# An Overview of Non-Isolated Hybrid Switched-Capacitor Step-Up DC-DC Converters
## 4 The Charge Interchange between Capacitors
### Different Types of Charge Interchange
Those concerns have limited the applications of what we now call \"pure switched capacitors\" circuits to low-power applications, but it has motivated the creation of another type of switched-capacitor circuits, which we call Resonant Switched Capacitors (RSC). Resonant Switched-Capacitor (RSC) circuits contain small inductors to avoid a spike current while making those currents sinusoidal. It is important to emphasize that avoiding inductors was the motivation to develop SC converters, and then the inclusion of inductors may sound contradictory, but inductors used in RSC converters are pretty small compared to energy storage inductors used in traditional converters. Figure 18 shows a comparison of two inductors used in traditional converters (such as the boost converter) and a small inductor (0.2 \(\mu\)H) used in an RSC converter. Some articles have reported air-core inductors or the use of the parasitic inductance on wires to produce the resonant charge interchange [42]. Figure 19 shows an equivalent circuit of two capacitors connected in parallel through a small resonant inductor, \(L_{\textit{res}}\); the current between the capacitors tends to be sinusoidal. We assume the switch opens when the current crosses zero to prevent the current from becoming negative. Figure 17: Basic non-resonant SC circuit, (**a**) schematic, (**b**) current between capacitors with full charge interchange, (**c**) current between capacitors with a partial charge interchange. Figure 18: Comparison between two energy storage inductors and a tiny resonant inductor. The shape depends on the damping factor, as explained in [42], but as we can choose the capacitance and the inductance (and try to minimize the parasitic resistance), the designer can ensure the current is similar to a sinusoidal. The resonant switching limits the peak current between capacitors; it produces a zero current switching and leads to better circuit efficiency. Conversely, the resonant period must be considered to avoid opening the resonant circuit before the current reaches zero. The main difficulty with the resonant-switched capacitor operation is that the current does not stop automatically when it reaches zero. It tends to produce a sinusoidal signal, typical in resonant circuits. It is important to open the switch exactly when the current reaches zero, which is difficult considering the resonant frequency may change due to the tolerance of components from one converter to another one. Another option is to use a diode to prevent the current from becoming negative [40; 41]. The efficiency improvement on resonant switched-capacitor circuits compared to pure switched capacitors can be explained by analyzing the efficiency from the stored energy point of view [43]. The efficiency of a power converter is usually analyzed from the power consumption point of view, but the current shape (which depends on parasitic components) is not easy to determine on switched-capacitor circuits; the other option is the energy-based analysis.