Combined PWM and PFM control for universal line voltage of a piezoelectric transformer off-line converter

A new control method for an AC/DC converter using a piezoelectric transformer is presented. The combination of pulse width modulation and pulse frequency modulation makes it possible to control the output voltage, and its adaptability to a worldwide range of input voltage variation. The AC/DC converter implemented on a printed circuit board is experimentally evaluated, and as a result, good controllability is successfully achieved. The highest efficiency of 86% was obtained at an input voltage of AC 90-270 V, output voltage of 17 V, load current of 0.57-1.4 A, and operating at frequencies around 107 kHz.