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Letter
Minimization of Passive Components
in Multi-level Flying Capacitor DC-DC Converter
Asmarashid Bin Ponniran＊a), Student Member,
Koji Orikawa＊, Member,
Jun-ichi Itoh＊, Member(Manuscript received Jan. 00, 20XX, revised May 00, 20XX)
In this paper, we propose a minimization design method of the inductance and the capacitance of the flying capacitors for a multilevel flying capacitor DC-DC boost converter (FCBC). The effectiveness of the small capacitances of the flying capacitors in threelevel and five-level FCBCs is investigated. We experimentally confirmed that the distortion of the voltage across the input source
and the input inductor is drastically reduced by increasing the number of levels. Thus, a small inductance value and a small
capacitance value can be used for the input inductor and the flying capacitors, respectively, in an n-level FCBC. Therefore, theminimization of passive components in multi-level FCBCs is achieved. Moreover, the achieved maximum efficiencies of thedesigned three-level and five-level FCBCs are 98.5% and 97.8%, respectively, at an output power of 1 kW.
Keywords: Flying capacitor boost converter, inductor, inductor current ripple, flying capacitor, flying capacitor voltage ripple

Fig. 1 shows the circuit configuration of a n-level FCBC. By
considering the n-level FCBC and a conventional 2-level DC-DC
boost converter are having the same input voltage, output voltage,
switching frequency and inductor current ripple, the inductance ofthe input inductor L(n-level) in the n-level FCBC is expressed by

1. IntroductionTypical conventional 2-level DC-DC boost converters require alarge input inductor, which is bulky and heavy. On the other hand,the flying capacitor boost converter (FCBC) requires only a small
inductance of the input inductor because the boost-up energy istransferred 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 theflying capacitor should be large in order to keep the flying capacitor
voltage half of the output voltage [1]. As a result, the low-voltagerating switching devices can be used. However, nowadays lowpower-loss and high-voltage-rating switching devices, such as SiCMOSFETs have been developed [2]. Thus, small capacitances suchas ceramics and films types can be applied due to the availability ofhigh-voltage-rating switching devices with low-power-loss feature.
As a result, the capacitance reduction of the flying capacitor can beachieved compared to a conventional 2-level FCBC. The 3-level
FCBC with a small capacitance of the flying capacitor has beendiscussed 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].
This paper establishes the minimization design method of thepassive components which are the inductance of the input inductor
and the capacitance of the flying capacitors in the n-level FCBC inorder to reduce the size and weight of the power converter. On theother hand, two prototypes of 3-level and 5-level FCBCs wereconstructed in order to confirm the validity of the minimization
design method for the inductance and the capacitance.

Lnlevel Lconventionaln 12....................................................... (1)where Lconventional is the inductance of the conventional 2-level DCDC boost converter and n is the number of level. Based on (1) theinductance is reduced when the level n is increased [1].

3. Minimum capacitance of flying capacitor in nlevel FCBCs
In the n-level FCBC, the flying capacitor voltage Vfcx(n-level) andthe flying capacitor voltage ripple Vfcx(n-level) are expressed byV fcxnlevel xVn 1 outVfc1nlevel ...................................................... (2)1x
2Vout Vfc2nlevel Vout Vfcxnlevel Voutn 1n 1n 1

2. Inductance reduction of the input inductor in nlevel FCBCsa) Correspondence to: Jun-ichi Itoh. E-mail: itoh@vos.nagaokaut.ac.jp＊Nagaoka University of Technology
1603-1 Kamitomioka-machi, Nagaoka, Niigata 940-2188, Japan© 200● The Institute of Electrical Engineers of Japan.

Fig. 1. Circuit configuration of n-level FCBC.

1

Minimization of passive components in multi-level FCBC (Asmarashid Bin Ponniran et al.)

V fcx nlevel  

Pin D......................................... (3)Vin C fcx ( nlevel ) f sw

where x is the x-th of flying capacitor, Vout is the output voltage, Pinis the input power, D is the duty ratio, Vin is the input voltage, Cfcx(nlevel) is the capacitance of the flying capacitor, fsw is the switchingfrequency. Besides, the minimum capacitance of the flying
capacitor in the n-level FCBC is expressed by

C fcx( nlevel)min 2n 1Vn  1Pin Dfcx ( n level )  max 2 xVout Vin f sw..... (4)where Vfc(n-level)-max is the maximum flying capacitor voltage which
is ideally equal to the withstand voltage of switching devices.

(a) Cfc1, 2, 3 = 1.1 F (calc., 1.13 F), (b) Cfc1, 2, 3 = 0.35 F (calc., 0.36 F)
Cout =1.5 F

4. Experimental Results

Cout =1.5 F

Fig. 2. Experimental waveforms of the flying capacitor voltage
ripples and the output voltage ripple in the 5-level FCBC.

The specifications of the experiment condition are as follows: theduty ratio is 0.25, the output voltage is 350 V, the switchingfrequency is 100 kHz and the output power is 1 kW. In this paper,the operations of 3-level and 5-level FCBCs prototypes with small
capacitances of the flying capacitors are experimentally confirmed.
The maximum efficiencies of the 3-level and 5-level FCBCs
prototypes with a small capacitance of the flying capacitors are
98.5 % and 97.8 %, respectively at the output power of 1 kW.
Fig. 2 shows the flying capacitor voltage ripples in the 5-levelFCBC. The voltage ripples are 10 V and 29 V when thecapacitances of the flying capacitors are 1.1 F and 0.35 F,
respectively. It is confirmed that the flying capacitor voltage ripples
are same when the same capacitances of the flying capacitors areused as shown by (3).
Fig. 3 (a) shows the experimental results of the kvconv which is theratio between the effective values of distorted components Vnfsw
against DC component VDC. It is confirmed that the ratio kvconv
converges into a constant value when the capacitance of the flying
capacitor is increased. Meanwhile, Fig. 3 (b) shows theexperimental results of the kiout which is the ratio between theeffective values of distorted components Infsw against DC
component IDC. The ratio of kiout is not affected on the variation of
capacitances of the flying capacitors although small capacitance isused. Thus, the validity of the small capacitance of the flying
capacitors in the 3-level and 5-level FCBCs are experimentallyconfirmed.
Fig. 4 shows the harmonics spectrum of vconv in the 3-level and5-level FCBCs. The voltage vconv in the 5-level FCBC obviously has
lower harmonic components compared to the 3-level FCBC when
the capacitance of the flying capacitor of 1.1 F is used. From theseresults, it is clarified that not only the small capacitance for theflying capacitor in fact the small inductance of the input inductor
can be considered in the n-level FCBC.

0.4

0.8
3-level

0.3

40kvconv 0.2

Vn1

3-level

0.6
25-level

nfsw

400.4

VDC

VDC: DC component
Vnfsw: Harmonic component

kiout 0.1

In 12nf sw

I DC

0.2

IDC: DC component
Infsw: Harmonic component

5-level

00012345012345

Capacitance of flying capacitor Cfc ( F)(a) vconv.

(b) iout.

Fig. 3. Ratio of the effective value of distorted components
against DC component in the 3-level and 5-level FCBCs.

300

300

200200100

10000Fig. 4.

5. Conclusion

Harmonic components of vconv in the 3-level and 5-level
FCBCs.
References

This paper established the minimization design method of thepassive components in n-level FCBCs which are the inductance ofthe input inductor and the capacitance of the flying capacitors. The
effectiveness of the small capacitance of the flying capacitors isevaluated by analyzing the distorted voltage across the input
voltage source and the input inductor and the distorted current to
the output side which includes the output capacitor and the load.
In a future work, converter volume estimation of a multi-level
FCBC with minimized input inductor and small capacitances of theflying capacitors will be conducted for weight and size reduction.

(1)

(2)

(3)

2A. B. Ponniran, K. Matsuura, K. Orikawa, and J. Itoh, “Size Reduction ofDC-DC Converter using Flying Capacitor Topology with Small Capacitance”,
IEEJ Journal of Industry Applications, Vol.3, No.6, pp.446-454 (2014).
L. Lorenz, “Power Semiconductor Devices- Development Trends andSystem Interactions”. in Power Conversion Conference - Nagoya, 2007. PCC'07. (2007).
Fan Zhang, Fang Zheng Peng, and Qian Zhaoming, "Study of the multilevel
converters in DC-DC applications". in IEEE 35th Annual Power Electronics
Specialists Conference, 2004. PESC 04. (2004).

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