International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 6 (2016) pp 3819-3827 © Research India Publications. http://www.ripublication.com Class D Series Resonant Inverter with PDM Scheme For Induction Heating Application S. Jaanaa Rubavathy* Research Scholar, Faculty of Electrical Engineering, Sathyabama University, Chennai, Tamil Nadu, India. Dr. P. Murugesan Professor, Department of EEE, S.A. Engineering College, Chennai, Tamil Nadu, India. transferred to it. For applications like steel melting, brazing and surface hardening high operating temperature is required for which the operating frequency must be selected based on work piece size and skin depth requirement [4]. The inverter topologies used in induction appliances are resonant inverters, like full-bridge, half bridge and single switch inverters [5-9]. In this study, Class D inverter topology is used for supplying the induction heating load. Recently, voltage source series resonant inverter (SRI) has been suggested for induction heating [10 &11] with various modulation schemes to vary the output power. In the conventional fixed frequency modulation techniques, ZVS is ensured only for a limited range of duty cycle[12]. Power frequency modulation (PFM) can also be employed for controlling the output power by controlling the switching frequency [13-16]. The two output fixed frequency control techniques employ asymmetrical duty-cycle or asymmetrical Pulse Width Modulation (PWM) control. It is based on an unequal duty-cycle operation of the switches in inverter [17]. A two output resonant inverter for cooking application was given by Jose [18]. Later this was modified into an inverter designed to give three outputs for induction heating cooking appliances [19] and now it is designed as two output solution for induction heating cooking applications with full bridge inverter topology and fixed frequency control which reduces the number of switching devices to 2n+2 instead of 4n [20]. Thus, the use of multi-output inverters is better for multiburner cookers: higher utilization of electronics, higher maximum power and it is possible to share some components of the converter. In addition to this, they also employ fixed frequency control technique [21 & 22], in particular Asymmetrical Voltage Cancellation Control (AVC). The Finite Element Method (FEM) and the heat generating rate was derived from the theoretical results [23]. Analysis and design of a new AC-AC resonant converter applied to domestic induction heating was employed using half bridge series resonant inverter, which can be operated with Zero Voltage Switching (ZVS) during both switch on and switch off transitions. Hence the output voltage was doubled and the current in the load was reduced for the same output power [24]. A quantitative analysis of domestic induction heating appliances was proposed by considering different parameters which led to efficiencies higher than 95% [25]. The direct AC-AC boost resonant converter not only reduces the component count, but also exhibits higher efficiency levels, making it appropriate for the domestic IH applications [26]. Abstract This paper proposes a unique topology of Class D series resonant inverter with output power control schemes for Induction Heating (IH) application. The main aim of this work is to reduce the steady state error so as to get better performance for induction heating applications under varying load conditions. The power control strategy allows the class D series resonant inverter to operate nearly at the resonance frequency and enhances the power quality of inverter output power waveform. The class D series resonant inverter here employs constant frequency Pulse Density Modulation (PDM) scheme to reduce the switching losses. It also compares the performance of closed loop output power control using conventional PID controller and Load Adaptive Fuzzy logic controller (LAFLC) for reducing steady state error. To verify the results, the proposed Class D inverter system is designed and simulated using MATLAB/SIMULINK simulation tool. The PDM method is implemented by using the PIC16F84A microcontroller. The proposed system is simulated and validated by experiments in the laboratory. Keywords: Pulse Density Modulation, PID controller, Fuzzy logic controller, Induction heating system, Class D series resonant inverter. Introduction Induction heating is employed in industries due to its various advantages like safety, cleanliness, non-contact and efficiency compared with the other classical methods of heating. Electrical energy supplied to the coil is converted to thermal energy in the work piece through the electromagnetic field, without any physical electrical connection to the work piece. The induced „eddy‟ current intensity is maximum at the surface of the work piece and decreases towards its center as a function on the ratio: thickness/skin depth. As the ratio increases, a greater proportion of the total power is dissipated in the outer surface; this phenomenon is called the skin depth. In general, the electromagnetic induction heating is a clean, non-contact and low loss heating method due to semiconductor devices based on Faraday‟s law and Joule‟s heating principle [1]. In order to have desired temperature level at work piece, IH load has to be controlled smoothly and accurately [2 & 3] by means of electric power that is 3819 International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 6 (2016) pp 3819-3827 © Research India Publications. http://www.ripublication.com Also a new power control based Phase Shift (PS) technique has been implemented in full-bridge inverter fed series resonant load circuit for IH applications, which ensures ZVS operation for all output power levels and load conditions [27]. The above literature, however, does not deal with the power control strategy. In PDM control method, Zero Voltage Switching (ZVS) condition is maintained throughout the operation, which satisfies the variable power requirement of the load with reduced switching losses. Switching losses are kept minimum due to its constant frequency mode of operation. This paper discusses the output power control of Class D resonant inverter fed induction heating system using PDM technique in open loop as well as in closed loop controls. In closed loop control, conventional PID controller performance is compared with load adaptive Fuzzy logic controller for Class D resonant inverter in terms of output power and temperature rise. The paper is organized as follows: In section 2, the description of Class D series resonant inverter topology is presented. In section 3, open loop simulation results of a Class D series resonant inverter are presented. In section 4, Closed loop simulation results with PID controller & Fuzzy Logic Controller (FLC) are presented and compared. In section 5, experimental results of the prototype are presented. Finally, the conclusions of the work are provided in section 6. Consider the equivalent circuit of series resonant inverter as shown in Figure 2. Let Io be the current through induction load, Vo the voltage across induction load, and Vc the voltage across resonant capacitor. Figure 2: Circuit model of series resonant inverter The voltage balance equation is given by Vin = I0R+ L dI/dt + Vc (1) Ic = Cr dVc/dt (2) Under resonant condition, (XL = XC), output power of converter is obtained across the resistor R, Pout = I02R /2 Description of class D inverter topology (3) And the quality factor of the coil is given by, One of the most popular resonant inverters used for induction heating application is Class D series resonant inverter. The circuit configuration of Class D series resonant inverter topology shown in Figure 1 offers more advantages. It is basically an AC-AC conversion circuit for producing high frequency AC (HFAC) from utility frequency AC for induction heating applications. It consists of an uncontrolled rectifier which is used to convert the given single phase AC input voltage (Vin) of 50Hz frequency into its equivalent DC voltage. The DC output obtained from the uncontrolled rectifier is then filtered by using a capacitive filter (C1). Pure DC is now applied to the Class D Series Resonant Inverter to produce HFAC, which is required for induction heating load and the induction pan gets heated up. The two MOSFET switches M1 & M2 are provided with anti-parallel connection of diodes, which are alternately operated with the high frequency PDM pulses generated by logic circuit. Q = ωsL / R (4) Hence, whenever there is a variation in load parameters, it is required to change the operating conditions like duty ratio and operating frequency of the inverter. Open loop control of Class-D Series Resonant Inverter The Class D series resonant inverter fed induction heating system is modeled and simulated by using the MATLAB/ SIMULINK software. The system data used for simulation are given in Table. 1. The Pulse Density Modulation (PDM) pulses for the MOSFET switches M1 & M2 are shown in Figure 3a. By controlling the duty period DPDM of the low frequency signal, the output power can be varied. The value of DPDM considered for this simulation is 80%. Figs. 3b & 3c illustrate the output voltage waveform and RMS voltage waveform for 0. 8 DPDM. The output current waveform and RMS current waveform for 0. 8 DPDM are presented in Figs. 3d & 3e respectively. The output voltage and the output current waveforms are discontinuous in nature, hence the output power and mean power are measured and they are given in Figs. 3f & 3g respectively. The average power output obtained is 80watts since the DPDM= 80%. The value of average output voltage increases with increase in DPDM. Figure 1: Class D Series Resonant Inverter 3820 International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 6 (2016) pp 3819-3827 © Research India Publications. http://www.ripublication.com Table 1: Design parameters 8 6 Pulses Parameter’s Name AC input voltage DC link capacitance Load Inductance Resonant Capacitance Load Resistance Duty ratio Switching Frequency Resonant frequency 2 1 0 -1 Class D inverter 50V (Peak) 90000e-5F 50. 92e-6H 0. 65µF 1Ω 80% 22kHz 20kHz Current (A) 4 Pulses -8 0 0.05 0.1 0.15 0.2 0.25 Time (s) 0.3 0.35 0.4 0.45 0.5 Figure 3d: Output current waveform with PDM technique for DPDM =0. 8 0 0.1 0.2 0.3 0.4 4.5 0.5 4 3.5 0 0.1 0.2 0.3 0.4 3 0.5 2 1 0 -1 0 0.05 0.1 0.15 0.2 Current (A) Pulses Pulses -2 -6 Time (s) 2 1 0 -1 0 -4 Time (s) 2 1 0 -1 2 0.25 0.3 Time (s) 0.35 0.4 0.45 2.5 2 1.5 0.5 1 0.5 0 0.1 0.2 0.3 0.4 0.5 0 Time (s) 0 0.05 0.1 0.15 0.2 0.25 Time (s) 0.3 0.35 0.4 0.45 0.5 0.45 0.5 Figure 3e: RMS current waveform Figure 3a: Switching pulses waveform 100 120 100 50 Power (W) Voltage (V) 80 0 60 40 20 -50 0 -100 0 0.1 0.2 0.3 Time (s) 0.4 -20 0.5 0 0.05 0.1 0.15 0.2 0.25 Time (s) 0.3 0.35 0.4 Figure 3f: Output power waveform with PDM technique for DPDM =0. 8 Figure 3b: Output voltage waveform with PDM technique for DPDM = 0. 8 100 35 90 30 80 70 60 Power (W) Voltage (V) 25 20 15 50 40 30 20 10 10 5 0 0 -10 0 0.05 0.1 0.15 0.2 0.25 0.3 Time (s) 0.35 0.4 0.45 0.5 0 0.05 0.1 0.15 0.2 0.25 Time (s) 0.3 0.35 0.4 Figure 3g: Mean power waveform Figure 3c: RMS voltage waveform 3821 0.45 0.5 International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 6 (2016) pp 3819-3827 © Research India Publications. http://www.ripublication.com Closed loop control of Class-D Series Resonant Inverter With PID Controller Simulink model of closed loop controlled Class D series resonant inverter fed induction heating system with PID controller is carried out by means of MATLAB simulink. Any change in load parameters causes dip in the inverter output voltage. Thus error e(s) is processed by the PID controller. The control signal obtained from the output of PID controller is designed with gains Kp=1e-4, Ki=5e-4 & Kd=1e-7 as follows: C(s) = {Kp + (Ki/s) + SKd}*e(s) (5) The PID controller regulates the output voltage with a large peak overshot, settling time, and the steady state error. The main drawback of this controller is the non-linear variation in IH load parameters. Hence to improve the dynamic response of the system, fuzzy-logic control scheme is proposed. The simulation is done for different values of set power. Output voltage, RMS value, and output current are shown in Figs. 4a, 4b, & 4c respectively. RMS output current, output power waveform, and mean power are shown in Figs. 4d, 4e, & 4f respectively. The RMS current is 3. 5 amperes. The RMS power is approximates to 60 watts. 8 6 Current (A) 4 2 0 -2 -4 -6 -8 0 0.05 0.1 0.15 0.2 0.25 Time (s) Figure 4c: Output current waveform for set power equal to 60 W Figure 4d: RMS output current waveform Figure 4a: Output voltage waveform for set power equal to 60W Figure 4e: Output power waveform for set power equal to 60 W 30 Voltage (V) 25 20 15 10 5 0 0 0.1 0.2 0.3 0.4 0.5 Time (s) Figure 4f: Mean power waveform for set power 60 W Figure 4b: RMS output voltage waveform 3822 International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 6 (2016) pp 3819-3827 © Research India Publications. http://www.ripublication.com Table 2: Rule base with 25 rules With fuzzy controllers In recent years, FLC has been widely used for many domestic and industrial applications because it is defined as a theory of vagueness & uncertainties. It is a non-linear control technique which is employed to the controling of inverter to improve the dynamic response of the system as suggested in the literature [28]. The centroid defuzzification technique is employed in this work. The output power is required to be varied in accordance with reference power set by the user. The output of FLC is used to vary the DPDM of MOSFET switches. FLC with 2 inputs (Error and Change in error) and one output are converted into 5 triangular functions as shown in Figs. 5a, 5b, & 5c respectively. And the Rule Base is given in Table. 2. The surface plot is shown in Figure 5d. e /Δe NB NS Z PS PB NB Z PB NB NB NS NS PS Z NS NS PB Z PB NB Z PB NB PS NB PS PS Z PS PB NS NS PB PS Z Figure 5d: Surface plot The fuzzy controlled Class D series resonant inverter system is also simulated using MATLAB simulink Fuzzy toolbar. Actual power is measured using P-Q block and it is compared with reference power. The error and its derivative are applied to the fuzzy block. The pulse is adjusted such that the actual power is equal to the set power. Output voltage waveform, RMS voltage, output current waveform, RMS output current and output power waveforms are shown in Figs. 5e, 5f, 5g, 5h, & 5i respectively. The mean power is shown in Figure 5j. The comparison of time domain specifications of PID & Fuzzy controlled systems is given in Table 3. It can be seen that fuzzy controlled system settles faster than PID controlled system. The steady state error in power is less with FLC. Figure 5a: Triangular membership function for error 80 60 Figure 5b: Triangular membership function for change in error Voltage (V) 40 20 0 -20 -40 -60 -80 0 0.1 0.2 0.3 0.4 0.5 Time (s) Figure 5e: Output voltage waveform for set power equal to 60W Figure 5c: Triangular membership function for control output 3823 International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 6 (2016) pp 3819-3827 © Research India Publications. http://www.ripublication.com 80 30 70 25 60 Power (W) Voltage (V) 20 15 10 50 40 30 20 10 5 0 0 0 0.1 0.2 0.3 0.4 0 0.05 0.1 0.15 0.2 0.25 Time (s) 0.3 0.35 0.4 0.45 0.5 0.5 Time (s) Figure 5j: Mean power waveform for set power 60 W Figure 5f: RMS output voltage waveform Table 3: Comparison of time domain specifications 8 Controllers Settling time (s) Steady state error (W) 4 PI Controller 1. 5 0. 8 0. 1 FLC 6 Current (A) 4 2 0 -2 Multiple Power Settings using FLC Simulation studies with multiple power settings are also performed. Set power of 60W upto 0. 25 sec and then 20W is used. FLC is adaptive for multiple set powers, whereas PID requires gain adjustment for different output levels. The output voltage waveform, RMS output voltage, output current waveform, and RMS output current are shown in Figures 5k, 5l, 5m, & 5n respectively. The output power waveform and the mean power waveform are shown in Figures 5o & 5p respectively. It is observed from Figure 5p that the power reduces from 60W to 20W. -4 -6 -8 0 0.1 0.2 0.3 0.4 0.5 Time (s) Figure 5g: Output current waveform for set power equal to 60W 3.5 3 80 60 2 40 1.5 Voltage (V) Current (A) 2.5 1 0.5 0 0 20 0 -20 -40 -60 0.1 0.2 0.3 0.4 0.5 -80 Time (s) Figure 5h: RMS output current waveform 0 0.05 0.1 0.15 0.2 0.25 Time (s) 0.3 0.35 0.4 0.45 0.5 Figure 5k: Output voltage waveform for set power equal to 60 W till 0. 25s and then 20W 100 40 80 30 40 Voltage (V) Power (W) 35 60 20 25 20 15 10 0 5 -20 0 0.1 0.2 0.3 0.4 0 0.5 Time (s) 0 0.05 0.1 0.15 0.2 0.25 Time (s) 0.3 0.35 0.4 0.45 0.5 Figure 5l: RMS voltage waveform for set power equal to 60 W till 0. 25s and then 20W Figure 5i: Output power waveform for set power equal to 60W 3824 International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 6 (2016) pp 3819-3827 © Research India Publications. http://www.ripublication.com full bridge uncontrolled rectifier was constructed by using four IN4007 diodes to give DC supply to the Class D inverter. Then Class D inverter was constructed by using two IRF840 MOSFETs for the conversion of DC into HFAC. The control logic for PDM technique was implemented by using a PIC 16F84 micro-controller, which gives driving pulses for MOSFETs 1 & 2. These control pulses were amplified using IR2110 drivers. The output pulses from the driver are shown in Figure 6b. The output voltage is shown in Figure 6c. And the Output voltage along with driving pulse waveform is given in Figure 6d. The 10cm heating coil of gauge 18 having 90 turns per coil and a resistance of 0. 0645 ohms was considered. Temperature was measured using industrial thermometer and it reached 500ºC in 125 seconds for class D inverter system. It could be noticed that the oscillograms were in agreement the simulation results. 8 6 Current (A) 4 2 0 -2 -4 -6 -8 0 0.05 0.1 0.15 0.2 0.25 Time (s) 0.3 0.35 0.4 0.45 0.5 Figure 5m: Output current waveform for set power equal to 60 W till 0. 25s and then 20W 3.5 3 Current (A) 2.5 2 1.5 1 0.5 0 0 0.05 0.1 0.15 0.2 0.25 Time (s) 0.3 0.35 0.4 0.45 0.5 Figure 5n: RMS current waveform for set power equal to 60 W till 0. 25s and then 20W 120 100 Power (W) 80 Figure 6a: Hardware layout of Class D inverter fed IH system 60 40 20 0 -20 0 0.05 0.1 0.15 0.2 0.25 Time (s) 0.3 0.35 0.4 0.45 0.5 Figure 5o: Power waveform for set power equal to 60 W till 0. 25s and then 20W 80 70 60 Power (W) 50 Figure 6b: Driving pulses of switches M1 & M2 (X-axis 100μs/div, Y-axis 10v/div) 40 30 20 10 0 -10 0 0.05 0.1 0.15 0.2 0.25 Time (s) 0.3 0.35 0.4 0.45 0.5 Figure 5p: Mean power waveform for set power equal to 60 W till 0. 25s and then 20W Experimental Results The laboratory model for class D Series resonant inverter fed induction heater system rated 100W was fabricated and tested to validate the PDM scheme. The hardware layout of Class D inverter fed IH system is shown in Figure 6a. The Figure 6c: Output voltage waveform (X-axis 100μs/div, Yaxis 30v/div) 3825 International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 6 (2016) pp 3819-3827 © Research India Publications. http://www.ripublication.com [6] [7] Figure 6d: Channel 2: Driving pulse (X-axis 100μs/div, Yaxis 10v/div), Channel 1: Output voltage(X-axis 100μs/div, Y-axis 50v/div) [8] Conclusion The open loop response of Class D resonant inverter, closed loop response of PID controlled and fuzzy controlled systems were simulated and the results were presented. Hardware implementation of Class D resonant inverter fed induction heating system was performed to study the controller response. 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