Closing a second feedback loop in DC/DC converters based on piezoelectric transformers

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 high frequencies, with lower losses. However, their optimum operating frequency exhibits a strong dependence on different parameters, such as temperature, load or 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 Hertzs. On the other hand, it is not only necessary to ensure the PT is driven at the proper frequency in terms of gain and efficiency, including zero voltage switching (ZVS) in the power stage, but also to regulate the output voltage. 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 performs the output voltage regulation.