Maximum power tracking of piezoelectric transformer HV converters under load variations

The problem of maximum power point tracking of high output dc voltage converters that apply piezoelectric transformers (PT) and voltage doublers was studied theoretically and experimentally. It was shown that the operating frequency of the PT, at which maximum power is reached, is a function of the load. Hence, under load variations, and to overcome parameter instability, there is a need for some frequency tracking mechanism that will help to lock the operating frequency to the optimum one. The proposed method to achieve frequency tracking is based on a phase locked loop (PLL). The PLL inputs are the phase of the input voltage driving the PT and the phase of the current flowing through one of the voltage doubler diodes. Theoretical analysis, verified by experiments, shows that when the phase shift of the diode current relative the phase of the input voltage is zero, the voltage gain of the system is at its maximum. By applying this approach, the system´s operation can be made independent of input voltage, load variations, temperature (within a permitted range), and the spread and nonlinearity of the PT parameters, as well as their drift with time.