圓盤壓電變壓器的理論與驗證

Abstract

本論文研究內容主要為研究創新壓電變壓器設計，利用單一極化方向同心圓壓電變壓器完成高效率T5日光燈管壓電換流器，另外一方面利用半月型圓盤壓電變壓器，點亮液晶顯示器背光模組中的冷陰極管所需的壓電變壓器。為了提高市場競爭力，本文中所使用的皆為單層壓電變壓器作為壓電式換流器主要元件。首先針對文中所使用的壓電變壓器之等效電路參數利用MATLAB軟體進行分析操作特性，觀察壓電變壓器的重要參數，瞭解電壓升壓比、匹配阻抗、最大效率、輸出電流、輸入阻抗變化…等參數都因頻率與負載阻抗變化，研究過程中還運用壓電變壓器的等效電路來快速瞭解電特性。為了達到整體壓電變壓器系統最佳化，在研究的過程中，本論文嘗試引入有限元素軟體模擬，觀察壓電變壓器的共振模態、應力分佈與電壓分佈情形，同時以量測所得壓電變壓器的特性來驗證理論推導計算的結果，其中如壓電變壓器隨不同的負載阻抗會有不同的升壓比，因此正好可符合各式燈管點亮前的高電壓所需，同時也藉由前述設計與模擬方法進而取得效率及成本的改善。 壓電變壓器最大瓦數的限制傳統上都以振動速度超過壓電材料的極限為標準，在此狀態下壓電結構的品質因素Q會開始下降，從電性參數來看，小訊號測量所測到的等效電路已經不符合在高瓦數驅動下的行為，本論文針對高瓦數下的壓電變壓器輸入阻抗變化作了詳盡的討論。本論文分析各式壓電變壓器得到設計壓電變壓器參數中必須要考量另外一種造成壓電變壓器瓦數上的限制，也就是機電耦合係數的影響，機械耦合必須考慮輸入端機械耦合參數以及輸出端耦合參數兩者相對關係，搭配前述分析模擬方法即可藉著輸入阻抗知道耦合係數的關係。 壓電式換流器存在切換損失，若不詳細考慮並進行適切設計，將會造成熱損失在開關中，本文中引入柔性切換加入設計考量，並且使用了小電感串接在壓電變壓器前面，利用小電感幫助壓電變壓器的零電壓切換情形，使得開關無升溫，再透過圓盤壓電結構的濾波特性對輸入非弦波電壓以及電流進行濾波，達到輸出電壓以及電流皆為弦波的高效能壓電式換流器。

The main thrust of this dissertation is to develop innovative piezoelectric transformers. A newly developed Uni-poled disk type piezoelectric transformer inverter for T5 was proposed in this thesis. On the other hand, crescent input electrode disk type piezoelectric transformer was used to light the LCD monitor backlight. As single-layer piezoelectric transformers have the chance to lower the manufacturing cost of piezoelectric transformer inverter and increase the competitive edge when compared to traditional magnetic coil transformer, single-layer piezoelectric transformers were used throughout this thesis. As the fastest way to understand the piezoelectric transformer electric characteristics was using the equivalent circuit of piezoelectric, MATLAB was first used to analyze the equivalent circuit so as to understand all piezoelectric transformer characteristic such as step up ratio, impedance match, max efficiency, output current, change of input impedance, etc. It was verified that these parameters vary with frequency and load. To design piezoelectric transformer system that can reach optimal state, finite element software was adopted to simulate the modal stress and voltage distributions. As a piezoelectric transformer varies its step up ratio with the load impedance, it fit the lighting lamp need. The simulation model developed provides us with a way to achieve improvement in efficiency and cost. Traditional limit of piezoelectric transformer power was defined by the vibration speed limited by the piezoelectric property. In that condition, piezoelectric structure quality factor will begin to decrease, which indicates that small power equivalent model is completely different from that of the high power operating condition. In this dissertation, the piezoelectric input impedance associated with high power condition will be detailed. Electromechanical coupling factor is yet another important factor reason that imposes the high power limit form various piezoelectric transformers, where the input and output coupling coefficient relation can be obtained by taking the correspoinding input impedance into considerations within the above-mentioned simulation model. Soft switching techniques were adopted to reduce the switching loss and to lower the waste heat in this thesis. The switching dead time between the on and off states was adjusted to make the end voltages of MOSFETs being charged and discharged to reach zero voltage right before switching, i.e., to drive the MOSFET in the so-called zero voltage switching (ZVS) condition. Furthermore, to simplify the condition that could make MOSFET reach zero voltage switching condition, a small inductance was connected in front the piezoelectric transformer. The Disk piezoelectric transformer filters out the higher order harmonics while keeping the output voltage and current a sine wave in order to achieve high driving efficiency. Combining all design parameters mentioned above, the dissertation developed innovative piezoelectric transformers based inverters that possess performance higher than traditional inverter.