以模態壓電變壓器研製液晶顯示器背光模組用換流器的理論與驗證

Theory and Experimental Verification of Modal Based Piezoelectric Transformer for LCD Backlight Inverters

黃耀田(Yao Tien Huang)

指導教授 ： 李世光

國立臺灣大學/工學院/應用力學研究所/博士(2006年)

https://doi.org/10.6342/NTU.2006.00556

摘要

本研究論文主要的研究內容在於開發創新的壓電變壓器，並利用所開發的創新壓電變壓器來完成液晶顯示器(Liquid Crystal Display)背光所需的壓電式換流器(Piezoelectric Inverter)製作。由於針對不同尺寸、不同用途的液晶顯示器，其所需要的換流器規格也會不同，因此本論文分別針對大尺寸液晶電視背光和中小型可攜式液晶顯示器背光的需求，分別開發出不同規格之壓電式換流器。對大尺寸液晶電視所需的壓電式換流器而言，在本論文中提出利用單層壓電變壓器來製作升壓元件的創新架構，所使用的單層壓電變壓器可以降低壓電式換流器的製作成本，因此可以提高市場競爭力。同時為了彌補單層壓電變壓器升壓比不足的限制，所以在本研究論文中所開發完成的壓電式換流器中，提出藉由系統內部的功率因子校正電路(Power Factor Correction, PFC)將一般市電電壓直接轉換成為可供系統使用的高直流工作電壓。對於中小型可攜式液晶顯示器背光用換流器，由於其電壓源通常來自電池，因此其所能提供的電壓仍為低直流電壓，所以為了增加壓電變壓器的升壓比，本論文採用了多層壓電變壓器來完成具有高升壓比的壓電式換流器。
在創新壓電變壓器的設計上，本研究論文導入模態感應子與致動器(Modal Sensor and Actuator)的設計理念到創新壓電變壓器的輸入電極形狀(Input Electrode)設計。透過壓電變壓器表面電極形狀的設計，可以使壓電變壓器具有在頻率域上過濾輸入電壓內部高頻成分的特性，以增進壓電變壓器的整體能量傳輸效率。為了能夠設計出適合不同壓電變壓器的模態電極形狀，本論文從傳統的壓電力學理論分析其機械結構的共振振形出發，在以機械結構振動的偏微分方程式(Partial Differential Equation, PDE)為理論基礎，利用正交基底將偏微分方程式轉換成常微分方程式(Ordinary Differential Equation, ODE)以求能表示出壓電變壓器在共振頻率附近的振動狀態。同時在經由對壓電變壓器作力學分析的過程，本論文分別設計出具有全模態濾波(Full Modal Filtering Effect)和準模態濾波(Quasi Modal Filtering Effect)特性的輸入電極形狀。透過準模態電極的使用，可以提供過濾輸入電壓內部奇數倍基頻的濾波效果，使壓電變壓器可以直接採用方形或是類似方形的梯形輸入電壓直接驅動。透過全模態電極的使用，可以提供完全的模態過濾效果，因此可以提升壓電變壓器的能量傳輸效率和增加壓電變壓器的最佳工作頻率範圍(Optimal Operating Region)。
為了降低壓電式換流器內部場效電晶體(MOSFET)的開關損失(Switching Loss)，在本論文中還引入了過去在研究切換式電源供應器(Switching Power Supply)所使用的軟切換技巧(Soft Switching Technology)來設計壓電換流器的驅動電路。過去在分析壓電變壓器時，最常使用的方析方式是利用等效電路的模型來分析壓電變壓器的特性，但在本研究論文中，為了分析場效電晶體是否可以達到零電壓切換的工作狀態，因此導入了壓電變壓器等效電路模型來作分析。基本上，壓電變壓器的等效電路是利用一個電容和電感來構成LC共振電路，並透過壓電變壓器內部的電感和調整換流器內切換電路的開關時間，進而讓負責切換的場效電晶體(MOSFET)達到零電壓切換(Zero Voltage Switch)的目的，因此可以降低開關損失(Switching Loss)，進而提升整體系統的能量傳輸效率。除此之外，為了簡化達到零電壓切換狀態的設計考量，本論文中採用一了個小電感串接在壓電變壓器前，並利用小電感幫助場效電晶體完成零電壓切換的工作狀態，而後再透過壓電變壓器的準模態電極來對輸入電壓的波形進行濾波。
綜合上述各種設計參數，本論文嘗試融合歷史上針對壓電變壓器所完成的力學與電學的分析方式，逐步完成足以突破過去世界各國對壓電變壓器所設下的專利藩籬，透過準模態電極和零電壓切換的開關設計，順利完成了驅動液晶顯示器背光冷陰極燈管所需使用之無線圈壓電式換流器。除此之外，本論文還提出如何採用具有全模態電極的多層壓電變壓器，來提升可攜式液晶顯示器用壓電換流器的整體系統效率的創新架構。

關鍵字

壓電變壓器 ； 模態電極 ； 液晶顯示器背光 ； 零切換

並列摘要

The main thrust of this dissertation is to develop innovative piezoelectric transformers so as to use these series of newly developed piezoelectric transformers for the development of piezoelectric technology based inverter for LCD monitor backlight. The specifications of the piezoelectric technology based inverter vary whenever the dimension and purpose of the LCD monitor is changed. Various types of piezoelectric transformers for large and mid sized LCD TV and portable LCD monitor were examined and studied. For large size LCD TV backlight module, a single-layer piezoelectric transformer was used as the component to amplify the input voltage to light cold cathode fluorescent lamp (CCFL). The single-layer piezoelectric transformer provides us with an opportunity to lower the manufacturing cost of the piezoelectric technology based inverter, which in turn then increase the competition edge when compared to traditional magnetic coil transformer. To overcome the limitation that the step-up ratio of a single layer piezoelectric transformer is low, internal power factor correction circuit (PFC) was used to convert. the AC voltage derived from regular electric outlet to high DC input voltage for the newly developed inverters. For portable LCD monitor, the DC voltage usually was derived form small battery, which leads to low DC input voltage. Therefore, to increase the step-up ratio of the piezoelectric transformer, a multi-layer piezoelectric transformer was used to complete the piezoelectric technology based inverter for portable LCD monitor.
By introducing the design thoughts of modal sensors and actuators into the design of piezoelectric transformer input electrode, a series of innovative piezoelectric transformers were developed. It was shown that these newly introduced input electrodes can provide modal filtering property needed to filter out unwanted high frequency voltage embedded within the input voltage waveform. To design modal electrode for various piezoelectric transformer, traditional piezoelectric theory was used to analyze the resonant vibration behaviors of the piezoelectric transformers. Traditionally the structural vibration of the piezoelectric transformer was modeled by using a partial differential equation (PDE). Adopting normal mode expansion, this PDE can be decomposed into a series of ordinary differential equations (ODE). Each ordinary differential equation can be used to represent mechanic vibration of the piezoelectric transformer when the driving frequency is near the resonant frequency of the piezoelectric transformer. Both full modal electrode and quasi-modal electrode were developed within this dissertation. By using the quasi-modal electrode, the voltage waveform with odd times piezoelectric transformer resonant frequency existed within the input voltage can be completely filtered out. By using the quasi-modal electrode, both rectified and trapezoidal input voltage can be used to drive the piezoelectric transformer directly without suffering efficiency loss. It was also shown that full modal filtering effect can be achieved by using full modal electrode. The experimental result and the theoretical analysis presented in this dissertation demonstrated that full modal electrode can improve the energy transfer efficiency and expand the optimal operating region of the piezoelectric transformer.
To lower the switch loss of MOSFET adopted in the piezoelectric technology based inverter, soft switching technology that was adopted inside the traditional switching power supply circuit was used to design the driving circuit needed for piezoelectric transformer based inverter. Traditionally, an equivalent circuit model was used to simulate the property of the piezoelectric transformer. To verify whether the MOSFET can reach zero voltage switching (ZVS) condition or not, the equivalent circuit model of the piezoelectric transformer must be used to calculate. The equivalent circuit of the piezoelectric transformer includes a capacitance and an inductance, both of which produce the LC resonant circuit. By controlling the internal inductance of the piezoelectric transfer and the dead time of the MOSFET switch, zero voltage switching condition was shown to be achievealbe and practical. It was also shown that the zero voltage switching condition can decrease the switching loss such tha t energy transfer efficiency was improved.
Furthermore, to simplify the conditions that can make MOSFET reach zero voltage switching condition, a small inductance was connected in front the piezoelectric transformer to make sure that the MOSFET work under the zero voltage switching condition. The quasi modal electrode was used to filter out the high frequency voltage that exists within the input voltage.
Combining all design parameters mentioned above, the dissertation attempts to combine traditional mechanic and electric analysis for piezoelectric transformer, which finally leads us to invent a series of approaches that can be used to circumvent all of the prior art. By using quasi-modal electrode and soft switching technology, a coil less piezoelectric based inverter for LCD TV backlight was completed. A full modal multi-layer piezoelectric transformer was also completed to light portable LCD monitor backlight. Finally, the full modal electrode was shown to improve the overall energy transfer efficiency of the piezoelectric based inverter.

並列關鍵字

Piezoelectric transformer ； Modal electrode ； LCD backlight ； ZVS

參考文獻

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[3]林柏丞，相位控制之壓電式背光反流器，國立台灣大學電子工程學研究所碩士論文，2005年7月。

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