Linear-Sinusoidal Air-Core Transformers Featuring Multicycle Passive Resonance for Wide-Input Low-Power Applications

In this article, a linear-sinusoidal air-core transformer (ACT) featuring multicycle passive resonance is proposed for wide-input low-power applications, which has the advantage of smaller inductance and coupling coefficient but an operating frequency of only hundreds of kilohertz. The proposed ACT has a simple structure, where the half-bridge <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CLLC</i> transformer is modified to a flyback coupling type and a half-wave rectifier with a freewheeling diode is employed. It achieves a combination of linear current injection and sinusoidal current feeding, and the operating frequency and turn-<sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> losses are significantly reduced via multicycle passive resonance modulation. Moreover, the leakage inductor of the ACT is multiplexed as the resonant inductor, providing soft switching for semiconductors. The article investigates the operating principle, voltage gain, turn-<sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> current, maximum resonance number, and soft switching of the proposed ACT. Then, according to the voltage gain range and loss characteristics of multicycle resonance, two voltage gain modes are designed for the wide input application, and the mode transition strategy and parameter design method are given. To verify the theoretical analysis, a 48 W prototype with an operating frequency of 120/240 kHz, an input voltage of 43–160 V, and an output voltage of 24 V is fabricated, achieving a peak efficiency of 92.56%.