Dongwon Kwon

Single-Inductor–Multiple-Output Switching DC–DC Converters

Emerging feature dense portable microelectronic devices pose several challenges, including demanding multiple supply voltages from a single miniaturized power-efficient platform. Unfortunately, the power inductors used in magnetic-based switching converters (which are power efficient) are bulky and difficult to integrate. As a result, single-inductor-multiple-output (SIMO) solutions enjoy popularity, but not without design challenges. This brief describes, illustrates, and evaluates how SIMO dc-dc converters operate, transfer energy, and control (through negative feedback) each of their outputs.

A single-inductor 0.35µm CMOS energy-investing piezoelectric harvester

Because miniaturized systems store little energy, their lifespans are often short. Fortunately, vibrations are consistent and abundant in many applications, so ambient kinetic energy can be a viable source. Vibrations induce the charges in piezoelectric transducers to build electrostatic forces that damp vibrations and convert kinetic energy into the electrical domain. The shunting switches and switched-inductor circuit of bridge rectifiers in [1-2] increase this output energy by extending the damping (i.e., harvesting) duration within a vibration cycle. Because the output voltages of bridge rectifiers clamp and limit the electrical damping forces built, switched-inductor converters in [3-4], whose damping voltages can exceed their rectified outputs, draw more power from vibrations. Still, electrical-mechanical coupling factors in tiny transducers are low, so electrical damping forces (i.e., voltages) remain weak. Investing energy into the transducer can increase this force, but unlike in [5-6], which demand multiple inductors and high-voltage sources, the system presented here invests energy with only one inductor at low voltages.

A 2-

A fundamental problem that miniaturized systems, such as biomedical implants, face is limited space for storing energy, which translates to short operational life. Harvesting energy from the surrounding environment, which is virtually a boundless source at these scales, can overcome this restriction, if losses in the system are sufficiently low. To that end, the 2-μm bi-complementary metal-oxide semiconductor switched-inductor piezoelectric harvester prototype evaluated and presented in this paper eliminates the restrictions associated with a rectifier to produce and channel 30 μW from a periodic 72- μW piezoelectric source into a battery directly. In doing so, the circuit also increases the systems electrical damping force to draw more power and energy from the transducer, effectively increasing its mechanical-electrical efficiency by up to 78%. The system also harnesses up to 659 nJ from nonperiodic mechanical vibrations, which are more prevalent in the environment, with 6.1±1.5% to 8.8±6.9% of end-to-end mechanical-electrical efficiency.

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Although the benefits of incorporating noninvasive intelligence (e.g. wireless micro-sensors) to state-of-the-art and difficult-to-replace technologies are undeniable, micro-scale integration constrains energy and power to the point lifetime and functionality fall below practical expectations, forcing technologists to seek energy and power from the surrounding environment. To this end, a piezoelectric energy harvester circuit is proposed. The 2µm CMOS design circumvents the need for (and losses and low-voltage restrictions associated with) a rectifier by extracting and transferring energy directly from the piezoelectric transducer to the battery via a switched inductor. Simulation results show that the proposed system can harvest 45nJ and 10nJ per period at 71% and 69% efficiency from 3V and 1.5V peak piezoelectric voltages, respectively.

m BiCMOS Rectifier-Free AC–DC Piezoelectric Energy Harvester-Charger IC

Microscale integration constrains energy and the lifetime microsystems like wireless sensors and biomedical implants can achieve to impractical levels. Harnessing ambient vibration energy from a small piezoelectric transducer, however, can viably keep an otherwise exhaustible battery charged. The problem is rectifying unpredictably small ac signals (which are prevalent in small volumes and with weak vibrations [1]) whose peak voltages fall below the rectified output level targeted, requires low-loss [2]–[3], no-threshold rectifiers. To fulfill these necessities, quasi-lossless LC energy-transfer networks that precede [4] or follow [5] the rectifier can extract all the energy stored in the piezoelectric capacitance and therefore overcome the basic threshold-voltage limitation, except the rectifier and its controllers headroom and quiescent current nonetheless limit the input voltage range of the system and dissipate power. The harvester- charger presented here, however, whose power-train simulation results were first reported in [6], (i) eliminates the rectifier core and its headroom limit by steering the inductor energy directly into the battery and (ii) increases the electrical damping force against which vibrations work, inducing the transducer to source more power.

A rectifier-free piezoelectric energy harvester circuit

The potential application space for miniaturized systems like wireless microsensors is expansive, from reconnaissance mission work and remote sensors to biomedical implants and disposable consumer products. Conforming to microscale dimensions, however, constrains energy and power to such an extent that sustaining critical power-hungry functions like wireless communication is problematical. Harvesting ambient kinetic energy offers an appealing alternative, except the act of transferring energy requires power that could easily exceed what the harvester generates in the first place. This paper reviews piezoelectric and electrostatic harvester circuits, describes how to design low-power switched-inductor converters capable of producing net energy gains when supplied from piezoelectric and electrostatic transducers, and presents experimental results from prototype embodiments. In the electrostatic case shown, the controller dissipated 0.91 nJ per cycle and the switched-inductor precharger achieved 90.3% efficiency to allow the harvester to net a gain of 2.47 nJ per cycle from a capacitor that oscillated between 157 and 991 pF. The piezoelectric counterpart harnessed 1.6 to 29.6 μW from weak periodic vibrations with 0.05-0.16- m/s2 accelerations and 65.3 μJ from (impact-produced) nonperiodic motion.

A single-inductor AC-DC piezoelectric energy-harvester/battery-charger IC converting ±(0.35 to 1.2V) to (2.7 to 4.5V)

Wireless microsensors that monitor and detect activity in factories, farms, military camps, vehicles, hospitals, and the human body can save money, energy, and lives. Miniaturized batteries, unfortunately, easily exhaust, which limit deployment to few niche markets. Luckily, harnessing ambient energy offers hope. The challenge is tiny transducers convert only a small fraction of the energy available into the electrical domain, and the microelectronics that transfer and condition power dissipate some of that energy, further reducing the budget on which microsystems rely to operate. Improving transducers and trimming power losses in the system to increase output power is therefore of paramount importance. Increasing the electrical damping force against which transducers work also deserves attention because output power is, fundamentally, the result of damping. This paper explores how investing energy to increase electrical damping can boost output power in electromagnetic, electrostatic, and piezoelectric transducers.

Harvesting Ambient Kinetic Energy With Switched-Inductor Converters

The potential application space for miniaturized systems like wireless microsensors is expansive, from reconnaissance mission work and remote sensors to biomedical implants and disposable consumer products. Conforming to microscale dimensions, however, constrains energy and power to such an extent that sustaining critical power-hungry functions like wireless communication is next to impossible. Harvesting ambient energy offers an appealing alternative, except the act of transferring energy requires power that could easily exceed what the transducer generates in the first place. This paper presents how to design low-power switched-inductor converters capable of producing net energy gains when supplied from low-power piezoelectric and electrostatic kinetic-harvesting sources.

Increasing Electrical Damping in Energy-Harnessing Transducers

Ensuring stable operation is perhaps one of the most challenging aspects of designing a power-supply circuit because the load is dynamically unpredictable and widely variable. Unfortunately, the nonlinear dynamics of switch-mode converters accentuate these difficulties, especially when considering conventional analytical techniques rely on abstract mathematics to describe ac switching events that convey little operational insight into the circuit. This paper proposes an operation-based signal-flow ac analysis approach that is both sufficiently general to apply to most switched-inductor converters in continuous- and discontinuous-conduction modes (CCM and DCM) and insightful enough to derive directly from the waveforms of the circuit, not from the system of differential equations that govern them. To this end, after presenting the technique, the paper applies it to a relatively simple topology like the buck converter to prove its efficacy and then to a more complex counterpart like the non-inverting buck-boost converter to show the ease with which a more complicated system can be analyzed.

Harvesting kinetic energy with switched-inductor DC-DC converters

Energy and power in tiny batteries are often insufficient to sustain the demands of a wireless microsystem for extended periods. Piezoelectric transducers are viable alternatives because they draw power from a vast tank-free supply of ambient kinetic energy in vibrations. Unfortunately, small devices alone seldom dampen vibrations enough to fully harness what is available, which is why investing energy to increase the electrical damping force that transducers impose is so important. This paper introduces and evaluates three investment schemes and 0.35-μm CMOS switched-inductor circuits that increase this force to generate more output power.