四元調變與三元調變方案在D類音頻放大器之分析

Abstract

隨著音頻產品應用於生活周遭，在音頻產品的體積越來越小下，消耗功率越來越大又相對於音頻產品的溫度不能過高的情形上，提出D 類音頻放大器來改善。 D 類音頻放大器上較傳統AB類的音頻放大器具有更高效能及更高功率的優勢， 而本論文提出D 類音頻放大器分別二種調變方法：四元調變方法與三元調變的分析，比較二種調變方法，可以清楚何種調變方法失真較低， 何種調變方法效率更好，更能節省功率消耗。但在D 類音頻放大器三元調變的作法會直接影響輸出低瓦數諧波失真的嚴重性。 在一個D類音頻放大器常見到輸出級加電感及電容的二階慮波器，IC外部的原件成本的問題，系統廠商須降低成本，大多變換電感的感值變小及規格變差的情形，將會容易造成D 類音頻放大器過電流保護功能下啟動，造成聲音的中斷或是開機瞬間的誤動作，這是使用者無法接受的問題，本論文也提出如何避免且有效改善此問題。 此論文模擬結果，在負載8ohm下輸出最大功耗43瓦數，效率能達94%。

In recent years, wildly used audio applications in various fields, such as portable cellular phones, tablets or TVs, currently demand small chip size with extremely high power output and relative low temperature dependence during its operation. This thesis not only proposes a Class-D Amplifier to fulfill high-power demand but also maintain good efficiency without increasing operation temperature. Compared to Class-AB amplifier, Class-D amplifier has advantages of high efficiency and high power output. In this thesis, we analyze and compare two modulation techniques, ternary modulation and quaternary modulation. From the comparison, we could clearly understand which modulation can achieve low harmonic distortion, offer efficiency, and save much energy loss. Ternary modulation will directly degrade THD performance when Class-D amplifier operates in low power condition. In commercial applications, Class-D amplifier always come with 2nd order filter design which is composed of inductor, capacitor, and resistor load. Audio system design house always decreases their BOM cost to maintain good profit. Recently, inductor’s value is often to be lower to save more cost. However, the change will causes unacceptable performance. In the worst scenario, occurrence of false alarm may interrupt music play or lead to start-up failure due to over-current protection. These issues cannot be acceptable for the customers. Design house will suffer from return loss. In this thesis, efficient and effective method to avoid inrush current is proposed. The design referred in this thesis is fully verified by Hspice simulations. Proposed technique can offer 94% efficiency and generated 43W output power at 8ohm loading condition.