Challenges of multi-channel interleaved bidirectional power converters and their digital solutions

Abstract

This thesis work presents the challenges of bidirectional interleaved converters and proposes possible solutions which can be easily implemented using commonly available digital controllers. Implementation of interleaved power converters using coupled inductors in opposite direction has the advantage of reducing the magnetization current and hence reduces the volume of inductor required. However, use of coupled inductors has two main challenges. The first is the circulating current through the coupled inductors due to unbalanced pole voltage. The second issue is the sampling problem which causes oscillation in the output. In this thesis, a simple solution is proposed which can be implemented using simple DSP. Independent PI controller for each channel is proposed to ensure balanced channel currents and sampling at higher frequency and averaging it over a switching period is used to minimize the sampling error. Four channel interleaved bidirectional power converters are considered in simulation using both coupled and uncoupled inductors. It has been found that a multi-channel interleaved converter which uses single stage pair coupling gives a better volume reduction as compared to either of uncoupled inductors, circular inter-cell coupling or multi-stage pair coupling. Experimental setup is made to test the proposed solutions. The control algorithm is implemented using Piccolo F28069 DSP from Texas Instruments. The power converter was operated in two modes: as DC/DC buck converter and DC/AC inverter mode both of them at 10 kHz switching frequency. Using sampling instances which are synchronized with PWM carriers resulted oscillation when it is applied to coupled-interleaved power converters. On the other hand, simultaneous sampling of all channels at sampling rate which is a multiple of both the number of channel and the switching frequency gives a better system response. Harmonics analysis is done using Fast Fourier Transform (FFT). All the results reveals that interleaved power converters decrease current ripples and increase ripple frequency which can significantly reduce the size of passive elements in the converter. This increases the power density of the converter, increases system reliability and modularity and improves efficiency. Such converters suits very well not only for high power high current application, but can also be good candidates for low power applications which may require high power density