A 3.6kW Single-ended Resonant Inverter for Induction Heating

Transcription

1 A 3.6kW Single-ended Resonant Inverter for Induction Heating Applications Jae-Eul Yeon*, Kyu-Min Cho**, Hee-Jun Kim*** * FAIRCHILD KOREA SEMICONCUCTOR ** YUHAN UNIVERSITY *** HANYANG UNIVERSITY , Fairchild Korea Semiconductor, 850gil, Pyeongcheon-ro, Wonmi-gu, Bucheon-si, Gyeonggi-do, KOREA Tel.: +82 / (32) Fax: +82 / (32) jaeeul.yeon@fairchildsemi.com URL: Keywords «Induction heating», «Single-ended resonant inverter», «Interleaved control», «Resonant converter» Abstract The single-ended (SE) resonant inverter is a type of class E parallel resonant inverter and popularly used in many IH applications due to its lower cost structure and relatively high efficiency. However its maximum power rating should be limited because the resonant voltage of SE resonant inverter increases as its power increases. Consequently the maximum power rating of SE resonant inverter should be limited up to around 2kW. This paper proposes an interleaved single-ended resonant inverter for the induction heating applications. The proposed inverter can provide two times higher output power than the conventional single-ended resonant inverter by the alternative operation of two inverters. A burst mode control enhances the power control range from very light load to full load and can improve efficiency. Introduction Since induction heating heats a heating object directly, it has many advantages over other heating methods associated with a traditional stove (electrical coils, burning gas). Some of these advantages include rapid heating, improved thermal efficiency, precise heat control, and a stove that is easy to clean. For induction heating, a high-frequency resonant inverter is required that converts the electrical energy into heat energy on a ferromagnetic metal object. Basically, two types of resonant inverters, a half-bridge (HB) inverter and a single-ended (SE) inverter can be considered. Of these two inverter types, the SE resonant inverter is generally more popular in induction heating applications due to its lower cost structure as well as its relatively high efficiency. However the SE resonant inverter has a critical drawback that its application should be having lower power rating than 2kW. Because the SE resonant inverter is a voltage resonant type thereby its working voltage is inevitably in proportion to the power rating; the higher power results in the higher working voltage. In the SE resonant inverter, the peak working voltage of IGBT is normally around 1100V under the condition with the input voltage is 220V, the switching frequency is around 20kHz, and the power rating is 2kW. Meanwhile, the breakdown voltage of IGBT in the SE resonant inverter is normally limited to around 1200V~1500V due to the trade-off between the breakdown voltage and the current rating; As the higher breakdown voltage of IGBT gets higher, its V CE(sat) also gets higher exponentially. In this paper, a 3.6kW interleaved SE resonant inverter for induction heating application is proposed and discussed with experimental results. The proposed inverter can provide twice of output power than conventional one by means of alternative switching operation of two inverters. Besides, a burst mode control allows wide power control range and better thermal performance thereby the proposed inverter EPE'15 ECCE Europe ISBN: and CFP15850-USB P.1

2 will also be more reliable. To verify the validity of the proposed inverter, experiments with an interleaved single-ended (SE) resonant inverter for induction heating were carried out and the test result and validity are presented in this paper Proposed interleaved SE resonant inverter for IH applications a) Single-ended Resonant Inverter for IH applications Because of many advantages including higher thermal efficiency, precise and rapid heat control, and so on, induction heating has become a popular heating method. For induction heating or cooking, a high-frequency resonant inverter is required that converts the electrical energy into heat energy on a ferromagnetic metal object. The SE resonant inverter has continued to gain popularity especially in table-top cooker and rice jar applications, and even for inverterised micro-wave ovens due to its lower cost structure and simple construction. The basic circuit diagram and the operation modes of SE resonant inverter are illustrated in Fig. 1. The rectifier, choke coil, and input capacitor, C in shown in Fig. 1 comprise a low-pass filter (LPF). Meanwhile, the working coil can be represented as a series combination of inductance, L r and resistance, R eq, which combine with capacitor (C r ) to form a resonant tank circuit. Mode I Mode II L r R eq Choke Coil i Lr i Cr C r V ac Rectifier C in + v cr - Q i Q + vce - Mode III Mode IV Fig. 1 The circuit diagram and operation modes of SE resonant inverter for IH application The operation of inverter is simply divided into 4 modes. During the mode I, the resonant current flows through the anti-parallel diode, thus the collector-emitter voltage (V CE ) of the switch, Q becomes zero. The switch should be turned on within this mode to achieve zero voltage switching (ZVS) turnon. When the switch is turned-off, a quasi resonance between L r and C r is begun. Since the switching voltage gradually rises due to this resonance (slower dv CE /dt condition), the ZVS turn-off is also achieved. In order to achieve ZVS turn-on and turn-off, the off-time has to be fixed. Through the zero voltage switching (ZVS) by voltage resonance, this inverter system provides the better efficiency. However, it needs high voltage IGBT as a switching device. Because the inverter is operated by only single IGBT and a very high resonant voltage is applied to the IGBT. EPE'15 ECCE Europe ISBN: and CFP15850-USB P.2

3 E S (a) On period (b) Off period Fig. 2. Equivalent circuits of SE resonant inverter during IGBT on and off period The equivalent circuits during the IGBT on and off period are illustrated in Fig. 2. In the SE resonant inverter in Fig. 1(a), i Leq flows through L eq, R eq, Q and C in and the energy is transferred to the load during the switch-on period as shown in Fig. 2(a). Thereby the voltage equation is: dileq Vin Leq Re qileq = 0 dt And, the inductor current is derived as: (1) E ileq( on)() t = 1 e R R t L When switch, Q, is turned off at t 2 in Fig. 1(b), the resonance between L r and C r is begun and the circuit can be considered as shown in Fig. 2(b). Therefore, the voltage and current equations are derived as below; (2) V s in { } ( ) 1 Ioff ( s) + L sioff ( s) Io + RIoff s = 0 sc (3) where, αt ( )() = cos( ω + θ) i t e A t Leq off E α Io A= + I ωl ω 2 2 O 1 αlio E θ = tan ωlio, (4) (5) (6) and, R eq α = (7) 2L R eq 1 ω = LC 2L 2 (8) The breakdown voltage of IGBT should be limited because both V CE(sat) and the tail current will be exponentially increased in accordance with increasing the breakdown voltage of IGBT. Consequently, 1200V-1500V IGBTs are generally used in SE resonant inverters and the maximum power rating of SE resonant inverter is inevitably limited. EPE'15 ECCE Europe ISBN: and CFP15850-USB P.3

4 b) Interleaved Operation with Two Single-ended Resonant Inverters (a) Circuit diagram v cr 1 = icr dt C r 3π 2ω (b) Theoretical waveforms Fig. 3. The proposed dual SE resonant inverter for induction heating applications The basic circuit diagram and the theoretical waveforms of the proposed interleaved inverter for induction heating applications are illustrated in Fig. 3 (a) and Fig. 3 (b) respectively. The proposed inverter in the Fig. 3(a) consists of two SE resonant inverters that work alternately with 180 degree of phase differences. The SE resonant inverter for IH application challenge requires high breakdown voltage and high current IGBT as a switching device. As the output power of SE inverter increases, the resonant voltage of inverter also increases and the higher breakdown voltage of IGBT is required. However, the performance of the IGBT in terms of conduction and switching performances get worse as its breakdown voltage increases. Therefore, the output power of SE resonant inverter has to be limited. Generally, the maximum power rating of SE inverter is limited up to around 2 kw. The proposed inverter composes two SE inverters working alternately as shown in Fig. 3(b) thereby it can provide two times of output power compare to conventional SE resonant inverter. EPE'15 ECCE Europe ISBN: and CFP15850-USB P.4

5 Experimental Results To verify the validity of the proposed inverter, an experiment was carried out with a prototype 3.6 kw interleaved SE resonant inverter as shown in Fig. 4. Fig. 5 demonstrates the experimental waveforms at middle load and full load conditions respectively. In the Fig. 5(a) for the middle load condition, the measured power is 1.8 kw and the peak voltage and the peak current of each inverter are around 940V and 28A respectively. In the Fig. 5(b) for full load condition, the measured power is 3.6 kw and the peak voltage and the peak current of each inverter are around 1.27 kv and 52A respectively. Although the maximum power of experimental set-up in this paper is 3.6 kw, 1350V and 20A IGBTs were used that is generally applied to the range of 1.8kW IH applications. Fig. 4. A 3.6kW Interleaved SE IH inverter set-up (a) Middle load (1.8kW) Fig. 5. Experimental waveform; V GE, V CE, and I Q (b) Full load (3.6 kw) The amplitude of the voltage resonance reads to ZVS turn-on in case of SE resonant inverter. If the negative peak of resonant voltage is smaller than the amplitude of the input voltage, ZVS turn-on can t be achieved and even excessively high charging current may damage IGBT. The amplitude of voltage resonance is directly in accordance with the switching current; the higher switching current results in the higher voltage resonance. Therefore, the light power condition with small duty ratio can be harsh to IGBT. Fig. 6 demonstrates the switching waveform at the light load condition. As shown in Fig. 6, IGBT can t be turned on under ZVS condition. Even excessively high charging current can destroy the IGBT due to excessively high turn on loss. EPE'15 ECCE Europe ISBN: and CFP15850-USB P.5

6 1 tu() t RC Ee Fig. 6. Switching waveform at the light load condition Fig. 7. Thermal performance of IGBT in accordance with power in SE resonant inverter Fig. 7 demonstrates that the thermal performance of IGBT in SE resonant inverter. The power range above P mid., the temperature of IGBT gets higher as the power increases. In contrast, the power range below P mid., the IGBT temperature gets much higher as the power decreases and IGBT can be destroyed by extremely high temperature. Generally, for the power below P mid., the burst mode control can be usefully used because the fast thermal response isn t required in case of IH applications. Fig. 8 shows an example of burst mode control with interleaved SE resonant inverter. (a) Burst duty ratio : 50% (b) Burst duty ratio : 25% (c) Burst duty ratio : 12.5% Fig. 8. Burst mode operation for the light load In general, the power consumption of induction heating system is relatively high. Thus, the function of power factor collection (PFC) is necessary. Basically, SE resonant inverter doesn t require a dedicated PFC circuit because the inverter works as PFC circuit itself. However, the larger chock coil is needed for lower EMI noise. The proposed is however much advantageous than conventional in terms of power factor and EMI because of interleaving operation. The input voltage and current waveform comparison between a conventional 1.8kW) and the proposed SE resonant inverter (3.6kW) are illustrated in Fig. 9(a) and Fig. 9 (b) respectively. Both cases show higher power factor over 0.95 without a dedicated PFC circuit. However the high frequency ripple of the input current in the EPE'15 ECCE Europe ISBN: and CFP15850-USB P.6

7 proposed inverter is much smaller than that in the conventional inverter which is more advantageous in terms of EMI characteristics. (a) conventional SE inverter (1.8 kw) Fig. 9. Input voltage and current waveforms (b) interleaved SE inverter (3.6 kw) Conclusion In this paper an interleaved single-ended resonant inverter for the induction heating applications was presented. The maximum power rating of the conventional SE resonant inverter should be limited by the resonant voltage because the resonant voltage of SE resonant inverter increases as its power increases. The proposed inverter can however provide twice of the power than conventional inverter by interleaving operation. Through an experiment with a 3.6 kw prototype inverter, the validity of the proposed inverter was verified. Additionally, the burst mode control provides wider power control range and interleaved operation is more beneficial to get higher power factor. References [1] Laska, T., Munzer, M., Pfirsch, F., Schaeffer, C., and Schmidt, T., The Field Stop IGBT (FS IGBT) A new power device concept with a great improvement potential ISPSD.2000, pp [2] New methods for extracting field-stop IGBT model parameters by electrical measurements Tang Yong ; Chen Ming ; Wang BoIndustrial Electronics, ISIE IEEE International Symposium on Digital Object Identifier: /ISIE Publication Year: 2009, Page(s): [3] T. Laska, M. Munzer, F. Pfirsch, C. Schaeffer, "The Field Stop IGBT (FS IGBT) -A New Power Device Concept with a Great Improvement Potential," ISPSD, May, 2000, pp [4] Alessandria, A. and Fragapane, L., A new top structure concept for a trench-gate emitter implant Field- Stop IGBT, SPEEDAM 2010, pp [5] T. Laska, A. Mauder, L. Lorenz : The Field Stop IGBT Concept with an Optimized Diode; Conference on Power Electronics and Intelligent Motion PCIM; Nurnberg, [6] E. Griebl, O. Hellmund, M. Herfurth, H. Huesken, M. Puerschel: LightMOS IGBT with integrated Diode for Lamp Ballst Applications, Conference on Power Electronics and Intelligent Motion, PCIM 2003, p. 79ff, [7] O. Hellmund, L. Lorenz, H. Ruething: 1200V Reverse Conducting IGBTs for Soft-Switching Applications; China Power Electronics Journal , pp , [8] Takahashi, H. ; Yamamoto, A. ; Aono, S. ; Minato, T., 1200V reverse conducting IGBT ISPSD '04. pp [9] D. J. Kessler and M. K. Kazimierczuk, Power losses and efficiency of Class-E power amplifier at any duty ratio, IEEE Trans. Circuits Syst I, Reg. Papers, vol. 51, no. 9, Sep. 2004, pp , [10] N-J Park, D-Y Lee, and D-S Hyun, Study on the new control scheme of class-e inverter for IH-jar application with clamped voltage characteristics using pulse frequency modulation, Electric Power Applications, IET Vol. 1, Issue 3, pp [11] D-Y Lee, and D-S Hyun, A new hybrid control scheme using active-clamped class-e inverter with induction heating jar for high power applications, Journal of Power Electronics, Vol. 2, No. 2, April 2002, pp EPE'15 ECCE Europe ISBN: and CFP15850-USB P.7