. Therefore, mono-directional MERS can consist of two MOSFETs and two diodes and a capacitor as Fig. 1(b). The circuit diagram of MERS is similar to full-bridge con- verter; however, there are two different points. First, MERS is connected in series to circuit. Since MERS is inserted between power supply and load, the MERS can control current flowing to the load. Second, the voltage of the DC capacitor of MERS is allowed to change dynamically and even becomes zero because the capacitor is not connected to DC power supply. B. Operation Principle MERS is usually used with an inductive load. Fig. 2 shows operation modes of MERS. When a pair of MOSFETs is turned on, current flows through in two ways as Fig. 2(a). Next, when the MOSFETs are turned off, the current charges the capacitor through diodes as Fig. 2(b). Current decreases gradually and it becomes zero. After this time, no current flows because of diodes as Fig. 2(c). The magnetic energy stored at the inductive load is absorbed to the capacitor, and converted to electrostatic energy. Next, when the MOSFETs are turned on, the electrostatic en- ergy of the capacitor raises current as Fig. 2(d). The voltage of the capacitor decreases gradually and it becomes zero. After this time, the current flows through the MOSFETs and the diodes which included in MOSFETs of the other pair, so it becomes 1051-8223/$20.00 © 2006 IEEE Authorized licensed use limited to: TOKYO INSTITUTE OF TECHNOLOGY. Downloaded on November 25, 2008 at 23:44 from IEEE Xplore. Restrictions apply. ISOBE et al.: VOLTAGE RATING REDUCTION OF MAGNET POWER SUPPLIES 1647 (a) (b) (c) (d) Fig. 2. Operation modes of MERS. The MERS has four modes to control current in each direction. (a) When the capacitor is not charged and a pair of MOSFETs is turned on, current flows through two ways. (b) When current flows and the MOSFETs are turned off, the current charges the capacitor through diodes. (c) When the current becomes zero and the MOSFETs are kept turned off, no current flows. (d) When the capacitor is charged and the MOSFETs are turned on, the capacitor discharges. Fig. 3. Circuit configuration of power supply using MERS for high-repetition pulsed magnet. A mono-directional MERS is connected in series to a DC power source. parallel conduction mode as Fig. 2(a). The magnetic energy is recovered from the electrostatic energy. III. M AGNET P OWER S UPPLIES U SING MERS Power supplies for magnet coils are suitable applications for MERS. Most of the energy given to the magnet is stored on magnetic field. Therefore, in high frequency applications, most of energy shuttles between the power source and the magnet in each cycle. By using MERS, the energy shuttles between the MERS and the magnet, and the power source supplies only losses at the magnet and switching devices, etc. A. Power Supply for High-Repetition Pulsed Magnet Fig. 3 shows circuit configuration of the power supply using MERS for high-repetition pulsed magnet. In this circuit, a mono-directional MERS is connected in series to a DC power source. By switching MOSFETs of the MERS, pulsed currents are generated from the DC power source. Moreover, the MERS absorbs and supplies magnetic energy of the magnet in each cycle. Fig. 4. Schematic waveforms of generated current i and capacitor voltage v by the circuit of Fig. 3. t and t mean times to charge and discharge the capacitor respectively. I and V mean peak values of i and v respectively. Mode symbols are referred to Fig. 2. Current waveform shown as dotted line indicates schematic current waveform when the MERS is not used. Fig. 4 shows schematic waveforms of generated current and voltage of the capacitor of the MERS. In mode (b), when MOS- FETs are turned off, the current decreases and the capacitor voltage increases. In mode (d), the current increases and the voltage decreases. These phenomena are part of LC resonance. Consequently and are given by (1) where is the inductance of the magnet and is the capaci- tance of the MERS. In general, it is much shorter than the time constant of the load. In mode (a), the current flows through the MERS and only the voltage of the power source is applied to the magnet. The current raised in mode (d) is equivalent to the current shut off in mode (b). Therefore, after some cycles, becomes as (2) where is the voltage of the power source and is the total resistance of the circuit. Since the electrostatic energy stored in the capacitor when mode is (c) is equivalent to the magnetic energy stored at the magnet when mode is (a), is given by (3) B. Power Supply for Alternating Magnetic Field Coil Fig. 5 shows the circuit configuration of a proposing power supply for alternating magnetic field coil. The power supply consists of a full-bridge inverter and a bi-directional MERS con- nected in series. MOSFETs , , , and are turned on and off at the same time with , , , and respectively. and are always opposite condition to and and and are also always opposite condition to and . The MERS absorbs and supplies magnetic energy also in this circuit. Consequently the full-bridge inverter supplies only losses. Fig. 6 shows schematic waveforms of generated current and voltage of the capacitor of the MERS. The current flowing Authorized licensed use limited to: TOKYO INSTITUTE OF TECHNOLOGY. Downloaded on November 25, 2008 at 23:44 from IEEE Xplore. Restrictions apply. 1648 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 16, NO. 2, JUNE 2006 Fig. 5. Circuit configuration of power supply using MERS for alternating mag- netic field coil. A bi-directional MERS is connected to a full-bridge inverter. Fig. 6. Schematic waveforms of generated current i and capacitor voltage v by the circuit of Fig. 5. Gate signals to U and Y of the full-bridge inverter and V and X of the MERS are also shown. Gate signals of MERS are shifted by t to those of the inverter. Current waveform shown as dotted line indicates schematic current waveform when the MERS is not used. through the magnet is decreased by the MERS and it becomes zero. The magnetic energy of the current is absorbed to the capacitor, and then used to raise the opposite direction current. The inverter supplies the maximum power when the inverter are controlled so that forward current flows when it outputs positive voltage and reverse current flows when it outputs negative voltage. The shifted time to realize that condition is given by (4) and are given by (2) and (3) respectively. C. Voltage Rating Reduction of Power Supplies Required DC voltage to raise current by a conventional voltage source type power supply is given by (5) where is the inductance of the coil and is peak current and is requested time to raise the current. On the other hand, from (2), required DC voltage with a power supply with MERS is given by (6) while is the resistance of the circuit. From (5) and (6), voltage reduction rate is given by (7) where is the time constant given by . This means that the voltage rating reduction is achieved more effectively in the con- dition that the time constant is much longer than the requested time to raise the current. Usually power supplies for high-repe- tition pulsed magnets and high frequency alternating magnetic field coils are under this condition. Using MERS can reduce voltage rating and capacity of power supply. However, total capacity of semi-conductor devices can not be reduced since MERS also consists of semi-conductor de- vices. The number of semi-conductor devices increases and total voltage ratings can not be reduced. The most important point of the voltage rating reduction is that DC capacitors of the power supply can be reduced by using MERS. Large DC capacitors are used in the power supply because energy storage is needed to generate pulsed output. The size of the capacitors relates to the voltage rating of the power supply. Therefore, voltage rating reduction causes decreasing of the DC capacitors. In general, the DC capacitors occupy large part of the power supply in volume. The DC capacitors store energy as large as several times energy of one cycle in order to maintain the voltage within the range of several percent. On the other hand, the capacitor of MERS only stores the energy of one cycle. The capacitor of MERS dis- charges all of its charge and the voltage becomes zero. There- fore, the total size of capacitors is reduced by using MERS. In other words, dividing off the capacitor of MERS from the capacitors of the inverter power supply can reduce the total size of capacitor. To discuss about it, the energy supplied to the coil is divided between the energy consumed at the coil and the en- ergy which makes a round trip between the power supply and the coil. Both these parts of energy pulsate and cause voltage fluctuations of the capacitors. The capacitors must maintain the voltage within a range; therefore the capacitors must store en- ergy as large as several times of both these parts of energy. By using MERS, the energy which makes a round trip does not af- fect to the capacitors but relates to the capacitor of MERS. The capacitors of the inverter must store energy as large as several times of only the energy consumed at the coil, and the capac- itor of MERS must store as large as the energy which makes a round trip. Therefore, the total size of capacitors which relates to stored energy is reduced. IV. E XPERIMENTS Some experiments are carried out to confirm operations of the power supply and theory about voltage rating reduction. A lab- oratory pilot device for alternating magnetic field coils is made. Circuit configuration is same as Fig. 5. Fig. 7 shows photos of experimental setup. A DC power supply which can generate variable DC voltage was used as DC voltage source. Two test coils which have different time constants were used for experi- ments. Table I shows parameters of these coils. Time constants in the table are calculated from coil parameters and the re- sistance of MOSFETs. In the experiments using the laboratory pilot device, a comparatively high frequency is used because the Authorized licensed use limited to: TOKYO INSTITUTE OF TECHNOLOGY. Downloaded on November 25, 2008 at 23:44 from IEEE Xplore. Restrictions apply. ISOBE et al.: VOLTAGE RATING REDUCTION OF MAGNET POWER SUPPLIES 1649 Fig. 7. Photos of the experimental setup. (a) Test coil is air-core coil made from polyester enameled copper wire of 0.7 mm in diameter. (b) Experimental circuit which have an inverter and a MERS with a small capacitor. TABLE I E XPERIMENTAL P ARAMETERS Fig. 8. Waveforms of current and voltage applied to the coil when t is 30 s . MERS generated about 360 V and applied it to the coil alternately. The current was controlled fast by that voltage. time constant of the laboratory scale circuit is much shorter than that of real applications. In the first experiment, target condition was set to 4 A with 30 rising time which is much shorter than the time constant in the coil A. By using conventional power supply, required voltage to achieve that condition is 206.7 V from (5). From (7), is estimated at 0.0489 and consequently the required voltage will be reduced to about 10 V by using MERS. The capacitor of MERS is determined at 0.2 to meet that condition from (1). In that condition, the maximum voltage of the capacitor will be 357 V from (3). Forward and reverse currents of 4 A were raised alternately at 5 kHz repetition rate. Experimental results confirmed that required voltage was reduced most effectively when the gate shift time is 30 calculated by (4). Fig. 8 shows wave- forms of current and applied voltage when is 30 .Itwas also confirmed that required DC voltage was reduced to 13.4 V extremely. The MERS generated about 360 V and controlled current by applying that voltage. Fig. 9. Measured voltage reduction rate with varying requested rising time T . Theoretical values of as a function of T by (7) are shown as dotted lines. In the second experiment, required voltage with varying requested rising time are measured by two coils which have different time constants. Fig. 9 shows voltage reduction rate from the measurement. These results confirms that measured roughly agree with theoretical values given by (7). V. C ONCLUSION This paper discussed the magnet power supplies using MERS. The power supplies can reduce their voltage ratings regardless of frequency by forced LC resonance. MERS can not reduce total semi-conductor capacity but total amount of DC capacitors which occupy large part of power supplies. Therefore, by using MERS, comparatively small scale power supplies can be designed. Voltage rating reduction rate is described by coil parameters and target condition. Large time constant and fast rising time can realize more effective voltage rating reduction. This indi- cates that large scale magnets which have large time constant are suitable applications. However quite large scale applications which have huge magnetic energy is not suitable because these applications do not use DC capacitors to store the energy. High frequency applications are also suitable because required cur- rent rising time is fast compared with the time constant. Experiments by the laboratory pilot device confirm opera- tions of this power supply and voltage rating reduction. R EFERENCES [1] Y. Watanabe, T. Adachi, H. Someya, S. Koseki, and S. Ogawa, “Com- parison of power supply systems for rapid cycling synchrotron,” in Proc. 2005 Japan Industry Applications Society Conf. (in Japanese), 2005, pp. 137–141. [2] K. Bürkmann, G. Schindhelm, and T. Schneegans, “Performance of the White circuits of the BESSY II booster synchrotron,” in Proc. EPAC 98, 1998, pp. 2062–2064. [3] T. Takaku, T. Isobe, J. Narushima, and R. Shimada, “Power supply for pulsed magnets with magnetic energy recovery current switch,” IEEE Trans. Appl. Supercond., vol. 14, no. 2, pp. 1794–1797, 2004. [4] T. Takaku, G. Homma, S. Kato, T. Isobe, S. Igarashi, Y. Uchida, and R. Shimada, “Application of magnetic energy recovery switch (MERS) to improve output power of wind turbine,” in Proc. 2005 Int. Power Electronics Conf. (IPEC-Niigata 2005), 2005, pp. 1280–1285. [5] J. Narushima, K. Inoue, T. Takaku, T. Isobe, T. Kitahara, and R. Shi- mada, “Application of magnetic energy recovery switch (MERS) for power factor correction,” in Proc. 2005 Int. Power Electronics Conf. (IPEC-Niigata 2005), 2005, pp. 737–743. Authorized licensed use limited to: TOKYO INSTITUTE OF TECHNOLOGY. Downloaded on November 25, 2008 at 23:44 from IEEE Xplore. Restrictions apply. . that can reduce voltage ratings of the power supplies is proposed. Circuit diagram and operation principles of magnetic energy recovery switch (MERS) are. Switch Takanori Isobe, Taku Takaku, Takeshi Munakata, Hiroaki Tsutsui, Shunji Tsuji-Iio, and Ryuichi Shimada Abstract A new concept of magnet power supplies