The Design And Application Of Piezoelectric Transformer's Driver And Control Circuit

Piezoelectric transformer(PT) is the third-generation transformer which makes use of piezoelectric and inverse piezoelectric effect.Compared with the electromagnetic transformer, piezoelectric transformer was applied in more and more fields based on the advantages of small size,no electromagnetic interference,high efficiency and some other merits. Piezoelectric transformer can be classified as step-up PTs and step-down PTs.The former PTs are fit for the device with high voltage and low current;the latter PTs can be used in AC-DC converters.The gain and efficiency of piezoelectric transformer are influenced by load, temperature and some other factors,so it is necessary to configure corresponding driver circuit.The driver circuits of step-up PTs and step-down PTs are researched in this paper,and are applied in CCFL and AC-DC converters respectively.The control strategy that combined PFM and PWM is introduced based the characters of step-up PT.The efficiency can reached max when the working frequency is consistent with resonant frequency by PFM control,and then the output voltage of piezoelectric transformer can be adjusted by PWM control.The driving control circuit based on closed loop feedback control with CCFL is designed.This circuit adopts the type of XC2S100E FPGA as controller and make use of full-bridge inverter circuit composed of MOSFET.The arithmetic is also compiled according to PFM and PWM control.It is tested that the driver circuit can adjust the working frequency and the output voltage of piezoelectric transformer automatically,so the goal of high efficiency and high output voltage can be achieved.To apply AC-DC converters to PTs,municipal electric power is converted to direct current(DC) voltage after rectifying and filtering,and this DC voltage supplies the PTs for input signal with suitable frequency through inverter circuit.The output signal of PTs becomes low direct current voltage after rectifying and filtering.This paper introduced the driver circuit of step-down PTs with fixed load that adopts PIC24F as controller,PWM control strategy and make use of half-bridge inverter circuit composed of MOSFET which bears high voltage.It is designed that capacitor is adopted to achieve step-down voltage,and this low voltage work as the power supply of PIC MCU.This solved the problem of the supply of controller.In this paper,the output power of designed AC-DC converters could reach above 5W.It is validated that the designed AC-DC converters can be used as adapter for mobile phones and can realize the goal of converting municipal electric power to low direct current voltage.