The self-powered power management circuit applying energy harvester for battery-free portable electronics is attracting a lot of attention. These harvested voltages are usually lower than the threshold voltages (V(TH)) of standard CMOS technologies. Therefore, a step-up DC-DC converter is required to convert the harvested energy to usable outputs. There are many challenges when implementing such a low input voltage DC-DC converter, including startup mechanism, low conversion efficiency, and process variation. This thesis proposes several techniques to solve these problems to develop an extremely low input voltage DC-DC converter.

This thesis is organized into 6 chapters. The first chapter describes the motivation and contribution of this thesis. Harvesting energy from the environment by using the thermoelectric generator (TEG) or the photovoltaic cells provides a solution for battery-free sensor networks or electronic healthcare systems. New startup mechanisms without using mechanical switch or large off-chip transformer could achieve low startup voltage while the circuit topologies could remain the same in operation mode. This means that we could apply all the developed control schemes to realize the DC-DC converter with high efficiency and good regulation performances.

Chapter 2 introduces two basic types of step-up DC-DC converters. The first type is capacitive step-up converter called "charge pump" and the other type is inductive step-up converter called "boost converter". These converters perform different characteristics and can be used in different application. We will also introduce the prior works on low-voltage startup mechanism. Both the operation principles and drawbacks of these circuits will be described.

Chapter 3 presents the implementation of 0.18-V startup step-up DC-DC converter. It applied an on-chip charge pump as a startup mechanism in startup mode to reduce the startup voltage of the boost converter significantly. In operation mode, the converter uses boost converter as the main DC-DC converter to achieve the high conversion efficiency and high output power. In the developed charge pump, all the switch transistors are forward body biased by using the inter-stage/output voltages. Therefore, the conduction loss can be reduced without large area overhead. To verify the circuit characteristics, the conventional zero body bias charge pump and the proposed forward body bias charge pump were fabricated with 65nm CMOS process.

Chapter 4 describes a 95-mV startup-voltage step-up DC-DC converter. Two techniques are applied to reduce the startup voltage. One is capacitor pass-on scheme and the other is fixed-charge VTH-tuned oscillator. Capacitor pass-on scheme solve the limitation of startup voltage in chapter 2. The fixed-charge VTH-tuned oscillator compensates for the die-to-die process variation by post-fabrication VTH trimming. This technique can make sure the clock generator can provide the clock signal to charge pump at low supply voltage.

Chapter 5 shows a low startup-voltage and fast startup dual-mode boost converter, which further improves the converter introduced in chapter 4. Comparing with the previous work, the startup-time, the minimum startup voltage, and the program time are greatly improved. (1) A startup by the boost converter instead of a charge pump reduced the startup-time to 4.8ms which is reduced to 1/56 and the shortest to date. (2) The proposed sub-1nW charge-pumped pulse generator (CPPG) enabled the lowest startup voltage of 80mV to date without mechanical switch. (3) The proposed threshold-voltage-tuned oscillator with hot-carrier injection (HCI) for CPPG to compensate for the die-to-die process variations reduced the program time to 3min which is 1/20 of previous version, thereby reducing the test cost.

Usually, the control circuit and switch driver in DC-DC converter are supplied either by input voltage or by output voltage. In energy harvesting applications, sometimes both the input/output voltages are both too small. If there is any process variation, the conduction loss changes dramatically and decreases the conversion efficiency. To compensate the conduction loss variation caused by process variation, we developed an adaptive local boost DC-DC converter. It adaptively changes the supply voltage of driver circuit to fix the voltage drop in power switches. Another merit of this technique is the conversion efficiency improvement over a wide output load. It achieves higher conversion efficiency comparing to conventional approaches. The circuit design and design methodologies of the adaptive local boost DC-DC converter are presented in chapter 6.

Finally, the thesis concludes in chapter 7.

本論文は「Circuit Design for Sub-0.5V DC-DC Converter」(和訳:0.5V以下動作のDC-DCコンバータに向けた回路設計)と題し、将来の環境エネルギー向け電源供給システムに適用可能な低電圧電源集積回路を指向し、低起動電圧かつ高変換効率を実現する昇圧型電源回路を提示するもので、全7章で構成されている。

第1章は「Introduction」(序論)であり、環境エネルギーを用いた電源システムの要求事項や課題について述べるとともに、本研究の背景を述べ、目的を明確化している。

第2章は「Analysis on Step-up DC-DC Converters, and Conventional Startup Techniques」(昇圧型DC-DCコンバータの分析と従来の起動技術)であり、容量型昇圧回路(チャージポンプ)、インダクタ型昇圧回路(ブーストコンバータ)の動作原理、主要な電力損失要素、各設計パラメータによる影響等について分析するとともに、近年の顕著な関連起動回路の研究について概説している。

第3章は「CMOS Implementation of 180 mV Startup DC-DC Converter」(180 mV起動・DC-DCコンバータのCMOS実現手法)と題し、順基板バイアス型低電圧チャージポンプ回路と低起動電圧電源システムを提案している。チャージポンプを起動回路として電源システムに統合し、CMOS技術で180 mV起動電圧のDC-DCコンバータを実証した。

第4章は「Startup Techniques for 95 mV Input DC-DC Converter」(95 mV DC-DCコンバータの起動技術)と題し、電源システムの出力容量に起動回路で電荷を貯め、メイン昇圧回路にパスオンするシステムを提案した。また、発振器の最低動作電圧を製造後補正技術で改良する技術も提案し、95 mV起動電圧のDC-DCコンバータ動作を65nm CMOSプロセスにおいてその有効性を実証した。

第5章は「80 mV Input, Fast Startup DC-DC Converter」(80 mV入力・高速起動のDC-DCコンバータ)と題し、チャージポンプ型パルス発生器及び起動モードと動作モードを兼ね備えたデュアルモードDC-DCコンバータを提案している。両技術を組み合わせる事で、80 mVの低電圧入力で高速起動が可能なDC-DCコンバータを65nm CMOSプロセスにおいて実証した。

第6章は「Adaptive Local Boost DC-DC Converter」(適応型局所昇圧DC-DCコンバータ)と題し、プロセスバラツキの影響を大きく受ける低電圧動作DC-DCコンバータの性能向上に向け、適応型局所昇圧手法を提案している。最終駆動トランジスタの駆動電圧を出力電流に応じて適応的に変化させ、高い電力効率を実現できる可能性を示した。

第7章は「Conclusions」(結論)であり、本研究の成果を要約し結論を述べている。

以上のように本論文は、環境エネルギーを利用した将来の低電圧電源システムを目指し、基板順バイアス型チャージポンプ回路、キャパシタ・パスオン手法、製造後しきい値調節型発振器およびチャージポンプ型パルス発生器を提案し、それらの有効性を集積回路の設計、試作、測定を通じて実証したものであって、電子工学上寄与するところが少なくない。

よって本論文は博士(工学)の学位請求論文として合格と認められる。