Design of Transformer Turn-ratio for Maximizing the ZVS Region of Dual Active Bridge Converter

This paper proposes a design the turn ratio of the high-frequency transformer for maximizing the zero-voltage switching region under varying output voltage conditions. The dual active bridge converter can achieve zero-voltage switching for all switches when the input-to-output voltage ratio is unity. However, when the input-to-output voltage ratio differs from unity, hard switching occurs, leading to increased switching losses. This issue is crucial in battery-connected systems with variable voltage, where maintaining a wide zero-voltage switching region is essential for achieving high-efficiency. Among the parameters related to the dual active bridge converter, the turn ratio of the high-frequency transformer is a crucial design parameter in determining the zero-voltage switching region, as it directly affects the voltage ratio between the primary and secondary sides. Conventional method designs the turn ratio based on the midpoint of the output voltage variation condition (i.e. battery voltage), but this approach does not guarantee a maximized zero-voltage switching region under varying conditions. In this paper, unlike the conventional method of evaluating the ZVS area of a DAB converter using the phase shift ratio and the primary/secondary voltage ratio, a method of evaluating the ZVS area using the power between the primary/secondary sides and the voltage ratio is proposed. Through mathematical analysis, the turn ratio that maximizes the zero-voltage switching region is analytically derived, and the effectiveness of the proposed design method is verified through experiments.