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Synchronous rectifiers (SRs) composed of MOSFETs have recently been employed to replace the conventional rectifiers with diodes. SRs are widely used in switched-mode power supplies with low output voltage and high output current for efficiency improvement. Owing to the high-efficiency characteristic, it is adequate to use an SR buck converter in a voltage regulator for powering a central processing unit. Normally, such SR buck converter must operate at fairly high switching frequency for miniaturizing a whole circuit and achieving a fast response. However, at the conditions of low output voltage, high output current, and high switching frequency, the influence of parasitic elements to circuit operation will become extremely obvious. Therefore, the design considerations concerning the ripple voltage and the stability of such SR converters should be carefully investigated and clarified. By establishing the equivalent circuit and using a state-space averaged method, the ripple ratio of output voltage and the static and dynamic characteristics of the SR buck converter with nonnegligible parasitic elements are obtained. Thus, the design criteria concerning the output ripple voltage and the stability are clarified.
Date of Publication: March 2010