Design and Control of Tunable Piezoelectric-transformer-based dc-dc Converter
The piezoelectric transformer (PT) is an alternative for magnetic transformers in power converters. Compared to the magnetic transformer, the PT exchanges electric potential with the mechanical force instead of the magnetic flux. Specifically, the tunable piezoelectric transformer (TPT) is a recently developed device that has input, output, and control ports. When connected with different impedance at the control port, the TPT has different input-to-output voltage gain characteristics. In this work, it is proposed to use this property for output voltage regulation while keeping constant switching frequency. The proposed TPT based converter is shown in Fig. 1. A half-bridge voltage inverter is used to generate an AC voltage with desirable frequency. An input inductor is utilized to filter out the high frequency harmonics entering the TPT. A diode bridge rectifier is applied at the output port of the TPT. Following the rectifier, an LC filter is used to reduce the ripple of output voltage. A variable capacitor, realized by a duty cycle controlled switched capacitor, is utilized at the control port to achieve the output voltage regulation at different load and/or input conditions. Fig. 2 shows the experimental waveforms of output voltage Vo (blue curve), duty cycle D of the control switch SCtrl (yellow curve), and its drain-to-source voltage of VdsCtrl(green curve), un-der different test conditions. When the input line voltage increases or the output load resistance increases, D increases so that Vo is regulated, and vice versa. It is also verified that zero voltage switching is achieved for the control switch since SCtrl is always turned on when VdsCtrl=0.