Broadband Impedance-Measurement Methods in Dynamic Analysis of Dual Active Bridge Converters

The output impedance of a switched-mode converter is an important parameter in the converter small-signal stability analysis and stability-enhancing control. One of the popular methods for obtaining the converter output impedance is to introduce a broadband signal such as the pseudo-random binary sequence (PRBS) to the converter duty cycle; the resulting output voltage and current are measured, and the output impedance is obtained with Fourier methods. However, such techniques have not been fully considered for dual active bridge (DAB) converters. This paper discusses the challenges of broadband impedance measurements on DAB converters, compares different implementation methods, and outlines guidelines for achieving accurate results. Unlike conventional dc-dc converters, the power of a DAB converter can be controlled by both the duty ratio and phase shift, thus allowing several methods to perform the impedance measurement. Yet, nonlinearity between the power transfer and phase shift and duty ratio introduces potential challenges to the measurement implementations. Experimental measurements based on a DAB converter are shown to demonstrate these challenges as well as the effectiveness of the proposed measurement guidelines.