## 3 Low-Power Electronic Converters
### Switching DC/DC Regulators
#### 3.3.2 Structure and Operational Principle of Step-Up Converters  
A simplified schematic configuration of a step-up converter is given in Figure 25, and timing diagrams of voltages and currents are given in Figure 26. In this circuit, when the switch is open, a current flows through the coil, inducing a voltage. When the switch is opened, the direction of the current is retained, and the output voltage is equal to the sum of the input voltage and the voltage across the coil. As a result, the output voltage is greater than the input voltage.  
The output voltage value is found similarly as for the previous circuit, having the form [46]  
\[V_{o}=\frac{t_{ON}+t_{OFF}}{t_{OFF}}V_{i}=\frac{T}{t_{OFF}}V_{i}. \tag{29}\]  
According to the formula in [15; 66], to determine the value of the coil, the following expression is obtained:  
\[L=\frac{V_{i}^{2}}{V_{o}}\bigg{(}1-\frac{V_{o}}{V_{i}}\bigg{)}\frac{T}{2I_{o, \min}}. \tag{30}\]  
Figure 26: Voltage and current timing diagrams of a step-up converter.  
Figure 25: Simplified circuit configuration of a step-up converter.  
Conventional Parallel-Plate Capacitors and Thin-Film Supercapacitors--Equivalent Circuits and Basic Parameters  
Based on the analysis presented above, piezoelectric elements that are suitable for sources (or generators) of electrical energy from spontaneous mechanical vibrations can be considered as current sources with a relatively large internal resistance. Moreover, mechanical vibrations in most cases cause voltages with a non-sinusoidal waveform, which lead to relatively complex analytical expressions for the electrical parameters. Under these operating conditions, to ensure the continuous operation of the electronic devices that use the produced voltage of the piezoelectric element as a power supply voltage, it is necessary to ensure the possibility of storing the electrical energy before it is applied to an electronic converter and load. This means that directly after rectifying the AC voltage produced, an inductor or capacitor can be connected. The inductors are used less often since at a large value of the inductances they have a relatively complex construction and, in some cases, are relatively large in size. Also, inductors are more difficult to implement in integrated circuits. Moreover, when environmental parameters change, inductors more easily change their electrical parameters. In most practical cases, conventional parallel-plate capacitors with a relatively large capacity with a small value of equivalent series resistance (ESR) or supercapacitors are used, as thin-film electronic elements are preferred for wearable electronic devices. Also, when using supercapacitors as energy storage elements, to limit the voltage across them, a limiter is connected in parallel, most often implemented with a Zener diode, whose anode is connected to the ground. To ensure the completeness of the review, this section of this paper presents an analysis of the structure, principle of operation, and basic parameters of conventional parallel-plate capacitors and thin-film supercapacitors, which are suitable for use as _energy storage elements_ in wearable electronic devices.