Magnetically Controlled Transformer With Variable Turns Ratio and Low Series Inductance: Analysis and Implementation Toward Its Application in SMPS

A comprehensive description of magnetically controlled transformers is presented in this article to assess their potential in isolated and resonant power converters. These converters benefit from implementing variable magnetics as the soft switching zones and voltage operating ranges are extended, along with fixed- or quasi-fixed-frequency operation. To represent the variation of the transformer's equivalent turns ratio and parallel–series inductance, the cantilever model is selected. Moreover, two novel transformer structures employing parallel premagnetization are introduced; only off-the-shelf magnetic cores are required for their implementation. In conjunction with small-signal analytical expressions, an adapted SPICE model is provided and compared to finite-element analysis (FEA) simulations. Additionally, the impact of the stray inductance of the structures is assessed, improving the understanding of this controllable magnetic device's constraints and tradeoffs. Experimental results align with the derived models and prove the proposed structures to provide significant variations in transformer equivalent turns ratio, keeping a relatively high parallel–series inductance ratio.