Half Bridge Series Resonant Converter For Domestic Induction Heating

Half Bridge Series Resonant Converter For Domestic Induction Heating

Anjana M P Jeena Joy Neena Mani.

M.tech student

Assistant Professor

Assistant Professor

1. A.College of Engineering M. A.College of Engineering M.A.College of Engineering Kothamangalam, Kerala, Kothamangalam, Kerala Kothamangalam,Kerala

   Abstract–The Domestic induction heating operation is based on a resonant inverter which supplies medium-frequency currents (20100 kHz) to an inductor, which heats up the pan. This paper presents the analysis of an ACAC resonant converter which is based on the half-bridge series resonant inverter applied to domestic induction heating. Only two diodes is used in this topology to rectify the mains voltage. The converter can operate with zero-voltage switching during both switch-on and switch- off transitions. Since half bridge topology is being employed, it doubles the output voltage, and therefore, the current in the load is reduced for the same output power. The simulation of this topology has been carried out in MATLAB/ SIMULINK at a switching frequency greater than the resonant frequency. A prototype has been implemented which works on three different frequencies. It has verified that the condition with minimum delay heats up faster. This topology also shows great improvement in efficiency (about 96%).

   Keywords High frequency; induction heating, Resonant converter; zero-voltage switching (ZVS).

   1. INTRODUCTION

      Domestic induction technology has become more popular in recent years due to features such as efficiency, safety, and accurate output power control, which outperform other traditional domestic heating technologies. Domestic induction hobs are now becoming a standard option, especially in Asia and Europe. The high efficiency of the induction hobs is attracting the attention of researchers devoted to highly efficient power electronic systems. Induction heating is a non-contact heating process. It uses high frequency electricity to heat materials that are electrically conductive. Since it is non-contact, the heating process does not contaminate the material being heated. It is also very efficient since the heat is actually generated inside the work piece. This can be contrasted with other heating methods where heat is generated in a flame or heating element, which is then applied to the work

      piece. For these reasons Induction Heating lends itself to some unique applications in industry.

      Induction cooker constitute a major domestic application of the induction-heating phenomena. In such devices, the desired heating is done in metallic vessels by varying the magnetic field, which in turn is generated by a planar coil fed by a power electronics inverter. Basically, a domestic induction arrangement consists of a planar multi-turn winding situated below a metallic vessel [2], [9] and supplied by a medium- frequency power source, normally operated between 20 and 100 kHz. The frequency is higher than 20 kHz is preferred to avoid the audible range and lower than 100 kHz to reduce switching losses. Increasing the frequency of operation of power converters is desirable, as it allows the size of circuit magnetics and capacitors to be reduced, leading to cheaper and more compact circuits. However, increasing the frequency of operation also increases switching losses and hence reduces system efficiency. One solution to this problem is to replace the "chopper" switch of a standard SMPS topology (Buck, Boost etc.) with a "resonant" switch [8], which uses the resonances of circuit capacitances and inductances to shape the waveform of either the current or the voltage across the switching element, such that when switching takes place, there is no current through or voltage across it, and hence no power dissipation.

      Fig.1.Induction cooking appliance block diagram

      The most used device is the insulated gate bipolar transistor (IGBT) because of the operating frequency range and the output power range, up to 3 kW. The main blocks of an induction cooking appliance are outlined in Fig. 1 [3]. The energy taken from the mains is filtered by an EMC filter, which prevents the device from inserting interferences, and it provides immunity to voltage transients. Afterward, the voltage is rectified and filtered, generating a dc bus. A low value of filter capacitor is chosen to get a high PF, and, as a consequence, a high-ripple dc bus is obtained. Then, the resonant inverter supplies variable-frequency current to the induction coil. This current produces an alternating magnetic field, which causes eddy currents and magnetic hysteresis heating up the pan.

      A topology giving efficient operation is discussed in this paper which utilizes the resonant inverter to heat up the pan. An experimental set up is also presented in this paper.

   2. HALF BRIDGE SERIES RESONANT INVERTER

      The power converter generally implemented in domestic IH appliances is a resonant inverter due to its improved efficiency and lower size. The main power circuit employs a half-bridge series converter switching at a high frequency. The half-bridge series resonant inverter is the most employed topology due to its simplicity, its cost-effectiveness, and the electrical requirements of its components [7]. The simplest half bridge inverter topology is shown in Fig 2. The switching circuit consists of an IGBT. Zero voltage/current turn-on switching is enabled by turning on the IGBT while the diode is in turn on period. The resonant circuit comprises of resonant inductance (Lr) and resonant capacitance (Cr) [10]. The capacitors, C1 and C2, are the lossless turn-off snubbers for the switches, S1 and S2. The resonant frequency fr of the converter is mainly determined by the inductance Lr and the capacitance Cr of the series capacitor.

      Fig. 2 Half bridge series resonant inverter

      Induction-coil-and-pan coupling is modelled as a series connection of an inductor and a resistor, based on the analogy of a transformer, and it is defined by the values of Leq and Req. [4] The switching frequency of the system is set higher than the resonance frequency, in order to avoid noise generated within the audio frequency band. The resonant load consists of the pan, the induction coil and the resonant capacitor. By connecting the IGBT switching circuit, S1 and S2 in parallel to diodes D1 and D2, current loss is minimized. When S1 is turned-off, D2 helps S2 stay on zero voltage/current before being turned on, thereby substantially reducing current loss (the same is the case with S1). There is no reverse-recovery problem as the voltage on both sides remains zero after the diode is turned off. However, as the switching circuit is turned off at around the upper limit of voltage and current, some switching loss results on turn-off. The capacitors C1 and C2, acting as turn-off snubbers connected in parallel to S1 and S2, keep this loss to a minimum. Upon turn-on the switching circuit starts from zero voltage/current, so these turn-off snubbers operate as lossless turn-off snubbers.

   3. AC-AC POWER CONVERTER

      This topology (see Fig. 3) consists of two

      bidirectional switches composed of IGBTs T1 and T2, and antiparallel diodes D1 and D2, respectively. Only

      fr

      2

      1

      Lr Cr

      two diodes is used in this topology to rectify the mains voltage Vac, shown as DH and DL, but only one of them is activated at the same time. This operation increases efficiency with regard to classical topologies based on a full-bridge diode rectifier plus a dc-link inverter. This topology is a seriesparallel resonant converter. Th inductorpot system is modelled as an equivalent series resistance Req and inductance Leq, as shown in Fig. 3.

      Fig.3 Ac-Ac power converter applied to domestic induction heating

   4. ANALYSIS

      Since the converter is having symmetry between positive and negative ac voltage supply, simplified analysis is possible. The circuit can been drawn for one of the half cycles since it is applicable to the other cycle too. The equivalent circuit for positive half cycle is as shown in Fig. Based on series resonance normalised equations can be written [1], [7].

      Fig.4. Pulses to both switches

      For zero voltage transitions three different states could be presented here based on turn-on and turn-off of both the switches. The equivalent circuit representing the 3 states are shown in Fig 5 (a, b, c). The first state begins when the lower switch is triggered OFF. In this moment, the antiparallel diode D1 conducts and T1 can be triggered ON ensuring ZVS switching-on conditions. The parallel resonant circuit is set by an equivalent capacitor Ceq, obtained from Cr and Cb and the load parameters Req and Leq.

      C )

      1 2

      eq Cr ( 1

      In the second state both switching devices conducts, although only the upper switch is triggered ON. The parallel resonant circuit is set by the load parameters in parallel with both resonant capacitors. The third state starts when the upper switch is triggered OFF. Then D2 starts conducting and T2 can be triggered ON achieving ZVS switch-on conditions. This state ends when the lower switch is deactivated. The resonant circuit is set by Cr in parallel with the series connection of Cb and the parallel connection of the load and the other one Cr.

      Where is the ratio between the input choke and the equivalent inductance of the inductor pot system and is the ratio between the dc-link and the resonant capacitors and. 0, sw, n are respectively the angular resonant frequency, the angular switching frequency, and the normalized angular switching frequency. Z0 is the equivalent impedance of the resonant circuit. And Qeq is the quality factor at the resonant frequency of the equivalent inductor-pot system. The pulses given to the switches are given as in Fig 4.[2]

      (a)

      (b)

      (c)

      Fig.5 Equivalent circuits at different states during positive half cycle. (a)State 1 (b) State 2 (c) State 3

   5. SIMULATION RESULTS

      The AC to AC converter fed domestic induction heater is simulated using MATLAB/SIMULINK and their results are presented here. The SIMULINK model of AC-AC converter is shown in Fig.6 and its corresponding waveforms is also shown.

      Fig.6 Simulink model of ac – ac converter

      The simulation has been carried out with the parameters as shown in table 1.

      Table1: Parameter values

      Parameters	Values
      Equivalent resistance, Req	7
      equivalent inductance, Leq	70µH
      Supply source voltage, Vac	230V
      resonant capacitor, Cr	450nF
      DC link capacitor, Cb	450nF
      input inductance, Ls	1.6mH
      equivalent series resistance, Rs	80m
      switching frequency	25kHz.

      The simulation was done with a duty cycle of 25% and is assumed to have a switching frequency of 25 kHz. The input voltage waveform is as seen in Fig 7. The input current through the inductor is obtained with amplitude of approximately 5A. The voltage and current waveforms through the upper and lower switches are shown in g 5.3 and 5.4 respectively.

      Fig.7 Input voltage waveform

      Fig.8 Input current through the inductor

      Fig. 9: The voltage and current waveforms through the upper switch

      Fig.10 : The voltage and current waveforms through the lower switch

      And the voltage and current obtained through the load Req and Leq is shown in fig. 11

      implemented. The block diagram of the model is as in Fig 13.

      Fig.11 voltage and current obtained through the load

      Thus with these following conditions and assumptions an output power of 3.6 kW is obtained. The waveform of power is also shown in Fig. 12.

      Fig.12: Output power waveform

      A prototype has been built with the same circuit parameters [6]. It is implemented so as to work at 3 different frequencies. This is done by giving different delays between the two switches, 100s, 500s and 10ms respectively. The control is given using microcontroller (PIC16F877A). Since there are two switches two driver circuit is also needed. One part in the prototype is this driver circuit. The other part contains the main ac ac power converter circuit. The coil is heated at all the three switching conditions representing three different frequencies. It has been observed that the coil heats up faster when the delay between both the switches (upper and lower) is low. A model representing the above discussed converter is

      Fig.13: Block Diagram representing the setup

      Here the supply 230V from the ac mains is step downed to 12V using a transformer for feeding the driver circuit, which mainly comprises of optocoupler, buffer IC and transistors. The pulses thus generated are given to the switches. The experimental setup is shown in Fig. 14.

      Fig.14 Experimental set-up of the ac-ac power converter

   6. CONCLUSION

This paper presents an ac – ac converter applied to domestic induction heating. An analytical analysis has been done the equivalent circuit during the positive half cycle. The three states have been discussed based on switching of the two switches. The converter can operate with zero-voltage switching during both turn-on and turn-off commutations. Besides, the output voltage is doubled compared to the classical half-bridge, reducing the current through the switching devices. As a consequence, the power converter efficiency is improved in the whole operating range. A prototype has been implemented in order to validate the analytical results. Since it is not designed for a particular power, it

could be shown as heating at fast rate with 3 different frequencies with fast heating at low time delay. This validates the feasibility of the power converter discussed above.

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