Pyungwoo Yeon

A Q-Modulation Technique for Efficient Inductive Power Transmission

A fully integrated power management ASIC for efficient inductive power transmission has been presented capable of automatic load transformation using a method, called Q-modulation. Q-modulation is an adaptive scheme that offers load matching against a wide range of loading (RL) and coupling distance (d23) variations in inductive links to maintain high power transfer efficiency (PTE). It is suitable for inductive powering implantable microelectronic devices (IMDs), recharging mobile electronics, and electric vehicles. In Q-modulation, the zero-crossings of the induced current in the receiver (Rx) LC-tank are detected and a low-loss switch chops the Rx LC-tank for part of the power carrier cycle to form a high-Q LC-tank and store the maximum energy, which is then transferred to RL by opening the switch. By adjusting the duty cycle (D), the loaded-Q of the Rx LC-tank can be dynamically modulated to compensate for variations in RL. A Q-modulation power management (QMPM) prototype chip was fabricated in a 0.35 μm standard CMOS process, occupying 4.8 mm 2. In a 1.45 W wireless power transfer setup, using a class-E power amplifier (PA) operating at 2 MHz, the QMPM successfully increased the inductive link PTE and the overall power efficiency by 98.5% and 120.7% at d23 = 8 cm, respectively, by compensating for 150 Ω variation in RL at D = 45%.

12.7 A power-management ASIC with Q-modulation capability for efficient inductive power transmission

A wide variety of applications can benefit from near-field wireless power transfer using coupled inductive links, such as wireless sensors and implantable microelectronic devices. The use of inductive power transmission is expected to see an explosive growth over the next decade as engineers try to cut the last cord from mobile electronics, small home appliances, and even electric vehicles [1]. The inductive link power transfer efficiency (PTE) is highly dependent of the loading of the receiver (Rx) coil, referred to as RL. As shown in Fig. 12.7.1a, magnetic resonance-based power transmission in the form of a 3-coil link has been proposed to maximize PTE for any given RL by transforming it to an optimal load, using k34 variable [2,3]. Alternatively, an off-chip matching circuit has been used to transform RL [4]. However, these methods need either an additional coil or a network of off-chip capacitors and inductors, which add to the size/cost of Rx. Moreover, in the above applications, RL can change drastically during operation and there is a need for Rx to dynamically compensate for a wide range of RL to maintain high PTE.

Fabrication and Microassembly of a mm-Sized Floating Probe for a Distributed Wireless Neural Interface

A new class of wireless neural interfaces is under development in the form of tens to hundreds of mm-sized untethered implants, distributed across the target brain region(s). Unlike traditional interfaces that are tethered to a centralized control unit and suffer from micromotions that may damage the surrounding neural tissue, the new free-floating wireless implantable neural recording (FF-WINeR) probes will be stand-alone, directly communicating with an external interrogator. Towards development of the FF-WINeR, in this paper we describe the micromachining, microassembly, and hermetic packaging of 1-mm3 passive probes, each of which consists of a thinned micromachined silicon die with a centered Ø(diameter) 130 μm through-hole, an Ø81 μm sharpened tungsten electrode, a 7-turn gold wire-wound coil wrapped around the die, two 0201 surface mount capacitors on the die, and parylene-C/Polydimethylsiloxane (PDMS) coating. The fabricated passive probe is tested under a 3-coil inductive link to evaluate power transfer efficiency (PTE) and power delivered to a load (PDL) for feasibility assessment. The minimum PTE/PDL at 137 MHz were 0.76%/240 μW and 0.6%/191 μW in the air and lamb head medium, respectively, with coil separation of 2.8 cm and 9 kΩ receiver (Rx) loading. Six hermetically sealed probes went through wireless hermeticity testing, using a 2-coil inductive link under accelerated lifetime testing condition of 85 °C, 1 atm, and 100%RH. The mean-time-to-failure (MTTF) of the probes at 37 °C is extrapolated to be 28.7 years, which is over their lifetime.

Wireless Power Transfer With Zero-Phase-Difference Capacitance Control

Wireless power transfer enables the frequent and ubiquitous charging of electronic devices. However, the variation of the efficiency and the received power with the transmission distance is an outstanding issue. To solve the problem of efficiency degradation of the magnetic resonance at short distances, zero-phase-difference capacitance control (ZPDCC), which is suitable for integration in large scale integrations (LSIs) is proposed in this paper. The proposed ZPDCC achieves adaptive capacitance control by a newly proposed control algorithm with a current-sensing circuit to control variable capacitors at a fixed frequency. Additionally, a theoretical analysis of the total DC-DC power transmission efficiency (ηTOTAL) including a power amplifier, coupled resonators, and a rectifier is demonstrated in this paper. The analysis indicates that the frequency (and capacitance) splitting of ηTOTAL is mainly due to the power amplifier; additionally, the efficiency of the power amplifier is maximized at the split peaks when the transmission distance (d) is short. A wireless power transfer system in magnetic resonance with ZPDCC is fabricated in a 3.3 V, 180 nm CMOS. By introducing ZPDCC, the measured ηTOTAL at 13.56 MHz increases 1.7 times from 16% to 27% at d=2.5 mm.

A Multicycle Q-Modulation for Dynamic Optimization of Inductive Links

This paper presents a new method, called multicycle Q-modulation,to modulate the quality factor (Q) of the receiver (Rx) coil and dynamically optimize the load impedance to maximize the power transfer efficiency (PTE) in two-coil links. A key advantage of the proposed method is that it can be easily implemented using off-the-shelf components without requiring fast switching at or above the carrier frequency, which is more suitable for integrated circuit design. Moreover, the proposed technique does not need any sophisticated synchronization between the power carrier and Q-modulation switching pulses. The multicycle Q-modulation is analyzed theoretically by a lumped circuit model, and verified in simulation and measurement using an off-the-shelf prototype. Automatic resonance tuning in the Rx, combined with multicycle Q-modulation helped maximizing PTE of the inductive link dynamically in the presence of environmental and loading variations, which can otherwise significantly degrade the PTE in multicoil settings. In the prototype conventional two-coil link, the proposed method increased the power amplifier plus inductive-link efficiency from 4.8% to 16.5% at (RL = 1 kW, d23 = 3 cm), and from 23% to 28.2% at (RL = 100 Ω, d23 = 3 cm) after 11% change in the resonance capacitance, while delivering 168.1 mW to the load (PDL).

Robust Wireless Power Transmission to mm-Sized Free-Floating Distributed Implants

This paper presents an inductive link for wireless power transmission (WPT) to mm-sized free-floating implants (FFIs) distributed in a large three-dimensional space in the neural tissue that is insensitive to the exact location of the receiver (Rx). The proposed structure utilizes a high-Q resonator on the target wirelessly powered plane that encompasses randomly positioned multiple FFIs, all powered by a large external transmitter (Tx). Based on resonant WPT fundamentals, we have devised a detailed method for optimization of the FFIs and explored design strategies and safety concerns, such as coil segmentation and specific absorption rate limits using realistic finite element simulation models in HFSS including head tissue layers, respectively. We have built several FFI prototypes to conduct accurate measurements and to characterize the performance of the proposed WPT method. Measurement results on 1-mm receivers operating at 60 MHz show power transfer efficiency and power delivered to the load at 2.4% and 1.3 mW, respectively, within 14–18 mm of Tx–Rx separation and 7 cm2 of brain surface.

Toward a distributed free-floating wireless implantable neural recording system

To understand the complex correlations between neural networks across different regions in the brain and their functions at high spatiotemporal resolution, a tool is needed for obtaining long-term single unit activity (SUA) across the entire brain area. The concept and preliminary design of a distributed free-floating wireless implantable neural recording (FF-WINeR) system are presented, which can enabling SUA acquisition by dispersedly implanting tens to hundreds of untethered 1 mm3 neural recording probes, floating with the brain and operating wirelessly across the cortical surface. For powering FF-WINeR probes, a 3-coil link with an intermediate high-Q resonator provides a minimum S21 of -22.22 dB (in the body medium) and -21.23 dB (in air) at 2.8 cm coil separation, which translates to 0.76%/759 μW and 0.6%/604 μW of power transfer efficiency (PTE) / power delivered to a 9 kΩ load (PDL), in body and air, respectively. A mock-up FF-WINeR is implemented to explore microassembly method of the 1×1 mm2 micromachined silicon die with a bonding wire-wound coil and a tungsten micro-wire electrode. Circuit design methods to fit the active circuitry in only 0.96 mm2 of die area in a 130 nm standard CMOS process, and satisfy the strict power and performance requirements (in simulations) are discussed.

Optimal design of a 3-coil inductive link for millimeter-sized biomedical implants

For wireless power transfer to multiple millimeter-sized implantable medical devices (IMDs), power delivered to a load should be over minimum operating power of the IMDs and power transfer efficiency (PTE) should be maximized to reduce specific absorption rate across a wide area of interest. We have demonstrated advantages of using a 3-coil inductive link to energize multiple IMDs and its PTE optimization to power up bonding-wire wound coils implanted in tissue environment, in contrast to earlier works that focused on 2-coil inductive link optimization methodologies to a single mm-sized IMD. HFSS simulation results show superiority of the 3-coil inductive link to wirelessly deliver power to multiple mm-sized coils. A 3-coil inductive link optimization method, considering the desired radius of the resonator coil, is presented. Measured PTE to the boding-wire wound coil in tissue media were 9.13% and 2.01% at 275 MHz and 131 MHz when resonator radii were 0.5 cm and 1 cm, respectively.

A multi-cycle Q-modulation technique for wirelessly-powered biomedical implants

This paper presents a new technique, called multicycle Q-modulation, which can be used in wireless power transmission (WPT) to modulate the quality factor (Q) of the receiver (Rx) and dynamically match the load impedance with that of the inductive link. A key advantage of the proposed Q-modulation method is that it can be easily implemented using off-the-shelf power management components without requiring any fast switching function at the carrier frequency, which is only feasible in integrated circuit implementation. The multi-cycle Q-modulation is analyzed theoretically by a lumped circuit model, and verified in simulation and measurement using an off-the-shelf receiver. In a conventional 2-coil link, the proposed method increases the power amplifier (PA) plus inductive link efficiency from 1.2% to 12.8% at RL = 1 kΩ, and from 17.8% to 24.5% at RL = 50 Ω with 170 mW power delivered to the load.

Wireless coil array sensors for monitoring hermetic failure of millimeter-sized biomedical implants

This paper presents an automated hermetic failure monitoring system design for multiple millimeter-sized biomedical implants using an inductive link array. 1 × 1 mm2 sized passive implants, wrapped with power receiving and data transmitting inductor-capacitor (LC) tank, and coated with parylene-C and polydimethylsiloxane (PDMS) were utilized for packaging failure monitoring, which can result in phase-dip disappearance or phase-dip frequency shift. The presented system can wirelessly monitor the phase-dip in the frequency and time domains through the readout coil array, as they are coupled with the LC sensors embedded in the implants. The amplitude of the phase-dip signal at ∼1.5 mm sensing distance is 0.25° and the standard deviation from the phase-dip center frequency is 135 kHz, which represents only 0.345% variation around the 116.3 MHz resonance frequency of the LC sensor.