## 7 Concluding Remarks  
This paper's study objects are the approaches and techniques for designing power management systems intended for converting electrical energy from energy harvesters. As sources of electrical energy, the capabilities of piezoelectric elements are considered, in which the mechanical energy of vibrations is converted into electrical voltage and current. A typical Si-based cantilever piezoelectric element can generate electrical power in the range of 800 \(\upmu\)W to 10-15 W, depending on factors such as the amplitude (1 g-10 kg) and frequency (10 Hz-200 kHz) of the applied mechanical vibrations, the size and geometry of the piezoelectric material (thickness of 0.8 \(\upmu\)m-800 \(\upmu\)m, linear dimensions of 10 mm-5 cm), and the specific piezoelectric coefficient of the material used (50-500 pC/N). They can also produce current outputs ranging from 100 nA to 100 \(\upmu\)A, making them valuable components for applications requiring energy harvesting from ambient vibrations. When using piezoelectric elements as sources of electrical energy, an AC voltage with a non-sinusoidal waveform is obtained. The voltage has an approximately rectangular form, and more often it has an impulse character, i.e., short, intermittent pulses. Also, due to the spontaneous nature of mechanical vibrations, the amplitude of the produced AC voltage varies in a wide range and for a single element can have values of up to several volts and, in some cases, even 10 V or higher. If the produced electrical energy is used to supply a mixed-signal processing circuit or a microcontroller, the continuity of operation is necessary (i.e., constant value of the supply voltage during the operation), and an increase in the clock frequency results in an increase in the current consumption. All these conditions require a \"short path\" from the source of the electrical energy to the energy storage elements and also to the load, which in most cases is a supercapacitor, used as electrical energy storage elements. In this case, in the discharge mode of the electric energy storage element, the output current can reach values greater than 1 mA. To satisfy these conditions, there is a wide variety of low-power management systems, which in some cases are only a voltage rectifier or electronic AC/DC converter, employing a microcontroller as a switching circuit. At the same time, the quiescent current for the switching circuit does not exceed 1 \(\upmu\)A at a relatively wide range of input voltage (from 1-2 V up to 20 V). In this paper, the authors analyze the basic stage and functional blocks of an energy harvesting system from the signal source, through the electronic converter to the electrical energy storage elements. The structure of suitable piezoelectric elements for harvesting electrical energy is analyzed; then, a review of various types of low-power electronic circuits is performed, defining their advantages and disadvantages, and finally, the structure and operation principles of parallel-plate capacitors and thin-film supercapacitors suitable for wearable devices are discussed. Furthermore, the focus of the authors' attention is on the various types of electronic circuits that can work with AC voltage obtained from piezoelectric elements and provide charging of a large-capacity electrolytic capacitor or supercapacitor. Since more and more electronic systems are implemented as laboratory monolithic integrated circuits, a comparative analysis was performed for them, indicating the most frequently used structural blocks with a definition of their basic electrical parameters and functional advantages. For all the circuits, the energy efficiency coefficient is greater than 60%, and for some of them, a value greater than 85% is achieved.