40 PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 97 NR 5/2021 Pavlo HERASYMENKO Institute of Electrodynamics of the National Academy of Sciences of Ukraine doi:10.15199/48.2021.05.07 Combined PS-PDM control method for voltage-source series- resonant inverter Abstract. This paper proposes a new control method for a voltage-source series-resonant inverter (SRI) of the induction heating system. The proposed method of control is based on combining the phase-shift (PS) and pulse-density-modulation (PDM) controls. The main feature of this control method is that the current regulation for pulse density in the range from 0.5 to 1 is provided by only the traditional PDM control and in the range from 0 to 0.5 – with the PS control and constant density equal to 0.5 of the PDM control. Theoretical analysis shows that using the proposed control method can reduce the peak-to-peak current fluctuation and increase the minimum peak current of the SRI, compared with the traditional PDM control. The results of numerical computations are in good agreement with the results of simulations. Experiments were carried out to confirm the practical applicability of the proposed control method for series-resonant inverters for induction heating application. Streszczenie. W artykule zaproponowano nową metodę sterowania falownikiem rezonansowym szeregowym ze źródłem napięcia (SRI) systemu ogrzewania indukcyjnego. Proponowana metoda sterowania opiera się na połączeniu sterowania przesunięciem fazowym (PS) i modulacji gęstości impulsów (PDM). Główną cechą tej metody sterowania jest to, że regulację prądu dla gęstości impulsów w zakresie od 0,5 do 1 zapewnia tylko tradycyjne sterowanie PDM, a zakresie od 0 do 0,5 - ze sterowaniem PS i stałą gęstością. Analiza teoretyczna pokazuje, że zastosowanie proponowanej metody sterowania może zmniejszyć wahania prądu międzyszczytowego i zwiększyć minimalny prąd szczytowy SRI w porównaniu z tradycyjnym sterowaniem PDM. (Połączona metoda sterowania PS-PDM dla falownika rezonansowego szeregowego ze źródłem napięcia) Keywords: control method, current fluctuation, induction heating, pulse-density modulation, phase-shift control, series-resonant inverter. Słowa kluczowe: sterowanie przesunięciem fazy PS, sterowanie gęstością impulsów PDM, nagrzewanie indukcyjne. Introduction A voltage-source series-resonant inverter (SRI) is well used for induction heating systems because of its simple power circuit topology and because many different control techniques are suitable to regulate its output current or power [1-11]. Various control techniques have been proposed to provide soft-switching operation modes of SRI transistors. In recent years, one of the frequently mentioned control techniques is pulse-density modulation (PDM) [8- 12]. This technique makes it possible to achieve zero- voltage and quasi-zero-current switching (ZVS and ZCS) operations of the SRI transistors. However, the use of PDM leads to peak-to-peak fluctuations of the SRI output current, and the lower the value of the quality factor of the series- resonant circuit, the higher the fluctuation. This can be a serious problem, as at low values of the output current of the SRI, transistors’ commutation can occur under NON- ZVS conditions. Various enhanced PDM techniques have been proposed to reduce the peak-to-peak current fluctuation [11-14]. Compared to the traditional PDM control technique, in which durations of an injecting interval and a free- wheeling interval are multiple of the SRI output voltage period, the enhanced PDM techniques operate with durations that are multiple to half-period of the SRI output voltage. In [13] an enhanced PDM technique is proposed, according to which only the duration of the free-wheeling interval is a multiple of the SRI output voltage period. This makes it possible to achieve smaller fluctuations in comparison to the traditional PDM control technique for the same pulse density. In [14-16], the authors proposed the enhanced PDM technique, according to which both the injection interval and the free-wheeling interval are multiple of the half-period of the SRI output voltage. This technique provides smaller fluctuations compared to the traditional PDM control technique and the enhanced PDM control technique proposed in [13]. But in this case, the SRI output voltage contains a DC component. To avoid the appearance of the DC component on the primary side of the matching transformer of the induction heating system, a blocking capacitor is required, and it should be paid careful attention to the choice of its capacity. Thus, this PDM technique is convenient when there is no need to use the matching transformer in the induction heating system. Also, modular converters topologies with PDM control have been proposed to reduce the peak-to-peak current fluctuation [16-18]. The use of the converters that have several identical channels with the interleaved control to reduce a current/voltage ripple is well known [19-20]. The more channels in the converter, the fewer the fluctuation. But the modular converter must have at least two channels, which is not always expedient in the case of low-power converters. In [21] the authors proposed the extended topology of the SRI, which combines the topologies of a half-bridge inverter and a full-bridge inverter. This converter topology has fewer transistors and provides the same peak- to-peak current fluctuations in comparison to the two- channel modular converter with the interleaved or stepped PDM controls. However, this topology has its drawbacks: the reverse recovery problem of anti-parallel diodes; not evenly discharging the capacitors of the half-bridge inverter topology. Still, there is necessary to have more transistor in comparison to a conventional full-bridge inverter topology. Combining PDM control with other control techniques makes it is possible to reduce the fluctuation. One of the control techniques which is reasonable to use with PDM is the phase-shift (PS) control. Because the phase-shift- controlled SRI operates at the frequency close to the operating frequency of the pulse-density modulated SRI, so it is easy to combine them. Hybrid PS-PDM power control is presented in [22], in which the PS and PDM control techniques operate simultaneously, which means that the phase-shift control is used between PDM patterns to provide more continuous power regulation, which in general does not allow to reduce the current fluctuations. Also, during a working process, which is presented in [22], the output voltage of the SRI contains a DC component. In [23], among other things, the author also discusses a tuning system which can simultaneously utilize pulse-density- modulation and phase-shift controls, which leads to more continuous power regulation, compared to the traditional PDM. However, the output frequency could be three and more times less than the operating frequency of the traditional PDM SRI. Thus, in this case, the matching transformer will be significantly larger.