GENERAL CLOSED-FORM ZVS ANALYSIS OF DUAL-BRIDGE SERIES RESONANT DC-DC CONVERTERS

Abstract

Switching behaviour analysis is an indispensable step for evaluating the steady-state performance of a bidirectional DC-DC converter and a prerequisite for soft-switching modulation design. For Dual-Bridge Series Resonant DC-DC Converters (DBSRCs), the commonly used fundamental harmonic approximation (FHA) does not usually provide predictions accurate enough for reliable analysis and design. This project discloses the exact closed-form solution for the zero voltage switching (ZVS) operation conditions of DBSRCs for the most general case, in which all modulation quantities i.e. phase shift, duty cycles and switching frequency – are included. The proposed approach relies on a geometrical analysis of the converter state-plane trajectory, and allows to analytically predict the ZVS or hard-switching (HS) state of any switch and for any given converter operating point. By inherently capturing the effects of all tank harmonics, the model disclosed in this project shows higher accuracy than conventional FHA-based approach, and translates into a practical tool for ZVS prediction and optimization at the converter design stage. Based on the derived analytical results, switching behaviour of DBSRCs with minimum RMS current trajectory (MCT) modulation is investigated, and an effective design choice of resonant to switching frequency ratio is presented which contributes to reduced switching losses and enhanced efficiency. Furthermore, a variable frequency modulation scheme is formulated, achieving ZVS operation of all transistors over a wide input/output voltage range and efficiency improvement versus MCT technique.

Existing System

In the existing system, conventional FHA (Fundamental Harmonic Approximation) -based approach is used.

Proposed System

In this project, closed-form expressions for ZVS operation condition of DBSRCs are disclosed for a generic PWM+PSM scheme. The switched currents are analytically calculated by employing the geometrical relationship provided by state-plane trajectory. Analytical results for half-bridge DBSRCs are presented which covers all possible modulation techniques, bidirectional power flow and step-down/up operation, allowing to predict the switch ZVS or hard-switching (HS) state for all possible operating points. Next, a number of modulation strategies are analyzed in the context of the developed results. Firstly, ZVS operation ranges of PSM and PWM+PSM are respectively discussed. Then, switching behaviour of DBSRCs with minimum RMS current trajectory (MCT) modulation is investigated and a design choice of the resonant-to-switching frequency ratio is developed which benefits reduced switching losses. A variable frequency modulation technique with optimal switching frequency range is also formulated, achieving ZVS operation of all power switches over a wide input/output voltage range. Compared with FHA analysis, the proposed model not only achieves better accuracy for ZVS prediction, but also translates into a practical tool for ZVS prediction and optimization at the converter design stage. As a final contribution the effect of parasitic resistances on the ZVS boundaries is discussed. A simplified compensation method is presented, which provides an accurate correction criterion useful in wide voltage range applications where the voltage conversion ratio may be significantly away from unity.

Architecture

Topologies of bidirectional Dual-Half-Bridge Series Resonant DC-DC Converter, (a) non-isolated, (b) isolated