翹翹板式微機電升壓轉換器設計及分析

Abstract

電壓轉換電路包含儲能元件與切換電路，利用儲能元件儲存輸入電能，再利用切換電路以高電壓／低電流的方式輸出，達成電壓提升的目的。傳統的電路儲能裝置包括電容與電感，由於其能量密度低於機械式的儲能裝置，因此Novoroski[1]首先提出微機電升壓轉換器（MEMS boost converter），希望藉由機械式的儲能裝置，進一步縮小升壓轉換器的體積。由於Novoroski所提出的微結構設計與切換電路較簡單，因此存在：能量轉換效率低、升壓倍率未最佳化、寄生電容等問題。在先前研究中，本實驗室提出翹翹板式（seesaw）的微機電升壓轉換器設計[2]，成功的解決能量轉換效率問題。本論文則針對升壓倍率最佳化、寄生電容問題，進行分析與探討。 本設計的儲能裝置是一靜電力驅動的可移動元件，升壓倍率的最佳化可藉由驅動訊號的設定（驅動電壓、電壓切換點）與移動元件的最大電容變化量所獲得。傳統的「吸附電壓」、「吸附位移」（pull-in voltage, pull-in displacement）分析方式並不適用於動態系統，因此本研究重新推導靜電力驅動元件的動態穩定條件，及相對應的最佳升壓倍率。 切換電路中的開關元件一般由單一MOS元件來實現，MOS元件中的寄生電容值遠高於CMOS-MEMS製程中可實現的MEMS電容值（相差>103），因此切換電路必須謹慎設計，否則將導致升壓機制失效。本研究提出以栓鎖電路（Latch）來取代單一MOS元件開關，進一步分析其可行性。 本研究利用Simulink中的Simscape建立系統的電路架構與微結構的運動方程式模型，驗證MEMS升壓電路的效能。由模擬結果顯示，輸入電壓低於靜電驅動元件的吸附電壓時有較佳的電壓升壓倍率。在本元件的設計中，在不考慮寄生電容的條件下，升壓倍率為5.74，較Novoroski的設計提升47.5%，能量轉換率92.73%，若考慮寄生電容，且的情形下，升壓倍率為1;採用栓鎖電路時，升壓倍率可達2.37。惟目前所設計的栓鎖電路耗能過大，MEMS升壓電路尚無法進入實作階段。

A boost converter contains an energy storage component and several switches in a circuit. The input energy is stored in the energy storage component and is converted into a high-voltage/ low-current using the accompanied switch circuits. The conventional energy components include capacitor and inductor, which energy density could be less than the mechanical energy storage component such as spring. Therefore, Novoroski [1] proposed MEMS boost converter, which uses mechanical energy storage element to further reduce the size of the boost converter. However, due to the relatively simple design in his MEMS device and switch circuits, Novoroski’s design suffers from the low efficiency of energy conversion, voltage conversion gain, and parasitic capacitance problems. In our pervious study, we proposed a MEMS seesaw boost converter which successfully solved the low energy conversion problem. In this thesis, we focus on the optimal voltage conversion and parasitic capacitance problem. The proposed MEMS seesaw boost converter is essentially an electrostatic actuator. The optimal voltage conversion gain can be obtained by studying the input voltage, switching point, and the corresponding capacitance variations. The conventional approach of the pull-in voltage and pull-in displacement analysis, which works for static systems, is inapplicable to this dynamic system. Therefore, we develop new criterions for the stability of a dynamic seesaw actuator and check the corresponding optimal voltage conversion gain. For simplicity, a switch in circuits is realized by a single MOS device, which may have parasitic capacitances that are 3-orders higher than that of MEMS devices realized by CMOS-MEMS processes. Therefore, the accompanied switch circuits should be carefully designed, otherwise, the MEMS boost converter would fail. In this thesis, we proposed using a Latch circuit to implement the switch function and analyzed its feasibility. We used the Simscape toolbox in the Simulink software to model the proposed MEMS device and accompanied circuits. Simulation results show that, the input voltage should be lower than the pull-in voltage of the electrostatic MEMS device to obtain the higher voltage conversion gain. In a conceptual design, when not considering the parasitic capacitance, the voltage conversion gain is 5.47, which accounts for 47.5% improvement from Novoroski’s design. The energy conversion efficiency is 92.73%. When considering the parasitic capacitance and using a single MOS device to implement the switch function, the voltage conversion gain is 1. On the other hand, when using the proposed Latch circuit design to implement the switch function, the voltage conversion gain is 2.37. Due to the proposed latch circuit design consuming too much power, the proposed MEMS seesaw boost converter is not ready for fabrication.