A Study Of Isolated Bidirectional DC/DC Converter Based On Digital Phase Shift Control

Isolated bi-directional DC/DC converters are applied in many fields. There are lots of topologies about them. In this paper, phase phift control isolated symmetrical half bridge (SHB)and asymmetrical half bridge(AHB) voltage source-current-source topologies are only studied because the topologies are fit for middle or low power and its circuit is simple. As important research content, the operation mode, the mathematic model of the topologies ,the operation characteristic and the scheme of digital control are studied in the two topologies. For SHB,based on the equivalent circuits and the switching function , a novel model-construction method is presented which the varying fast state variable is substituted into the varying slow sub-system ,then a descended order state space average model (SSAM) for large inertia sub-system can be gotten.. This method will make the analysis and calculation simple and the simulation time is shorten. Based on the simplified state space average model, a small signal model (SSM)of the topology is constructed and the compensator or controller design method is proposed according to the automation theory.. For the solution of stable state value ,there are two method. One of them is direct analysis method based on the circuit topology. The other is based on the small signal model. The same results can be gotten through the two different method and prove that the model is precise. The simulation and experimental results are presented to verified the theory analysis. In this topology, when the voltage between the energy transport inductance is not mismatch, a large current peak will appear. A multi-windings transformer can be used to avoid this negative state. The isolated (AHB) bi-directional converter is varied from the SHB topology, but its circuit is more simple. For this topology, there is a large current peak when the voltage between the energe transfer inductance mismatch. The larger the voltage error is ,the large the current peak is . In order to restrict the peak current , an improved phase shift control method can be applied. Based on this control method, a model construction method based on state space average is proposed. As a prototype with the topology of phase shift control isolated SHB or AHB, some actual problems such as the choice of switching devices, starting current, circulating current and efficiency and so on. There is a high voltage peak when the switching device is turning off under boost model. The principle of the high voltage peak is analyzed and the passive or active clamp circuit are presented to restrict the voltage peak. Experimental results shows starting current is larger than the normal current. In order to reduce the starting current , soft-start should be taken. An improved ramp input soft-start method and soft-start with flyback converter are presented. For the SHB bi-directional converter, it's very important for the topology to ensure the voltage capacitor sharing the input voltage in order to prevent the bias-magnetic phenomenon. A voltage error feedback control and integrated-magetic sharing circuit are presented .The latter is applied in isolated symmetrical half-bridge topology and the sharing voltage result is very good. The experimental results show the efficiency of phase-shift control bi-directional converter is not high. In this paper, the influence factors of efficiency are analyzed and the results shows reactive power and the circulating current are the key factors. In this paper, based on DSP TMS320C240 ,digital control method is adopted. A direct digital phase-shift method based on DSP full comparison is presented .Only by software program ,the phase-shift waveform can be gotten and the dead-time can be set. As bi-directional converter , there are two model: Boost mode and Buck mode. In Boost mode constant voltage control is needed and in Buck mode, constant current control is needed. According to the mathematic model of control system, a fit controller is designed and good control feature is achieved .In order to realize the reliable shift between the two modes, a reasonable shift method is proposed. A prototype, ration power 1kW, is made. The experimental result shows the model ,analysis and the design is correct.