Brad Bryant (Committee Member), Malcolm Daniels (Committee Member), Marian Kazimierczuk (Advisor), Kuldip Rattan (Committee Member), Raymond Siferd (Committee Member)
Doctor of Philosophy (PhD)
Current-mode control is a commonly adopted method of regulation for pulse-width modulated (PWM) dc-dc power converters in industry, but is not well understood. The advantages of current-mode control over the voltage-mode control include inherent overload and short circuit protection, faster response, line-noise rejection, and multiple converter paralleling. Current-mode controlled system consists of (1) an inner-current loop and (2) an outer-voltage loop, which sets the reference voltage to the inner loop. To ensure stable operation of the multi-loop converter, all the sequential loops in the circuit should be stable with sufficient degree of stability. The research in this dissertation is focused on the relative stability of the inner-current loop in peak current-mode (PCM) controlled PWM dc-dc converters operating in CCM.
The operating principle of peak current-mode control is presented. The inner-current loop dynamics of a peak current-mode controlled dc-dc converter is investigated using perturbation theory. Considering its mixed-signal (analog and digital) behavior, the current loop is modeled using sample-and-hold theory. Taking the discrete nature of the inner-current loop into account, a closed-loop transfer function for the current loop is derived in z-domain and an equivalent-hold approximation is used to derive an approximate closed-loop transfer function in the continuous s-domain using modified Pad´e approximation. A general expression for the loop gain of the inner-loop, independent of the converter topology, is derived. Using the loop gain, a measure of relative stability of the inner loop is developed. Expressions for amount of slope compensation required at maximum duty cycle, for the inner loop to be marginally stable and to achieve a specified margin of stability, are derived. Also, expressions for maximum duty cycle at a given amount of slope compensation, for the inner loop to be marginally stable and to obtain a specified margin of stability, are derived. The control current expressions for the inner loop of peak current-mode controlled converters without and with slope compensation are derived. A procedure to design the inner-current loop is developed. Saber Sketch simulation and experimental results are presented to validate the presented theory. The dynamic behavior of the inner-current loop of peak current-mode controlled PWM dc-dc buck converter operating in CCM is analyzed.
The critical path power stage transfer functions, the relevant inner-current loop transfer functions, and the control-to-output transfer function of peak current-mode controlled PWM dc-dc buck converter operating in CCM are derived. The presented model is validated using experimental Bode plots.
Department or Program
Ph.D. in Engineering
Year Degree Awarded
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