Size Reduction of DC-DC Converter using Flying Capacitor Topology with Small Capacitance

Abstract

The purpose of the present paper is to analyze the input inductor design and to establish the relationship between the capacitance of the flying capacitor and the output voltage ripple in order to reduce the size and weight of the flying capacitor DC-DC boost converter (FCBC). The inductance of the input inductor is designed by considering the maximum input current ripple, and the experimental results are used to confirm that the input current ripple is within the designed value. Furthermore, according to the design specifications, the required inductance of an input inductor is approximately 25% of that of a conventional two-level DC-DC boost converter, and the required inductor core volume is approximately 35% of that of a conventional two-level DC-DC boost converter. Moreover, the capacitance of the flying capacitor and the output voltage ripple are confirmed to be independent of each other. Theoretically, this is because the time constant of the output capacitance and the output resistance of the FCBC is larger than the switching period of the switching frequency. This finding is confirmed by the simulation and experimental results of the present study. On the basis of this finding, the capacitance of the flying capacitor can be estimated and designed without considering the output voltage ripple. Moreover, the achieved maximum efficiency of the designed FCBC is 98.5% of the output power at 1 kW.

Minimization of Passive Components in Multi-level Flying Capacitor DC-DC Converter

“…The 3-level FCBC with a small capacitance of the flying capacitor has been discussed in Ref. (1). However, the minimum capacitance design method and its influence in the n-level FCBC have not been discussed by other researchers (2) (3) .…”

“…On the other hand, the flying capacitor boost converter (FCBC) requires only a small inductance of the input inductor because the boost-up energy is transferred from the flying capacitor to the output side. Therefore, reductions of the inductance and the core volume are achieved (1) . Besides, in a conventional 3-level FCBC, the capacitance of the flying capacitor should be large in order to keep the flying capacitor voltage half of the output voltage (1) .…”

“…Therefore, reductions of the inductance and the core volume are achieved (1) . Besides, in a conventional 3-level FCBC, the capacitance of the flying capacitor should be large in order to keep the flying capacitor voltage half of the output voltage (1) . As a result, the low-voltage-rating switching devices can be used.…”

Minimum flying capacitor for N-level capacitor DC/DC boost converter

“…The FCBC requires only a small input inductor in order to suppress the inrush current from a power supply to a flying capacitor [1][2][3][4]7]. In addition, one important feature of the FCBC is that the boost-up energy is transferred from the flying capacitor to the output side, which means that the input inductor can be designed with a small inductance and a reduced core volume of the inductor [2,4].…”

“…In addition, one important feature of the FCBC is that the boost-up energy is transferred from the flying capacitor to the output side, which means that the input inductor can be designed with a small inductance and a reduced core volume of the inductor [2,4]. Consequently, the overall size and weight of the input inductor can be greatly reduced as compared to a conventional two-level DC/DC boost converter [1,2,5,7].…”