Bidirectional DC-to-DC converter for solar battery backup applications

The application of battery backup systems automatically leads to inverter structures with relative low input voltage levels compared to the DC-link voltage. To guarantee the required system capability the power electronic system has to maintain the resulting high input current ratings. This leads to a problematic design with noticeable efficiency restrictions. To improve the overall efficiency of the inverter and to reduce the input capacitor, a possible solution is the parallel operation of several converter stages. Here, a special disadvantage has to be marked: The parallel operation of conventional DC-to-DC converters with voltage source characteristics mostly used requires special control mechanism to ensure correct power distribution. Contrary to this behavior, current sourced converter types can act in parallel without any special measurements. The solution proposed in this paper is capable to improve the parallel operation of power stages in conventional converters with modern current sharing techniques. Due to the reduced current in each of the paralleled output stages, the efficiency will increase significantly. The ripple of the input current is shared between the different stages, which helps to reduce the input capacitor in each string. In addition, the resulting frequency rises with the number of used stages when multiphase operation is used. This leads to a more silent (EMC) design. Another big problem, the output rectifier operating at the high DC-link voltage, can be disburdened significantly by using the suggested current sourced inverter structure. In this paper a bidirectional 12V / 500 W current sourced inverter for solar battery applications is described. Due to the special design, it could be built up by the usage of cheap mass components. The power stage operates at 50 kHz, supplying a 400 V DC-link.