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1794 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 14, NO. 2, JUNE 2004 Power Supply for Pulsed Magnets With Magnetic Energy Recovery Current Switch Taku Takaku, Takanori Isobe, Jun Narushima, and Ryuichi Shimada Abstract—In this paper, we propose a power supply with magnetic energy recovery current switch for pulsed magnets, such as the synchrotron accelerator bending magnets, magnetizer. The switch which consists of four MOSFET elements and one capacitor, generates a fast pulsed current with low voltage, and it improves the power factor. The switch absorbs the magnetic energy stored in the inductance of the load into the capacitor. And in next time on, it regenerates the energy to the load. In addition, this switch operates in zero-voltage switching and zero-current switching, and the switching loss is very small. In order to turn on the load current at high speed in the circuit with an inductance, high voltage of several times higher than the voltage which main- tains steady current. Therefore, by adopting this switch in the power source for pulsed power supply, high-speed pulsed current is efficiently generated by recovering the magnetic energy which has been stored in the inductance to the load in the next time on. As an application of DC circuit, a semiconductor Marx-generator which generates the high voltage pulse composed of a multistage magnetic energy recovery is described. Index Terms—Magnetic energy, power supply, pulsed current. I. I NTRODUCTION I N A circuit containing the inductance , in order to raise cur- rent faster than time constant , it is necessary to apply high voltage of several times higher than resistance voltage which keeps steady current. Therefore, power supply for pulsed magnets requires high voltage higher than and large current, and the power factor is bad. In addition, efficiency is poor when the magnetic energy which has been stored in the inductance is dissipated by resistance. We have already proposed a bi-directional magnetic energy recovery switch [1], [2]. This switch absorbs magnetic energy which has been stored in the inductance of the circuit and re- covers it to the load. The switch generates itself the high voltage which compensates inductance voltage. By adopting this switch in the power sources for pulsed power supply, a high-speed pulsed current is efficiently generated. Thereby, high speed and high repetition pulsed currents are realizable only with a low voltage power source to a load with inductance. The function of this switch is equal to a serial capacitor for power factor correc- tion. The advantage of this switch is that the power factor cor- rection of power source is possible regardless of the frequency even in DC. Recently, it is expected that a power MOSFET of SiC semiconductor becomes available. It realize high-speed and low loss power converters. It is expected that it is applied not Manuscript received October 21, 2003. The authors are with the Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan (e-mail: ttakaku@nr.titech.ac.jp). Digital Object Identifier 10.1109/TASC.2004.831107 Fig. 1. Circuit diagram of bi-directional magnetic energy recovery current switch. The switch is inserted in series between power source and load. only to a matrix converter but to a high-speed and high-repeti- tion pulse power supply, the drive of a high frequency electric motor, induction heating, and an electric power system field as applicable fields with this new current switch. The basic configuration of this switch is shown in Fig. 1. Four MOSFETs are connected in two parallel arms. Each arm con- sists of two MOSFETs connected in series. Four MOSFETs are connected in reverse direction each other both in series and par- allel connection. The middle points of series are connected to a capacitor. Although it is the same composition as a single phase full bridge, it is a new point that the usage differs. II. C IRCUIT C ONFIGURATION A. Operation Principle Fig. 2 shows a case when the electric current flows from A to B. In this case, on/off control only of S1 and S3 is done, S2 and S4 are kept turned off. When S1 and S3 are turned on, current flows in parallel. Next, when S1 and S3 are turned off, the mag- netic energy of the load is absorbed by the capacitor through diodes, and the capacitor voltage is increases gradually. When the capacitor is completely charged, the switch is blocked. After this time, S1 and S3 are turned on, the capacitor discharges the electrostatic energy to the load, and the capacitor voltage grad- ually decreases. When the capacitor voltage becomes zero, cur- rent flows in parallel again. The magnetic energy is recovered from the electrostatic energy. Also, in the case that a current flows from B to A, the switch is controlled by turning on or off S2 and S4. The polarization of the capacitor remains the same regardless of the direction of current. 1051-8223/04$20.00 © 2004 IEEE Authorized licensed use limited to: TOKYO INSTITUTE OF TECHNOLOGY. Downloaded on November 26, 2008 at 00:02 from IEEE Xplore. Restrictions apply. TAKAKU et al.: POWER SUPPLY FOR PULSED MAGNETS WITH MAGNETIC ENERGY RECOVERY CURRENT SWITCH 1795 Fig. 2. Operation of the bi-directional magnetic energy recovery current switch. (a) Current flowing in parallel. (b) Load energy is absorbed by the capacitor. (c) Off condition. (d) Capacitor energy goes to load. The current raised after the switch is turned on is equivalent to the current which was flowing just before turned off. In steady state, the current is determined by the resistance and the voltage of the circuit. The voltage charged in the capacitor is given by (1) where is the current which was flowing just before turned off. B. Soft Switching Ideal semiconductor valve devices, such as MOSFETs or IGBTs, the switching operation is done in a moment, so the switching loss is zero. There is transition time in actual semiconductor devices, and the switching loss is generated. However, this magnetic energy recovery switch achieves zero voltage switching and zero current switching so that the switching loss is reduced. In order to confirm this, switching loss of a magnetic recovery switch and hard switching was compared. Fig. 3 shows turn-off waveforms of one MOSFET element in this switch. Switching frequency is 1 kHz. The rise time of current is delayed compared with hard switching which does not use a switch, because the circuit becomes equivalent to a series circuit. The switch is turned on with zero current switching. Also Fig. 4 shows turn-off waveforms. The magnetic energy stored in inductance is absorbed into the capacitor, the voltage applied to a MOSFET element does not rise so rapidly. The switch is turned off with zero voltage switching. Turn-on and turn-off switching losses calculated from Figs. 3 and 4 is shown in Table I, from which, it is confirmed that switching losses of the magnetic energy recovery switch were drastically reduced. Fig. 3. Turn-on waveforms of experimental results. Fig. 4. Turn-off waveforms of experimental results. TABLE I T URN -O NAND T URN -O FF S WITCHING L OSSES III. H IGH R EPETITION P ULSED P OWER S UPPLY A. Pulsed Current in High Repetition This switch can be applied to a power supply which sup- plies high-speed and high-repetition pulsed current to induc- tance load. By absorbing and recovering the magnetic energy for the load using resonance, the inductance of the load is equivalently zero, and the fast current pulse can be realized. When S1 and S3 are turned off, the magnetic energy is absorbed to the capacitor. By choosing suitable capacitor from (1), high voltage can be generated to the capacitor. The high voltage is applied on the load when S1 and S3 are turned on again, and the current, which is determined by the resistance and the voltage, is turned on and off faster than the time constant of the circuit inductance and resistance. On the circuit of Fig. 1, a simulation and an experiment which switches S1 and S3 simultaneously were carried out. Current Authorized licensed use limited to: TOKYO INSTITUTE OF TECHNOLOGY. Downloaded on November 26, 2008 at 00:02 from IEEE Xplore. Restrictions apply. 1796 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 14, NO. 2, JUNE 2004 Fig. 5. Simulation waveforms of current pulses generation. Power source voltage V = 100 V , C =1mF , L =1mH , R =0:2 , f =20Hz . Fig. 6. Experimental waveforms of current pulses generation. V =10V , C =0:1 F , L = 175 H , R =3:0 , f = 1 kHz . and capacitor voltage waveforms is shown in Fig. 5. Mea- sured capacitor voltage is shown in Fig. 6. gradually rises with the switch repeated turn on and off, and it reached 70 V which was 7 times the source voltage. The maximum voltage of is given by (2) where is determined by (3) where is power source voltage. Measured value is lower than the theoretical value given by (2), presumably because the cur- rent decreases by on-resistance of MOSFET and switching loss. B. Current Waveform Control It is required to control the current in time for operation of a synchrotron accelerator or a plasma control system. By control- ling S1 and S3 using PWM, the current waveform can be freely Fig. 7. Experimental waveforms of voltage and current controlled by PWM. Fig. 8. Magnetic energy recovery type Marx-generator. controlled. Fig. 7 is experiment waveforms with a model circuit. When S1 and S3 are turned on simultaneously, load current in- creases by the capacitor discharge. When S3 is turned off, the capacitor does not discharge and the current is kept constant be- cause the switch acts as freewheel diode. When both S1 and S3 are turned off, the energy which was stored in the inductor is charged in the capacitor and current decreases. IV. M AGNETIC E NERGY R ECOVERY T YPE M ARX -G ENERATOR A. Magnetic Energy Recovery Type Marx-Generator A magnetic energy recovery type Marx-generator is multi- staged magnetic energy recovery switches as shown in Fig. 8. Like the conventional Marx-generator, capacitors are used in parallel in charging and in series in discharging. First, all MOSFETs are turned on with enough time, the flowing current is given by (3). and magnetic energy is stored in the inductor. When all switches are turned off, the Authorized licensed use limited to: TOKYO INSTITUTE OF TECHNOLOGY. Downloaded on November 26, 2008 at 00:02 from IEEE Xplore. Restrictions apply. TAKAKU et al.: POWER SUPPLY FOR PULSED MAGNETS WITH MAGNETIC ENERGY RECOVERY CURRENT SWITCH 1797 TABLE II P ARAMETERS OF M AGNETIC E NERGY R ECOVERY T YPE M ARX -G ENERATOR magnetic energy stored in the inductor charges capacitors in parallel. The time required for charging is given by (4) where is the number of stage. The voltage charged in one capacitor is (5) Next, if switches are turned on, capacitors are discharged in se- ries and the voltage of capacitors become zero. The time required for discharging is (6) The voltage applied to the load is given by (7) and the current is raised to . In steady state, is deter- mined by the resistance and the voltage source as . Exploiting the advantage of Marx circuit that capacitors charges in parallel and discharges in series, this magnetic energy recovery type Marx-generator is more advantageous to obtain high voltage of several times higher than that of one stage magnetic energy recovery switch. And, it can be said that it is effective for the high repetition pulsed power supply by adding features of magnetic energy recovery current switch to a conventional Marx circuit. B. Experimental Results Using previously presented circuit, four-stage magnetic en- ergy recovering switch was made. The circuit parameters are given in Table II. Fig. 9 shows experimental waveforms of current and voltage of the load. The repetition rate is 2 kHz. Since the capacitors are charged with a DC power source, the voltage is 24 V right after Fig. 9. Experimental waveforms of current and voltage of the load. When the switch is turned on, the capacitor discharges stored energy and current increases quickly. Next the switch is turned off, current decreases and the capacitor is charged. the switching started. And the discharge voltage rises gradually by repeated switching. In steady state after time passed enough, high-voltage pulse of 1250 V was applied to the load. The load current rapidly increased by the discharge of the capacitor, and it reached 2 A in about 70 . When all MOSFETs are turned off, cur- rent rapidly decreased, and the capacitor was charged at 300 V. Moreover, the charge time of a capacitor is about 4 times longer than the discharge time. From these results, it is confirmed that the proposing circuit is effective for fast pulsed current source. V. S UMMARY The application of a magnetic recovery switch to the pulsed power supply was proposed. This switch can flow pulsed cur- rent in high repetition, without being concerned with induc- tance of load. By absorbing and recovering the magnetic energy stored in inductance of load, the power factor of power source is improved. The magnetic energy recovery type Marx-generator which consists of semiconductor elements was proposed. It was shown that a high voltage and large current pulse can be easily generated only with a low voltage power source. R EFERENCES [1] R. Shimada et al., “Development of magnetic energy recovery current switch,” in 2003 National Convention Record IEE Japan, 2003, no. 4, pp. 102–103. [2] K. Shimada et al., “Bi-directional current switch with snubber regen- eration using P-MOSFETs,” in Proc. International Power Electronics Conference, Apr. 2000, no. 3, pp. 1519–1524. Authorized licensed use limited to: TOKYO INSTITUTE OF TECHNOLOGY. Downloaded on November 26, 2008 at 00:02 from IEEE Xplore. Restrictions apply. . al.: POWER SUPPLY FOR PULSED MAGNETS WITH MAGNETIC ENERGY RECOVERY CURRENT SWITCH 1795 Fig. 2. Operation of the bi-directional magnetic energy recovery current. a power supply with magnetic energy recovery current switch for pulsed magnets, such as the synchrotron accelerator bending magnets, magnetizer. The switch