Bidirectional High Current DC/DC Converters for Capacitive Deionisation Water Treatment

Abstract

This thesis proposes three new DC/DC topologies and related technologies to control salt removal from water sources using the capacitive deionisation (CDI) technique. These technologies are critical in improving energy utilization, higher product yield and water recovery, and simpler design. A lossless bidirectional current sensing circuit is proposed. The proposed circuit avoids the use of conventional current shunt and extracts the current information from the winding resistance of the inductor (DCR) without introducing excessive conduction loss. Moreover, this improved DCR current sensing circuit has a high bandwidth and a low error over the entire range, even near the zero-crossing. A successful application of the proposed circuit is demonstrated in a 5-phase interleaved Buck/Boost bidirectional converter. The same converter has been used for CDI cell characterization. A time-domain analysis of the three-phase interleaved LLC topology is presented. The proposed analysis method reveals various facts that cannot be explained with the conventional Fundamental Harmonic Analysis (FHA) methods, including the number of resonant frequencies. The theory also gives a more accurate prediction of the gain-frequency-power relationship and the soft-switching conditions. Extensive simulations and experiments validate the correctness of the theory. Two new switch-capacitor two-phase interleaved flyback converters are proposed, which can invert the polarity of the input voltage and efficiently supply a high current while inheriting all the advantages of the Buck and Boost counterparts, such as the intrinsic current sharing, high conversion ratio, lower current ripple, and reduced switching loss. The operating principle, key waveform, simulation, and experimental results are presented. Finally, a new two-phase interleaved bipolar four-quadrant converter is proposed. Without sacrificing efficiency, it enables high-current discharge at extremely low cell voltage and features seamless transition. The proposed converter combines the switch-capacitor flyback converter with the switched-capacitor Buck converter in a creative manner so that the input and the output share a common ground reference while featuring a bipolar output, which can simplify the wiring when connecting more units in parallel. A switching pattern is proposed to enable a seamless transition between different operation modes. An auxiliary switching network is introduced to correct the loss of natural inductor current balancing in the transition mode