Research On Secondary-side LC Resonant DC-DC Converters

The development of DC/DC converters tends to be high frequency, high power andhigh density. There exist many research results showing that traditional hard-switchingDC-DC converters have largely limited its frequency and power. Resonant DC/DCconverters have been developing to address these issues. The ZVS characteristics ofprimary-side LLC resonant converter have become a hot topic, however, there are fewstudies on the secondary-side resonant circuit. Aiming at this problem, threesecondary-side resonant circuits are proposed in this article.Firstly, a novel LC resonant push-pull DC/DC converter is proposed based ontraditional push-pull circuit, and a frequency control with fixed turn-on time and variabledead time is adopted. No needing to leave a small gap for the transformer because of asmall rate (leakage inductance/magnetizing inductance), and that also reduces theoperating excitation current. In this circuit, the leakage inductance acts as resonantinductance, so no extra inductance need to be inserted. Working principles, gaincharacteristics, and efficiency characteristics are elaborated. An experimental prototypeis built, and the results have verify the correctness of working principles.Secondly, a bridge-type LC resonant push-pull circuit is proposed on the basis ofthe former circuit. Bridge-type structure reduces the voltage stress of power switches bya half. Blocking capacitors can effectively inhibit the useless resonance, which isconducive to improve the converter’s efficiency. The working principles are elaborated.Soft switching characteristics and the gain curve are discussed. A test prototype is built,and experimental results verify the feasibility and effectiveness of the proposedconverter.At last, a four-transistor voltage-doubler resonant converter is proposed with thesame frequency control method. All operating modes are analyzed in detail. The voltagegain is exhaustively derived. A experimental prototype is also built, and its results alsoverify the correctness of the convert.