Comparing piezoelectric transformer working with PLL and with nonlinear load approaches in DC-DC converter

A current trend in power electronics is the integration of power supplies. In this field, electromagnetic transformers may be replaced by piezoelectric transformers (PT). In a PT, the input electrical energy is transferred to the output through a mechanical wave using the direct and converse effects of piezoelectric materials. It has been proved that PT exhibit suitable characteristic, good integration capability and several advantages (high insulation, gain ...), but it also shows the difficulty to work with variable load. It is very difficult to make it functioning at its maximum power point because the resonance frequency, which corresponds to a maximum power, depends strongly on the load. In this paper, two methods of maximum power point tracking (MPPT) are presented: the first one is the classical method of frequency regulation with a phase locked loop (PLL), and the second one is a new method, it is based on a phase regulation between the output voltage and the mechanical vibration velocity of the PT. This is an innovative non-linear processing technique derived from the so-called ´synchronized switch damping´ technique (SSD) that was developed to address the structural damping problem. A comparison of the performances of these two kinds of MPPT is presented. The study and comparisons are carried out with the same transformer in two DC-DC converters.