Member, IEEE O Lucía

Member, IEEE P Maussion

Member, IEEE E Dede

Senior Member, IEEE. J M Burdío

Induction Heating Technology and its Applications: Past Developments, Current Technology, and Future Challenges

Keywords: Induction heating, resonant power conversion, electromagnetic analysis. I. Enabling technology Application Power Electronics Modulation and Control Algorithms Magnetic Components Industrial High power. Improved reliability. Assembly-line read. Low-high operating frequency. Multi-zone control algorithm. Improved interface and communications. Variable load and power ranges. Multi-load management. Temperature control. High efficiency. Variable shape. Optimized heat distribution. Domestic management. Temperature control. High efficiency. Heat non-ferromagnetic materials. Flexible cooking surfaces. Medical Low power. High quality factor resonant tank. High operating frequency. Accurate power and temperature control. Frequency selection. Local heating. Controlled magnetic field interactions. Ferromagnetic fluids

Induction heating technology is nowadays the heating technology of choice in many industrial, domestic, and medical applications due to its advantages regarding efficiency, fast heating, safety, cleanness, and accurate control. Advances in key technologies, i.e. power electronics, control techniques, and magnetic component design, have allowed the development of highly reliable and cost-effective systems, making this technology readily available and ubiquitous. This paper reviews induction heating technology summarizing the main milestones in its development, and analyzing the current state-of-art of induction heating systems in industrial, domestic and medical applications, paying special attention to the key enabling technologies involved. Finally, an overview of future research trends and challenges is given, highlighting the promising future of induction heating technology.

INTRODUCTION

Induction heating (IH) provides contactless, fast, and efficient heating of conductive materials. It is becoming one of the preferred heating technologies in industrial [START_REF] Lozinskii | Industrial Applications of Induction Heating[END_REF], domestic [START_REF] Moreland | The induction range: its performance and its development problems[END_REF], and medical applications [START_REF] Stauffer | Magnetic Induction Heating of Ferromagnetic Implants for Inducing Localized Hyperthermia in Deep-Seated Tumors[END_REF], among other applications, due to its advantages when compared with other classical heating techniques such as flame heating, resistance heating or traditional ovens or furnaces.

Fig. 1 shows a typical arrangement of an induction heating system [START_REF] Brown | Theory and Application of Radio-Frequency Heating[END_REF][START_REF] Cable | Induction and Dielectric Heating[END_REF][START_REF] Rudnev | Handbook of induction heating[END_REF][START_REF] Davies | Conduction and Induction heating[END_REF] in a longitudinal flux configuration. An ac source is used to supply an alternating voltage to an induction heating coil. The coil generates an alternating magnetic field, in which the induction target, i.e. the load, is immersed. As a consequence, the induction target is heated by means of two physical phenomena: eddy currents and magnetic hysteresis [START_REF] Brown | Theory and Application of Radio-Frequency Heating[END_REF]. Eddy currents oppose to the magnetic field applied to the induction target, and they produce the heating by Joule effect. This is commonly the main heat source in IH processes. In addition to this, magnetic hysteresis creates additional heating in ferromagnetic materials. The typical operating frequencies of these systems ranges from line frequency, e.g. industrial and high power applications, up to a few MHz's, typical of medical systems.

Among the advantages of IH, the following ones are commonly recognized:

Fast heating: IH technology directly heats the induction target, reducing wasted heat and significantly reducing heating times thanks to high power densities and without any thermal inertia. Efficiency: Modern efficient designs of the coil and the power converter allows obtaining efficiency values higher than 90%, significantly improving conventional heating techniques. Moreover, since only the induction target is heated, the heat loss through the ambient and surrounding elements is minimized and high temperatures can be reached. Controlled heating: The power applied by the IH system and the location can be accurately controlled through the appropriate design of the coil and the power converter and its control. As a consequence, advanced features can be implemented such as local heating, predefined temperature profiles, etc. Improved industrial process: IH consistency and repeatability improves the quality process and maximizes the productivity of the process. Moreover, since IH is a contactless heating process, the induction target is not affected by the heating tool, i.e. the coil, and the quality is ensured.

Cleanness and safety: Since IH heats directly the induction target, the temperature of the surroundings of the heating area is lower, avoiding burning other materials, such as spilled food in the case of domestic IH. Moreover, there is no local pollution unlike fossil fuel furnaces.

These advantages, and the progress in IH technology achieved in recent years, have boosted applications of IH that can be classified into industrial, domestic, and medical applications. The main enabling technologies that have allowed this progress are power electronics, modulation and control algorithms, and magnetic component design. Table summarizes the main differential characteristics of the main IH applications and the enabling technologies involved.

Although all IH applications share the main fundamental principle, they have differential characteristics that have to be addressed through the technologies involved in each design. Industrial applications require usually higher output powers and higher reliability, constraining the power converter topology selection. Besides, since they have intensive industrial usage, assembly-line readiness and an improved interface and communications are required. In addition to this, the inductor design needs to adapt to the different shapes of induction targets, e.g. shafts, gears, etc., and provide the required heat profile.

By contrast, domestic IH systems that are usually implemented in induction cookers require low cost and highly efficient implementations due to the limited cooling capabilities. Besides, the wide load range due to the different materials, geometries and required output powers is the main challenge from the control point of view. The inductor system must be designed also to obtain high-efficiency and be able to heat non-ferromagnetic materials typical of some cultures. Finally, medical applications require specific but vital requirements to IH systems. These systems are usually low power, but they require very accurate control of the heating process, including temperature and localization.

The next lines give a brief outline of the evolution of IH systems and its applications.

II. HISTORY OF INDUCTION HEATING SYSTEMS

IH technology has been in constant evolution following industrial and technological advances since the end of XIX century [START_REF] Mühlbauer | History of Induction Heating and Melting[END_REF]. The principle of induction heating was discovered by Michael Faraday when he discovered induced currents by a magnet. James C. Maxwell developed later the unified theory of electromagnetism and James P. Joule described the heat produced by a current in a conductor, establishing the fundamental principles of induction heating.

The first industrial applications of the IH phenomenon were identified in 1887 by Sebastian Z. de Ferranti, who proposed IH for melting metals, filling the first patent on industrial applications of IH. Later, in 1891, F.A. Kjellin presented the first fully functional induction furnace. The first major advance came when Edwin F. Northrup implemented the first high-frequency induction furnace at Princeton in 1916. Nearly at the same time, M.G. Ribaud developed highfrequency IH technology using spark-gap generators and, later, Valentin P. Vologdin developed IH generators using machine generators and vacuum tubes. These were the beginnings of modern high-frequency induction heating systems. During WWII and later, automotive and aircraft industry boosted the use of IH technology not only for melting metals but also for advanced material treatment, significantly increasing the IH technology penetration on industrial processes. The second major revolution of IH technology came along with the development of solid-state generators. These generators took advantage of new power semiconductor technology, mainly thyristors, to implement highly reliable power converters. Later, the development of higher frequency power devices, such as the power bipolar junction transistor (BJT) and the power metal-oxide-semiconductor field-effect transistor (MOSFET), enabled the design of higher efficiency power converters, making IH the technology of choice in many applications. The grade of performance and efficiency achieved in IH systems, together with further advances in semiconductor technology and the introduction of the successful insulated-gate bipolar transistor (IGBT), expanded the applications of IH technology beyond the industrial environment. Since late 80s, many domestic applications of IH have appeared, and nowadays induction heating cookers are well-established in many countries. Moreover, since late 80s and, specially, in the XXI century, a special interest of IH for medical applications has appeared due to its advantages in terms of precise and local heating for hyperthermia treatment.

Currently, IH technology is evolving fast towards highly reliable and efficient systems, allowing implementing highly versatile systems that makes IH ubiquitous. Next section summarizes the state-of-art of the enabling technologies that makes possible such systems.

III. ENABLING TECHNOLOGIES

Advances in key technologies have made possible the development of IH technology [START_REF] Stansel | Induction Heating[END_REF]. This section summarizes the current state-of-art of the main enabling technologies commonly involved in IH systems: power electronics, modulation and control algorithms, and magnetic component design. A selection of references summarizing the state-of-art is given in next subsections, and they are also summarized in Table II according to the application that they focus on.

A. Power electronics

Fig. 2 shows the main power conversion scheme present in most of the single coil IH systems. Firstly, an electromagnetic compatibility (EMC) filter ensures that the power converter complies with the electromagnetic standards. After that, an ac-dc converter provides a dc-bus to supply the inverter block. The rectifier stage can be either a noncontrolled stage, i.e. diode rectifier, or a controlled one. The latter implementation is used to provide an additional degree of freedom for the control system, and can be implemented either as a controlled rectifier [START_REF] Yilmaz | Medium-frequency induction melting furnace as a load on the power system[END_REF] or as a diode rectifier plus a dc-dc converter [START_REF] Pham | Phase angle control of highfrequency resonant currents in a multiple inverter system for zone-control induction heating[END_REF]. Depending on the applications, some IH systems also include a power factor corrector block in order to increase voltage and ensure sinusoidal input current [START_REF] Kawaguchi | A comparative evaluation of DCM control and CCM control for soft-switching PFC converter[END_REF].

The dc-ac power converter, also known as inverter, is the most important one and has to supply medium frequency currents to supply the inductor [START_REF] Dawson | A comparison of load commutated inverter systems for induction heating and melting applications[END_REF]. The operating frequency is usually higher than 20 kHz in order to avoid audible noise and rises up to 1 MHz depending on the application. Currently, most IH systems feature either voltage source or current source resonant inverters [START_REF] Kazimierczuk | Resonant Power Converters[END_REF] in order to obtain efficient and high-power-density implementations. The induction heating load is usually modeled as an equivalent resistor Req and inductor Leq (Fig. 3 (a)), which may be connected in series or parallel, depending on the model, and additional external inductors and/or capacitors are added to complete the resonant tank. The resonant inverter topology used can be classified either considering the type or resonance used or the number of switching devices.

Considering the resonant tank, the most used configurations are the second-order series resonant (Fig. 3 (b)) and parallel resonant circuits (Fig. 3 (c)), and the third-order LLC series-parallel resonant circuit (Fig. 3 (d)). The series resonant RLC circuit [START_REF] Esteve | Improving the efficiency of IGBT series-resonant inverters using pulse density modulation[END_REF], [START_REF] Lucía | Efficiency oriented design of ZVS half-bridge series resonant inverter with variable frequency duty cycle control[END_REF] is commonly used in voltage source inverters and ensures zero mean current through the inductor, due to the series capacitor, and zero voltage switching (ZVS) conditions above the resonant frequency, i.e. zero voltage across the device during the switching process that ideally leads to zero switching losses. By contrast, the parallel-resonant RLC circuit [START_REF] Steigerwald | A comparison of half-bridge resonant converter topologies[END_REF] is used in current source inverters, achieving reduced current through the switching devices and zero current switching (ZCS), i.e. switching with zero current through the device that leads to ideally zero switching losses. For this reason, this topology is chosen when high inductor current is demanded in order to reduce stress in the power devices. Finally, the series-parallel LLC circuits combines the benefits of parallel resonance with additional load short-circuit protection [START_REF] Huerta | Design of the L-LC resonant inverter for induction heating based on its equivalent SRI[END_REF], [START_REF] Chudjuarjeen | An improved LLC resonant inverter for induction-heating applications with asymmetrical control[END_REF], making it one of the most used topologies for high power industrial applications.

TABLE II. SELECTED REFERENCES CLASSIFIED ACCORDING THE FOCUS ON APPLIACIONS AND ENABLING TECHNOLOGIES

Enabling technology

Application

Power Electronics Modulation and Control Algorithms Magnetic Components

Industrial [START_REF] Lozinskii | Industrial Applications of Induction Heating[END_REF], [START_REF] Brown | Theory and Application of Radio-Frequency Heating[END_REF], [START_REF] Cable | Induction and Dielectric Heating[END_REF], [START_REF] Esteve | Improving the efficiency of IGBT series-resonant inverters using pulse density modulation[END_REF][START_REF] Huerta | Design of the L-LC resonant inverter for induction heating based on its equivalent SRI[END_REF][START_REF] Fujita | A new zone-control induction heating system using multiple inverter units applicable under mutual magnetic coupling conditions[END_REF][START_REF] Yilmaz | Medium-frequency induction melting furnace as a load on the power system[END_REF][START_REF] Chudjuarjeen | An improved LLC resonant inverter for induction-heating applications with asymmetrical control[END_REF][START_REF] Ahmed | High-frequency soft-switching ac conversion circuit with dualmode PWM/PDM control strategy for high-power IH applications[END_REF][START_REF] Rodriguez | Nonresonant and resonant frequency-selectable induction-heating targets[END_REF][START_REF] Rodriguez | A multilevel inverter topology for inductively coupled power transfer[END_REF][START_REF] Stansel | Induction Heating[END_REF][START_REF] Dawson | A comparison of load commutated inverter systems for induction heating and melting applications[END_REF][START_REF] Koertzen | Design of the half-bridge series resonant converters for induction cooking[END_REF][START_REF] Kamli | A 50-150 kHz half-bridge inverter for induction heating applications[END_REF][START_REF] Dede | 25-kW/50-kHz generator for induction heating[END_REF][START_REF] Esteve | Induction heating inverter with simultaneous dual-frequency output[END_REF][START_REF] Kazimierczuk | Resonant Power Converters[END_REF][START_REF] Steigerwald | A comparison of half-bridge resonant converter topologies[END_REF][START_REF] Pham | Phase angle control of highfrequency resonant currents in a multiple inverter system for zone-control induction heating[END_REF][START_REF] Pham | Estimating method of heat distribution using 3-D resistance matrix for zone-control induction heating systems[END_REF][START_REF] Pham | Dynamic analysis and control for resonant currents in a zone-control induction heating system[END_REF].

[25], [START_REF] Pham | Estimating method of heat distribution using 3-D resistance matrix for zone-control induction heating systems[END_REF], [START_REF] Fujita | Pulse-density-modulated power control of a 4 kW, 450 kHz voltage-source inverter for induction melting applications[END_REF][START_REF] Fujita | Control and performance of a pulse-density-modulated series-resonant inverter for corona discharge processes[END_REF][START_REF] Egalon | Multiphase system for metal disc induction heating: Modeling and RMS current control[END_REF][START_REF] Park | A power-control scheme with constant switching frequency in class-D inverter for induction-heating jar application[END_REF][START_REF] Nguyen Kien | Resonant control of multi-phase induction heating systems[END_REF][START_REF] Navarro | High-level synthesis for accelerating the FPGA implementation of computationallydemanding control algorithms for power converters[END_REF].

[26], [START_REF] Egalon | Multiphase system for metal disc induction heating: Modeling and RMS current control[END_REF], [START_REF] Dughiero | A translational coupled electromagnetic and thermal innovative model for induction welding of tubes[END_REF][START_REF] Caricchi | Axial-flux permanent-magnet generator for induction heating gensets[END_REF][START_REF] Benato | Proximity effect and magnetic field calculation in GIL and in isolated phase bus ducts[END_REF][START_REF] Candeo | Multiphysics modeling of induction hardening of ring gears for the aerospace industry[END_REF][START_REF] Aliferov | Coupled magneto-thermal FEM model of direct heating of ferromagnetic bended tubes[END_REF][START_REF] Tudorache | Magneto-thermal-motion coupling in transverse flux heating[END_REF][START_REF] Fishman | Induction heating device and process for controlling temperature distribution[END_REF][START_REF] Uchida | Induction heating method and unit[END_REF][START_REF] Hurley | Induction heating of circular ferromagnetic plates[END_REF][START_REF] Kazimierczuk | High-Frequency Magnetic Components[END_REF].

Domestic

[2], [START_REF] Forest | Frequency-synchronized resonant converters for the supply of multiwindings coils in induction cooking appliances[END_REF][START_REF] Forest | Principle of a multiload/single converter system for low power induction heating[END_REF][START_REF] Lucía | A versatile power electronics test-bench architecture applied to domestic induction heating[END_REF][START_REF] Burdío | A two-output series-resonant inverter for induction-heating cooking appliances[END_REF][START_REF] Lucía | Series-resonant multiinverter for multiple induction heaters[END_REF][START_REF] Lucía | Multiple-output resonant matrix converter for multiple induction heaters[END_REF][START_REF] Lucía | Efficiency oriented design of ZVS half-bridge series resonant inverter with variable frequency duty cycle control[END_REF][START_REF] Sarnago | Class-D/DE dual-modeoperation resonant converter for improved-efficiency domestic induction heating system[END_REF][START_REF] Sarnago | High efficiency ac-ac power electronic converter applied to domestic induction heating[END_REF][START_REF] Fujita | Latest developments of high-frequency series load resonant inverter type built-in cooktops for induction heated all metallic appliances[END_REF][START_REF] Ahmed | Boost-half-bridge edge resonant soft switching PWM high-frequency inverter for consumer induction heating appliances[END_REF][START_REF] Kifune | Quasi-series-resonant-type softswitching phase shift modulated inverter[END_REF][START_REF] Ogura | Performance evaluation of edge-resonant ZVS-PWM highfrequency inverter using trench-gate IGBTs for consumer induction cooking heater[END_REF][START_REF] Sugimura | Highfrequency resonant matrix converter using one-chip reverse blocking igbt-based bidirectional switches for induction heating[END_REF][START_REF] Wang | Induction-heated cooking appliance using new quasi-resonant ZVS-PWM inverter with power factor correction[END_REF][START_REF] Saha | Commercial frequency AC to high frequency AC converter with boost-active clamp bridge single stage ZVS-PWM inverter[END_REF][START_REF] Jung | Dual half bridge series resonant inverter for induction heating appliance with two loads[END_REF][START_REF] Sarnago | High efficiency parallel quasi-resonant current source inverter featuring SiC MOSFETs for induction heating systems with coupled inductors[END_REF][START_REF] Acero | Domestic Induction Appliances[END_REF][START_REF] Koertzen | A comparative study of single switch induction heating converters using novel component effectivity conceps[END_REF].

[33], [START_REF] Lucía | Efficiency oriented design of ZVS half-bridge series resonant inverter with variable frequency duty cycle control[END_REF], [START_REF] Acero | Domestic Induction Appliances[END_REF], [START_REF] Nguyen-Quang | Single phase matrix converter for radio frequency induction heating[END_REF][START_REF] Artigas | Power measurement by output-current integration in series resonant inverters[END_REF][START_REF] Barragán | Efficiency optimization in ZVS series resonant inverters with asymmetrical voltage-cancellation control[END_REF][START_REF] Burdío | Asymmetrical voltage-cancelation control for full-bridge series resonant inverters[END_REF][START_REF] Lucía | Series resonant multi-inverter with discontinuous-mode control for improved light-load operation[END_REF][START_REF] Lucía | Load-adaptive control algorithm of half-bridge series resonant inverter for domestic induction heating[END_REF][START_REF] Sarnago | Modulation scheme for improved operation of a RB-IGBT based resonant inverter applied to domestic induction heating[END_REF][START_REF] Lucía | Real-time FPGA-based hardware-in-the-loop simulation test-bench applied to multiple output power converters[END_REF][START_REF] Navarro | Synchronous FPGA-based implementations of digital pulse width modulators[END_REF][START_REF] Kawaguchi | A comparative evaluation of DCM control and CCM control for soft-switching PFC converter[END_REF][START_REF] Tian | A half-bridge series resonant inverter for induction cookers using a novel FPGA-based control strategy[END_REF][START_REF] Paesa | Adaptive Simmering Control for Domestic Induction Cookers[END_REF][START_REF] Jiménez | FPGA-based test-bench for resonant inverter load characterization[END_REF][START_REF] Primiani | Rigorous electromagnetic model of an induction cooking system[END_REF][START_REF] Jiménez | FPGA-based gain-scheduled controller for resonant converters applied to induction cooktops[END_REF].

[42], [START_REF] Kazimierczuk | High-Frequency Magnetic Components[END_REF], [START_REF] Forest | Principle of a multiload/single converter system for low power induction heating[END_REF], [START_REF] Acero | Domestic Induction Appliances[END_REF], [START_REF] Primiani | Rigorous electromagnetic model of an induction cooking system[END_REF], [START_REF] Acero | Modeling of planar spiral inductors between two multilayer media for induction heating applications[END_REF][START_REF] Acero | Analytical equivalent impedance for a planar circular induction heating system[END_REF][START_REF] Acero | Simple resistance calculation in litz wire planar windings for induction cooking appliances[END_REF][START_REF] Carretero | Computational modeling of two partly-coupled coils supplied by a double half-bridge resonant inverter for induction heating appliances[END_REF][START_REF] Acero | Mutual impedance of small ring-type coils for multi-winding induction heating appliances[END_REF][START_REF] Acero | Analysis and modeling of planar concentric windings forming adaptable-diameter burners for induction heating appliances[END_REF][START_REF] Carretero | Frequencydependent modeling of domestic induction heating systems using numerical methods for accurate time-domain simulation[END_REF][START_REF] Acero | Magnetic vector potential based model for eddy-current loss calculation in round-wire planar windings[END_REF][START_REF] Wojda | Winding resistance of litz-wire and multistrand inductors[END_REF][START_REF] Wojda | Analytical optimization of solid-roundwire Windings[END_REF][START_REF] Bartoli | Modeling Litz-wire winding losses in high-frequency power inductors[END_REF].

Medical [START_REF] Stauffer | Magnetic Induction Heating of Ferromagnetic Implants for Inducing Localized Hyperthermia in Deep-Seated Tumors[END_REF], [START_REF] Landi | On the energy conversion efficiency in magnetic hyperthermia applications: A new perspective to analyze the departure from the linear regime[END_REF][START_REF] Tai | A compact half-bridge heating system for magnetic nanoparticle thermotherapy applications[END_REF][START_REF] Tai | The design of a half-bridge series-resonant type heating system for magnetic nanoparticle thermotherapy[END_REF].

Industrial and domestic techniques applied.

[43], [START_REF] Gomez-Polo | Analysis of heating effects (magnetic hyperthermia) in FeCrSiBCuNb amorphous and nanocrystalline wires[END_REF][START_REF] Cano | An induction heater device for studies of magnetic hyperthermia and specific absorption ratio measurements[END_REF][START_REF] Candeo | Numerical FEM models for the planning of magnetic induction hyperthermia treatments with nanoparticles[END_REF][START_REF] Barba | Synthesizing distributions of magnetic nanoparticles for clinical hyperthermia[END_REF][START_REF] Barba | Coupled field synthesis in magnetic fluid hyperthermia[END_REF][START_REF] Stauffer | Practical induction heating coil designs for clinical hyperthermia with ferromagnetic implants[END_REF][START_REF] Barba | Magnetic field synthesis in the design of inductors for magnetic fluid hyperthermia[END_REF].

L eq R eq C r L eq R eq C r L eq R eq C r L r
All these three resonant configurations can also include a transformer to provide isolation and additional voltage gain [START_REF] Huerta | Design of the L-LC resonant inverter for induction heating based on its equivalent SRI[END_REF].

According to the number of switching devices, the inverter topologies commonly used in IH are the full-bridge [START_REF] Dede | 25-kW/50-kHz generator for induction heating[END_REF], halfbridge [START_REF] Koertzen | Design of the half-bridge series resonant converters for induction cooking[END_REF], [START_REF] Kamli | A 50-150 kHz half-bridge inverter for induction heating applications[END_REF], and single-switch [START_REF] Koertzen | A comparative study of single switch induction heating converters using novel component effectivity conceps[END_REF] resonant inverters. Fig. 4 shows the voltage source full and half-bridge in the series resonant form, and a ZVS single-switch inverter. The full-bridge topology is commonly used for output powers higher than 5 kW, and it is the standard choice for industrial systems. By contrast, the half-bridge topology is the preferred one for domestic induction systems up to 5 kW, whereas the single-switch inverter is used in small IH generators and domestic systems up to 2 kW.

Apart from the single output topologies previously detailed, modern designs include multiple-coil systems intended to improve the heat distributions [START_REF] Rodriguez | Nonresonant and resonant frequency-selectable induction-heating targets[END_REF]. These multicoil systems have been applied to both industrial [START_REF] Pham | Phase angle control of highfrequency resonant currents in a multiple inverter system for zone-control induction heating[END_REF][START_REF] Pham | Estimating method of heat distribution using 3-D resistance matrix for zone-control induction heating systems[END_REF][START_REF] Pham | Dynamic analysis and control for resonant currents in a zone-control induction heating system[END_REF] and domestic applications [START_REF] Forest | Frequency-synchronized resonant converters for the supply of multiwindings coils in induction cooking appliances[END_REF], [START_REF] Lucía | Series-resonant multiinverter for multiple induction heaters[END_REF], [START_REF] Lucía | Series resonant multi-inverter with discontinuous-mode control for improved light-load operation[END_REF], and require the development of multiple-output inverters in order to obtain cost-effective solutions. Fig. 5 summarizes some of the approaches proposed. Fig. 5 (a) and (b) detail the schematic of a dual full-bridge inverter [START_REF] Burdío | A two-output series-resonant inverter for induction-heating cooking appliances[END_REF] and a dual half-bridge inverter [START_REF] Forest | Frequency-synchronized resonant converters for the supply of multiwindings coils in induction cooking appliances[END_REF], [START_REF] Jung | Dual half bridge series resonant inverter for induction heating appliance with two loads[END_REF], respectively. These topologies have similar operation to their single-output counterpart, but with limited output power control due to additional ZVS constrains. Fig. 5 (c) shows a half-bridge topology featuring frequency selectable resonant loads [START_REF] Forest | Principle of a multiload/single converter system for low power induction heating[END_REF]. Each resonant tank is tuned at a different resonant frequency, allowing selecting among them with the inverter operating frequency. Finally, Fig. 5 (d) shows the series resonant multi-inverter topology [START_REF] Lucía | Series-resonant multiinverter for multiple induction heaters[END_REF], [START_REF] Lucía | Series resonant multi-inverter with discontinuous-mode control for improved light-load operation[END_REF] designed to supply IH systems with a high number of coils with a reduced number of switching devices, providing independent output power control in each load. Additionally, alternative conversion schemes have been recently studied for IH systems. Direct ac-ac conversion has been studied by some authors because of its benefits in terms of reduced component count and electromagnetic emissions (EMI), and improved efficiency. Fig. 6 summarizes some of the proposals made by several authors. In Fig. 6 (a) [START_REF] Nguyen-Quang | Single phase matrix converter for radio frequency induction heating[END_REF], several switching devices are combined to build bidirectional switches in order to implement a full-bridge direct ac-ac power converter. The main drawback is the increased number of switches, which increases the cost and complexity of the power converter. The power converter proposed in [START_REF] Sugimura | Highfrequency resonant matrix converter using one-chip reverse blocking igbt-based bidirectional switches for induction heating[END_REF], Fig. 6 (b), follows the same procedure but using the recently developed reverse-blocking (RB) IGBTs to implement a half-bridge direct ac-ac converter with similar advantages and drawbacks than the previous implementation. In order to overcome these limitations, the proposals shown in Fig. 6 (c) and (d) were presented. On one hand, Fig. 6 (c) proposes a direct ac-ac half-bridge power converter featuring high-frequency rectifier diodes, which avoids the use of additional switching devices [START_REF] Sarnago | High efficiency ac-ac power electronic converter applied to domestic induction heating[END_REF]. This power converter also increases the voltage applied to the load, increasing further the efficiency. Fig. 6 (d) shows a multipleoutput proposal where a common ac-ac converter block composed of the switching devices Smh and Sml is used to supply a large set of loads and, therefore, reducing the relative impact of the ac-ac block cost and complexity [START_REF] Lucía | Multiple-output resonant matrix converter for multiple induction heaters[END_REF].

B. Modulation and control algorithms

Modulation and control algorithms have to accurately control the power converter in order to obtain the desired performance. More specifically, accurate output power (and as consequence, temperature) and current control are required, with appropriate dynamics. The main issues to address, which are common to nearly any IH application, include the management of highly variable output power and IH loads and, more recently, the implementation of multi-coil systems.

In order to obtain an apropriate output power control from a static poing of view, different modulation types have been succesfully proposed for single phase systems: square wave [START_REF] Lucía | Efficiency oriented design of ZVS half-bridge series resonant inverter with variable frequency duty cycle control[END_REF], [START_REF] Lucía | Load-adaptive control algorithm of half-bridge series resonant inverter for domestic induction heating[END_REF], [START_REF] Sarnago | Modulation scheme for improved operation of a RB-IGBT based resonant inverter applied to domestic induction heating[END_REF], asymmetrical control [START_REF] Chudjuarjeen | An improved LLC resonant inverter for induction-heating applications with asymmetrical control[END_REF], [START_REF] Lucía | Efficiency oriented design of ZVS half-bridge series resonant inverter with variable frequency duty cycle control[END_REF], [START_REF] Saha | Commercial frequency AC to high frequency AC converter with boost-active clamp bridge single stage ZVS-PWM inverter[END_REF], [START_REF] Barragán | Efficiency optimization in ZVS series resonant inverters with asymmetrical voltage-cancellation control[END_REF], [START_REF] Burdío | Asymmetrical voltage-cancelation control for full-bridge series resonant inverters[END_REF] or pulse density modulation (PDM) [START_REF] Esteve | Improving the efficiency of IGBT series-resonant inverters using pulse density modulation[END_REF], [START_REF] Ahmed | High-frequency soft-switching ac conversion circuit with dualmode PWM/PDM control strategy for high-power IH applications[END_REF], [START_REF] Fujita | Pulse-density-modulated power control of a 4 kW, 450 kHz voltage-source inverter for induction melting applications[END_REF], [START_REF] Fujita | Control and performance of a pulse-density-modulated series-resonant inverter for corona discharge processes[END_REF], [START_REF] Park | A power-control scheme with constant switching frequency in class-D inverter for induction-heating jar application[END_REF], [START_REF] Saha | Commercial frequency AC to high frequency AC converter with boost-active clamp bridge single stage ZVS-PWM inverter[END_REF], [START_REF] Lucía | Load-adaptive control algorithm of half-bridge series resonant inverter for domestic induction heating[END_REF]. Square wave and asymmetrical controls allow to change the output power in the complete operation range by either controlling the switching frequency or the duty cycle of the control signals. PDM, also known as burst mode, has the advantage of controlling the inverter at a fixed switching frequency by controlling the on-time of the inverter. The use of a phase-locked loop (PLL) is also common in the control schemes of IH inverters such as in [START_REF] Chudjuarjeen | An improved LLC resonant inverter for induction-heating applications with asymmetrical control[END_REF], [START_REF] Tian | A half-bridge series resonant inverter for induction cookers using a novel FPGA-based control strategy[END_REF]. As an alternative to this idea, papers [START_REF] Pham | Phase angle control of highfrequency resonant currents in a multiple inverter system for zone-control induction heating[END_REF], [START_REF] Pham | Dynamic analysis and control for resonant currents in a zone-control induction heating system[END_REF] divide the control into two parts: direct phase control thanks to a PLL and indirect RMS amplitude control through a proportional-integral (PI) controler, as it can be seen in Fig. 7. This solution is possible

v bus S 1 S 2 S 3 S 4 C s C s C r L eq R eq v o v l i l + - + - v bus S 1 S 2 C s C s L eq R eq v o v l i l C r C r + - - v bus S 1 L eq R eq v o =v l i l C r + -
in the case of one dc-dc converter per phase but the RMS calculation slows the dynamic response.

One of the issues for the future of IH is the load adaptive capabilities and some solutions have been proposed in [START_REF] Egalon | Multiphase system for metal disc induction heating: Modeling and RMS current control[END_REF], [START_REF] Cano | An induction heater device for studies of magnetic hyperthermia and specific absorption ratio measurements[END_REF], [102]. An adaptive simmering control of the temperature for a domestic induction cooker is presented in [START_REF] Paesa | Adaptive Simmering Control for Domestic Induction Cookers[END_REF]. Parameters are updated online, depending on the estimates provided by a multiple-model reset observer (MMReO). This observer consists of a reinitialized reset observer and of multiple fixed identification models, as it can be seen in Fig. 8. Besides, a fixed robust quantitative feedback theory based controller is designed for comparison purposes. Such a control scheme achieves quick heating up and accurate temperature control whatever the amount of water, with or without a lid. Besides all the single phase architectures, multi-phase or multi-coil configurations could be helpful to increase either flexibility or power or even both simultaneously. Control becomes even more important since the currents have to be perfectly controlled in form, amplitude and phase, due to the coupling effects between the coils and with the load, whatever the configuration, transverse flux or longitudinal flux. Moreover, as the load characteristics changes with temperature, the controller task becomes heavy. An original control scheme is described and tested in [START_REF] Pham | Phase angle control of highfrequency resonant currents in a multiple inverter system for zone-control induction heating[END_REF][START_REF] Pham | Estimating method of heat distribution using 3-D resistance matrix for zone-control induction heating systems[END_REF][START_REF] Pham | Dynamic analysis and control for resonant currents in a zone-control induction heating system[END_REF] for a six-coil system called zone controlled induction heating (ZCIH) in a longitudinal flux configuration. Paper [START_REF] Pham | Dynamic analysis and control for resonant currents in a zone-control induction heating system[END_REF] presents the use of a current control method based on a circuit model using real and imaginary (Re-Im) current/voltage components instead of the current amplitude and phase angle control in the classical approaches. State feedback decoupling provides separate current control on each phase with high dynamics but with a high computation burden due to numerous high rank matrix transformations. The six currents remain in phase in this application where no dc-dc converter is needed unlike in [START_REF] Pham | Phase angle control of highfrequency resonant currents in a multiple inverter system for zone-control induction heating[END_REF].

v bus S 1 S 2 S 4 S 5 C s C s C s C s C r1 L eq1 R eq1 v o1 i l1 S 3 S 6 C s C s v l2 C r2 L eq2 R eq2 i l2 + - + - v bus S 1 S 2 C s C s i l1 S 3 C s L eq2 R eq2 i l2 L eq1 R eq1 v l2 v l1 C r2 C r1 D 1 D 2 + - + - v bus S 1 S 2 C s C s v o v l i l1 L eq1 R eq1 C r1 L eq2 R eq2 C r2 L eqn R eqn C rn + - R eq,4 L eq,4 C r,4 C r,3 L eq,3 R eq,3 … … … S 1 S 3 S n-1 S 2 S 4 S n S h S l C s C s V bus R eq,n L eq,n C r,n R eq,2 L eq,2 C r,2 C r,n-1 L eq,n-1 R eq,n-1 C r,1 L eq,1 R eq,1 i o i l,1 i l,3 i l,n-1 i l,2 i l,4 i l,n v l,2 v l,4 v l,n v l,1 v l,3 v l,n-1 v o i bus c h c l c 1 c 2 c 3 c 4 c n c n-1 + - + + + + + + - - - - - - T h D h T l D l T 2 D 2 T 1 D 1 T 3 D 3 T 4 D 4 T n-1 D n-1 T n D n v bus S 1 S 2 S 3 S 4 C r L eq R eq v o i l + - C s C s C s C s v bus S 1 S 2 C s L eq R eq v o v l i l C r + - - Req4 Leq4 Cr4 C r3 Leq3 Req3 … … … S 1 S 3 Sn-1 S2 S4 Sn S mh
In order to obtain appropriate dynamics, resonant control of the inductor currents has been presented in [START_REF] Nguyen Kien | Resonant control of multi-phase induction heating systems[END_REF] versus other classical control techniques for comparison purposes. With a highly reduced power electronic structure, the amplitudes and phases of the currents have to be precisely controlled in a CSI feeding a multi-coil IH system in transverse flux configuration. The innovation lies in the current phase control in order to fit a required temperature profile for metal treatment. A resonant controller is implemented on each of the phases of this multi-coil induction heating system in order to achieve very fast and accurate control. Currents in the other phases are considered as disturbances that are compensated by each resonant controller.

All these modulation and control algorithms have been implemented either using analog [START_REF] Artigas | Power measurement by output-current integration in series resonant inverters[END_REF] or digital techniques. Nowadays, digital implementations using digital signal processors (DSPs) or field-programmable gate arrays (FPGAs) due to their advantages in terms of configurability and performance. In particular, several design methodologies [START_REF] Navarro | High-level synthesis for accelerating the FPGA implementation of computationallydemanding control algorithms for power converters[END_REF], monitoring [START_REF] Jiménez | FPGA-based test-bench for resonant inverter load characterization[END_REF] and control architectures [START_REF] Lucía | A versatile power electronics test-bench architecture applied to domestic induction heating[END_REF], [START_REF] Navarro | Synchronous FPGA-based implementations of digital pulse width modulators[END_REF], [START_REF] Jiménez | FPGA-based gain-scheduled controller for resonant converters applied to induction cooktops[END_REF], and accelerated simulation techniques [START_REF] Lucía | Real-time FPGA-based hardware-in-the-loop simulation test-bench applied to multiple output power converters[END_REF] for IH systems have been proposed taken advantages of FPGAs.

C. Magnetic component design

The most important magnetic component in an IH system is the inductor-load system [START_REF] Hurley | Induction heating of circular ferromagnetic plates[END_REF], [START_REF] Kazimierczuk | High-Frequency Magnetic Components[END_REF]. Significant efforts have been paid to the modeling [START_REF] Dughiero | A translational coupled electromagnetic and thermal innovative model for induction welding of tubes[END_REF], [START_REF] Benato | Proximity effect and magnetic field calculation in GIL and in isolated phase bus ducts[END_REF], design, and optimization using either analytical [START_REF] Acero | Modeling of planar spiral inductors between two multilayer media for induction heating applications[END_REF] or finite element analysis (FEA). From an application point view, the most important aspects are the electrical equivalent parameters extraction [START_REF] Forest | Frequency-synchronized resonant converters for the supply of multiwindings coils in induction cooking appliances[END_REF], [START_REF] Forest | Principle of a multiload/single converter system for low power induction heating[END_REF], [START_REF] Acero | Analytical equivalent impedance for a planar circular induction heating system[END_REF], [START_REF] Carretero | Frequencydependent modeling of domestic induction heating systems using numerical methods for accurate time-domain simulation[END_REF] the efficiency optimization [START_REF] Acero | Simple resistance calculation in litz wire planar windings for induction cooking appliances[END_REF], [START_REF] Acero | Magnetic vector potential based model for eddy-current loss calculation in round-wire planar windings[END_REF][START_REF] Wojda | Winding resistance of litz-wire and multistrand inductors[END_REF][START_REF] Wojda | Analytical optimization of solid-roundwire Windings[END_REF][START_REF] Bartoli | Modeling Litz-wire winding losses in high-frequency power inductors[END_REF], and the heat distribution optimization [START_REF] Pham | Estimating method of heat distribution using 3-D resistance matrix for zone-control induction heating systems[END_REF], [START_REF] Candeo | Multiphysics modeling of induction hardening of ring gears for the aerospace industry[END_REF], [START_REF] Aliferov | Coupled magneto-thermal FEM model of direct heating of ferromagnetic bended tubes[END_REF].

In the industrial applications of IH, two different types of flux inductors exist, depending on the arrangements of the coil and the load and on the flux direction: transversal flux in Fig. 9 (a) or longitudinal flux in Fig. 9 (b).

The advantages of classical induction heating systems can be improved by multiphase inductors. Indeed, achieving metal temperature homogeneity with a single inductor on a large scale is impossible, particularly in transverse flux heating. A solution to overcome this constraint consists in using multicoil systems but with an increased complexity.

Conventional control solutions for industrial multiinductor systems [START_REF] Caricchi | Axial-flux permanent-magnet generator for induction heating gensets[END_REF], [START_REF] Tudorache | Magneto-thermal-motion coupling in transverse flux heating[END_REF] are based on several inductors with mobile magnetic screens and mobile flux concentrators (Fig. 10). These additional devices adjust the magnetic fields produced and adapt the system to different formats of material and changes in position in order to reach the desired temperature gradients. The processing lines are then subject to Fig. 7.

Inductor current control circuit [START_REF] Pham | Phase angle control of highfrequency resonant currents in a multiple inverter system for zone-control induction heating[END_REF]. Significant progresses [START_REF] Fujita | A new zone-control induction heating system using multiple inverter units applicable under mutual magnetic coupling conditions[END_REF], [START_REF] Pham | Phase angle control of highfrequency resonant currents in a multiple inverter system for zone-control induction heating[END_REF][START_REF] Pham | Estimating method of heat distribution using 3-D resistance matrix for zone-control induction heating systems[END_REF][START_REF] Pham | Dynamic analysis and control for resonant currents in a zone-control induction heating system[END_REF] have been made in this domain by considering a static multi-inductor system without any moveable devices such as yokes or magnetic screens. Nevertheless, these multi-coil systems involve mutual coupling between inductors themselves and between the inductors and the load that have to be taken into account in the control scheme [START_REF] Pham | Dynamic analysis and control for resonant currents in a zone-control induction heating system[END_REF], [START_REF] Carretero | Computational modeling of two partly-coupled coils supplied by a double half-bridge resonant inverter for induction heating appliances[END_REF][START_REF] Acero | Mutual impedance of small ring-type coils for multi-winding induction heating appliances[END_REF][START_REF] Acero | Analysis and modeling of planar concentric windings forming adaptable-diameter burners for induction heating appliances[END_REF]. The corresponding architectures often present rather complex solutions with one dc-dc converter (or one rectifier) plus one resonant inverter per phase as in [START_REF] Fujita | A new zone-control induction heating system using multiple inverter units applicable under mutual magnetic coupling conditions[END_REF], [START_REF] Pham | Phase angle control of highfrequency resonant currents in a multiple inverter system for zone-control induction heating[END_REF][START_REF] Pham | Estimating method of heat distribution using 3-D resistance matrix for zone-control induction heating systems[END_REF][START_REF] Pham | Dynamic analysis and control for resonant currents in a zone-control induction heating system[END_REF]. In [START_REF] Fishman | Induction heating device and process for controlling temperature distribution[END_REF], it is proposed to manually change the coil connections for load-generator adaptation. Some decoupling transformers between the different phases are added in [START_REF] Uchida | Induction heating method and unit[END_REF], which are certainly bulky and costly. It is possible to take advantage of simpler structures by an optimization with constraints, based on a prior determination of the mapping of induced current distribution inside the heated material. It necessitates the global power density calculation in order to achieve the correct temperature profile through the setting of currents in the 3 phases in [START_REF] Egalon | Multiphase system for metal disc induction heating: Modeling and RMS current control[END_REF]. In that case, amplitudes and phases of the inductor currents have to be determined and controlled.

Fig. 11 shows an example of several water-cooled inductors for industrial applications, whereas Fig. 12 shows several flat inductors with different shapes and sizes for domestic induction heating.

This Section has covered the main enabling technologies in IH systems. Next Section will explain the main applications of IH with a special focus on the differential characteristics and the implemented technologies.

IV.

INDUCTION HEATING APPLICATIONS Among the many applications of induction heating, industrial, domestic and medical are the most important in terms of installed power and economic importance. The next subsections details the main characteristics of these IH applications.

A. Industrial applications

Industrial applications of IH started in the early 1900s with metal melting and were later extended to the automotive and aircraft industries. Current applications have been extended to many manufacturing processes including preand post-heating, melting, forging, surface treatment, sealing, bonding, annealing, and welding, among others.

The use of induction heating improves the speed of the process, its precision, efficiency, and repeatability, which are key features needed for the automation of the industrial processes. Fig. 14 shows two complete induction heating installation for joints hardening (a) and tube welding (b). Other industrial applications can be seen in Fig. 13 (a) and (b), for high power level, or in Fig. 13 (c) and (d), for lower power levels. The method in Fig. 13 (c) consists in accelerating the polymerization of the glue by heating the metal parts to be glued by induction. The temperatures required are generally low (150 to 300°C). Fig. 13 (d) is an elegant means of sealing aluminum covers on jars of food product. Inductive heating of the aluminum film increases the temperature of the sealing product applied to the side of the cover in contact with the jar. Depending on the final application and the material to heat, the power converter operating frequency is significantly different, varying from a few Hz for high power systems, typical for metal melting, to several hundreds of kHz, for surface heat processing. As a consequence, semiconductors used in industrial heating converters are thyristors, operating at frequencies up to 3 kHz, for power ratings of several MWs, while IGBTs, operating at frequencies up to 150 kHz for power ratings up to 3 MW are normally used. MOSFETs are used for higher frequencies, up to several hundreds of kHz and output powers lower than 500 kW. Fig. 15 summarizes the semiconductor device utilization area depending on the switching frequency, the power level, and the type of resonant tank, series or parallel, used.

Certain applications require more advanced topologies that aim to improve the heating characteristics or expanding into new fields of application. This is the case of the dual frequency generators [START_REF] Esteve | Induction heating inverter with simultaneous dual-frequency output[END_REF] used to feed the inductor, sequentially or simultaneously, with two different frequencies (Fig. 16) to achieve different penetration depths. One frequency is typically set in the medium frequency range (3 to 10 kHz), and the other one in the high frequency range (200 to 400 kHz). These types of generators are used for hardening work pieces with irregular surface geometry like gears.

It is important to note that control and interoperability of industrial IH systems are extremely important. On the one hand, the power converter operation must be accurately controlled to obtain the desired results in the IH target. On the other hand, the IH system must be interconnected to the complete assembly line using industrial protocols such as Profinet, Interbus, Profibus, etc.

B. Domestic applications

The main domestic applications of IH are the induction heating appliances [START_REF] Acero | Domestic Induction Appliances[END_REF]. IH cookers take advantage not only of improved heating times and efficiency, but also lower surface temperatures, which implies better security and cleanness, since food does not get burnt. Fig. 17 shows the main structure of an IH appliance (a) and a detail of the inductor-pot system (b). The main characteristics of this application is the low-profile compact design, and the highly variable IH target, which can change in shape, materials, and position very easily. Efficiency is also a key design parameter due to the limited cooling capabilities. For this reason, large efforts have been paid to improve the power converter and inductor efficiency [START_REF] Lucía | Efficiency oriented design of ZVS half-bridge series resonant inverter with variable frequency duty cycle control[END_REF][START_REF] Sarnago | Class-D/DE dual-modeoperation resonant converter for improved-efficiency domestic induction heating system[END_REF][START_REF] Sarnago | High efficiency ac-ac power electronic converter applied to domestic induction heating[END_REF], [START_REF] Ahmed | Boost-half-bridge edge resonant soft switching PWM high-frequency inverter for consumer induction heating appliances[END_REF], [START_REF] Kifune | Quasi-series-resonant-type softswitching phase shift modulated inverter[END_REF], [START_REF] Sarnago | High efficiency parallel quasi-resonant current source inverter featuring SiC MOSFETs for induction heating systems with coupled inductors[END_REF]. Nowadays, two technological trends are present in IH appliances. Both of them follow the same power conversion diagram shown in Fig. 17 (c), but differ in the inverter topology. Designs for Europe and America are usually specified for output powers up to 4 kW, and the power converter topology usually chose is the series resonant halfbridge inverter [START_REF] Lucía | Efficiency oriented design of ZVS half-bridge series resonant inverter with variable frequency duty cycle control[END_REF], [START_REF] Ogura | Performance evaluation of edge-resonant ZVS-PWM highfrequency inverter using trench-gate IGBTs for consumer induction cooking heater[END_REF]. By contrast, appliances for Asiatic countries are usually designed with 2 kW output power and the preferred topology is the ZVS single-switch quasiresonant inverter [START_REF] Wang | Induction-heated cooking appliance using new quasi-resonant ZVS-PWM inverter with power factor correction[END_REF]. In addition to this, this technology also offers the capability to heat high-conductivity materials [START_REF] Fujita | Latest developments of high-frequency series load resonant inverter type built-in cooktops for induction heated all metallic appliances[END_REF], such as aluminum or copper, usually employed in these countries. Taking into account the output power and cost restrictions, switching frequencies usually ranges from 20 kHz up to 100 kHz. The lower limit is set in order to avoid acoustic noise emissions, not desirable in domestic applications, whereas the higher limit is imposed by the switching losses of the power devices.

These appliances feature also advanced control techniques including smart pot recognition [START_REF] Jiménez | FPGA-based test-bench for resonant inverter load characterization[END_REF] and adaptive control strategies [START_REF] Paesa | Adaptive Simmering Control for Domestic Induction Cookers[END_REF] to control not only the output power but also the pot temperature. It provides the user with advanced functionalities that cannot be achieved with conventional cookers, significantly improving the user performance.

Currently, research trends are focused on higher efficiency power converters and flexible cooking surfaces technology [START_REF] Esteve | Improving the efficiency of IGBT series-resonant inverters using pulse density modulation[END_REF], [START_REF] Sarnago | High efficiency ac-ac power electronic converter applied to domestic induction heating[END_REF], [START_REF] Sarnago | High efficiency parallel quasi-resonant current source inverter featuring SiC MOSFETs for induction heating systems with coupled inductors[END_REF], [START_REF] Acero | Magnetic vector potential based model for eddy-current loss calculation in round-wire planar windings[END_REF], where a special effort is required to design multiple-output power converters and compact coils [START_REF] Forest | Frequency-synchronized resonant converters for the supply of multiwindings coils in induction cooking appliances[END_REF], [START_REF] Lucía | Series-resonant multiinverter for multiple induction heaters[END_REF], [START_REF] Acero | Mutual impedance of small ring-type coils for multi-winding induction heating appliances[END_REF]. The total active surface concept (Fig. 18), still under development, is an example of modern development where the user can place any pot, with any shape, anywhere in the cooking surface, redefining completely the cooking experience. This new concept requires the development of high-efficiency high-performance multiple output power converters to satisfy the required specifications with a costeffective solution. Besides, the control and modulation strategies become more complex since accurate multi-load management is required. This is a remarkable example of how advances in enabling technologies leads to improved induction heating systems present in the market.

C. Medical applications

Nowadays, the third major area of application of IH technology is related to its medical applications. Initially, IH was applied in the manufacturing and sterilization of many surgical instruments, since it is a clean, fast, and portable heat source. However, in recent years IH has also been introduced in some minimally-invasive therapies [START_REF] Stauffer | Magnetic Induction Heating of Ferromagnetic Implants for Inducing Localized Hyperthermia in Deep-Seated Tumors[END_REF].

Hyperthermia is a cancer treatment therapy based on heating the target tumor temperatures over 50 ºC. This local treatment removes cancer tissue while minimizing the damage to surrounding healthy cells. IH is therefore a good alternative for hyperthermia treatments because it is a contactless heating technique, i.e. less invasive, and provides accurate power control. In order to precisely deliver the power the tumor, usually a ferromagnetic material is placed in the area to be treated. Modern research trends investigate about the use of fluids with ferromagnetic nanoparticles in order to obtain precise heat distributions [START_REF] Gomez-Polo | Analysis of heating effects (magnetic hyperthermia) in FeCrSiBCuNb amorphous and nanocrystalline wires[END_REF][START_REF] Cano | An induction heater device for studies of magnetic hyperthermia and specific absorption ratio measurements[END_REF][START_REF] Candeo | Numerical FEM models for the planning of magnetic induction hyperthermia treatments with nanoparticles[END_REF][START_REF] Barba | Synthesizing distributions of magnetic nanoparticles for clinical hyperthermia[END_REF][START_REF] Barba | Coupled field synthesis in magnetic fluid hyperthermia[END_REF]. These techniques require the design of accurate power converters and control [START_REF] Landi | On the energy conversion efficiency in magnetic hyperthermia applications: A new perspective to analyze the departure from the linear regime[END_REF][START_REF] Tai | A compact half-bridge heating system for magnetic nanoparticle thermotherapy applications[END_REF][START_REF] Tai | The design of a half-bridge series-resonant type heating system for magnetic nanoparticle thermotherapy[END_REF], and specific inductor designs [START_REF] Stauffer | Magnetic Induction Heating of Ferromagnetic Implants for Inducing Localized Hyperthermia in Deep-Seated Tumors[END_REF], [START_REF] Stauffer | Practical induction heating coil designs for clinical hyperthermia with ferromagnetic implants[END_REF]. Due to the typical low resistivity nature of IH loads for medical applications, parallel resonant inverters are applied in order to minimize the current through the inverter. Besides, the operating frequencies are set to be higher than 300 kHz up to several MHz to obtain appropriate equivalent electrical parameters, being the MOSFET the power device commonly implemented.

Future research include more precise and homogeneous heating, and temperature monitoring in the cancer and surrounding tissues, along with combining this technique with other therapies to improve results.

V. FUTURE CHALLENGES

Although IH systems have reached certain maturity, there are still some issues to address to further increase its performance. Moreover, advances in enabling technologies and applications continuously open new research trends and industrial interest. Among the many topics of interest, significant efforts are expected in these topics in the next years:

High-efficiency IH systems: the improvement of semiconductor technology and the development of widebandgap devices, together with advanced topologies, are allowing the design of higher efficiency systems. Such systems not only exhibit higher efficiency, but also improved performance and reliability.

Multi-coil IH systems: Multi-coil systems offer higher flexibility, performance, and heat distribution, and are essential for some high precision/flexibility industrial and domestic applications. These systems represent a major breakthrough in IH technology, requiring the development of multiple-output power converters, advanced control techniques, and special inductor design paying very high attention to coupling effects.

Advanced control systems: IH systems require robust control algorithms to adapt the power converter operation to different IH loads and operating points. Current research lines pursue the use of adaptive algorithms and real-time identification systems to improve the system performance. This is one of the key issues of multi-coil systems as well as real time temperature measurement and optimal control and stability study. Generalized averaging methods and envelope models will help to fix the characteristics of the current transients in these systems. Special applications: although the process parameters in many industrial and domestic applications are already well-known, there are still some applications that need further research and optimization to find viable solutions. Among these special applications, the following are worth to be highlighted: heating of low-resistivity materials, accurate heating of biological tissues for medical applications, faster design of IH systems for complex IH load geometries, and accurate 3D FEA simulation of the complete IH system.

VI. CONCLUSIONS

This paper has reviewed the current state-of-art of induction heating systems, focusing on the enabling technologies involved and the main applications, and providing an exhaustive list of papers.

From a design point of view, there are three main enabling technologies to tackle when designing an IH system: power converter, modulation and control architecture, and the inductor design. Extensive research has been carried out in recent years, leading to improved IH systems that have enabled widening IH systems usage to many industrial, domestic, and medical applications.

The future of IH systems is promising, with new technologies and applications fostering research in each one of the three enabling technologies aforementioned. The development of higher efficiency and performance IH systems is expected to be the focus of industrial interest and researches, further increasing the general interest in IH systems.

  I

Fig. 1 .

 1 Fig. 1. Typical arrangement of an induction heating system in a longitudinal flux configuration: (a) general view and (b) top view.

Fig. 2 .Fig. 3 .

 23 Fig. 2.Typical power conversion flow in an IH system.

Fig. 4 .

 4 Basic voltage source inverter topologies applied to IH: (a) fullbridge series resonant inverter, (b) half-bridge series resonant inverter, and (c) a single-switch ZVS quasi-resonant inverter.

Fig. 5 .Fig. 6 .

 56 Fig. 5. Multiple-output inverters for multi-coil IH systems: (a) dual-full bridge, (b) dual output half-bridge, (c) frequencymultiplexed half bridge, and (d) series resonant multi-inverter.

Fig. 8 .Fig. 9 .

 89 Fig. 8. Multi-model reset observer for x

Fig. 10 .

 10 Fig. 10. Transverse flux induction heating with moving parts

Fig. 11 .

 11 Fig. 11. Detail of several industrial water-cooled transverse flux inductors. Courtesy GH Electrotermia S.A.

Fig. 12 .Fig. 13 .

 1213 Fig. 12. Domestic induction heating inductors of different size and shape. Courtesy BSH Home Appliances Group.

Fig. 14 .

 14 Fig. 14. Industrial applications of induction heating: IH joints hardening facility for the automotive industry. Courtesy GH Electrotermia S.A.

Fig. 15 .Fig. 16 .Fig. 17 .

 151617 Fig. 15. Power devices used in industrial IH applications.

Fig. 18 .

 18 Fig. 18. Total active surface IH Appliance.

TABLE I .

 I DIFFERENTIAL CHARACTERISTICS OF INDUCTION HEATING APPLICATIONS FOR EACH ENABLING TECHNOLOGY.