A five-element multi-resonant (LLC − LC) dc-dc converter can attain the wide range output power and voltage regulations by a practical-range pulse frequency modulation (PFM). It is a drawback, however, the power conversion efficiency deteriorates under the light load condition due to the conduction power loss in the LC anti-resonant circuit. As a solution to this technical problem, the pulse density modulation (PDM) is applied to the LLC −LC dc-dc converter as the submode power regulation scheme. In this paper, the essential performances of the new prototype of the PFM/PDM dual mode controlled LLC − LC dcdc converter are presented with the experimental verification. The comparison between the LLC − LC dc-dc converter and the conventional LLC dc-dc converter is described in terms of efficiency as well as output voltage and power regulations characteristics, and the effectiveness of the LLC − LC dc-dc converter is evaluated from a practical point of view. I. INTRODUCTIONAn LLC dc-dc converter utilizing parasitic parameters in the high-frequency (HF) transformer has excellent characteristics of a low EMI noise, soft-switching operations over the widerange output power variations, and high efficiency [1]-[5].One of the practical applications of the LLC dc-dc converters could be the front-end dc-dc converter in a plasma radio frequency (RF) power generator for semiconductor manufacturing machinery. Fig.1(a) and (b) illustrate the typical electric power conversion process of the plasma power generator. The plasma chamber has a wide range of load power variations, thereby the wide-range output power and voltage regulations are demanded for its power converters, especially in the frontend dc-dc converter.The conventional concept of the power conversion scheme for the wide-range voltage and power regulations consists of the two-stage power conversion [6]. The first stage of this power processing is the buck-boost dc-dc converter which regulates the dc voltage and power by pulse-amplitudemodulation (PAM). This power converter is followed by a HF-link dc-dc converter operating as the electric transformer. Although the power factor correction can be ensured together with PAM, the power conversion efficiency might be degraded in the light load condition due to the number of circuit components, which is also another disadvantage with respect to cost effectiveness.
An LLC resonant circuit-based full-bridge dc-dc converter with an LC anti-resonant tank for improving the performance of pulse-frequency-modulation (PFM) is proposed in this paper. The proposed resonant dc-dc converter, named as LLC-LC converter can extend a voltage regulation area below the unity gain with a smaller frequency variation of PFM by the effect of the anti-resonant tank. This advantageous property contributes for protecting over-current in the case of the short-circuit load condition as well as the start-up interval in the designed band of switching frequency. The circuit topology and operating principle of the proposed converter is described, after which the design procedure of the operating frequency and circuit parameters is presented. The performances on the soft switching and the steady-state PFM characteristics of the LLC-LC converter are evaluated under the open-loop control in experiment of a 2.5 kW prototype, and its actual efficiency is compared with a LLC converter prototype. For revealing the effectiveness of the LLC-LC resonant circuitry, voltages and currents of the series and anti-resonant tanks are analyzed respectively with state-plane trajectories based on calculation and experiment, whereby the power and energy of each resonant tank are demonstrated. Finally, the feasibility of the proposed converter is evaluated from the practical point of view. Keywords-LLC resonant converter, anti-resonant tank, LLC-LC resonant converter, pulse frequency modulation (PFM), over-current protection (OCP), buck and boost voltage regulations, zero voltage soft-switching (ZVS), zero current soft-switching (ZCS). Nomenclature f s : Switching frequency ω s : Angular switching frequency (ω s = 2πf s) M : Input / output voltage conversion ratio (M = V o /V in) a : High-frequency transformer windings turns ratio (a = N 1 /N 2) f rs : Series resonant frequency (f rs = 1/(2π √ L s C s)) f rm : LLC resonant frequency (f rm = 1/(2π (L s + L m)C s)) F rs : Switching frequency normalized to series resonant frequency (F rs = f s /f rs)
An LLC multi-resonant DC-DC converter with antiresonant tank is presented in this paper. The five-element multiresonant (LLC-LC) dc-dc converter can realize the wide range regulations of output voltage and power by pulse frequency modulation (PFM) under soft switching conditions. The resonant dc-dc converter proposed herein is suitable for renewable and sustainable energy applications such as battery chargers interfaced with a photovoltaic power generation system. In this paper, the design guideline of circuit parameters is originally described. Experiment results on the soft-switching performances and steady-state characteristics including the power loss analysis are demonstrated, thereby the effectiveness is verified from a practical point of view.
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