IEEE PEDS 2011, Singapore, 5-8 December 2011 A Series-Resonant Half-Bridge Inverter for Induction-Iron Appliances N. Sanajit* and A. Jangwanitlert ** * Department of Electrical Power Engineering, Faculty of Engineering, Mahanakorn University, Bangkok Thailand, 10530 ** Department of Electrical Engineering, Faculty of Engineering, King Mongkut s Institute of Technology Ladkrabang, Bangkok Thailand, 10520 Abstract- This paper presents a series-resonant half-bridge inverter for induction iron appliances. The series-resonant inverter is implemented to provide Zero Current Switching (ZCS) for all the switches at turn off conditions and Zero Voltage Switching (ZVS) at diode turn on. The main features of the proposed inverter are simple PWM control strategy and high efficiency. The operation mode of the inverter will be evaluated corresponding to the duty cycle of the switch. The experimental results verify the advantages of the proposed topology. The paper is organized as follows. In Section II, the converter is chosen. The operation principle of the synthesized converter is presented in Section III. The experimental results are indicated in sec IV. Finally, conclusions of the work are provided in Section V. Index Terms- ZVS, ZCS, induction iron, half-bridge inverter, resonant I. INTRODUCTION INDUCTION heating system can apply for many things such as induction hardening, induction melting, induction cooking, etc [1]. One thing that is no paper to propose for induction heating systems is induction iron. Therefore, this is the first time to propose the induction iron appliance. Induction iron is low-power inductionheating systems with a maximum output power usually less than 1.5- kw per load. An induction iron appliance is basically made up of a flat-type inductor coil, with insulator, in which the iron tray to be heated is placed. Between the iron tray and the coil, an insulator is placed in Fig. 1. The heat is generated at the iron tray s bottom due to eddy currents and hysteresis losses. These induced currents are caused by an alternating magnetic field generated by a medium frequency (20 100 khz) current through the coil. Iron tray-inductor coupling is usually modeled as the series connection of an inductor and a resistor, based on the transformer analogy. The values of the equivalent inductance and resistance depend on the operating frequency and the required maximum power. An arrangement of an induction iron is shown in Fig. 2. An induction iron takes the energy from the mains voltage, which is rectified by a bridge of diodes. In this paper, a bus filter is designed to allow a large voltage ripple to obtain the high input power factor. Then, the inverter topology supplies the high-frequency ripple current to the induction coil. The main inverter topologies used in induction heating system are the resonant inverters, including full-bridge, halfbridge and single-switch inverters. In this case, induction iron appliance including is possible to use a half- bridge inverter as shown in Fig. 3. The purpose of this work is to get a series-resonant inverter for using it in power electronic converters mainly for induction iron. The operating frequency is fixed, but the duty cycle control can be adjusted, based on power desired and load conditions. The required fixed-frequency control has some additional advantages, as reducing the electromagnetic noise spectrum and avoiding the acoustic noise [2] due to different operating frequencies which cause low-frequency interferences amplified by the iron. Fig. 1 Iron tray with Coil Fig. 2 Typical arrangement of an induction iron appliance. II. CHOICE OF CONVERTER The half-bridge, series resonant converters were selected above the single-switch topologies due to the following reasons [3]: The voltage across the semiconductors is clamped. Even though two switches are needed, at least half the voltage blocking capability is required. Due to switching is done at a duty ratio of 50%, feedback is not needed. Anti-parallel composite-switches must be used, as shown in Fig. 3, consisting of a singular-switch (S) and an anti-parallel diode (D). Isolated gate bipolar transistors (IGBTs) are shown because they are well suited for this medium frequency application. Although an isolated gate drive is needed, an opto-isolator can be used due to the 50% duty ratio. The 978-1-4577-0001-9/11/$26.00 2011 IEEE 46
switching power loss in the semiconductors could be a disadvantage and is investigated. khz. The following changes were made to the converters described by The resonant capacitors (C 1 +C 2 ) // C) are used, as shown in Fig. 3, so that current is drawn from the supply during each half cycle of the switching period, enhancing the power factor. Fig. 3 Half-bridge, series resonant circuit. The are generally two possible switching schemes for this converter to obtain the desired output power control without varying the input voltage, i.e. load commutation and forced commutation. In this paper, the load commutation should be applied. Load Commutation A typical i CS waveform is shown in Fig. 4. The power is decreased by decreasing switching frequency (f s ) below the resonant frequency (f r ) where f 1, ( ) / / r = CT = C1+ C2 C (1) LC T Giving the following advantages and disadvantages: + No turn-off power loss for the singular-switches (ZCS) and no turn-off power loss for the anti-parallel diodes (ZVS) as shown in Fig.4. - Maximum power is obtained at the upper limit of the switching frequency. - Turn-on power loss for the singular-switches and turnoff power loss and reverse recovery current for the antiparallel diodes. III. OPERATION PRINCIPLES Fig. 5 shows possible current paths of the inverter output voltage (v AB ) and load current (i L ) which are only one cycle (see Fig. 2 or Fig. 6). Also, the operation waveforms are shown in Fig 6. To simplify it, the following assumptions are provided: The resonant capacitors are ideal Inductance L is working coil and work piece Resistance R is work piece all devices are no losses. V i C 1 C 2 S 1 S 2 (a) Mode 1 D 1 D 2 R C L Fig. 4 Singular-switch and diode current for load commutation. To be easily operated, the load commutated converter was applied. The elimination of the reverse recovery current of the diodes [3,4]and the maximum power was obtained at the upper point of the switching frequency range, usually about 30 47
IV. EXPERIMENTAL RESULTS The experimental results are done for the main components as given below: C 1 =C 2 = 12 uf; C = 0.8 uf, L = 35.62 uh, R = 26 mohm, Maximum input voltage V i = 310 V. Fig. 5 Operation mode. This inverter can be operated under soft switching conditions as shown in Fig. 4, and its output power cannot be regulated continuously due to the fact that the input voltage (V i ) has a high ripple (as shown in Fig.7) following by the ac input voltage. However, the duty cycle control for switches is fixed. The duty cycle D is equal to 0.35. The results from each voltage and current of switch per cycle are shown in Fig.8 that is indicated as the soft switching (ZVS and ZCS like Fig. 4). The characteristics of voltage and current waveform of the previous cycle is look like the same as the next cycle, but the amplitude of voltage and current is different. Therefore, the inverter output voltage and load current waveform are shown in Fig. 9. That is indicated that switch is operated as the load commutation. Fig. 6 Operation waveform. There are 4 switching modes in a switching period as described in the following. Mode 1: (t 0 -t 1 ) [Fig. 5(a)]: Switch S 1 conducts the load current i L. The output voltage across the load (inverter output voltage) is V i /2. The zero current of switch S 1 is zero at turn off. In this mode, the zero current switch (ZCS) of switch S 1 is achieved. Mode 2: (t 1 -t 2 ) [Fig. 5(b)]: switch S 1 is off. The voltage across S 1 is zero to prepare the diode D 1 going to a zero voltage switch (ZVS) at turn on. The diode D 1 conducts the load current i L in the opposite direction of mode 1. The output voltage across the load is still equal to V i /2. Fig. 7 High input ripple voltage (Vi) (upper) and current (below). Mode 3: (t 2 -t 3 ) [Fig. 5(c)]: Diode D 1 is off. Switch S 2 conducts the same side of load current i L and S 2 achieves the ZCS condition. The output voltage across the load is -V i /2. When the switch conducts, the voltage across of switch S 2 is zero. Mode 4: (t 3 -t 4 ) [Fig. 5(d)]: switch S 2 is off. The voltage across S 2 is zero to prepare the diode D 2 going to a zero voltage switch (ZVS) at turn on. The diode D 2 conducts the load current i L in the opposite direction of mode 3. The output voltage across the load is still equal to -V i /2. The next cycle will be repeated mode 1 and so on. Fig. 8 Voltage across switch (upper) and current of each switch (below). 48
Fig.9 The inverter output voltage (v AB) (square wave) and load current (i L ) (almost sine wave). In addition, the experiment shows the heat from the iron sheet that is compound of the iron. Fig.10 shows the average absorbed temperature results from the thick of the iron sheet. The more thickness of iron sheet had, the more time for getting high temperature had. Moreover, Fig. 11 shows the average emitted temperature results from the thickness of the iron sheet. The more thickness of iron sheet had, the more time for reducing high temperature to low temperature had. In this case, the upper limit of temperature is 90 o C while lower limit of temperature is 30 o C. To use appropriately in this research, 4-mm thickness of iron sheet was used. Furthermore, the test of heat, which is from iron sheet, is compared between the induction iron and ordinary electric iron. The right hand side of Fig. 12 shows the distributed- Fig. 12 Distributed temperature over the iron sheet from thermo-scan. temperature over the iron sheet of induction iron and the left hand side of this Fig. 12 shows the distributed temperature over the iron sheet of ordinary electric iron. It can be observed that the yellow color is over the iron sheet of induction iron. This indicates the temperature is almost the same over the iron sheet. However, the yellow color shows on the iron sheet of ordinary electric iron that indicates only topical coil, which is in body of ordinary electric iron. It describes the temperature over the iron sheet is too much different In Fig. 13, the output power of ordinary electric iron is compared with that of induction iron. When the output power from induction iron is tested at 385 watts, the output power from the ordinary electric iron is 880 watts, the induction iron can save more energy per hour. The energy per hour from ordinary electric iron is 0.117 kw/h. but the induction iron is 0.092 kw/h. The efficiency of induction iron is 92% as shown in Fig. 14. Ordinary Electric Iron vs. Induction Iron Output Power (W) 1000 800 600 400 200 0 0 20 40 60 80 100 Temperature (C) Po (W) Ordinary Electric Iron Po (W) Induction Iron Fig. 10 The average temperature absorbed from the thick of iron sheet. Fig. 13 Output power of both irons. Efficiency 92.5 92 91.5 91 90.5 90 89.5 89 88.5 88 87.5 0 20 40Temp (C) 60 80 100 Fig. 11 The average temperature emitted from the thick of iron sheet. Fig. 14 Efficiency of induction iron. 49
V. CONCLUSION In this paper the operation principle of the prototype induction iron is introduced together with the switching frequency lower than resonant frequency, power desired and ZCS and ZVS algorithm incorporated in it. Its performance characteristics are verified by the experimental results for a prototype induction iron rated at 385 W. The induction iron is designed to replace ordinary electric iron. The prototype has been developed and the performance is good. It can provide the heat like ordinary electric iron, it can reduced to 1/2 of the power for the same amount of heating capability. REFERENCES [1] N.S. Baytndtr, O. Kukrer and M. Yakup, DSP-based PLL-controlled 50-100 khz 20 kw high-frequency inducton heating system for surface hardening and welding applications, IEE Proc-Electr. Power Appl., Vol. 150, No. 3, May 2003. [2] A. Y. Goharrizi, M. Farasat, S. H. Hosseini, A. K. Sadigh, A Half bridge inverter with soft switching auxiliary circuit for induction-cooking applications, The Proceedings of the 1 st IEEE Power Electronic & Drive Systems & Technologies Conference, 2010, 80-84. [3] H. W. Koertzent, J. D. Van Wyk and J. A. Ferreira, Design of the halfbridge, series resonant converter for induction cooking, The Proceeding of the 26 th IEEE Power Electronics Specialists Conference, 1995, pp. 729-735. [4] Y. Lu, K. W. E. Cheng, K.W. Chan, Z.G. Sun, and S.W. Zhao, Development of a Commercial Induction Cooker, The proceedings of the 3 rd International Conference on Power Electronics Systems and Applications, 2009. 50