Double Frequency Modulation And Parallel Operation For Resonant Converters

The global energy shortage and environmental pollution urge people to seek for more efficient and environment friendly energy supply and consuming methods. The energy network, DC micro-grid, DC distribution grid and electric vehicle are among the many green ways that researchers and industries are focusing. Power electric equipment is key to realizing those visions, and especially high performance unidirectional and bidirectional DC/DC converters play important roles in those applications. Resonant DC/DC converters are ideal candidates because of the full range zero voltage switching (ZVS) realization which significantly elevates the efficiency. However, in order to achieve the aforementioned application, for the resonant converters, the efficiency needs to be further increased, the size and weight need to be reduced, and the stable and flexible parallel method needs to be provided. In this paper, the double frequency modulation for single phase inverter is transplanted to full bridge LLC resonant converter, deriving a series of high efficiency high power density double frequency resonant converters. Based on the Envelope model of LLC resonant converter, by adopting the resonant current control, the stability and dynamic characteristics of the paralleled resonant converters are improved.First, inspired by the double frequency modulation for single phase inverter, the asymmetrical double frequency modulation for full bridge LLC resonant converter is proposed. Because the total switch output capacitance taking part in every switching is halved, the required magnetizing current is lower, which reduces the resonant current for high frequency applications. Thus the conduction loss is reduced and the sizes of the resonant components are smaller. Moreover, the double frequency LLC resonant converter for high input voltage is proposed. The sizes of resonant components are reduced, and high step-down conversion ratio is achieved. A series of double frequency converters for high input voltage are derived, which extends the applications of the double frequency modulation.Second, the paper proposes a double frequency bidirectional resonant converter. For high power applications, IGBT is widely used, but the tail current limits the switching frequency, which leads to large sizes and heavy weight of passive components. The proposed double frequency modulation doubles the resonant tank frequency, reduces the sizes of the transformer and resonant components while keeping the safe operation of IGBTs. The symmetry of the resonant tanks on the primary and secondary sides results in symmetrical voltage and power output characteristics for different transmitting directions. That effectively makes it easier to design and control the bidirectional converter. The complete close-loop control strategy is provided, especially for start-up and power transmitting direction change.In order to use the resonant converter in higher power applications, the stability analysis of the paralleled resonant converters is provided. The resonant current control is introduced to improve the stability and bandwidth of the parallel system as well as the modules. Based on transformation between the time-domain signal and the rotary vector, the Envelope model of the LLC converter is analyzed. The paper reveals the sensitivity of the small-signal characteristics to the operating points and the reason of the poor dynamic performance with the conventional single voltage loop control. The principle of the resonant current loop is analyzed, which unveils that the essential function is reducing the system orders. The complete design guideline is provided based on a 16kW parallel system to increase the practical value of the design method.The proposed converters and control strategy are all analyzed theoretically and verified experimentally. Moreover, the resonant current control is applied to 6kW modularized DC supply engineering project. The configuration, control strategy, start-up, fault handling and the autonomic digital current sharing control and communication protocols are illustrated. The modularized power supply has been in mass production.