Abstract: In order to remedy some defects caused by only one regulating variable in the current DC/DC converters,a new concept to add a control variable in the converters is proposed. In this paper, the chosen control variable is the operating frequency f of the converter, and make f decrease according to a set law when the voltage of the primary power supply increases, so as to solve some problems such as severe heating of the switching transistors, etc. when the frequency is invariable. The main problem to realize the idea is to construct a variable frequency type of pulse width modulator ( PWM ) by a general PWM. Now taking the general modulator 1842/3/4/5 and 1842A/3A/4A/5A as examples to discuss how to construct the variable frequency type of PWM, and show the realizable frequency characters are given. Both of the linear transform circuits of voltages are designed to transform the voltage of the primary power supply to the input voltage of the PWM. The representations for calculating parameters of elements in these circuits are derived. The experiment results are consistent with designs.

Key words: variable frequency type of pulse width modulator, linear transfer circuits of voltages