This paper analyzes a high-power (50 kW) high-frequency (150 kHz) voltage-fed inverter with a series-resonant load circuit for industrial induction heating applications, which is characterized by a full bridge inverter made of insulated-gate bipolar transistor and a power control based on pulse density modulation (PDM). This power control strategy allows the inverter to work close to the resonance frequency for all output-power levels. In this situation, zero-voltage switching and zero-current switching conditions are performed, and the switching losses are minimized. An additional improvement of inverter efficiency is achieved by choosing appropriate values of the modulation index. Results are verified experimentally using a prototype for induction hardening applications. A comparative study between the PDM and the classical power control by frequency variation will be made.

This paper presents a 100-kW 100-kHz insulated-gate bipolar transistor (IGBT) series resonant inverter for induction heating applications that uses an improved power control scheme based on the standard pulse density modulation (PDM). This standard power control is a good solution for the design of high-frequency inverters because the output power factor is near to unity in a wide range of output power, resulting in great reduction of switching losses and electromagnetic noise. However, the output current can be in discontinuous mode, particularly for resonant loads of low quality factor or for low output power or low-load operation. This output current fluctuation produces a high output current ripple that can lead to an increase in power losses and loss of accuracy of the response of the frequency tracking control. The proposed control strategy, which is called enhanced PDM (EPDM), provides twice less output current ripple, thus resulting in an improved inverter behavior in terms of frequency tracking accuracy and energy efficiency. Experimental tests have been made, in order to compare the EPDM strategy with standard power control schemes.

This paper analyzes a high-power (100-kW) high-frequency (50-kHz) voltage-fed inverter with a series resonant load circuit for industrial induction heating applications which is characterized by a full-bridge inverter composed of isolated-gate bipolar transistors and a new power control based on phase-shift (PS) control. This power control circuit incorporates a load-adaptive variable-frequency controller and automated blanking time management in order to allow the inverter to work in zero-voltage switching for all output power levels and load conditions. An important improvement of the inverter reliability is achieved by choosing an appropriate and novel switching sequence for the PS inverter. The results are verified experimentally using a prototype for induction hardening applications. A comparative study between the proposed and standard PS power control will be made.

This paper presents a comparison of the behavior of the intrinsic diode of silicon (Si) and silicon carbide (SiC) MOSFETs. The study was done for 1200 V Si and SiC MOSFETs. The data sheet from manufacturers shows the characteristics of MOSFET' intrinsic diode when gate source voltage (V_GS) is 0 V. There are applications where the MOSFET' intrinsic diode is used while V_GS is different than 0 V. One of these applications is induction heating, where depending on the load and the regulation system, the diode can conduct a significant part of the inverter current. In most applications which use the MOSFET' intrinsic diode, the turn ON of the intrinsic diode happens at V_GS = 0 V. After a blanking time, the MOSFET' gate is activated waiting for the direction change of current in the circuit. Therefore, most of the current through the MOSFET' intrinsic diode occurs with a V_GS different of 0 V. This paper shows the direct output characterization of Si and SiC MOSFET' intrinsic diode under different gate voltages. The gate resistor (R_G) is an important parameter of the characterization. Depending on the input capacitance of the Si or SiC MOSFET, different R_G are needed. The turn-on and turn-off behaviors are obtained when RG is optimized for each Si and SiC MOSFET. This has result in the turn-off robustness of intrinsic diode with optimum R_G. This paper presents a surprising result for the reverse characteristic of Si and SiC MOSFETs for the same current at different V_GS. The technology of Si MOSFET has different behavior depending on the manufacturer. The technology of SiC MOSFET presents a very similar behavior to low-voltage Si MOSFETs.

The induction surface hardening of parts with nonuniform cylindrical shape requires a multifrequency process in order to obtain a uniform surface hardened depth. This paper presents an induction heating high power supply composed by a single inverter circuit and a specially designed output resonant circuit. The whole circuit supplies simultaneously both medium- and high-frequency power signals to the heating inductor. An initial study is made to select the most appropriated topology for this application. The resonant output circuit is analyzed, and a design procedure is presented. The selected inverter operation is described and simulated. Simulations are experimentally verified on a 10-kW dual-frequency resonant inverter operating at 10 and 100 kHz using MOSFETs of silicon (Si) and silicon carbide (SiC) technology. A comparative study is presented based on the measurements of power losses and the energy efficiency of the inverter using both types of MOSFETs.

This article presents the analysis of a converter based on an LLC resonant output inverter and its optimal design used in induction heating applications. The new optimal design method improves several operating parameters that leads to an optimization of the dimensioning of the components of the converter. Additionally, this converter achieves an output power factor that can be considered optimal since it allows to minimize the reactive power of the resonant circuit components and reduces the rms values of the output current of the inverter and the current of its switching devices in relation to that found in traditional designs. A complete study of the circuit based on classic models is carried out to introduce simple rules for the design of this type of inverter for induction heating applications and to control its output power based on a phase shift (PS) system. Since the inverter is made with silicon carbide MOSFET transistors, an efficiency greater than 99% is reached. The experimental results were obtained from the test of a 12 kW 20 kHz converter for induction heating application.