| In order to save energy and protect the environment, how to use energy in an efficient way has attracted more and more research efforts. Energy storage system is needed in the renewable energy systems to deal with the imtermitent nature of renewable energy sources like photovoltaics, wind power and fuel cell, in which bidirectional DC/DC converter is a key component. Besides that, bidirectional DC/DC converter is also adopted in electric vehicles, hybrid electric vehicles and many other application fields. Since the bidirectional DC/DC converter can regulate the output power and make the system stable, research about the high efficiency, high reliability and high power density bidirectional DC/DC converters has become more and more popular in recent years.Different kinds of bidirectional DC/DC converters are proposed in recent years, and the dual active bridge converter is the most popular among all these topologies.It has the advantages of simple structure, high voltage and high power level, high power density and easy to achieve ZVS. However, it suffers from narrow soft switching range, high circulating energy and high turn off current, which results in lower conversion efficiency. LLC resonant converter is a proposing candidate to solve these problems, which can achieve ZCS and ZVS. The bidirectional operation capability of LLC resonant converter is analyzed in detail in this paper.A bidirectional full-bridge LLC resonant converter is proposed firstly, which has two different control methods:the synchronous control and the asynchronous control between the primary side switches and the secondary side switches. The synchronous control method is simple, and zero voltage switching (ZVS) can be achieved, which contributes to higher efficiency. However, zero current switching (ZCS) is lost, and the existence of circulating energy limits the voltage gain range.With the asynchronous control method, ZVS, ZCS and buck/boost capability can be achieved for the proposed bidirectional LLC resonant converter, so the voltage gain range is wider. Besides that, switching frequency is always below or equal to the resonant frequency, and the switching loss will be lower. When the converter is in bidirectional operation, forward mode and backward mode can be automatically switched by regulating the switching frequency. The bidirectional full-bridge LLC converter is attractive in the applications of energy storage systems.In order to further wider the output voltage gain range of bidirectional LLC converter without reducing the conversion efficiency, a hybrid full-bridge three-level bidirectional LLC resonant converter is proposed. Pulse width and amplitude modulation (PWAM) is used to regulate the output voltage instead of frequency modulation, and ZVS and ZCS can always be achieved. Three different control schemes are proposedand and there are three operation modes respectively, by switching the operation mode between each other, the whole output voltage gain range is very wide and can be modulated continuously and seamlessly.Besidesthe bidirectional LLC converter, aunidirectional power supply circuit between the energy source and load is also needed in the distributed generation system. When the bidirectional converter and the power supply converter are operating independently, the control method is complex and more components are used. In order to reduce cost and increase the power density, a novel three-port LLC resonant converter is proposed in the paper, which integrates the energy source, energy storage component and load. ZVS and ZCS can be achieved in the proposed converter. Four different operation modes are proposedto deal with different input power and load condition, which will keep the output power constant and make the operation of system more stable. |
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