## 1 Introduction  
In the last few years, a large number of wearable electronic devices have appeared in the specialized scientific literature. Moreover, these devices have various practical applications. For example, most of them are for medical purposes [1; 2; 3], to determine various biometric parameters, as well as to study the behavior of the human body. There are also wearable electronic devices designed to determine, for example, the location, speed, and acceleration of various vehicles [4; 5; 6; 7; 8] or monitor electrical and non-electrical parameters in an industrial environment by wireless sensor nodes (WSNs) [9; 10; 11]. All the wearable devices listed above require a certain _power supply device_ (PSD) with the following general technical specifications: First, the power supply sources have to provide a constant value of the DC output voltage and current under variations in the ambient temperature and the levels of the input signals, produced by piezoelectric, electromagnetic, electrostatic, or triboelectric elements. Second, the power supply devices have to work with a high coefficient of efficiency (for example, higher than 50%) or with relatively low power dissipation (for example <0.01 W). Third, the power sources have to include functional blocks for short circuits or thermal overload protection of the output transistors. Also, the sources have to provide charging of a low-power rechargeable battery or supercapacitor, and the use of an additional battery (or backup primary battery) has to be implemented only in cases where the energy of the input source is insufficient. The review of recent achievements in the field of the design of wearable electronic devices shows a wide variety of technical solutions for the implementation of stand-alone power sources with the specific electronic circuitries of _Power Path Management Systems or Circuits_ (PMSs or PMCs). Some of these use commercially available application-specific integrated circuits that include functions for both AC-to-DC voltage conversion and DC/DC regulation providing a charge signal to low-power rechargeable batteries or supercapacitors. Others use custom-designed power supplies that are a functional block of a specialized electronic module implemented as a monolithic integrated circuit. For both power source implementations, additional recommendations are needed to select appropriate circuit configurations and sizing formulas depending on the parameters of the input signal source and the load in the output. For both variants of the implementation of power sources, additional recommendations are needed for the selection of appropriate circuit configurations and formulas for the selection of component values of a certain circuit depending on the parameters of the input signal source and the output load. In order to satisfy the above-mentioned need, an analysis of published review papers [12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22] has been performed. Moreover, the published review articles can be divided into two groups in view of the trends related to the continuous improvement in the approaches to designing electronic circuits, as well as the synthesis of new materials for the construction of piezoelectric elements. The first group of publications [12; 13; 14; 15; 16] reported in international scientific journals covers the period of the last ten years. In these publications, the focus of the authors' attention is mainly on the structure and principle of principle of piezoelectric elements. Also, the methods and technologies for building piezoelectric elements are examined in great depth, discussing various types of materials and the criteria for choosing the optimal option depending on specific applications. When considering the electronic circuits specially designed for piezoelectric energy harvesters, in most cases, the input stages implemented on the basis of voltage rectifiers with a series-connected inductor and electronic switch to increase the voltage amplitude are considered.