應用導通時間延伸技術於三階降壓轉換器之改良型PFM控制策略

Abstract

本論文提出一種新穎的脈衝頻率調變（PFM）控制技術，應用於三階降壓型轉換器，旨在於寬廣的工作占空比與負載範圍內維持卓越的效率。由於此類交錯訊號轉換器在使用傳統的固定導通時間（COT）控制下，於轉換比（M）大於 0.5 的不連續導通模式（DCM）中無法隨負載調整其開關頻率，使得輕載條件的切換頻率與切換損失較高，本文提出延長導通時間（Ext.-Ton）的方法以應對此問題。此外，考量到三階降壓轉換器在 M > 0.5 區域採用 COT 控制時可能產生的不穩定性，以及 M 接近 0.5 時存在零電流漣波點，本文選用 ICAPQOT 控制作為主要調變策略，以無需外加斜坡的方式維持系統穩定運作。進一步地，本文亦提出一種基於 VLX 節點的感測技術，能於全占空比範圍內有效平衡飛行電容電壓，適用於連續導通模式（CCM）與不連續導通模式（DCM）。預模擬結果顯示，在 CCM 下的開關頻率為 2 MHz，而於 DCM 下則具可變頻操作，並於 20 mA 至 1 A 負載範圍內達到超過 90% 的效率。本晶片採用台積電標準 0.18 μm CMOS 製程實現。最終量測結果來自一顆結合離散功率級的控制器晶片，與IQCOT控制相比，展現 200%的暫態響應的回穩時間改善。並且與傳統固定導通時間（COT）控制相比， 於Vo/Vin = 1.8/2.5 且 Io = 20 mA 條件下達成 428% 的電感電流切換頻率下降，驗證了 ICAPQCOT 控制與延長導通時間模式之有效性。

This work presents a novel control technique of Pulse Frequency Modulation (PFM) on a three-level buck converter, to keep remarkable efficiency within a wide range of duty cycle and loading. Since this interleaved signal converter with conventional constant on-time control cannot scale its switching frequency with load under discontinuous conduction mode when the conversion ratio(M) is larger than 0.5, leading to higher switching loss at light-load condition. Therefore, an on-time extension (Ext.-Ton) method is applied. Moreover, due to the instability when applying conventional constant on-time (COT) control to a three-level buck converter for M>0.5 and the presence of a zero-ripple point around M ≈ 0.5, the ICAPQOT control is selected as the primary modulation technique to maintain stable operation without the need for external ramp. Additionally, a VLX-based sensing technique is introduced to ensure effective flying capacitor voltage balancing in both CCM and DCM among full duty cycle. The pre-simulation results demonstrate a switching frequency of 2MHz in CCM and a variable frequency in DCM, achieving efficiency greater than 90% when load current ranging from 20mA to 1A. The chip is fabricated using a TSMC standard 0.18um CMOS process. The final measurement result is obtained using the controller chip combining with a discrete power stage, showing a 200% improvement in transient response compared to the IQCOT control. Furthermore, in comparison with COT control, a 428% reduction in switching frequency is achieved at Vo/Vin=1.8/2.5 and Io=20mA. These findings confirm the effectiveness of both the ICAPQCOT control and Ext.-Ton mode.