具有直流校正功能之漣波調變定導通時間控制架構之穩定度分析

Abstract

近年來，對於改善切換式電源供應器之輕載效率在多方面的應用上有新的需求。針對直流直流轉換器，漣波調變定導通時間(RBCOT)控制機制近來已備受關注。在輕載之情形下，大部分的電源供應器經常操作在不連續導通模式下。當轉換器操作在不連續導通模式下時，漣波調變定導通時間控制機制會隨著負載電流減少而自然地降低轉換器之切換頻率，也因此會減少能量在切換時的造成的損失。相對於傳統固定頻率可變導通時間操作，此控制方式稱為定導通時間可變頻率操作。 一具有基本漣波調變定導通時間控制機制的轉換器通常伴隨者輸出電壓準位誤差。因此，加入直流準位誤差校正電路至基本漣波調變定導通時間控制機制用以改善此問題。此改善的控制機制稱為準位校正漣波調變定導通時間 (OCRBCOT) 控制且為本篇論文主要探討之議題。目前已有一些期刊在準位校正漣波調變定導通時間穩定度控制的議題上做探討，但大部分都在探討轉換器操作在連續導通模式下時、亦就是操作在重載時的情形。本篇論文將會在不連續導通模式下探討相同的議題。 在準位校正漣波調變定導通時間控制機制下，由於輸出電壓之切換頻率漣波會影響脈衝寬度調變之操作過程，因此無法使用傳統小訊號模型加以分析。因此改用分析時域波形的方法來研究此議題。不連續導通模式下之穩定度條件將會被建立以利於設計者來分析此控制機制。利用相同的步驟分析此控制機制加入適應性電壓位置特徵之後的穩定度條件，並可發現加入適應性電壓位置機制後可改善系統的穩定度。

In recent years, there has been a new mandate to improve light-load efficiency for switching power supplies for many applications. Ripple-based Constant on-time (RBCOT) control for DC-DC power converters scheme has received much attention for this reason. A converter is normally designed to operate in discontinuous conduction mode (DCM) under light-load condition. Under DCM condition, RBCOT scheme naturally reduces converter switching frequency as load current is reduced and therefore increasing the light-load efficiency. This is a constant on-time variable-frequency control scheme, as opposed to conventional constant-frequency variable on-time scheme for DC-DC converters. A converter with a basic RBCOT control scheme is usually plagued with an output voltage offset. Therefore, an offset correcting circuit is often added to the basic RBCOT scheme to correct the situation. This scheme is abbreviated as OCRBCOT and is the focus of this thesis. There have been publications on the issue of control stability of OCRBCOT under heavy-load condition in which the converters are operated in continuous conduction mode (CCM). In this thesis, efforts were directed toward the investigation of the same issue under DCM operation. In an OCRBCOT control scheme, switching-frequency ripple of the output voltage is involved in the process of pulse-width modulation and therefore, conventional small-signal model cannot apply. A time-domain approach is taken to address the issue. A DCM stability criterion is established which is practically useful for designers of OCRBCOT converters. Same approach is also used to establish the stability criterion for converter using OCRBCOT scheme along with adaptive voltage positioning (AVP) features. It’s also found out that the addition of AVP feedback loop improves the feedback stability.