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Study of Toroidal Core Multilimb Transformer (TCMLT) for High-Power DC Application

Multimodule isolated converters incorporating excellent features of galvanic isolation, bidirectional power flow, and simple control are widely adopted in medium-voltage (MV) high-power dc–dc applications. However, the converter size and dc-link capacitor voltage balancing are significant concerns in galvanically isolated cascaded dc–dc converters. This article proposes a new design of an integrated transformer implementing toroidal cores named toroidal core multilimb transformer (TCMLT), employed in a dc–dc modular isolated bidirectional dual-active-bridge (IB-DAB) resonant converter topology. The modular design structure of the proposed transformer can be conveniently constructed and implemented for more number of participating modules conforming the compactness constraint. Moreover, an external coupling winding is implemented in the TCMLT structure to resolve the dc-link voltage balancing issue. This coupling winding method substitutes the expensive complex control circuits, which consequently results in cost reduction and reliability enhancement. Besides, the reluctance model of the proposed structure is analyzed. Based on the reluctance model, a common mode–difference mode (CM-DM) electrical circuit is realized for the proposed scheme. Furthermore, unlike the conventional approach of implementing U, UI, and EI cores for multilimb transformer (MLT) structure with separate limbs, a synthesis method of TCMLT in toroidal form is proposed. The comparative study of the proposed design with the conventional U-core MLT confirms that the proposed TCMLT offers less leakage inductance with significant size reduction. Finally, to validate the efficacy of TCMLT, a three-limb laboratory-scale prototype is employed in a 6-kW cascaded IB-DAB resonant converter topology.