Research On Key Technologies Of Bidirectional CLLLC Resonant DC/DC Converter

Isolated bidirecjuyoutional DC/DC converters can realize direct current power conversion, and has the advantages of high efficiency and power density. Meanwhile, it has a broad application in many fields such as electric vehicle, energy storage system, power quality regulation, and renewable power generation. The LLC resonant converter as high power isolated DC/DC converter has significant advantages considering its nature soft-switch feature. However, the traditional LLC resonant converter can be only used in unidirectional operation. Therefore, the author proposed to add an LC resonant circuit into LLC resonant converter in this thesis, which forms CLLLC resonant network, called bidirectional CLLLC resonant DC/DC converter, abbreviated as CLLLC resonant converter. It enables bidirectional operation. The symmetric circuit topology can guarantee the consistency of bidirectional operation. Moreover, it keeps natural soft-switching characteristic as LLC resonant converter regardless of operation direction. In this article, detailed study has been conducted in the aspect of soft switching, light load operation, starting operation and application in DC micro-grids.This paper firstly analyzes the resonant frequency characteristics and gain feature of CLLLC resonant converter using first harmonic approximation, studies the influence of resonant network parameter change on converter gain. Then the function of using gain characteristics for the parameter design in CLLLC resonant converter is discussed. After the working principle of dead-time for the CLLLC resonant converter is described, time domain analysis is applied to derive state equations in different operation mode, then the disturbance of the resonant current waveform caused by secondary side parasitic capacitance and the problem of floating output voltage when light loaded are analyzed. Aimed at the problem that secondary side parasitic capacitance causing the failure of converter soft switching, an improved parameter design method is proposed, which can eliminate the disadvantageous of parasitic capacitance effectively and maintain soft-switching characteristic in the bidirectional operation.Aimed at the low efficiency and the floating output voltage of CLLLC resonant converter when light loaded, a burst mode control strategy based on Bang-Bang charge control is proposed. The referred burst mode control combines Bang-Bang charge control with busrt mode operation, lowers equivalent switching frequency while guarantees the realization of soft-switching, and further improves converter efficiency when light loaded. Bang-Bang burst mode also makes full use of Bang-Bang charge control fast dynamic response and controllable input power characteristic, not only limits output voltage, but also accelerates process switching between burst mode and normal mode while improves transition effect.CLLLC resonant converter will charge output filter capacitance during start-up procedure, thus produce large impulse current. In the article aiming at the starting operation of the converter and a soft-starting control strategy based on phase-shift control is proposed. CLLLC resonant converter possesses the characteristic of gain increasing monotonously with the duty ratio when using phase-shift control. Therefore, regulating the duty ratio in phase-shift control during the start-up procedure can promote output voltage to rated value smoothly and quickly, which avoids the impulse starting current effectively.The application of CLLLC resonant converter in direct current micro-grid is then discussed. After analyzing the condition of power flow in the demonstrated DC micro-grid, a power flow transition control based on mode judgment for the CLLLC resonant converter is proposed. The proposed control strategy can judge converter operation direction precisely, and make the converter switching accurately and quickly among different operation directions and working modes. Finally, prototype experimental results show that CLLLC resonant converter can regulate the power flow reliably and effectively as the interface between two DC buses, maintain power balance in DC micro-grids.