From the environmental point of view, low power consumption and low noise have been required in recent years and new soft switch-mode power supply which has been developed to meet these two requirements will be presented in this paper. The noise level of this new switch-mode power supply is lower than Class-B of CISPR by 30dB or more and the leakage current which tends to be increased in a low-noise power supply can be suppressed to 0.18mA or less to improve the efficiency about by 5, compared with conventional power supply. This paper will report the operating principle of the SMZ system, operational analysis, oscillating frequency characteristics, efficiency characteristics, noise data etc, and also will describe the mechanism how the noise produced.
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