Research On Bi-directional DC-DC Converter

Bi-directional DC-DC Converter (hereinafter, "BDC") is the two-quadrant operating DC-DC converter. The power can flows in either direction, depending on the location of the DC source and the load. The BDC can perform two or more functions, so it is one of the useful methods to reduce the size, the weight and the cost of the power system. Three types BDC is proposed in the dissertation, and the related problems are researched. I. Push-pull Forward Phase Shifted Bi-directional DC-DC Converter The paper proposed the phase shifted BDC based on the cell of the push-pull-forward converter. The research work covers four aspects. i) The freewheeling current of the push-pull forward converter is detailed from the point of energy conversion. The analytical expressions of the input current and the value of clamped capacitor are given. ii) The operating principle of the push-pull forward converter is analyzed, which has the advantages of lower current ripple of output inductor, the realizing of ZVS during the duty ratio of 0.5 to 1. iii) A separated charging and the discharging channels ZCT scheme is proposed, which reduces dramatically the current stress in the main switches. iv) The phase shifted BDC based on the cell of the push-pull-forward converter is proposed. The simple topologies make it suitable in medium power BDC. II. Two-stage Bi-directional DC-DC Converter The non-isolated BDC and the "DC transformer" connecting in cascade forms the two-stage BDC. The research work covers four aspects. i) The state-space-average model is different in either power direction of BDC. The controlled model of BDC and the single voltage loop PID controller is proposed. ii) The operating principle of the "DC transformer" with push-pull forward cell is proposed. It has the advantages of wide frequency band, high power density. iii) The two-stage BDC with non-isolated BDC and DC transformer is proposed. The two-stage BDC has the advantages of ZVS switching, high power density, convenient to optimum design and convenient to modularize. The two-stage BDC is the good candidate in the situation of large voltage convert ratio. iv) The concept of the "DC transformer" can be applied in the situation of high frequency linked AC-AC transformer. Three structures of AC-AC transformer are given, which reduce the volume and the weight of the power system, are suitable for all shaped low frequency input voltage and all types of load. III. Forward-flyback hybrid bi-directional DC-DC converter The forward-flyback hybrid bi-directional converter is composed of the forward converter and the flyback converter; the windings of the forward and the flyback converter are in series on one side and parallel on the other. The coupled inductor and the forward transformer transfer the power together in which, the coupled inductor in flyback BDC, and the leakage inductance in phase shifted BDC vice verse. The proposed BDC has no high voltage spike problem, which is inherent in isolated Buck/Boost BDCs. It is easy to realize soft switching by proper control and design. The built-in flyback converter acts as the filter inductor, so the current ripple is small. Further more, the hybrid structure makes it suitable for high power situation. The paper detailed the steady state operation principles. The experimental results verify the analysis. Based on the principles of the active clamp forward-flyback BDC, a family of BDCs is proposed.