Efficacy of STC in High Frequency Full-Bridge DC–DC Converter

Abstract

In full-bridge dc–dc converter, the transformer is used to provide requisite voltage translation and safety isolation. For control of secondary-side variable(s) the control-actuation takes place at its primary. Converter's thermal deign presumes that the average flux in core in an integral switching-cycle is zero and the magnetizing current is negligible to ensure optimum core loss and, particularly, power loss in primary switching devices at turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> would be optimally small. The choice of control function should not disturb this basic precondition. Though, proportional plus integral control is mostly used, complex super twisting control (STC) is being rigorously tried as an apt alternate. The selection of its two gains is based on worst-case dynamic process behavior. This article addresses that such process-behavior-based gain-selection may not work well, particularly, when the actuation is through a transformer having nonlinear excitation characteristics. Using practical approach, this article elaborates that an optimally designed transformer, driven by high-gain STC, introduces dc bias in core leading to higher switching losses, poor operating duty-cycle of the system and the worst is, it could drive the transformer to saturation inviting reliability issues. The article further details with practical results an alternate high-gain control function for a high-frequency full-bridge dc–dc converter that generates superior effectiveness.