This paper presents a proposal when the converter works at light load condition, the use of the control circuit so that it can automatically select the operating frequency among a group of pre-set frequency values according to the duration of turn-on time of converter power transistor. This can realize high power density, reduce costs, increase transient response speed as well as achieve the purpose of improving the power efficiency of the converter. Experimental results show that while the converter outputs voltage and stable drive power, the low load efficiency is improved by nearly 24.0%, so the method is particularly suitable for battery-powered portable instrumentation equipment, which can maintain a longer battery usage time.