Mitchell Kline

Capacitive power transfer for contactless charging

The simplicity and low cost of capacitive interfaces makes them very attractive for wireless charging stations. Major benefits include low electromagnetic radiation and the amenability of combined power and data transfer over the same interface. We present a capacitive power transfer circuit using series resonance that enables efficient high frequency, moderate voltage operation through soft-switching. An included analysis predicts fundamental limitations on the maximum achievable efficiency for a given amount of coupling capacitance and is used to find the optimum circuit component values and operating point. Automatic tuning loops ensure the circuit operates at the optimum frequency and maximum efficiency over a wide range of coupling capacitance and load conditions. An example interface achieves near 80% efficiency at 3.7 W with only 63pF of coupling capacitance. An automatic tuning loop adjusts the frequency from 4.2 MHz down to 4MHz to allow for 25% variation in the nominal coupling capacitance. The duty cycle is also automatically adjusted to maintain over 70% efficiency for light loads down to 0.3 W.

In-Air Rangefinding With an AlN Piezoelectric Micromachined Ultrasound Transducer

An ultrasonic rangefinder has a working range of 30 to 450 mm and operates at a 375-Hz maximum sampling rate. The random noise increases with distance and equals 1.3 mm at the maximum range. The range measurement principle is based on pulse-echo time-of-flight measurement using a single transducer for transmit and receive. The transducer consists of a piezoelectric AlN membrane with 400-μm diameter, which was fabricated using a low-temperature process compatible with processed CMOS wafers. The performance of the system exceeds the performance of other micromechanical rangefinders.

An ultrasonic rangefinder based on an AlN piezoelectric micromachined ultrasound transducer

An ultrasonic rangefinder has a working range of 30mm to 450mm and operates at a 375 Hz maximum sampling rate. The worst-case systematic error less than 1.1 mm. The rms noise is proportional to the square of the distance and equals 1.3mm at the maximum range. The range measurement principle is based on pulse-echo time of flight measurement using a single transducer for transmit and receive consisting of a piezoelectric AlN membrane with 400 µm diameter which was fabricated using a low-temperature process compatible with processed CMOS wafers. All circuits are low voltage, enabling integration in standard low voltage circuit technology.

A transformerless galvanically isolated switched capacitor LED driver

The design and test of a capacitor-isolated LED driver, suitable for screw-in, residential lighting applications, is reported. The design relies on a pair of high voltage isolation capacitors, comprising part of a series resonant tank. The series resonant tank is integrated with a balanced ladder step-down switched capacitor front-end, enabling the series resonant conversion stage to function conveniently with any line voltage, while still preserving the efficient voltage regulation capability of the resonant stage. Dimming and power control are effected with a low frequency PWM control loop. The tested prototype delivers 15.5 W at 425 mA at rated power into a string of 12 LEDs at 92% efficiency. Efficiency exceeding 85% is maintained over more than a 10:1 dimming range, and also over a wide range of line voltages.

Silicon MEMS Disk Resonator Gyroscope With an Integrated CMOS Analog Front-End

We present a 2-mm diameter, 35-μm-thick disk resonator gyro (DRG) fabricated in <;111> silicon with integrated 0.35-μm CMOS analog front-end circuits. The device is fabricated in the commercial InvenSense Fabrication MEMSCMOS integrated platform, which incorporates a wafer-level vacuum seal, yielding a quality factor (Q) of 2800 at the DRGs 78-kHz resonant frequency. After performing electrostatic tuning to enable mode-matched operation, this DRG achieves a 55 μV/°/s sensitivity. Resonator vibration in the sense and drive axes is sensed using capacitive transduction, and amplified using a lownoise, on-chip integrated circuit. This allows the DRG to achieve Brownian noise-limited performance. The angle random walk is measured to be 0.008°/s/√(Hz) and the bias instability is 20°/h.

Quadrature FM gyroscope

A dual mass vibratory gyroscope sensor demonstrates the quadrature frequency modulated (QFM) operating mode, where the frequency of the circular orbit of a proof mass is measured to detect angular rate. In comparison to the mode-matched open loop rate mode, the QFM mode receives the same benefit of improved SNR but without the penalties of unreliable scale factor and decreased bandwidth. A matched pair of gyroscopes, integrated onto the same die, is used for temperature compensation, resulting in 6 ppb relative frequency tracking error, or an Allan deviation of 370 deg/hr with a 70 kHz resonant frequency. The integrated CMOS electronics achieve a capacitance resolution of 0.1 zF/rt-Hz with nominal 6 fF sense electrodes.

Epitaxially-encapsulated quad mass gyroscope with nonlinearity compensation

We present an epitaxially-encapsulated 2×2 mm2 quad-mass gyroscope (QMG). Relative to the earlier QMG which measured 8×8 mm2 and required an external vacuum package and getter [1], this device is 16x smaller in area and is vacuum-sealed at the wafer-level. Due to the devices small size, high quality factor (Q) and large oscillation amplitude are required to achieve low noise. However, the devices high Q (85,000) makes it highly sensitive to mechanical nonlinearity, resulting in amplitude-frequency dependence and instability of the oscillator loop at large amplitudes. To overcome these problems, we demonstrate electrostatic compensation of the mechanical nonlinearity, enabling 10x greater amplitude and therefore scale factor (SF). Together with closed-loop amplitude control and quadrature compensation, this enables angle-random walk of 0.42 mdeg/s/VHz, comparable to the best QMG published to date. Closed-loop amplitude control and quadrature null are used to achieve a bias instability of 1.6 deg/hr.

Piezoelectric micromachined ultrasonic transducers in consumer electronics: The next little thing?

This paper describes air-coupled piezoelectric micromachined ultrasonic transducers (PMUTs) for consumer electronics applications including time-of-flight range-finding, proximity and presence sensing, and gesture recognition. These applications require sensors that are small size, low-cost, and ultra-low-power, all of which are characteristics of PMUTs.

A stackable switched-capacitor DC/DC converter IC for LED drivers with 90% efficiency

A stackable switched-capacitor (SC) converter IC for a hybrid-SC-resonant LED driver using off-chip ceramic capacitors is presented in this paper. The IC can be configured to handle a range of input voltages through chip-stacking in the voltage domain. The tested driver delivers 17.6W at 470mA to the LED load with 90% peak efficiency from a rectified 120VAC line (170VDC) and maintains >85% efficiency over a rectified voltage range from 16VDC to 180VDC.

A 7ppm, 6°/hr frequency-output MEMS gyroscope

We report the first frequency-output MEMS gyroscope to achieve <;7 ppm scale factor accuracy and <; 6°/hr bias stability with a 3.24mm2 transducer. By implementing continuous-time mode reversal in an FM gyro, the rate signal is modulated away from DC, making the system insensitive to the resonant frequency of the transducer. The scale factor is almost entirely ratiometric, depending primarily on the mechanical angular gain factor of the transducer and the accuracy of the timing reference. Scale factor sensitivity to variations in quality factor, electro-mechanical coupling coefficients, and circuit drift is significantly reduced compared to conventional open-loop and force-rebalance operating modes.