Abstract—Resonant converters find a very wide application in

Induction heating, which requires high frequency currents. Series and Parallel resonant inverters are employed for this purpose. This paper gives a comparative analysis of series and parallel resonant inverter fed ferromagnetic load based on experiments carried out and finally concluded which is the suitable inverter for Induction heating application, whether it is Series Resonant Inverter (SRI) or Parallel Resonant Inverter (PRI) based on the experimental results. The results of SRI are compared with that of PRI and presented in this paper.

Keywords—Series Resonant Inverter, Parallel Resonant Inverter, Induction Heating.

I. INTRODUCTION NDUCTION heating is a well known technique to produce very high temperature for applications such as steel melting,

brazing and surface hardening. In each application appropriate frequency must be used depending on the work piece geometry and skin depth requirement [1, 2]. This technique requires high frequency current supply that is capable of inducing high frequency eddy currents in the work piece and thus results in the heating effect [1]. A large number of topologies have been developed in this area among them current-fed and voltage-fed inverters are most commonly used [3]. One of the most important advantages of current-fed inverter is the short circuit protection capability. However the current source inverter can only be controlled by using phase-controlled rectifier for adjusting the DC link. This differs from voltage source inverter which has various controls.

Recent developments in switching schemes and control methods have made the voltage source resonant inverters widely used in several applications require output power control. In pulse frequency modulation (PFM) the output power can be controlled by varying the switching frequency and it is operated at under zero voltage switching scheme [4]. The pulse density modulation (PDM) scheme regulates the output power by varying the period in which the inverter supplies high frequency current to the induction coil [5]. The phase shift (PS) control technique in [6] varies output power by shifting the phase of the switch conduction sequences while the asymmetrical duty cycle (ADC) control technique

A. Suresh is Research Scholar Sathyabama University, Chennai, India S. Rama Reddy is Professor, Jerusalem College of Engineering, Chennai,

India.

employs an unequal duty cycle operation of the switches in the convertor [7]. The asymmetrical voltage cancellation (AVC) is proposed in [8] where the author’s describes voltage cancellation for conventional fixed-frequency control strategies. In induction heating application the output power control using the mentioned techniques in fixed frequency and optimum duty cycle for ZVS operation are rather difficult due to variation of parameters in resonance load. In [9] AVC is implemented in full bridge series resonant inverter. The SRI needs an output power for matching the output power to the load and also it imposes restriction on bandwidth. The foresaid drawbacks in SRI can be overcome in PRI which is been justified by simulation results presented in the current paper.

II. PARALLEL RESONANT INVERTER The PRI is shown in Fig1a.The inverter consists of two

switches S1 and S2 with blocking diodes D1 and D2, a resonating capacitor C, a DC inductor L1 and an induction coil that comprises of a series combination of resistance R and coil inductance L. The full bridge inverter is based on the use of quasi square wave control technique. This control method uses the phase lock loop to automatically adjust the inverter’s operating frequency to a small constant leading phase angle when load parameters change.

Fig 1a A parallel class D current source inverter

Fig 1b Changing series circuit to parallel circuit

To simplify the calculation of necessary circuit parameters, series combination of Lcoil and Req is transferred to its equivalent parallel configuration of Lcoil and Rp as shown in Fig1b.The Rp is given as

Series and Parallel Resonant Inverter Fed Ferromagnetic Load-A Comparative Analysis

A. Suresh and S. Rama Reddy

I

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(1)

where ω is the system switching frequency. The total impedance ZTotal of the resonant circuit in Fig1a can be expressed as,

(2)

and the resonant frequency is given as , (3)

Note that the inverter is designed to operate such that the switching frequency is slightly higher than the resonant frequency for maximum output power. The parallel impedance at resonant frequency ωo is

(4)

And the average output power P is provided as

(or) (5)

III. SIMULATION RESULTS PRI FED FERROMAGNETIC LOAD Fig.2.a shows the simulink circuit of current source inverter

for ferromagnetic load. For induction heating class D inverter is used. This inverter converts DC input power into AC output power. This conversion is achieved by turning on and off alternately switches 1 and 2.

Fig.2.a Circuit Diagram

Fig.2.b shows the AC input voltage. Driving pulses and output of MOSFET1 are shown in Fig 2c. Driving pulses and output MOSFET2 are shown in Fig 2d. Output voltage of the inverter is shown in Fig 2e.Output voltage is sinusoidal due to resonance action.

Fig.2.b AC input voltage

Fig.2.c. Vgs and Vds across switch1

Fig.2.d. Vgs and Vds across switch2

- Fig.2.e Output voltage

Fig.3.a Circuit Diagram of open loop system

The open loop operating system makes use of PRI. The output voltage of the inverter is sinusoidal, in the case of low damping factor and the operating frequency near resonant frequency to achieve maximum power across output and for soft- switching operation as shown in Fig2. But the harmonic effect of the voltage and current are not considered while the output contains harmonics in this technique. The AC output of the inverter and the RMS value are shown Fig.3.b and Fig.3.c respectively.

Fig.3.b AC input voltage with disturbance

Fig.3.c Inverter Output voltage

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Fig.3.d Rectifier Output voltage

IV. CLASS D SRI A class D inverter is generally used to energize the

induction coil to generate high frequency magnetic induction. Fig.4 shows the class D inverter system for induction heating. The class D inverter consists of two switches S1 and S2, a resonant capacitor C1, inductor L1 and an induction coil. Induction coil is represented as parallel combination of inductor L2 and resistor R. Fig.5 represents the equivalent circuit model of the class D inverter.

The currents in positive and negative half cycles are as follows:

(6)

(7)

Fig.4: Class D inverter system for IH

Fig.5. Equivalent circuit

V. SIMULATION RESULTS OF SRI FED FERROMAGNETIC LOAD

Class D inverter simulink circuit is shown in Fig.3a. For induction heating class D inverter is used. This inverter converts DC input power into AC output power. This conversion is achieved by turning on and off alternately. Switches 1 and 2 .The voltage across the load is measured with the help of voltage measurement block and the output current is measured with the help of current measurement block and they are observed using a scope. Driving pulses given to the MOSFET are shown in Fig. 3b. Output voltage and current are shown in Fig. 3c. Both are sinusoidal due to the presence of L and C. The variation of output with the variation in the input is shown in Fig. 3d. The output voltage increases with the increase in the input voltage.

The circuit of open loop system is shown in Fig.6.The rectifier output voltage increases due to the step rise in input.

The output voltage of the inverter is measured with the help of voltmeter and it is observed using a scope. Similarly the current through the load is measured with the help of ammeter and it is observed using a scope. Input voltage and output voltage with a disturbance are shown from Fig.6.b to 6.f

Fig. 6.a: Circuit diagram

Fig.6.b: Output voltage and current

Fig. 6.c: Circuit diagram of open loop system

Fig.6.d: Input voltage with disturbance

Fig. 6.e: Output voltage with disturbance RMS Value)

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Fig.6.f: Rectifier output voltage

VI. EXPERIMENTAL RESULTS The Experimental set up is as shown in Fig.7. Parameters

are listed in Table.1 are presented from Fig.8 to Fig.10. The variation of output power with input is shown in Fig8. The variation of output voltage with input voltage is shown in Fig9. Variation of output power with resistance is shown in Fig10.

Fig.7 Experimental Set Up

TABLE I SYSTEM PARAMETERS

Parameter Symbol

Value

Switching Frequency

f 35 KHz

Resonant Capacitor C 100 KpF

Induction Coil Inductor

L 12 µH

DC Inductor L1 6 µH

Fig.8 Input Voltage v/s Output Power

Fig.9 Input Voltage v/s Output Voltage

Fig.10 Output Resistance v/s Output Power

VII. CONCLUSION From Fig.8 and Fig.9, it is observed that output power and

output voltage of Parallel resonant inverter are higher than that of series resonant inverter. Therefore Parallel Resonant Inverter is a better choice for Induction Heating application.

REFERENCES [1] Chudjuarjeen.S,et.al, “ Full bridge current fed inverter with automatic

frequency control for forging applications”,IEEE tecon 2004,Vol 4,pp.128-131,Nov 2004.

[2] C.Chunwattanapraniti,et.al “Half bridge current fed inverter power supply for Forging Applications,”25th electrical Engineering Conference,Thailand,pp.97-101,2002.

[3] E.J.Davies,J. and Simpson,P.,1979,Induction Heating Handbook,McGraw-Hill,UK.

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[4] M.K.Kazimierczuk and D.Czarkowski, Resonant Power Converter.New York:Wiley,1995.

[5] Viriya,et.al, “Analysis of High frequency Induction cooker with variable frequency power control,” Power Conversion Conference,2002.PCC Osaka2002.Procedings of the Vol.3,5-2 April 2002 Page(s)1507-1512 Vol.3

[6] Nam-Ju Park,et.al “A power control scheme with constant switching frequency in class- D inverter for induction heating jar application,”IEEE Tran.Ind Electronics,vol54,no 3 pp1252-1260, June,2007.

[7] L.Grajales and F.C.Lee, “Control system design and small-signal analysis of a phase shift controlled series resonant inverter for induction heating,”in Proc.IEEE Power Electron.Spec.Conf, 1995, pp.450-456.

[8] J.T.Matysik “A new method of integration control with instantaneous current monitoring for class D series resonant converter,” IEEE Tran.Ind Appl, vol53, no 5 pp1564-1576, Oct, 2006.

[9] H.Sugimura, H.Muraoka,T.Ahmed,S.Chandhaket ,E.Horaki, M.Nakanoda and H.W.Lee, “ Dual mode phase shifted ZVS-PWM series load resonant high frequency inverter for induction heating super heated steamer,”J.Power Electron,vol 4, no 3, pp 138-151,Jul 2004.

[10] A.Suresh and S.Rama Reddy , “Parallel resonance based current source inverter for induction heating,” European journal of scientific research Vol 58 No 2 ,pp 148-155,Aug 2011.

[11] A.Suresh and S.Rama Reddy , “Comparison of simulation and experimental results of class D inverter fed induction heater,” Research journal of applied sciences,Engineering and Technology 2(7) ,pp 635-641 ,Oct 2010.

A. Suresh is a research scholar at Sathyabama University, Chennai. His area of interest is Induction Heating. He has a decade of teaching experience in engineering college and a life member in ISTE.

Dr. S. Rama Reddy obtained his ME degree from Anna University, Tamil Nadu, India, in 1989. He has pursued research in the area of resonant converters in 1995. He has 2 years of industrial experience and 18 years of teaching experience. He is a life member of IE, IETE, ISTE, SSI, and SPE. He has published 20 papers in the area of Power Electronics and FACTs.

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