This letter presents an active rectifier with SAR-assisted coarse-fine adaptive digital delay compensation technique for biomedical implantable devices. Both on- and off-delays are compensated by adjusting the comparator offset. In order to achieve accurate zero-voltage switching against process, voltage, and temperature variations, a two-step coarse-fine digital tuning method is introduced. The coarse-tuning phase and the fine-tuning phase are controlled by a 7-bit SAR controller and a 3-bit bidirectional counter, respectively. Meanwhile, the tuning frequency is designed to be much smaller than the switching frequency to minimize the power consumption. The proposed rectifier has been fabricated in 0.18- $\mu \text{m}$ CMOS process with a core area of 0.203 mm ² and a maximum output power of 54 mW. With the input ac voltage ranging from 1.5 to 2 V, the measurement results show that the peak power conversion efficiency and voltage conversion ratio are 92.6% and 95.7% for the load resistor of 2 $\text{k}\Omega $ , respectively.

This article presents a resistor-array-tuning-based hybrid adaptive digital delay compensated rectifier for wireless power transfer systems. A no-crossing problem and nonlinearity problem for conventional delay-compensations are analyzed. In order to enhance the robustness of digital control, the on-delay and off-delay are compensated with different compensation schemes. The on-delay is compensated with a comparator-offset-adjusted technique, while the off-delay is compensated with a pulsewidth modulation (PWM)-based technique to guarantee falling zero-voltage crossing and adjust off-delay time linearly. Compared with the previous works, the delay time is adjusted by tuning the voltage drop across resistor arrays instead of changing the injected offset current of push–pull comparator so as to improve the power conversion efficiency (PCE) and loop stability. Implemented in a 0.18-$\mu$m BCD process with a total area of 0.446 mm$^{2}$, the rectifier delivered a maximum output power of 54.5 mW. With a 13.56 MHz ac input voltage in the range of 2.5 and 3.3 V, the measurement results show the peak VCR of 96.1$\%$ and peak PCE of 93.1$\%$ for the load resistance of 2 k$\Omega$ and 500 $\Omega$, respectively.

This paper presents an energy-efficient wireless power receiver for implantable electrical stimulation applications, which can achieve one-step adiabatic bipolar-supply that is generated by a hybrid single-stage dual-output regulating (SSDOR) rectifiers. The structure using only four switches overcomes the disadvantages that the two output voltage values in the traditional dual-output rectifiers are close to each other. A constant-current (CC) controlled adiabatic dynamic voltage scaling (DVS) technique is proposed to minimize the voltage headroom of the stimulating drivers and improve the stimulation efficiency significantly. In addition, the receiver adopts only one general constant on-time (COT) low-frequency control to adjust the stimulation current, reducing both the power consumption and the complexity of the control circuits. The proposed receiver has been fabricated in a 0.18 μm BCD process with ±6 V voltage compliance and 2.5 mA maximum stimulating current. With a current range from ±1.5 mA to ±2.5 mA, the measured maximum average headroom voltage is only 80 mV and the peak total efficiency of the receiver is 85.6%. The functionalities of the proposed receiver have been successfully verified through in vitro experiments.

This paper presents a novel charge balancing (CB) with a current-control (CC) mode and a voltage-control (VC) mode for implantable biphasic stimulators, which can achieve one-step accurate anodic pulse generating. Compared with the conventional short-pulse-injection-based CB, the proposed method could reduce the balancing time and avoid inducing undesired artifact. The CC operation compensates the majority stimulation charge at high speed, while the VC operation guarantees a high CB precision. In order to eliminate the oscillation during the mode transition, a smooth CC-VC transition method is adopted. In addition, a digital auxiliary monitoring loop is introduced against the variations of the tissue-electrode interface impedance during the stimulation process to meet long-term CB requirement. The proposed stimulator has been fabricated in a 0.18 μm BCD process with 10 V voltage compliance, and the measured CB precision is less than 3 mV. The functionalities of the proposed CB have been verified successfully through in vitro experiments.

Polyimide thin-film materials are widely used in aerospace and particle gas pedals, etc., but the phenomenon of secondary electron emission occurred under vacuum conditions. The graphene-coated polyimide materials were prepared for this phenomenon to suppress secondary electron emissions. The graphene coating was prepared on the polyimide surface through chemical vapor deposition (CVD). Scanning electron microscope images (SEM), X-ray photoelectron spectrometer images (XPS), Raman spectroscopy, atomic force microscopy (AFM), and other analytical methods were used to characterize the properties of the prepared materials. The C1s XPS fine spectra and Raman curve analyses showed that the material has an abundant sp2 hybridized structure, and the sp2 structure can reduce secondary electron emissions. The C, O, and N contents in the tested samples were 65.85, 20.47, and 13.68 at.%, respectively. It was examined that the graphene coating had an inhibitory effect on the secondary electron emissions of polyimide materials, and the secondary electron emission yield (SEY) was significantly reduced. The results of secondary electron tests showed that the maximum SEY (δmax) of the polyimide material decreased from 1.72 to 1.52 after the preparation of the graphene coating. The mechanism of using a graphene coating to reduce the SEY of polyimide was analyzed from experimental and theoretical perspectives. The results of this study can provide research ideas for polyimide thin film materials in aerospace, particle gas pedals, and other applications.