Closing a second feedback loop in a dc-dc converter based on a piezoelectric transformer

Nowadays, piezoelectric transformers (PT) are a good alternative to substitute magnetic materials in ac/dc and dc-dc converters. They have high isolation voltage and operate at higher frequencies than magnetics, with lower losses. However, their optimum operating frequency exhibits a strong dependence on different parameters, such as temperature, load, or even voltage level applied. This is usually an inconvenience, because this drift affects PT gain and efficiency, which can vary enormously within a few hundred of hertz. On the other hand, not only is it necessary to ensure the PT is driven at the proper frequency--in terms of gain and efficiency, including zero voltage switching in the power stage--output voltage must also be regulated. In this paper, two simple feedback loops are implemented in a PT-based dc-dc converter. One of them adjusts the switching frequency to obtain the best gain and efficiency. The other one performs the output voltage regulation. Index Terms--Piezoelectric transformers (PTs).