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A New AC Current Switch Called MERS with Low On-State Voltage IGBTs (1.54 V) for Renewable Energy and Power Saving Applications Ryuichi Shimada ∗ , Jan A. Wiik ∗ , Takanori Isobe ∗ , Taku Takaku † , Noriyuki Iwamuro † , Yoshiyuki Uchida ‡ , Marta Molinas § and Tore M. Undeland § ∗ Tokyo Institute of Technology, N1-33, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8550, Japan, Email: rshimada@nr.titech.ac.jp † Fuji Electric Device Technology Co., Ltd, 4-18-1, Tsukama, Matsumoto, Nagano 390-0821, Japan ‡ Curamik Electronics KK, Assorti Takanawa, 3-4-13 Takanawa, Minato-ku, Tokyo 108-0074, Japan § Norwegian University of Science and Technology, Institutt for elkraftteknikk, 7491 Trondheim, Norway Abstract— Emergence of new power electronics configurations have historically been one of the important drivers for improve- ment of the IGBT technology. Development of new IGBTs is said to be a trade-off between saturation voltage, short-circuit capability and switching losses. With the common applications requiring high switching frequency and short-circuit capability, the saturation voltage performance has not been fully optimized. This paper describes a new configuration called the Magnetic Energy Recovery Switch (MERS). It is characterized by using simple control and low switching frequency, where saturation voltage is the main contributor to losses. The semiconductor requirements of this configuration have led to the development of a new low on-state voltage IGBT. Application in the area of wind power conversion shows potential for efficiency improvements. Additionally, due to the soft-switching nature of the MERS application, series connection of the new IGBTs in variable frequency induction heating application is shown to be easy without voltage sharing problems. I. I NTRODUCTION Emergence of new power electronics configurations have historically been one of the important drivers for improvement and development of the IGBT technology. Since the introduc- tion of the IGBT in the early 1980s have need for higher power and reduced losses been given main attention. Several technologies have resulted, such as various trench structures and field stop layer. In a majority of the application areas, high frequency switching and need for short circuit capacity have been impor- tant requirements. In motor drive applications, usually there is no internal output impedance, meaning that a short-circuit at the inverter terminals is a direct short-circuit of the inverter transistors [1]. As a result, turn-off of the IGBTs must be managed in the case of a short circuit without being destructed. Several trade-offs exist in the development of IGBTs, some of them being switching losses, short circuit capability and on-state losses. With the typical existing applications, low switching losses and high short circuit capability have been prioritized. This paper looks at a new power electronics configuration called the Magnetic Energy Recover Switch (MERS). The Fig. 1. Configuration of the MERS. configuration is characterized by low switching speed, reduced need for short circuit capability and simple control. The special features of the configuration have led to the development of a new type of IGBT with lower conduction losses. The characteristics of the MERS are first discussed. This is fol- lowed by a description of the newly developed low saturation voltage IGBT. Application examples of the MERS are then given, where the advantages of the new IGBT compared to the existing ones are discussed. II. M AGNETIC E NERGY R ECOVERY S WITCH (MERS) A. Configuration The configuration of the Magnetic Energy Recovery Switch (MERS) is shown in Fig. 1 and consists of 4 forced com- mutated switches and a small dc-capacitor. The configuration is similar to that of a single phase full bridge, but the control is different and the size of the capacitor is several times smaller. The configuration was first suggested as a bi- directional current switch with snubber re-generation using power-MOSFETs [2]. The ability to control the flow of reac- tive power and the phase of the current was further investigated and the name Magnetic Energy Recovery Switch (MERS) proposed [3] [4]. By putting the MERS in series between an inductive load and the power source can the reactive power (or magnetic energy) be ”recovered” to the load. 1-4244-1532-2/08/$25.00 ©2008 IEEE 4 Proceedings of the 20th International Symposium on Power Semiconductor Devices & IC's May 18-22, 2008 Oralando, FL Authorized licensed use limited to: TOKYO INSTITUTE OF TECHNOLOGY. Downloaded on November 25, 2008 at 23:45 from IEEE Xplore. Restrictions apply. (a) (b) (c) Fig. 2. Possible current paths through the MERS when current is going in upward direction. Fig. 3. Example of resulting voltage and current curves when controlling the MERS. B. Operation principle The operation principle of the MERS is based on controlling the path of the current through the device (Fig. 2). By doing this can either the phase of the current or the size of the injected voltage be controlled. Examples of resulting curves are shown in Fig. 3. Three modes of operation are indicated. The not-continuous and the dc-offset mode results by turning two and two switches on/off in pairs while the active by- pass mode results when actively by-passing the capacitor for a part of the period. The resulting current waveform depends on the application; however, it will always be 90 degrees to the injected series voltage. In principle, the MERS can act as a variable capacitor. The size of the capacitive injected series voltage can be varied from zero to rated voltage within the current rating of the device. This also means that the series voltage injection capability stays constant even with varying frequency. Another important feature is the simplicity of the control. When operated in dc- offset mode can a phase shift of the gate signals directly control the phase of the current [5]. C. Applications Several types of applications have been investigated from low power to high power range as shown in Fig. 4. With the ability to control the phase of the current, the size of the (a) (b) (c) Fig. 4. Example of MERS applications. (a)Light intensity control of discharge lamps. (b)Voltage and power factor control of induction motor. (c)Series compensation in transmission system. load voltage or the power factor can also easily be controlled. This is attractive in cases where only voltage control is needed and variable frequency is not important. One such application is light intensity control of discharge lamps such as fluorescent and high intensity discharge lamps [6]. With a high efficient configuration, energy savings can be achieved with low cost. Voltage and power factor control of induction motors is another application [7]. The voltage of the induction motor can be controlled to improve efficiency. Additionally, the flow of reactive power to other loads can be controlled. In the high end power scale of MERS applications is the se- ries compensation in transmission systems. By controlling the injected series voltage, the flow of power can be controlled and increased. Several technologies already exist; however, MERS has advantages such as large operating range, simple control as well as good characteristics seen from a semiconductor perspective. Further two promising applications are that of wind power conversion and induction heating. These applications will be discussed further in section IV. D. Device characteristics To facilitate the success of the MERS application, it is important to consider the special characteristics seen from a semiconductor device perspective. Device related characteris- tics are: • Line frequency switching: one switch is only turned on and off once during a fundamental cycle meaning low 5 Authorized licensed use limited to: TOKYO INSTITUTE OF TECHNOLOGY. Downloaded on November 25, 2008 at 23:45 from IEEE Xplore. Restrictions apply. Fig. 5. IGBT chip trade-off. Fig. 6. Cross-section diagram of a conventional trench FS-IGBT. switching losses. • Soft-switching: Turn-on is performed at zero current. Zero voltage turn-off is achieved when operating in not- continuous mode. • Losses are similar to as if the current continuously goes through one diode and one active switch, meaning on- state voltage of the devices are of great importance. • High short circuit capability of the devices is not needed due to having one MERS device in each phase, shorting of input or output terminals will not lead to a short of the transistors. Additionally, the capacitor is smaller than a normal voltage source converter. Based on these characteristics, a new low on-state voltage IGBT has been developed, as described in the next section. III. D EVELOPMENT OF NEW LOW ON - STATE IGBT S A new IGBT with low saturation voltage has been developed for the MERS configuration [8]. IGBT development concerns the trade-offs shown in Fig. 5. The conditions can be changed if the IGBT is applied to new circuit configurations. For the MERS configuration, high speed switching charac- teristics are not required because of line frequency switching (up to 50 or 60 Hz in typical). On the other hand, a ma- jority of IGBT applications are commonly used with high frequency switching with PWM control technique (several kHz). Moreover, large short circuit capability is not required. Consequently, saturation voltage can be prioritized, as well as the forward voltage of FWD (free wheeling diode). Fig. 7. Appearance of 1200 V/150 A trench FS-IGBT chip for MERS. The IGBT and FWD chips were designed optimally for the MERS applications based on the chips used for a conven- tional 1200 V Field-Stop (FS) IGBT module with trench gate structure [9]. Fig. 6 shows cross-section diagram of the FS- IGBT. A distinctive feature of the conventional IGBT is low resistance and high speed switching due to thin silicon wafer with less than 150 μm for 1200 V. The trench gate structure also contributes to low resistance and high switching speed. For the MERS applications, design optimization was applied to the conventional IGBT as follows: 1) Improvement of bipolar characteristics by increasing minority carriers injection from the p-collector region. 2) Reduction of the resistance of n- region by improving placement of electrodes. For the FWD, lifetime control was reduced and this resulted in decreasing forward drop voltage. Fig. 7 shows appearance of the newly designed IGBT, which has 150 A current rating and dimension of 12.8 mm × 12.8 mm. Comparison of the V CE(sat) - I C characteristics of the conventional trench FS-IGBT and the new IGBT for MERS are shown in Fig. 8. Low saturation voltage (1.54 V) at rated current was achieved, where the conventional IGBT with same rating has 2.10 V saturation voltage. The FWD voltage was also improved to 1.2 V from 1.75 V. 1200 V - 150 A IGBT module for MERS using the developed IGBT and FWD chips were fabricated and tested. Test results confirmed isolation voltage and switching charac- teristics. IV. A PPLYING LOW ON - STATE IGBT STO MERS A. Loss reduction of wind power conversion system With the trend going toward full-converter solutions for wind power applications, a lot of attention is given to reducing cost and improving the efficiency of the power electronics converter. MERS has been suggested as a solution combined with a diode bridge rectifier to achieve a good generator utilization even with a high efficiency [10] [11] [12] [13]. The conventional solution today uses an active rectifier with hard-switching and resulting high switching losses. Due to the high frequency harmonics in the voltage output waveform, a filter is needed. Several configurations exist, with 2-level and 6 Authorized licensed use limited to: TOKYO INSTITUTE OF TECHNOLOGY. Downloaded on November 25, 2008 at 23:45 from IEEE Xplore. Restrictions apply. Fig. 8. V CE(sat) - I C characteristics of the conventional trench FS-IGBT and the trench FS-IGBT for MERS. 3-level topologies being common. A 3-level configuration is shown in Fig. 9(a). Permanent magnet generators applied to wind power gen- eration have typically a high synchronous reactance, (in the area of 1 per unit). With a pure diode bridge solution, the terminal voltage of the generator will drop as the current increases. As a result, the maximum output power is limited and the armature loss is increased for a given power output. By inserting a capacitor in series between the generator and the diode bridge, the voltage drop across the synchronous reactance can be cancelled and the output power increased. A requirement for such a compensator is that it shall be able to operate during variable speed and not cause any resonant problems. The configuration of the system when using the MERS is shown in Fig. 9(b). The concept can be further explained by the use of phasor diagrams, as shown in Fig. 10. There is a large voltage drop across the synchronous reactance, X S . Without any compen- sation, the output voltage will drop and the power output is limited (Fig. 10(b). On the other hand, the MERS can cancel the voltage drop across the synchronous reactance and control the phase of the current along the internal voltage such that maximum output power results (Fig. 10(c)). By controlling the size of the MERS voltage, the output power can be adjusted for a given speed and dc-voltage. Experiments were conducted on a 50 kW permanent magnet generator designed in the image of a large scale wind power generator. The specifications are given in Table I and a picture of the generator and MERS is shown in Fig. 11. The MERS in the experiments used the new IGBTs. Time trends for a case with 39kW output are shown in Fig. 12. There is some distortion in the generator terminal voltage; however, the distortion in the current is significantly lower due to the large synchronous reactance. (a) (b) Fig. 9. Wind power conversion system configurations. (a)Conventional 3- level PWM rectifier. (b)MERS and diode rectifier. TABLE I S PECIFICATIONS OF EXPERIMENTAL SET - UP . Generator Rated power 50 kW No-load voltage 306 V Rated frequency 50 Hz Pole number 116 Synchronous inductance 4.5 mH Armature resistance 0.18 Ω MERS IGBT voltage rating 1200 V IGBT current rating 150 A Capacitor rating 2000 μF The losses of the experimental system have been analyzed. The left part of Fig. 13 (case 1) shows the loss distribution of the generator side converter using the low loss IGBTs. By using the new IGBTs, the MERS losses were improved with 23.5 percent compared to using conventional IGBTs. Due to the low voltage utilization of the devices in the experiment, the losses are comparatively high. The relative losses for a real scale system have been esti- mated and also included in the figure. By increasing the dc-link voltage, and as a result the no-load voltage of the generator, the efficiency can be improved. High voltage utilization of the MERS device should be possible due to switching the current off when the voltage across the capacitor is low. This means the surge voltage will be low compared to that of a traditional PWM converter. Two cases were investigated. In case number 2, the dc-link voltage was set to 1100 V, which is a suitable 7 Authorized licensed use limited to: TOKYO INSTITUTE OF TECHNOLOGY. Downloaded on November 25, 2008 at 23:45 from IEEE Xplore. Restrictions apply. (a) (b) (c) Fig. 10. Phasor diagram illustrating the purpose of the MERS. (a) Equivalent circuit of the power conversion system with MERS. (b) Phasor diagram when the MERS is not activated. (c) Phasor diagram when the MERS is compensating the whole voltage drop across the synchronous reactance. Fig. 11. Picture of PM-generator and MERS used in experimental system. voltage level for grid side inverter with 1700 V IGBTs. In case number 3, the dc-link voltage was further increased such that the peak MERS voltage reached 1000 V, meaning close to optimal voltage utilization of the MERS IGBTs. Significant loss reductions can be found in both cases. The conventional solution using a PWM rectifier can in principle perform the same operation as that of the MERS system. The optimization of such a system is complex due to trade off between switching frequency and size of generator side filter. Several efficiency estimations and specifications exist, with one manufacturer specifying a recent converter system efficiency of 97.7 percent [14]. By assuming equal loss sharing between active rectifier and grid side inverter, the resulting losses of active rectifier is 1.15 percent (included as Fig. 12. Time trends for a case with 355 V dc-link voltage and 39 kW power output. case 4 in Fig. 13) . The losses can be found to be significantly higher than that of the MERS case. In summary, this indicates that the MERS configuration can contribute to energy savings and promote renewable energy conversion. B. Induction heating 1) Controllable frequency induction heating: Induction heating is widely used for industrial heating especially in the steel industry. Many high power induction-heaters, up to several mega watts, are installed to hot strip mills in the steel industry [15]. This heating method is efficient compared to other heating methods like gas furnace, because the substance is heated directly by electric power. Moreover, induction heating is a promising heating method to produce value added products because it has high potential for high performance heat control. In general, induction heating uses a high frequency ac power supply and a capacitor is connected in shunt or series with the load to compensate reactive power because of low load power factor as shown in Fig. 14(a). Therefore this type of converter can reduce ratings of power electronics components; however, frequency can not be controlled. Moreover, high power in- duction heating often uses natural commutated current source inverter using thyristors and parallel connected capacitors to turn off thyristors. In this case, it is challenging to operate the inverter when the load condition is changed dynamically or under no load condition. By using the MERS as an ac power supply to induction heating, the frequency can be controlled. This new feature adds another controllable property to induction heating, making this heating method more attractive for various industrial fields. 2) High frequency inverter using MERS configuration: The basic configuration of the proposed converter is shown in Fig. 14(b). A diode rectifier is connected to the dc capacitor 8 Authorized licensed use limited to: TOKYO INSTITUTE OF TECHNOLOGY. Downloaded on November 25, 2008 at 23:45 from IEEE Xplore. Restrictions apply. Fig. 13. Relative losses for the experimental system and estimated large scale system. (1) Experimental system (2) 1100V dc-link voltage (3) 1000V peak MERS voltage (4) Conventional system. of MERS through a dc inductor. The MERS configutration is, in this case, the same as a full-bridge inverter; however, the dc capacitor size is small and this results in the capacitor voltage being changed dynamically from zero to peak voltage as for the other MERS applications. Dc power is supplied from the diode rectifier through the dc inductor to the capacitor. Fig. 15 shows operating modes of the MERS type inverter and Fig. 16 shows schematic waveforms. From the point of MERS, current flowing to the load is the same as for the other MERS applications; however, there is no power supply connected to the load side ac circuit. Active power consumed at the load is provided by the dc current link to the capacitor. Fig. 16 also shows the voltage across and current flowing through a switch. The current starts to flow in the reverse diode when the voltage across the IGBT is zero. The current changes polarity with zero voltage naturally, and the current is shut down immediately with zero voltage. Therefore, every switching is performed under zero voltage and/or zero current condition. This means the switching losses and EMI can be reduced. Moreover, this soft-switching realizes the following advantages: 1) Surge voltage caused by turning off appears with almost zero static voltage, while voltage source type converter needs to carry the surge voltage on top of a full rated dc link voltage. This gives some advantages regarding device voltage rating. 2) Series connection of devices can be achieved with com- paratively small snubber circuit and/or not complicated (a) (b) Fig. 14. Circuit configuration of MERS inverter with diode rectifire for controllable frequency induction heating. (a) (b) (c) Fig. 15. Operational states of MERS inverter. gate control technique because of low dv/dt characteris- tics. 3) Optimized IGBT for soft-switching can be used. This can reduce conduction losses due to low saturation voltage. To use this converter in the soft-switching condition, zero voltage period of the capacitor is needed. This is realized under the not-continuous mode. 3) Development of a 90 kVA induction heating power supply: To demonstrate and investigate the proposed method, a 90 kVA 150 - 1000 Hz controllable frequency power supply for steel strip induction heating was developed. Circuit diagram 9 Authorized licensed use limited to: TOKYO INSTITUTE OF TECHNOLOGY. Downloaded on November 25, 2008 at 23:45 from IEEE Xplore. Restrictions apply. Fig. 16. Schematic waveforms of load voltage and load current. Applied voltage and flowing current of V-arm device are also shown. and overview of this power supply are shown in Fig. 17. The optimum designed IGBTs were used. Three IGBTs were con- nected in series to evaluate performance of series connection of IGBTs. Experiments using test facilities for induction heating of steel were conducted. Capacitor of the MERS was selected based on typical electrical parameters and a maximum fre- quency of 1000 Hz for achieving soft switching condition. Experiments confirmed that frequency can be controlled dy- namically. Fig. 18(a) shows time-trends for the case of 1000 Hz and 100 A. Waveforms of applied voltage and current of the coil included some harmonic distortions; however, there are no obvious problems related to heating characteristics. One of the important characteristics of this configuration is series connection of IGBTs. Voltage sharing of three connected IGBTs as shown in Fig. 18(b) was mesured. For canceling the effect of different stray capacitance, small capacitors (0.47 μF) were connected in parallel to each IGBT. Voltages across three IGBTs are also shown in Fig. 18(a). Good voltage sharing can be seen even though no special technique is applied to the gate drivers. This indicates the potential for simple series connec- tion of IGBTs in the MERS circuit, enabling construction of large scale variable frequency induction heating. (a) (b) Fig. 17. 90 kVA prototype power supply for controllable frequency induction heating. (a)Circuit diagram. (b)Overview of the power supply cabinet installed in a test facility. V. C ONCLUSION A new power electronics configuration called the Magnetic Energy Recovery Switch (MERS) has been discussed. Im- portant characteristics are simple configuration, typically low switching frequency and simple control, where the saturation voltage is the main contributor to losses. The special semi- conductor requirements of this application have led to the development of a new low saturation voltage IGBT (1.54 V). Experiments using the new IGBT have been performed for a wind power application and a variable frequency induction heating application. It is shown that the losses in the wind power application can be reduced significantly when applying the MERS configuration and the new IGBTs. Additionally, simple series connection of the devices is demonstrated in the induction heating application. R EFERENCES [1] T. Rogne, N. Ringheim, B. Odegard, J. Eskedal, and T. Undeland, “Short-circuit capability of IGBT (COMFET) transistors,” Industry Applications Society Annual Meeting, pp. 615 – 619, 1988. 10 Authorized licensed use limited to: TOKYO INSTITUTE OF TECHNOLOGY. Downloaded on November 25, 2008 at 23:45 from IEEE Xplore. Restrictions apply. (a) (b) Fig. 18. Series connection of IGBTs. (a)Time-trends of voltage sharing of three IGBTs for the case of 1000 Hz and 100 A. (b)Configuration. [2] K. Shimada, T. Takaku, T. Matsukawa, and R. Shimada, “Bi-directional current switch with snubber regeneration using p-mosfets,” Proceedings of the International Power Electronics Conference: IPEC-Tokyo 2000, vol. 3, pp. 1519–1524, 2000. [3] R. Shimada, T. Takaku, and T. Isobe, “Development of magnetic energy recovery current switch,” 2003 National Convention Record I.E.E. 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