Byungcho Choi

A method of defining the load impedance specification for a stable distributed power system

By applying the loop gain analysis technique, a forbidden region for the polar plot of the ratio of impedances at the interface between two cascaded power subsystems is determined. A method of transforming the forbidden region into a load impedance specification for a given source impedance is developed. The method assures system stability and minimal performance degradation of the distributed power system, while allowing impedance overlap at the interface. >

Control strategy for multi-module parallel converter system

A systematic control-loop design procedure for a multimodule converter system for high-current, low-voltage applications is presented. A small-signal model of the system is derived using the PWM (pulse width modulation) switch model and the small-signal model of the current mode control. The small-signal model for the multimodule converter system is simplified to a equivalent single-module model. The control-loop design is implemented using the single-module model. A three-loop control strategy for the multimodule converter system with a secondary output filter is developed. Significant improvements of small-signal performance and module-failure response are achieved using additional feedback from the intermediate filter stage. The small-signal analysis of the three-loop controlled converter is performed, focusing on the effects of the local voltage feedback on the closed-loop performance of the system. It is shown that local voltage feedback minimizes any detrimental effect of the resonance between the power stage filter of each converter module and the common output filter.<<ETX>>

Three-loop control for multimodule converter systems

A three-loop control scheme for multimodule power converter systems with a secondary LC filter is presented. In addition to output voltage and inductor current feedback, the control scheme employs feedback from the output capacitor of each module to improve the dynamic performance of the system, particularly the transient response in the event of failure of a module. The superiority of the three-loop control over the conventional two-loop current mode control is demonstrated by both small-signal analysis and large-signal simulations. >

Intermediate line filter design to meet both impedance compatibility and EMI specifications

This paper presents simple, noniterative, and practical design procedures for intermediate line filters intended for distributed power applications. The design procedures realize a line filter which simultaneously meets both impedance compatibility requirements and EMI specifications. Design examples are given for both single-stage and two-stage filters. >

Dynamic analysis and control design of LCC resonant converter

The dynamics of LCC resonant power converters are investigated using a small-signal model which is accurate up to the switching frequency. It is shown that the beat frequency dynamics is not only related to the operating region but also heavily dependent on the output filter design. The authors concentrate on frequency control of LCC resonant converters. Compensator designs are discussed, taking into consideration the strong impact of beat frequency dynamics. The frequency-domain analyses are verified using accurate large-signal simulations.<<ETX>>

Analysis and design of a forward-flyback converter employing two transformers

This paper presents the steady-state analysis and design of a forward-flyback converter that employs two transformers and an output inductor. By utilizing two separate transformers, the proposed converter allows a low-profile design to be readily implemented while retaining the merits of a conventional single-transformer forward-flyback converter with secondary center tap. By using an output inductor, the proposed converter efficiently reduces the output ripple to an acceptable level. The design and performance of the proposed converter are confirmed with experiments on a 100 W prototype converter.

Averaged Modeling and Switching Instability Prediction for Peak Current Control

This paper attempts to resolve some fundamental issues related to averaged modeling of peak-current controlled PWM converters. One issue to be addressed is the ability of averaged models to predict switching instability (also referred to as subharmonic oscillation) under peak-current control. It will be shown that, contrary to the widely held belief, such instability can be predicted by properly defined averaged models, and that incorporation of an additional second-order transfer function representing the so-called sample-and-hold effect is not necessary for this purpose. The averaged models capable of predicting such instability are developed by combining standard state-space averaged models with a properly defined duty-ratio constraint. A circuit approach based on general topological properties of PWM converters and a mathematical approach based on KBM ripple estimation algorithm to the derivation of the duty ratio constraint are reviewed and are shown to yield identical results. Implementation of the averaged models in circuit simulation programs using iterative method is also presented

Average Current Mode Control for LLC Series Resonant DC-to-DC Converters

An average current mode control scheme that consistently offers good dynamic performance for LLC series resonant DC-to-DC converters irrespective of the changes in the operational conditions is presented in this paper. The proposed control scheme employs current feedback from the resonant tank circuit through an integrator-type compensation amplifier to improve the dynamic performance and enhance the noise immunity and reliability of the feedback controller. Design guidelines are provided for both current feedback and voltage feedback compensation. The performance of the new control scheme is demonstrated through an experimental 150 W converter operating with 340 V to 390 V input voltage to provide a 24 V output voltage.

Control Design and Loop Gain Analysis of DC-to-DC Converters Intended for General Load Subsystems

DC-to-DC converters are usually intended for general applications where the load impedance characteristics are unknown or undefined. This paper establishes the control design procedures for DC-to-DC converters in the absence of any prior knowledge on their load impedance. The proposed control design can be universally adapted to all the DC-to-DC converters regardless of the impedance characteristics of their actual load. This paper also presents the loop gain analysis of the converter combined with an actual load whose impedance characteristics are only available afterward. A graphical analysis method is proposed, which enables us to predict the loop gain of the converter in the presence of an arbitrary load impedance. The validity of the analysis method is demonstrated using a current-mode controlled buck converter coupled with an inductive load, capacitive load, and converter load. Theoretical predictions are verified with both computer simulations and experimental measurements.

Control design and closed-loop analysis of a switched-capacitor DC-to-DC converter

This paper presents the theoretical and practical details involved in the control design and closed-loop analysis of a step-down switched-capacitor DC-to-DC power converter. The state-space averaging technique is applied to extract the small-signal dynamics of the power stage, and a graphical loop gain method is used to design the feedback compensation and analyze the closed-loop performance of a switched-capacitor converter. The results of the control design and closed-loop analysis are substantiated by experiments on a prototype switched-capacitor converter.