具快速響應轉導斜波相位交錯與電流平衡之適應性延伸導通時間控制多相位降壓電源轉換器

Abstract

由於現代可攜式行動裝置晶片的進步，對於電源管理IC的要求也越來越嚴格，處理器瞬間負載電流變化導致較大的輸出電壓下降，因此，微處理器的電壓調整器(VRs)必須提供超快速的暫態響應，以減少大量的輸出電容。多相位固定導通時間(COT)控制降壓轉換器具有高電流驅動能力、快速暫態響應、較佳的輕載效率和低輸出電壓漣波而被廣泛應用於高負載迴轉率之系統供電，本論文提出許多技術實現高切換頻率雙相位COT控制降壓轉換器。 由於固定導通時間控制法為非固定切換頻率操作，因此在多相位操作中相位交錯技術是很關鍵的，本論文提出基於轉導斜波的相位交錯技術，當負載電流發生瞬變時，所提出之技術可立即自動開啟兩相位操作以恢復輸出能量損失，無需任何使用者預設之臨界電壓，暫態期間兩相位導通時間線性重疊以減少輸出電壓下降和輸出電容要求。 多相位操作時並聯的相位之間會平均分配負載電流，但是由於元件之間的不匹配以及轉換器的佈局或擺放位置的不對稱性，相位之間的電流可能會大不相同，因此，本論文提出一個基於取樣原理的平均電流均衡機制來達成精準的相位電流平衡；此外，採用適應性擴展導通時間機制來克服由寄生電阻引起之切換頻率偏移；最後，本論文提出一種自動相位增減技術，相位增減控制電路可根據負載需求自動開啟或關閉工作相位數目，透過改變相位數目顯著地提升輕載時轉換器之效率。 綜合上面所述，本文提出之降壓轉換器可實現快速的暫態響應、準確的相位電流平衡、達到較佳的輕載效率並有效減少切換頻率偏移，本文提出之控制架構採用0.18 µm CMOS 製程實現積體電路，具有12 M赫茲高速切換頻率，可達成負載電流發生瞬變時自動開啟兩相位操作。晶片量測結果在1安培負載電流以迴轉率1 A/µs變化下，轉換器能夠在0.6 µs範圍內調節輸出電壓回穩態值，且電壓下降不超過50 m伏特，硬體量測切換波型用以驗證本論文所提出之技術，且主要技術皆會在本論文中被詳細討論。

With the advancement of modern mobile chip and portable devices, the requirements for power management integrated circuits (PMICs) are more stringent. The processor consumes large dynamic current with ultra-fast slew rate and cause large output voltage (VOUT) droops. Therefore, the voltage regulators (VRs) for microprocessors must provide ultra-fast transient response to reduce the bulky output capacitors. The multiphase constant-on time (COT) controlled buck converter is widely used in high slew rate load powering due to its high current-driving capability, fast transient response, better light load efficiency and low output ripple. The thesis adopts several techniques to realize high switching frequency dual phase buck converter with fast transient response. Since the naturally non-constant frequency modulation behavior of COT control, the phase interleaved topology is the crucial part in multiphase operation. The thesis performs a gm-ramped phase-interleaving method. When load transient occurs, the proposed technique can automatically turn on two phases immediately to recover the energy loss without any user predefined load transient threshold voltage. Two-phases on-time periods linearly overlap to reduce the output voltage deviation and output capacitance requirement. Multiphase operation is desired to distribute the load current among paralleled phases. However, because of component mismatch, as well as asymmetric layout or position of converters, their phase currents might be significantly different. Therefore, the thesis proposes a sample and hold based current balancing method to achieve phase current-balancing. Besides, an adaptive-extended on-time control (AETC) mechanism is adopted to overcome the steady state switching frequency variation caused by parasitic resistances. Lastly, the thesis presents an auto phase shedding and adding mechanism. The phase shedding control circuit can automatically enable or disable the operating phase based on the load demand. The technique can substantially improve light load efficiency by phase number change. As a conclusion, this control scheme achieves fast transient response, accurate phase current-balancing, better light load efficiency and mitigate the switching frequency deviation. The proposed circuit is fabricated in 0.18-µm CMOS process with 12 MHz high switching frequency. The measurement results show that, during 1 A load current step changes with 1 A/µs slew rate, the converter is able to regulate the output voltage within 0.6 µs with less than 50 mV undershoot. Experimental switching waveforms are shown to support the described techniques and analysis and major features are also discussed.