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High frequency inverter design for large-signal characterization of domestic
induction heating load
Abstract –
Current research topics in the field of domesticinduction heating entail analytical study of inductor-potcoupling behavior which must be verified with experimentalresults. Small-signal measurements are easily achieved up to 1MHz with commercial LCR meters. On the other hand, largesignalexperimentation requires a custom made measurementtest bench. Whatever the approach selected to design thissystem, the instrumentation will be divided into a signalgenerator, a data acquisition block and a post-processingmethod for model parameter extraction. In order to separateinfluence of main factors determining load electrical parametervalues a good solution could be sinusoidal current excitationwith selectable frequency and amplitude. The scheme proposedis the utilization of a high frequency resonant half-bridgeinverter operating slightly above resonance under Zero VoltageSwitching conditions to acquire near sinusoidal current. Outputpower control is carried out through of a variable DC bus andresonant conditions are fixed by means of an adjustableresonant capacitor. This paper enumerates the designchallenges that such inverter involves in this application andwhich are the solutions adopted. Final prototype covering therequired operation range has been designed, constructed andevaluated up to 1.5 MHz for a given active power of 3.5 kW atresonance with power capability of 16.8 kVA at Megahertzfrequencies.
I. INTRODUCTION
In domestic induction heating systems heat is directlyproduced at the bottom of the pot through generation ofmedium-frequency magnetic field by means of a planarinduction coil. Thanks to induced Eddy currents andferromagnetic hysteresis losses phenomena this process canbe safely achieved with small energy losses, reduced thermalinertia and low hob temperature [1].In order to get the alternating magnetic field it is necessaryto make current to flow through the inductor by using aDC/AC inverter which aimed to system global efficiencyimprovement is usually arranged as a resonant stage. Typicalemployed topology is the series-resonant half-bridge inverter.Output power is traditionally regulated by means of acontrol mode exhibiting Zero Voltage Switching (ZVS) suchas frequency control or Asymmetrical Duty Cycle (ADC)control.The inductor-pot couple impedance represents the load forthe power converter. It depends on several parameters such asgeometry, kind of winding wire (solid, Litz or twisted),temperature, pot material and current value.Typical load modeling has been usually carried out bymeans of a simple series connection of Req and Leq. Resistiveterm can be split up into several contributions representingeffective energy transferred to pot and inductor losses.For those applications where first harmonic approximationis possible, it is sufficient to evaluate load at switchingfrequency for simulation or calculation. Sometimes, suchapproach is not possible and a complete load description isrequired in order to get not only an energetic agreement butalso reliable time waveforms reproduction. This point iscritical when evaluating switching performance of invertercontrol strategies.Several studies have been carried out in order to obtaingeneralized analytical expressions describing small-signalload parameters behavior as a function of frequency whichhave been checked with precision impedance analyzersmeasurements [2]. Results describe techniques for inductoroptimization in terms of efficiency maximization.Such load frequency dependence can be described using asimple network of frequency dependent component or acomplex network composed of constant components. Thesecond approach is much more appropriated for simulation.Characterization is the process of measuring the basicelectrical parameters of the load such impedance as a functionof its determining factors. Later, model parameters can beidentified with those measurements.More complete load models are required for precisesystem design so other factors apart from frequency such asnon-linearity or temperature influence must be incorporated.Such experimental characterization entails the design of acustom-made large-signal test bench. In order to reducedistortion factor influence at measurements, a one-frequencyamplitude-controllable large-signal generator is required.System must be completed with a data acquisition hardwareand a post-processing algorithm for model parameterextraction.R (f) eq L (f) eqZ (f) eq R LCZ (f) eq(a) (b)Fig. 1. (a) Single frequency load model, (b) Exact linear load modeling.1-4244-0136-4/06/$20.00 '2006 IEEE 2706The presented application extends frequency range ofinterest from 1 kHz to 1 MHz and delivered active power atload up to 3500 W covering most part of typical domesticinduction operation conditions. In order to avoid powerdissipation problems associated with linear amplification thesystem proposed is based on the utilization of a voltage-fedseries-resonant half-bridge inverter for load exciting as shownin Fig. 2. Similar solutions are adopted in other fieldsrequiring RF power sources [3].Operating conditions are selected to achieve Zero VoltageSwitching (ZVS) at turn-on. Frequency of operation isselected close to resonance in order to get low currentdistortion being preferable class DE switching conditions inorder to minimize losses [4, 5]. Therefore, power control isperformed independently of switching frequency by means ofa variable DC-bus while triggering waveforms aresynthesized using a Field Programmable Gate Array (FPGA)which guarantees precision required at highest frequencyvalues [6]. Finally, an adjustable resonant capacitor bankenables tuning over the frequency range of interest.As load impedance is previously unknown in bothresistance and quality factor (Q) values a wide assortment ofsituations must be taken into account. These aspects includea large range of input bus voltage and a variety of turn-offand peak current values at switches. As a result, large dv/dtvalues are also possible. This paper shows the mainchallenging points in designing a versatile inverter wheresimultaneous high output power and wide frequency rangeare required.