零電壓切換中央驅動雙輸出壓電變壓器用換流器的理論與實驗

Abstract

本研究論文針對液晶顯示器背光模組發光源-冷陰極燈管（Cold Cathode Fluorescent Lamp, CCFL），提出一種單層中央驅動雙輸出壓電變壓器，並以壓電變壓器之基礎力學特性為本，分析中央驅動雙輸出壓電變壓器包含共振頻率、升壓比、匹配負載阻抗、最大能量傳輸效率、阻抗分析等整體特性。此種型式之壓電變壓器的應用可以有兩種方式：第一，由於兩端輸出具有對稱特性，因此兩端電極可以同時驅動兩支冷陰極燈管；第二，又因為兩端輸出為就結構上看來是同向極化的關係，使兩輸出端產生反相之輸出訊號，可以產生一較大之輸出電壓，因此也可以驅動一等效負載阻抗較大之冷陰極燈管。此外結合壓電變壓器的力學分析、等效電路分析和切換式電源供應的思路，提出了一最佳化之串聯電感於壓電變壓器前，此串聯電感與壓電變壓器之輸入靜態電容所形成的諧振槽介於壓電變壓器結構共振頻率之1倍頻∼3倍頻之間，由於此諧振槽之共振頻率並不在壓電式換流器之操作頻率上，因此可以解決過去必須使用較大電感之問題。並且利用調整開關切換的盲時間（Deadtime）之方法搭配串聯一最佳化電感，設計出零電壓切換中央驅動雙輸出壓電變壓器用換流器。最後本研究論文以單層中央驅動雙輸出壓電變壓器的設計，製作出一套完整的零電壓切換之壓電式換流器，並成功以單一變壓器驅動U型及兩支300mm、500mm、680mm冷陰極燈管且燈管電流皆控制在5mArms。綜合言之，藉由本文所得成果，在未來以單層中央驅動雙輸出壓電變壓器的優勢以取代傳統電磁式變壓器驅動液晶顯示器背光模組是可以被期待的。

The main thrust of this thesis is to develop a single layer center-driven dual-output piezoelectric transformer based inverter for lighting CCFL (cold cathode fluorescent lamp) in LCD (liquid crystal display) backlight applications. Considering the basic mechanical properties of piezoelectric transformer, center-driven dual-output piezoelectric transformer were modeled and analyzed as vibrating at the 3/2λ mode. This thesis not only built the equivalent circuit of center-driven dual-output piezoelectric transformers but also derived the equivalent relationship between the electrical component model and the corresponding mechanical properties. All important properties of center-driven dual-output piezoelectric transformer which include the input impedance, voltage gain, resonant frequency, and optimal load condition are presented in this thesis. In practical applications, two typical configurations of center-driven dual-output piezoelectric transformer were used. The first configuration is two CCFLs connect onto the two output sections of the piezoelectric transformer and shows that the two CCFLs can be lit in sequence or simultaneously. The second configuration involves connecting the two output electrode to the two ends of a single extra-long CCFL for large size LCD TV backlight, where the floating connection configuration can provide larger driving voltage because two output sections was polarized along the same directions from the geometrical view in order to light the extra long CCFL. Combining mechanical analysis and equivalent electrical circuit with the notion of switching power supply, the optimum series inductance connected in series with piezoelectric transformer for ZVS (Zero-voltage-switching) and filtering was proposed in this thesis. The resonant tank composed of series inductance and the input static capacitance of piezoelectric transformer located between the first and the third resonance frequency of the piezoelectric transformer structure, where high-order current harmonic was used for ZVS in order to greatly reduce the value needed for the series inductance. By adjusting the deadtime of power switch and putting optimum series inductance, ZVS on center-driven dual-output piezoelectric transformer based inverter was then designed in this thesis. The final experiment measured the properties of center-driven dual-output piezoelectric transformer in high power condition, In addition, the power transfer efficiency that include the switching loss and piezoelectric transformer loss within the inverter composed of half bridge driver, series optimum inductance and center-driven dual-output piezoelectric transformer was also measured. The ZVS on center-driven piezoelectric dual-output transformer based inverter was successfully completed. Moreover, by using single transformer to light a U-type and two CCFLs (300mm or 500mm or 680mm) was also successful demonstrated while controlling the lamp current at arround 5mArms. This thesis successful verified that center-driven dual-output piezoelectric transformer can be used to light CCFL. In summary, combining all advantages of the center-driven dual-output piezoelectric transformer investigated in this thesis; the center-driven dual-output piezoelectric transformer possesses the potential to replace traditional electromagnetic transformer in the future.