Design considerations of Piezoelectric transformers with voltage-mode rectifiers for DC/DC converter application

Piezoelectric transformers (PT) are widely adopted in recent year due to many advantages, including safety, no EMI problem, low housing profile, and high power density, etc. The characteristics of the PT are well-known when the load impedance is a pure resistor. When PTs are used in AC/DC or DC/DC converters, there are always a rectifier circuit block connected after the PT. The rectifier circuit block sometimes can be equivalent to a resistor, but not always true. Thus, this paper starts from the model of the voltage-mode rectifier directly. Then, three design constraints are discussed in detail, including the minimum input voltage, maximum mechanical current and the constraint of the PT configuration. Different from the typical optimal loading condition of the PT, the concept of the maximum mechanical current leads to a new optimal efficiency condition that is suitable for voltage regulation. Further, the ability of the regulation and the efficiency are found to be a trade-off. This paper proposes a simple criterion to obtain the good regulation and good efficiency at the same time. Finally, the completed design procedure is discussed by a given specification. The PT derived from the proposed design procedure can guarantee both good efficiency and enough range for voltage regulation.