Please cite this article as: V. Matko, M. Milanović, Temperature-Compensated Capacitance-Frequency Converter with High Resolution, Sensors and Actuators: A Physical (2014), http://dx.

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This review presents various ways of detection of different physical quantities based on the frequency change of oscillators using piezoelectric crystals. These are influenced by the reactance changes modifying their electrical characteristics. Reactance in series, in parallel, or a combination of reactances can impact the electrical crystal substitute model by influencing its resonant oscillation frequency. In this way, various physical quantities near resonance can be detected with great sensitivity through a small change of capacitance or inductance. A piezoelectric crystal impedance circle and the mode of frequency changing around the resonant frequency change are shown. This review also presents the influence of reactance on the piezoelectric crystal, the way in which the capacitance lost among the crystal's electrodes is compensated, and how the mode of oscillators' output frequency is converted to lower frequency range (1-100 kHz). Finally, the review also explains the temperature-frequency compensation of the crystals' characteristics in oscillators that use temperature-frequency pair of crystals and the procedure of the compensation of crystals own temperature characteristics based on the method switching between the active and reference reactance. For the latter, the experimental results of the oscillator's output frequency stability (f out = ±0.002 ppm) at dynamical change of environment temperature (0-50 • C) are shown.

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This paper investigates a control approach for achieving reliable zero-voltage switching transitions within the entire operating range of a conventional nonisolated bidirectional dc-dc converter that utilizes synchronous rectification. The approach is based on operation in the discontinuous conduction mode with a constant reversed current of sufficient amplitude, which is achieved by load-dependent variation of the switching frequency. This paper focuses on the obtained resonant voltage transitions and provides analytical models for determining the reversed current and timing parameters that would ensure safe, reliable and highly efficient operation of the converter. In addition, the proposed approach solves the synchronous transistor's spurious turn-on and body diode reverse recovery induced issues, does not require any additional components or circuitry for its realization, and can be entirely implemented within a digital signal controller. The effectiveness and performance of the presented control approach was confirmed in a 1-kW experimental bidirectional dc-dc converter that achieved 97% efficiency over a wide range of output powers at switching frequencies above 100 kHz.

The paper presents a Switched-Capacitor Boost DC-DC Converter (SC-BC) which can be used in energy harvesting applications using thermoelectric generators (TEGs) with low output voltage, low power and a significant internal resistance. It consists of a switching capacitor circuit, where MOSFETs are used as switches, and a boost stage. The converter is a modification of a previously presented scheme in which diodes are used in the switched capacitor stage. A higher voltage gain and an increased efficiency can thus be achieved. The model of the converter was developed considering the internal resistance of the TEG and boost stage inductor. A comparison with the diode based converter is shown, with consideration of the TEG internal resistance. Calculation is presented of the main passive components. A control algorithm is also proposed and evaluated. It is based on a linearization approach, and designed for output voltage and inductor current control. The operation of both converter and control are verified with the simulation and experimental results.

This paper deals with a wireless power transfer system where a novel structure of transmitting/receiving double DD coils is applied. This system uses two identical double D (DD) transmitter coils stacked on each other to transfer power to two stacked receiver coils. The power is transmitted simultaneously and independently through both transmitter coils to the receiving coils. The magnetic field of the first coil does not interfere with the second coil. Both transmitter and receiver coils are placed on each other and occupy the same footprint, so there is no need for increased space. This can lead to an interesting wireless power transfer system—from single load to double the load and higher power transfer density.

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