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

Abstract

A comprehensive description of magnetically controlled transformers is presented in this paper to asses 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 pre-magnetization 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 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 trade-offs. 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.

Dual Active Bridge Converter With Variable Transformer for Wide Voltage and Wide Load Range Operation

“…Fig. 11 illustrates the relation between K, G, and N sr for the DUT, consistent with the expressions derived in [22]. As shown, the VT's turns ratio gain can be increased by adjusting N sr , allowing it to shift its value from 1.37 to a maximum of 1.46.…”

“…A. EMPLOYED STRUCTURE [22,Fig. 3] shows the selected VT structure, it consists of three U-U ferrite core sets and five windings, in which winding w p , with N p turns, is tightly coupled to winding w s1 , which has N s1 turns, and loosely coupled to winding w s2 , which has N s2 turns.…”

“…This condition allows neglecting the induced voltage on the auxiliary windings and the oscillations on ℜ v U owing to the flux's variations through the transformer primary and secondary windings, ϕ AC1 and ϕ AC2 . For a detailed analysis, modelling and characterization procedure of the VT, refer to [22].…”

“…Nonetheless, it is possible to substantially enhance the turns ratio gain, as described in [22], by employing modified core arrangements that increase the coupling among the VT windings. This results in a higher value of K without altering the ratio between the turns of the secondary winding.…”

“…In this context, the magnetically controlled transformer introduced in [22] is a suitable alternative. Unlike the tapchanger solution, the change in the equivalent turns ratio is continuous and is given by the control of the transformer's equivalent series and parallel inductance by means of a DC current.…”