Three-Level DC-DC Converters: Topology Derivation And Control Strategy

The three-level (TL) topology derivation is a method to reduce the voltage stress of each switch in the converter by increasing the number of switches. The method and its modification make the converter have higher input voltage and/or higher output voltage, and thus widen the application range of the converter.Several TL converters are classified from papers of recent years, the basic method of TL topology derivation is obtained, which can be used to deduce the other kinds of TL topologies. The analysis reveals that single-switch converter can not be converted into TL topology by the basic method because of its asymmetry. Using the modified method, 6 kinds of TL topologies of single-switch converter are obtained. The deep research reveals that these TL topologies can have less value of the inductor by using interleaving modulation strategy compared with conventional modulation strategy on the condition that the current ripple of the inductor are the same.Some drawbacks such as high voltage stress, difficulty of realizing soft-switching, and magnetic aberration were presented in the conventional push-pull converter. The push-pull TL converter can be deduced by using the basic TL topology derivation and by special derivation only used in this topology, in which, the voltage stress of each switches is input voltage. The scheme of realizing soft-switching can be found, at the same tune, the concepts of the leading switches and the lagging switches are presented. Two kinds of soft-switching methods, zero-voltage-switching (ZVS) and zero-voltage and zero-current-switching (ZVZCS), can be classified on the base of realizing soft-switching of the lagging switches. These methods all can be used to solve the magnetic aberration hi the conventional push-pull converter.In DC-DC converters above a few kilowatts, the ZVS full-bridge (FB) converter is a preferred topology. Its main advantages are soft-switching while operating at constant switching frequency and a simple control. In order to be applied under more high voltage, the FB converter can also be converted to TL topology. Using the basic TL topology derivation method, FB TL converter can be deduced. With the increase of the switches, more modulation strategies of the FB TL converter can be presented. With the best modulation strategy which was found on condition that it must have largest energy transfer ratio, smallest current ripple of the inductor, and realizing ZVS of each switches, the converter overcomes the drawbacks presented by the conventional ZVS FB converter, such as high voltage stress of each switches, large current ripple of the inductor, and severe parasitic ringing on the rectifier diodes, so it can get higher efficiency, faster respond speed, and wider application area. The operation principle of the proposed converter is analyzed and verified by a 3kW, 50kHz experimental prototype.Experiments and research results show that the scheme of the TL topology derivation is reasonable, adjusted modulation strategy makes some other severe problems in conventional topologies easy to solve or makes the converter more competitive.