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.