壓電背光換流器驅動冷陰極管之分析與控制電路設計

Abstract

壓電變壓器(Piezoelectric transformer, PT)具有體積小，高能量密度，高效率以及低電磁干擾等優點。除此之外，由於壓電變壓器的電壓增益會隨負載阻抗而改變，能夠提供冷陰極管(cold cathode fluorescent lamp, CCFL)所需的高啟動電壓，而且壓電變壓器的形狀符合薄型化的需求，因此特別適合運用平面顯示器的冷陰極管背光模組上。然而，壓電變壓器狹窄的頻寬使的它需要有回授電路來加以控制。 相位控制因為有追蹤工作點的優點而被壓電變壓器採用作為回授控制。相位控制是運用鎖相迴路來鎖定輸入電壓與輸出電壓的相位延遲，其鎖定的相位可以藉由假設燈管之輸出電壓為定值而分析得出。然而在控制迴路之上，除了壓電變壓器本身之外，還有許多電路延遲會造控制相位的誤差，尤其是為了達成零電壓切換(Zero-Voltage-Switching, ZVS)所加的電感。根據等效電路，壓電變壓器有相當大的輸入電容，因此需要一串聯電感來補償此電容性負載，以產生零電壓切換所需的延遲電流。這個電感同時也兼具濾波器的功能。它與壓電變壓器的輸入電容組成一帶通濾波器，提供壓電變壓器基頻弦波的輸入電壓。 利用準模態壓電變壓器所具有的濾波特性，可以改變串連電感的設計。由於不需再負擔濾波的功能，串聯電感可以設計在高頻諧波共振，利用加強高頻諧波電流來達成零電壓切換。達到高頻共振所需要的串聯電感較小，也同時減小了輸入端LC共振電路在基頻產生的影響。 本文設計了一個控制系統來控制壓電變壓器驅動冷陰極管。這個控制系統包含了上述兩項設計，利用鎖相迴路來追蹤壓電變壓器的工作點，並採用高頻諧波零電壓切換來減少控制迴路上的相位延遲。除此之外，亮度的調節採用的是間歇式調光。間歇式調光可以在維持壓電變壓器的輸出條件下提供大範圍的亮度變化。而利用壓電變壓器增益變化的特性，本系統也設計了啟動電路來提供冷陰極管所需的啟動電壓。最後，此控制系統被實現為積體電路，並透過實驗驗證。根據實驗，此壓電背光換流器有約85%的轉換效率。

Piezoelectric transformer (PT) has the advantages of high power density, small size, high efficiency and low EMI. Moreover, it is suitable for the cold cathode fluorescent lamp (CCFL) backlight inverter because of the thin requirement of flat panel display (FPD) and load dependent voltage gain for CCFL striking. However, piezoelectric transformer has the drawbacks of the narrow bandwidth and high sensitivity. A specific feedback control is required for the piezoelectric transformer. Phase control is beneficial on tracking the operating point of piezoelectric transformer. It fixes the phase delay of piezoelectric transformer by using a Phase-Lock-Loop (PLL), and the control phase is analyzed with the constant lamp voltage assumption. However, circuit delays in the control loop increase the control errors. Piezoelectric transformer has a large equivalent input capacitor and hence an inductor is usually used in series to generate lagging current for zero-voltage-switching (ZVS). This inductor and the input capacitor also create a band pass filter. As a result, it provides variant phase error near the operating frequency. A quasi-modal piezoelectric transformer is introduced to take over the filter function. Thus a high order harmonics ZVS design is presented to achieve ZVS with smaller inductance. It provides the ZVS current with high order harmonics of input voltage. Thus the LC tank has less influence at fundamental frequency. A control system for piezoelectric transformer based CCFL backlight inverter is designed. It includes a PLL for tracking the operating point and a high order harmonic ZVS design for small phase delay. Furthermore, burst mode control is utilized for dimming function. It can provide a wide dimming range with steady voltage gain and load for piezoelectric transformer. Finally, a start-up circuit utilizing the variant voltage gain of piezoelectric transformer is designed for the high striking voltage of CCFL. A control IC including these control strategies is designed for verification of the concept, and the conversion efficiency of the piezoelectric transformer based CCFL backlight inverter is about 85%.