Hyung-Min Lee

An inductively powered scalable 32-channel wireless neural recording system-on-a-chip for neuroscience applications

There has been considerable effort devoted to developing technology for interfacing with the central nervous system in laboratory animals and humans [1–2]. Even though these efforts have led to marvelous technological advancements in circuits and systems, some of the resulting devices may find little use in their application domain, because the specifics of the targeted applications or the realistic needs of the end users may not be taken into account.

An Integrated Power-Efficient Active Rectifier With Offset-Controlled High Speed Comparators for Inductively Powered Applications

We present an active full-wave rectifier with offset-controlled high speed comparators in standard CMOS that provides high power conversion efficiency (PCE) in high frequency (HF) range for inductively powered devices. This rectifier provides much lower dropout voltage and far better PCE compared to the passive on-chip or off-chip rectifiers. The built-in offset-control functions in the comparators compensate for both turn-on and turn-off delays in the main rectifying switches, thus maximizing the forward current delivered to the load and minimizing the back current to improve the PCE. We have fabricated this active rectifier in a 0.5-μm 3M2P standard CMOS process, occupying 0.18 mm2 of chip area. With 3.8 V peak ac input at 13.56 MHz, the rectifier provides 3.12 V dc output to a 500 Ω load, resulting in the PCE of 80.2%, which is the highest measured at this frequency. In addition, overvoltage protection (OVP) as safety measure and built-in back telemetry capabilities have been incorporated in our design using detuning and load shift keying (LSK) techniques, respectively, and tested.

A Power-Efficient Wireless System With Adaptive Supply Control for Deep Brain Stimulation

A power-efficient wireless stimulating system for a head-mounted deep brain stimulator (DBS) is presented. A new adaptive rectifier generates a variable DC supply voltage from a constant AC power carrier utilizing phase control feedback, while achieving high AC-DC power conversion efficiency (PCE) through active synchronous switching. A current-controlled stimulator adopts closed-loop supply control to automatically adjust the stimulation compliance voltage by detecting stimulation site potentials through a voltage readout channel, and improve the stimulation efficiency. The stimulator also utilizes closed-loop active charge balancing to maintain the residual charge at each site within a safe limit, while receiving the stimulation parameters wirelessly from the amplitude-shift-keyed power carrier. A 4-ch wireless stimulating system prototype was fabricated in a 0.5-μm 3M2P standard CMOS process, occupying 2.25 mm2. With 5 V peak AC input at 2 MHz, the adaptive rectifier provides an adjustable DC output between 2.5 V and 4.6 V at 2.8 mA loading, resulting in measured PCE of 72 ~ 87%. The adaptive supply control increases the stimulation efficiency up to 30% higher than a fixed supply voltage to 58 ~ 68%. The prototype wireless stimulating system was verified in vitro.

A High Frequency Active Voltage Doubler in Standard CMOS Using Offset-Controlled Comparators for Inductive Power Transmission

In this paper, we present a fully integrated active voltage doubler in CMOS technology using offset-controlled high speed comparators for extending the range of inductive power transmission to implantable microelectronic devices (IMD) and radio-frequency identification (RFID) tags. This active voltage doubler provides considerably higher power conversion efficiency (PCE) and lower dropout voltage compared to its passive counterpart and requires lower input voltage than active rectifiers, leading to reliable and efficient operation with weakly coupled inductive links. The offset-controlled functions in the comparators compensate for turn-on and turn-off delays to not only maximize the forward charging current to the load but also minimize the back current, optimizing PCE in the high frequency (HF) band. We fabricated the active voltage doubler in a 0.5-μm 3M2P std . CMOS process, occupying 0.144 mm2 of chip area. With 1.46 V peak AC input at 13.56 MHz, the active voltage doubler provides 2.4 V DC output across a 1 kΩ load, achieving the highest PCE = 79% ever reported at this frequency. In addition, the built-in start-up circuit ensures a reliable operation at lower voltages.

A Power-Efficient Wireless Capacitor Charging System Through an Inductive Link

A power-efficient wireless capacitor charging system for inductively powered applications has been presented. A bank of capacitors can be directly charged from an ac source by generating a current through a series charge injection capacitor and a capacitor charger circuit. The fixed charging current reduces energy loss in switches, while maximizing the charging efficiency. An adaptive capacitor tuner compensates for the resonant capacitance variations during charging to keep the amplitude of the ac input voltage at its peak. We have fabricated the capacitor charging system prototype in a 0.35-μm 4-metal 2-poly standard CMOS process in 2.1 mm2 of chip area. It can charge four pairs of capacitors sequentially. While receiving 2.7-V peak ac input through a 2-MHz inductive link, the capacitor charging system can charge each pair of 1 μF capacitors up to ±2 V in 420 μs, achieving a high measured charging efficiency of 82%.

Fully integrated power-efficient AC-to-DC converter design in inductively-powered biomedical applications

In this paper we have reviewed several types of integrated AC-to-DC converters which have been widely used for inductively-powered applications. The limitations for achieving high power conversion efficiency (PCE) in each AC-to-DC converter have been considered in order to design highly power-efficient converters for applications that are in need of higher power levels with very low heat dissipation, such as implantable microelectronic devices (IMD). In this paper, we also propose a fully integrated active voltage doubler with offset-controlled high speed comparators, which provides much lower dropout voltage and far better PCE at high frequency (HF) range compared to its passive counterparts. Our active voltage doubler was fabricated in a 0.5-µm 3M2P standard CMOS process, occupying 0.144 mm2 of chip area. With 1.72 V peak AC input at 13.56 MHz, the active voltage doubler provides 2.4 V DC output across the 1 k7 load, achieving high PCE of 78%.

Advanced wireless power and data transmission techniques for implantable medical devices

Short-range wireless power and data transmission offers a viable mean to power up implantable medical devices (IMDs) with a wide range of power levels and communicate with external units across the skin. To optimize wireless power transfer (WPT), it is key to improve efficiencies in every stage of the power delivery path from external power sources to the IMD, while maintaining robustness and safety against load variations, coil misalignments, and nearby conductive objects. This paper reviews various mechanisms for WPT with focus on link structures and circuit techniques for wirelessly-powered IMDs. Moreover, advanced IMDs require wireless data telemetry (WDT) for wideband bidirectional data communication in the presence of the strong power carrier interference. This paper also discusses several modulation schemes and transceiver circuits for low-power, carrier-less, and robust WDT.

Energy management integrated circuits for wireless power transmission

Wireless power transmission is one of the few viable techniques to power up implantable medical devices (IMDs) across the skin without any direct electrical contact between the energy source and the IMD. There are also other wirelessly powered applications with various levels of power requirements from nanowatts in wireless sensors and radiofrequency identification (RFID) tags, milliwatts in near-field communication (NFC), watts in mobile electronics, and kilowatts in electric vehicles. High power transfer efficiency (PTE), robustness against nearby objects and coil misalignments, and extended power transfer range are highly desired in all of these applications.

Power-Efficient Wireless Neural Stimulating System Design for Implantable Medical Devices

Neural stimulating implantable medical devices (IMDs) have been widely used to treat neurological diseases or interface with sensory feedback for amputees or patients suffering from severe paralysis. More recent IMDs, such as retinal implants or brain?computer interfaces, demand higher performance to enable sophisticated therapies, while consuming power at higher orders of magnitude to handle more functions on a larger scale at higher rates, which limits the ability to supply the IMDs with primary batteries. Inductive power transmission across the skin is a viable solution to power up an IMD, while it demands high power efficiencies at every power delivery stage for safe and effective stimulation without increasing the surrounding tissue’s temperature. This paper reviews various wireless neural stimulating systems and their power management techniques to maximize IMD power efficiency. We also explore both wireless electrical and optical stimulation mechanisms and their power requirements in implantable neural interface applications.