International Journal of Power Electronics and Drive Systems (IJPEDS) Vol. 12, No. 1, Mar 2021, pp. 258~272 ISSN: 2088-8694, DOI: 10.11591/ijpeds.v12.i1.pp258-272  258 Journal homepage: http://ijpeds.iaescore.com Soft start-up strategy of pulse-density-modulated series- resonant converter for induction heating application Pavlo Herasymenko, Volodymyr Pavlovskyi Department of Transistor Converters, Institute of Electrodynamics, Kyiv, Ukraine Article Info ABSTRACT Article history: Received Jun 16, 2020 Revised Jan 21, 2021 Accepted Feb 2, 2021 This paper presents a soft start-up strategy of pulse-density-modulated series- resonant converter for induction heating application. The pulse-density modulation (PDM) technique is widely used in converters based on voltage- source series-resonant inverters (SRIs) to control the output current or power. However, during a start-up process, PDM has some disadvantages both in inrush current limiting and providing a zero-voltage switching operation of SRI transistors. In the paper, different PDM techniques are considered and basic moments of PDM using within the start-up process are analyzed. A new soft start-up strategy of PDM converter for induction heating application is proposed. The main features of the proposed strategy include an interleaved or a stepped PDM control, an initial combination of PDM at the beginning of the start-up process, and an operating algorithm during the start-up process. The proposed strategy was verified by a 2.5 kW experimental setup of the pulse- density-modulated interleaved converter with an operating frequency from 50 kHz up to 100 kHz. Experimental results confirm the effectiveness of the proposed start-up strategy and show that the maximum current amplitude within start-up processes exceeds the maximum steady-state current amplitude by no more than 30%. Keywords: Phase-locked loops Pulse-density modulation Pulse-density-modulated converter Series-resonant inverter Start-up strategy This is an open access article under the CC BY-SA license. Corresponding Author: Pavlo Herasymenko Department of Transistor Converters Institute of Electrodynamics 03057, 56 Peremohy avenue, Kyiv, Ukraine Email: herasymenko@ieee.org 1. INTRODUCTION Modern MOSFETs and IGBTs allow to create powerful high-frequency transistor converters. Such converters are increasingly used in various applications, in particular in induction heating systems. For these systems transistor converters based on the voltage-source or the current-source inverter are widely used. A start-up process is very important for transistor converters, especially to limit their inrush current. Furthermore, hard-switching operation modes of transistors under the start-up process may arise due to features of some control techniques. Therefore, the start-up process can be a difficult problem in power converters. Depending on which control technique is used to regulate the output current of the inverter, it is possible to obtain different switching modes of the inverter transistors both during the start-up process and in the steady-state mode. Many different control techniques have been proposed to regulate the output current or power of these converters, some of which make it possible to provide zero-voltage switching (ZVS) and/or zero-current switching (ZCS) operation modes. ZVS/ZCS makes it possible to substantially reduce power losses in the converters with high- frequency operation modes. The advantage of converters based on the voltage-source series-resonant inverter (SRI) is that they have a simple power circuit configuration and regulation of its output current or power can