Research On Impedance Measurement Techniques In DC Distributed System

DC distributed system offers a number of advantages, such as high reliability, design flexibility, good modularity and maintainability, has been widely used in different power supply systems. The stand-alone power electronic modules are well-designed to be individually stable with good dynamic performance, but after system integration, the interaction between modules may cause system stability degradation. To analyze the stability, the impedance criterion was proposed by Middlebrook. Therefore, how to extract the impedance of power converters becomes a crucial issue to verify the stability. This dissertation focus on the impedance measurement techniques in dc distributed system.The canonical small-signal model of different DC/DC converters viewed as a linear two-port model is used to derive the closed-loop impedances with different control modes. The effect of time delay caused by digital control on output impedance is analyzed in a digitally controlled synchronous buck converter in voltage mode.The factors such as the amplitude of injected signal, the sweep period, measured points and grounding problem which influenced the measurement accuracy are analyzed. The measurement of parasitic parameters in the converter is proposed. Then the practical issues and characteristics of input/output impedance measurements are introduced. Due to the pulsating and discontinuous nature of input current in most types of buck converter, the measured data of input impedance has an attenuation characteristic and the reason is explained. Based on a synchronous buck prototype, comparisons of the measured data and the theoretical input/output impedance are made to verify the measurements.The H-Bridge circuit and chopper circuit used to implement current injection in the measurements in high-power system are presented. The features and design considerations of the two injection circuits are discussed and the chopper circuit test setup for measuring impedance is realized. The laboratory measurements demonstrate the effectiveness of the injection setup.The measurement techniques can be used to measure the dc interface of AC/DC converter too. The output impedance of PWM rectifier is derived and the application is verified by comparing the measured and theorical impedance. Based on the impedance measurements, this thesis discusses the stability analysis of source and load interactions in dc distributed system. A dc cascade system with a PWM rectifier as the source subsystem and a synchronous buck as the load subsystem is used to measure the stability margin. Meanwhile, a cascade-connected synchronous buck converter system is considered to judge the stability by impedance measurements.