Yasuyuki Naito

A self-biased 5-to-60V input voltage and 25-to-1600µW integrated DC-DC buck converter with fully analog MPPT algorithm reaching up to 88% end-to-end efficiency

Energy harvesting is seen as an enabling technology for autonomous wireless sensing in automotive applications. This technology may rely on piezoelectric or electrostatic energy conversion using the energy available during the tire impact with the road. The power management system has to transfer harvested power to the load battery (few μW to mW) with the highest possible efficiency. An electrostatic harvester can generate voltages up to 40V even at low accelerations [1]. Hence, a high voltage (HV) DC-DC converter is needed to maximize the energy transfer from the harvester to the load battery. However, HV buck converters including maximum power point tracking (MPPT) algorithms have been shown only for much higher ranges of power, in the order of W [2]. On the other hand, all reported converters working in the sub-mW power range are not able to sustain high input voltages and do not include an integrated MPPT algorithm [3, 4]. Until this work, no solution in literature is able to interface with these electrostatic energy harvesters. Figure 4.6.1 shows the block diagram of the system, designed and fabricated in TSMC 0.25μm BCD CMOS (60V option). The IC is able to interface a vibrational harvester by means of few external components: a rectifier, its load capacitor CIN, and an inductor L. The converter is consisting of a power train and its control circuits. The power train is implemented with the external inductor L, and the integrated power switches MP and MN. The integrated control circuits have to provide the correct gate voltages VGN and VGP for allowing the converter to maximize its output power. All circuits are biased by means of an integrated current reference and are supplied by the load battery voltage VBAT and the DC input VIN.

A High Voltage Self-Biased Integrated DC-DC Buck Converter With Fully Analog MPPT Algorithm for Electrostatic Energy Harvesters

This paper presents a high voltage and low power inductive DC-DC buck converter for interfacing electrostatic energy harvesters. The system, implemented in TSMC 0.25 μm BCD, can convert input voltages from 5 to 60 V to recharge the energy storage system connected to the output. The system includes also a Maximum Power Point Tracking algorithm for matching the equivalent source resistance of harvester and AC-DC converter. Due to power consumption constrains, a conventional implementation of this algorithm, based on ADC, DAC and complex digital processing is replaced by a much simpler fully analog design. Measurements have shown a peak end-to-end efficiency of 88% with 99.8% MPPT peak efficiency.

RF-MEMS switching devices using vertical comb-drive actuation in the CMOS process

Radio frequency micro-electro-mechanical system (RF-MEMS) switching devices using vertical comb-drive actuation toward low-voltage actuation, fast response are presented in this paper. The switching devices, which comprise comb-drive electrodes, are actuated entirely by the electrostatic forces applied not only for the down-state but also for the up-state. The cost-effective MEMS process compatible with the complementary metal oxide semiconductor (CMOS) process is presented in this paper as well. The fabrication process is composed by adapting the CMOS 0.18 µm back end of line (BEOL) process on 200 mm wafers. The MEMS process in the CMOS process enables the realization of passive devices integrated with active devices, which is effective for size and cost reduction. Two metal interconnection layers in the BEOL process are used for the MEMS process. Interconnection aluminum and inter-layer dielectric tetraethoxysilane (TEOS) are used as MEMS structural material and sacrificial material, respectively. The chemical mechanical polishing (CMP) process is implemented to planarize the sacrificial material surface. The structures were fabricated using a simple low-cost two-mask process. The characteristics of switching capacitors, C-V, RF performance, switching speed and continuous drive cycles are measured on the fabricated devices. The capacitance ratio for the down-state/up-state is Cdown/Cup = 5.4. The characteristics of switching speed response/actuation voltage in the down-state and up-state are 4.5 µs/5 V and 8.0 µs/5 V, respectively. The switching speed is stable up to 107 cycles in spite of the fact that the unipolar voltage speed is stable up to 107 cycles.