# An Overview of Non-Isolated Hybrid Switched-Capacitor Step-Up DC-DC Converters
## 1 Introduction
### SC Controlled by Transistors, the Elementary Principle  
One realization of a voltage doubler similar to the digital-signals-driven SC converters can be seen in Figure 8. Instead of using a digital signal as in Figure 6, two transistors generate the pulses for capacitor \(C_{1}\). Figure 8a shows the more basic circuit with an auxiliary pulsating signal. Figure 8b shows how the pulsating signal can be produced with two transistors that connect the low side of capacitor \(C_{1}\) either to the positive or to the negative side of the input voltage source.  
Diodes and transistors are basically switches. Figure 9 shows the same topology with switches in all semiconductors, this does not modify the original idea, and sometimes in dc-dc conversion, switches are used instead of diodes since they have lower conduction losses. Figure 8 shows what we can consider as the conversion from the digital signals-driven SC circuits to the modern SC circuits based on the transistors.  
Let us consider the circuit in Figure 9a, which can be classified as a pure SC voltage doubler. The circuit in Figure 9a is equivalent to the one shown in Figure 8b, but it has only switches or transistors, four transistors instead of two diodes and two transistors. Switches \(s_{1}\) and \(s_{3}\) are driven with the same switching signal, a square signal (50% of duty cycle), and switches \(s_{2}\) and \(s_{4}\) are also driven with the same firing signal, which is the complementary function to the first one. Two equivalent circuits are produced in this operation, as shown in Figure 9b,c.  
Figure 9b shows the switching state or equivalent circuit of the converter when \(s_{1}\) and \(s_{3}\) are on (\(s_{1}=s_{2}=1\)) while \(s_{2}\) and \(s_{4}\) are off (\(s_{2}=s_{4}=0\)). Figure 9c shows the other switching state when \(s_{1}\) and \(s_{3}\) are off (\(s_{1}=s_{2}=0\)) while \(s_{2}\) and \(s_{4}\) are on (\(s_{2}=s_{4}=1\)).  
Figure 8: Switched-capacitor voltage doubler, the simplest configuration of the converter in Figure 6, (a) with an auxiliary pulsating signal and (b) based on two transistors and two diodes.  
Figure 7: A multiple auxiliary input SC voltage multiplier.  
When the circuit is in the state shown in Figure 9b, capacitor \(C_{1}\) gets charged to the input voltage, and in the other state (see Figure 9c), \(C_{1}\) gets in series with the input voltage source charging \(C_{2}\) with twice the input voltage. The load resistance discharges the capacitor \(C_{2}\), but if the capacitance value of the capacitors and the switching frequency are large enough, it is possible to design a circuit with a relatively small output voltage ripple.  
The parallel interconnection of the capacitors is the main difference between the operation of switched-capacitor circuits and traditional converters (such as the buck or boost); switched-capacitor circuits are intended to avoid inductors for which the energy interchange must be done with the unique energy stored components on the circuits, the \"Capacitors\".  
The circuit shown in Figure 9 is only a member of a larger family in which many combinations can be configured and extended. Pure SC topologies have been studied since the 1970s when the most traditional topologies were introduced. They were used as voltage multipliers to produce a larger voltage than their input. The first classification we may introduce to pure switched-capacitor circuits is that there are SC circuits in which input signals drive the switches, for example, in AC-DC voltage multipliers, and converters in which switches drive the operation.