IEEJ Journal of Industry Applications
Vol.3 No.6 pp.446–454
DOI: 10.1541/ieejjia.3.446

Paper

Size Reduction of DC-DC Converter using Flying Capacitor Topology
with Small Capacitance
Asmarashid Bin Ponniran∗
Koji Orikawa∗Student Member,
Member,Koichi Matsuura∗
Jun-ichi Itoh∗a)Non-member
Member(Manuscript received Dec. 24, 2013, revised May 14, 2014)

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 thedesigned value. Furthermore, according to the design specifications, the required inductance of an input inductor isapproximately 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 periodof 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.
Keywords: inductor size, flying capacitor, output voltage ripple, flying capacitor boost chopper

1.

Consequently, the inductance and core volume of the input inductors can be reduced (5)–(9) . Moreover, by considering multilevel structure topologies on the FCBC, the inductance and core volume of the input inductors can also be
reduced (1)–(4) . However, the interleaved circuit topology andmultilevel circuit structure introduce circuit complexity. Furthermore, such topologies require additional inductors, capacitors, and switching devices, as well as other components
and devices. Nevertheless, if a very-high-eﬃciency DC-DC
boost converter is critical, regardless of size and weight, such
topologies might be an option (1) (4)–(9) .
Therefore, for single-phase operation and simplicity of theconverter circuit, the FCBC oﬀers a good option due to theneed for fewer passive components and switching devices. In
addition, by considering appropriate designs of the passive
components and devices, the sizes and volumes of the components and devices can be minimized.
As in the case of a conventional DC-DC boost converter,
the output voltage in the FCBC can be controlled by controlling the duty ratio of the switching signals. One importantfeature of the FCBC is that the boost-up energy is transferredfrom the flying capacitor to the output side, which meansthat the input inductor can be designed with small inductanceand a reduced inductor core volume (1) (4) (10) . Consequently, theoverall size and weight of the input inductor can be greatly
reduced as compared to a conventional DC-DC boost converter (3) (10) . Moreover, the power loss in terms of copper loss
is also reduced as a result of fewer winding of the input inductor. Generally, passive components and devices in the FCBC,

Introduction

In general, as a power converter, the DC-DC boost converter is important in high-power-capacity applications, suchas solar electric systems and electric vehicle systems. Thesetypes of applications typically demand high conversion eﬃciency with low weight, volume, and cost of the converter.
Nevertheless, if a conventional DC-DC boost converter isconsidered, the bulkiness of the input inductor must be taken
into consideration because a typical conventional DC-DC
boost converter requires large inductors for energy storage,
especially for continuous-current-mode operation of the input current. Although conventional DC-DC boost converters
are evolving due to these drawbacks, the flying capacitor DCDC boost converter (FCBC) has become a possible option.
The FCBC with an input inductor has several advantages overa conventional DC-DC boost converter. The FCBC requires
only a small input inductor and the suppression of an input
inrush current due to the existing of an input inductor compared to a conventional FCBC without an input inductor (1)–(4) .
In addition, a number of researchers have reported thatinterleaved circuit topologies can be considered for conventional DC-DC boost converters. As a result, the input current
can be reduced based on the number of interleaved circuits.
a) 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
c 2014 The Institute of Electrical Engineers of Japan.
446

Flying Capacitor Topology with Small Capacitance（Asmarashid Bin Ponniran et al.）i.e., the input inductor, the flying capacitor, and the output
capacitor, must be designed correctly because these components will have a significant impact on converter operationand eﬃciency. However, this has not been clarified in previous studies (1)–(3) (10)–(12) .
Moreover, establishing the relationship between the capacitance of the flying capacitor and the output voltage ripple is also important. Unfortunately, this has not been discussed in other studies (1) (3) (4) (13) (14) . Converter circuit components/devices and electrical parameters are significantly related. Therefore, procedures for device estimation and design must be established. Generally, the output voltage ripple
should be considered as a factor that influences the estimationof the capacitance of the flying capacitor in the FCBC (15) (16) .
Hence, the parametric relationship between the capacitanceof the flying capacitor and the output capacitance from theoutput voltage ripple must to be analyzed.
In the present paper, an FCBC is proposed considering asmall capacitance of the flying capacitor in order to reduce
the size of the power converter. Principally, a small capacitance of the flying capacitor causes a large voltage ripple
across the flying capacitor. In contrast, the voltage stressincreases when a small capacitance of the flying capacitor
is used. However, low-power-loss and high-voltage powerdevices, such as SiC-MOSFETs, have recently been developed. Thus, downsizing is a higher priority than the use of
low-voltage-stress switching devices. Moreover, ideally, thevoltage stress of the switching devices in the FCBC becomes
half that of a two-level DC-DC boost converter. Based on
this argument, a small capacitance of the flying capacitor in
the FCBC is possible to be considered.
Moreover, the discussion is focused on issues related to the
input inductor and flying capacitor. In particular, the authors
focus on the input inductor design and establishing the relationship between the capacitance of the flying capacitor andthe output voltage ripple.
First, the principle of the FCBC is described. Then, the input inductor design in terms of inductance and inductor core
volume is discussed. The parametric relationship between
the flying capacitor and output capacitor is discussed considering the output voltage ripple. Experimental and simulation
results are analyzed and discussed in order to confirm the validity of the design parameters.
2.

(a) Conventional DC-DC boost converter.

(b) Three-level FCBC.

Fig. 1. DC-DC boost converters

Fig. 2. Switching pattern of the three-level FCBCβ=

1
· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (2)1−DFigure 2 shows the switching patterns of the FCBC. The
switching signal is generated by two carrier signals with half
the switching period T delay between the signals. Half of theswitching period T delay is needed in order to balance thecharging and discharging times of the input inductor in one
switching period T . The charging time of the input inductor
is determined by the duty ratio D and both carrier signals.
In the FCBC, the duty ratio D is a value in the range of0 ≤ D ≤ 1. If the duty ratio D is set at 0.25, meaningthat only 25% of the switching period T is activated. If the
FCBC topology is referenced, M3 and M4 will receive only
ON pulse widths at 25% of the switching period T . Meanwhile, the D1 and D2 pulse widths are opposite the M4 andM3 pulses width, respectively, as shown in Fig. 2.
Figure 3 shows the operation mode of the FCBC in circuit
diagram form. Table 1 summarizes the operation mode andthe range of β in the FCBC. The maximum required output
voltage of the three-level FCBC in the present paper is twicethat of the input voltage. Therefore, only the boost ratio in
the range of 0 < β ≤ 2 is considered.

Principle of the FCBC

Figure 1(a) shows a conventional DC-DC boost converter.
In a conventional DC-DC boost converter, the boost input inductor volume is large. Figure 1(b) shows a three-level FCBC
with a function for output voltage control and unidirectional
operation. Basically, the FCBC consists of two diodes, twoswitches (MOSFET), an input inductor L, a flying capacitor C f c , and an output capacitor Cout . The input inductor allows the output voltage of the FCBC to be controlled independently by controlling the duty ratio D. In addition, if the
input inductor is not considered, the maximum output voltage is double the input voltage Vin . The relationship between
Vin and the output voltage Vout is expressed as follows:
Vout = βVin · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (1)where the boost ratio β is expressed as
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Flying Capacitor Topology with Small Capacitance（Asmarashid Bin Ponniran et al.）Table 2. FCBC Specifications(a) Mode I(b) Mode II(c) Mode III

Fig. 3. Operation modes of three-level FCBC
Table 1. Operation mode and boost ratio

3.

Inductor Design

Fig. 4. Relationship between VL × T L (p.u.) and duty
ratio DIn this section, the inductor design principle is discussed.
The parameters of the FCBC circuit are determined according to the specifications shown in Table 2. Note that the inductor design is discussed under the assumption of a constantflying capacitor voltage. The validity of this assumption is
discussed in Sect. 5.
3.1 Inductance of an Input Inductor
In order to determine the appropriate inductance of an input inductor L, thedesired inductor current ripple ΔIL is required. For design
purposes, it is useful to express the inductance of the input
inductor L in terms of the desired inductor current ripple ΔIL .
Ideal characteristics of the switching devices are considered,
whereby ripples on the output voltage Vout and flying capacitor voltage V f c are ignored due to the independent relationship between the inductor current ripple and capacitance variation of the flying capacitor, as discussed in Sect. 5.3. Generally, the inductance of an input inductor L can be designedby considering the inductor current ripple ΔIL as expressed
as follows:
TL
ΔIL = VL
· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (3)Lwhere T L is the charging time of the inductance of an input
inductor, and VL is the inductor voltage. The inductor current
ripple is maximum when the product of T L and VL is maximum, as expressed by Eq. (3).
In a conventional DC-DC boost converter, the maximum
inductor current ripple occurs when the duty ratio D is 0.5(β = 2). Under this condition, the charging time of the input
inductor becomes half the switching period T , and the inductor voltage is equal to half the output voltage Vout . Therefore,
the inductance of an input inductor Lconventional for a conventional DC-DC boost converter can be expressed as follows:
Lconventional =the duty ratio D in the FCBC. Hence, the inductance of an
input inductor LFCBC in terms of the desired inductor current
ripple ΔIL can be expressed as follows:
LFCBC =

Vout T 1· · · · · · · · · · · · · · · · · · · · · · · · · · · · · (5)4 4 ΔIL

Referring to Eq. (5), the inductor voltage in the FCBC becomes half that of a conventional DC-DC boost converter ora quarter of the output voltage Vout . Meanwhile, the charging
time of the input inductor in the FCBC becomes half that ofa conventional DC-DC boost converter or one quarter of theswitching period T .
By considering an FCBC and a conventional DC-DC boost
converter having the same output voltage, switching frequency and inductor current ripple ΔIL , the ratio of the inductances of the input inductor between the FCBC and theconventional DC-DC boost converter can be expressed as follows:
LFCBC= 0.25 · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (6)Lconventional
Thus, referring to Eq. (6), the FCBC requires approximately 25% of the inductance of an input inductor, as compared to the conventional DC-DC boost converter, as shownin Fig. 1(a).
Therefore, the inductance of an input inductor can greatly
be reduced for the FCBC topology (4) . Theoretically, the inductance of an input inductor is proportional to the numberof windings. Therefore, the copper loss of an input inductor
is also reduced.
3.2 Inductor Core Volume
The core volume of an
input inductor is discussed based on the area product theory (17) . In this section, the input inductor core volumes for the
FCBC and conventional DC-DC boost converters are compared. The inductor core volume Vol as a function of constant
Kvol and the area product A p can be expressed as follows (17) :Vout T 1· · · · · · · · · · · · · · · · · · · · · · · · (4)2 2 ΔIL

In the FCBC, the maximum inductor current ripple occurs
for duty ratios of 0.25 (β = 1.33) and 0.75 (β = 4). Figure 4
shows the relationship between the product of T L and VL and
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Flying Capacitor Topology with Small Capacitance（Asmarashid Bin Ponniran et al.）Vol = Kvol A p

0.75

  ⎤0.75⎡⎢⎢⎢ 2W 104 ⎥⎥⎥⎥⎥⎥= Kvol ⎢⎢⎣⎢
· · · · · · · · · · · (7)Bm JKu ⎦Table 3. Time constant and switching period

where Bm is a flux density. Moreover, the energy-handlingcapability of an inductor core W is related to the area productA p . The energy-handling capability of an inductor core W
can be expressed as follows:W=

1
L × I 2 · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (8)2Table 4. Charging and discharging states of the input inductor, the flying capacitor, and the output capacitor according to the operation mode of the FCBC

where I is a maximum inductor current.
The inductor core volume Vol can be estimated by considering the energy required for an inductor, as expressed by
Eq. (8). Thus, the inductance required by the FCBC is approximately 25% of that required by the conventional DCDC boost converter. Furthermore, Eq. (8) can also be expressed by the following equations for the FCBC and conventional DC-DC boost converter, respectively:
1
1
LFCBC × I 2 = (Lconventional × 25%) × I 2
2
2· · · · · · · · · · · · · · · · · · · · (9)1
Wconventional = Lconventional × I 2 · · · · · · · · · · · · · · · · (10)2WFCBC =

be satisfied:

If the inductance Lconventional of a conventional DC-DC
boost converter is considered as a reference, the required input inductor core volume of the FCBC is approximately 35%of that of the conventional DC-DC boost converter, as expressed by the following equations:

Rout Cout =Based on the simulation and experimental setup, three output capacitances and an output resistance are selected, as
shown in Table 3. The switching frequency f sw is set to
100 kHz, which is equivalent to a switching period of 10 μs.
Referring to Table 3, it is obvious that all of the time constants
Rout Cout for the numerous output capacitances are greaterthan the switching period T . This condition is in agreementwith Eq. (13). Therefore, the output voltage should be constant. Furthermore, the minimum time constant Rout Cout forthese three conditions is 16.5 times greater than the switching period T . Thus, based on this analysis, the time constant
Rout Cout is always greater than the switching period T as expressed by Eq. (13). This condition leads to the capacitanceof the flying capacitor and the output voltage ripple always
being independent of each other.
Theoretically, the peak-to-peak measurement of the output voltage ripple of a conventional DC-DC boost converter
can be calculated by referring to the output capacitor current waveform, whereby the change in the output capacitor
charge Q must be considered (15) (16) . This principle can also be
considered in order to establish the relationship between thecapacitance of the flying capacitor and the output voltage ripple. Table 4 shows the charging and discharging conditions of
the passive components/devices, i.e., the input inductor, flying capacitor, and output capacitor according to the operationmode, as shown in Fig. 3.
In order to clarify the charging and discharging conditionsof the output capacitor, the converter operation modes shownin Fig. 3 are referenced. By referring to the operation modes
shown in Fig. 3, the output capacitor only discharges during
Mode I and charges during Modes II and III. Hence, the total
charge Qtotal from these charging and discharging processes
for one switching period is equal to zero. This condition can

0.75

Wconventional
VolFCBC=· · · · · · · · · · · · · · · (11)
Volconventional
WFCBC
VolFCBC = 35% × Volconventional · · · · · · · · · · · · · · · · · (12)Thus, not only is the inductance reduced, but the inductor
core volume is also greatly reduced. Consequently, the overall size and weight of the converter are reduced.
4.

2
Vout
1
Cout >· · · · · · · · · · · · · · · · · · · · · · (13)P
f sw

Relationship between the Capacitance of the
Flying Capacitor and the Output Voltage Ripple

Principally, the time constant Rout Cout , which consists of
the output resistance Rout and output capacitance Cout , canbe considered in order to establish the relationship between
the capacitance of the flying capacitor and the output voltage
ripple. Based on observations, if the output voltage ripple
is always constant, even though the capacitance of the flying
capacitor varies, the time constant Rout Cout should be greaterthan the switching period 1/ f sw , as expressed by Eq. (13).
Meanwhile, if the output voltage ripple varies depending on
the capacitance variation of the flying capacitor, the time constant Rout Cout should be either equal to or less than the switching period 1/ f sw . This principle can be proven mathematically in order to establish whether the relationship between
the capacitance of the flying capacitor and the output voltage ripple is independent or dependent. Moreover, the time
constant Rout Cout can be expressed as a function of the output
voltage and output power. Therefore, in order to estimate anddesign the capacitance of the flying capacitor without considering the output voltage ripple, the following equation should
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Flying Capacitor Topology with Small Capacitance（Asmarashid Bin Ponniran et al.）Table 5. Specifications of the simulation and experiment

Table 6. Specifications of the output voltage ripple measurement
Fig. 5. Simulation waveforms of the output capacitor
current, the output voltage, the flying capacitor voltage,
and the inductor current

be expressed as follows:
Qcharge = Qdischarge · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (14)Figure 5 shows the simulation results for the waveforms of
the output capacitor current, ICout , the output voltage, Vout ,the flying capacitor voltage, V f c , and the inductor current, IL .
The specifications of this simulation condition are as follows:
output voltage, Vout = 350 V, input voltage, Vin = 262.5 V,
output capacitance, Cout = 1.5 μF, capacitance of the flying capacitor, C f c = 0.35 μF, and flying capacitor voltage,
V f c = Vout /2. These simulation results indicate that the peakto-peak output voltage ripple can be determined by referringto the output capacitor current waveform, as shown in Fig. 5.
The change in the output capacitor charge can be calculated
as follows (15) (16) :Vout
DT = Cout ΔVout · · · · · · · · · · · (15)
ΔQdischarge =RoutFig. 6. Experimental waveforms of the inductor voltage
and inductor current

5.

In this section, the experimental results and experiment
specifications are shown. The details of the experiment specifications are depicted in Table 5. Practically, the selectedinductance of an input inductor is 200 μH, which is based on
the design principle, as described in Sect. 3. The maximum
output power for the conditions of the experiment and thesimulation is 1 kW. However, Table 6 also lists the experiment specifications for the observation of the relationship between the capacitance of the flying capacitor and the output
voltage ripple.
5.1 Input Inductor Design and Inductor Current Ripple
Table 5 lists the experiment specifications. For the design specifications of this prototype, the maximum inductor
current ripple, ΔIL-max , is designed at 1.1 A, as expressed by
Eq. (5). Moreover, based on the experimental results, the inductor current ripple is 1.1 A, as shown in Fig. 6. Therefore,
the simulation and experimental results are in good agreement, and the inductor current ripple design is confirmed.
However, the potential for an unbalanced flying capacitor
voltage must be considered for the stable operation of theconverter (11) (18)–(20) .

By considering this principle, the output capacitance Cout canbe estimated in terms of the output voltage ripple ΔVout in the
FCBC. Based on Eq. (15), the output voltage ripple ΔVout canbe expressed as follows:
ΔVout =

Vout D· · · · · · · · · · · · · · · · · · · · · · · · · · · · (16)Rout Cout f sw

If the boost ratio β and output power Pout are considered,Eq. (16) can be rewritten as follows:
ΔVout =

Experimental Results

Pout (β − 1)
· · · · · · · · · · · · · · · · · · · · · · · · · · (17)βVout Cout f sw

Based on Eq. (17), the output voltage ripple is proportional
to the output power and inversely proportional to the output
voltage, the output capacitor, and the switching frequency.
Thus, based on Eq. (17), the capacitance of the flying capacitor and the output voltage ripple are independent. Thisfinding is confirmed by the simulation and experimental results. Therefore, it is confirmed that the capacitance of theflying capacitor can be estimated without relying on the output capacitance or output voltage ripple.
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Flying Capacitor Topology with Small Capacitance（Asmarashid Bin Ponniran et al.）The inductance and core volume of the input inductor aredesigned by considering the maximum inductor current ripple as expressed by Eqs. (5) and (12). Theoretically, if thesame specifications of the inductor design are considered, theconventional boost DC-DC converter requires an inductanceof the input inductor of approximately 800 μH, whereas the
FCBC requires an inductance of only approximately 200 μH.
Moreover, the copper loss is reduced due to the small inductance of the input inductor. Hence, the FCBC needs only
approximately 25% and 35% of an inductance and an inductor core volume of the input inductor, respectively comparedto the conventional DC-DC boost converter. Therefore, thereductions in the inductance and inductor core volume of the
input inductor are reflected in the reduction of the size andweight of the converter.
5.2 Relationship between the Capacitance of the Flying Capacitor and the Output Voltage Ripple
In thissection, only the conditions with an output capacitance of
1.5 μF and several capacitances of the flying capacitor is considered in the discussion of the relationship between the capacitance of the flying capacitor and the output voltage ripple,
as shown in Fig. 7.
Figure 7 shows the details of the experimental waveformsof the output voltage ripple for an output capacitance of
1.5 μF and various capacitances of the flying capacitor, i.e.,
1.1 μF, 0.6 μF, 0.35 μF, and 0.11 μF, respectively. Table 6 lists
the specifications of the experiment. In this prototype, film
capacitors are used for the output capacitor and the flying
capacitor. The experimental results confirm that the peak-topeak output voltage ripple is 5.6 V for all capacitances of theflying capacitor, because the capacitance of the flying capacitor and the output voltage ripple are independent, as indicatedby Eq. (16).
Figure 8 shows a summary of the experimental results of
the relationship between the capacitance of the flying capacitor and the output voltage ripple with a fixed output capacitance and various capacitances of the flying capacitor. Table 6
lists the experiment specifications. In order to clarify thisphenomenon, small output capacitances (1.5 μF, 6 μF, and9 μF) and small capacitances of the flying capacitor (0.11 μF,
0.35 μF, 0.6 μF, and 1.1 μF) are selected. The output voltage ripple is confirmed to remain unchanged with respect tothe capacitance variation of the flying capacitor, as shown in
Fig. 8.
Therefore, these experimental results indicate that the capacitance of the flying capacitor and the output voltage ripple are independent of each other. Hence, the capacitanceof the flying capacitor can be estimated without considering
the output voltage ripple, as indicated by Eqs. (16) and (17).
However, due to the influence of parasitic components andthe ESR of the output capacitor, the measured output voltage ripples diﬀer slightly from the simulation results. The
errors between the simulation and experimental results are
0.3 V, 0.05 V, and 0.06 V for output capacitances of 1.5 μF,
6 μF and 9 μF, respectively, as shown in Table 7.
5.3 Relationship between the Inductor Current Ripple and the Flying Capacitor Voltage Ripple for Various
Capacitances of the Flying Capacitor
Figure 9 shows
the relationship between the inductor current ripple and theflying capacitor voltage ripple for various capacitances of(a) Cout = 1.5 μF, C f c = 1.1 μF(b) Cout = 1.5 μF, C f c = 0.6 μF(c) Cout = 1.5 μF, C f c = 0.35 μF(d) Cout = 1.5 μF, C f c = 0.11 μF

Fig. 7. Experimental waveforms of the output voltage,
the flying capacitor voltage, and the inductor current

the flying capacitor, i.e., 1.1 μF, 0.6 μF, 0.35 μF, and 0.11 μF.
Based on these relationships, the inductor current ripple isconfirmed to remain unchanged with regard to the capacitance variation of the flying capacitor. The flying capacitor
voltage ripple depends on the capacitance variation of theflying capacitor, where the flying capacitor voltage ripple becomes high when a small capacitance of the flying capacitor
is selected. For further observation, the experimental waveforms of the inductor current ripple and flying capacitor voltage ripple are referenced in order to describe the relationship between the inductor current ripple and the capacitancevariation of the flying capacitor, as shown in Fig. 7. Therefore, based on these experimental results, the inductor current
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Flying Capacitor Topology with Small Capacitance（Asmarashid Bin Ponniran et al.）Fig. 10. Relationship between the converter eﬃciency
characteristics and the capacitance variation of the flying
capacitor

voltage between the input and output voltages in Mode II, as
shown in Fig. 3. As a result, the inductance of an input inductor can be estimated without considering the voltage ripple on
the flying capacitor, as described in Sect. 3.
5.4 Converter Eﬃciency for Several Output PowerLevelsFigure 10 shows the converter eﬃciency characteristics. Several output power levels are considered for theseexperimental results. The output voltage is fixed at 350 V.
Several capacitances of the flying capacitor are considered
for the eﬃciency measurement of the constructed prototype
circuit. The achieved maximum eﬃciency is 98.5% for an
output power of 1 kW. During this achieved maximum eﬃciency condition, the capacitances of the flying capacitor are
1.1 μF and 0.6 μF. Moreover, for the same output power condition and a capacitance of the flying capacitor of 0.11 μF,
the eﬃciency is decreased to 98.2% due to the high value of
the ESR of the capacitor, as compared to that of the otherflying capacitors. Based on these eﬃciency characteristics,
the converter eﬃciency decreases when the output power isdecreasing, as shown in Fig. 10.
5.5 Power Loss Analysis based on Output Power Levels
The distribution of the converter power losses is analyzed based on the converter output power. The power losses
are distributed into seven parts, i.e., diode conduction loss,
MOSFET conduction loss, MOSFET switching loss, inductor copper loss, equivalent series resistance (ESR) loss of theflying capacitor and the output capacitor, no-load loss, andother losses. Moreover, due to the complexity of the estimation of the power losses, other losses such as wiring loss,
iron loss, and ringing loss are included in the ‘other losses’
category, as shown in Fig. 11. In addition, the diode reverserecovery loss is not considered in the present paper because
SiC Schottky barrier diodes without reverse-recovery loss areused in the prototype circuit.
Figure 11 shows the details of a power loss distribution of
the FCBC for several output power levels for a capacitanceof the flying capacitor of 1.1 μF and an output capacitance of
6 μF are considered. The total power loss of 100% is based on
the total power loss when the converter power is increased at
1 kW. Hence, the total power loss is considered as a reference
for this power loss distribution analysis.
Based on this power loss distribution analysis, one of the
major power losses is dominated by the diode conduction loss
for converter power levels of 1 kW and 800 W. However, it

Fig. 8. Relationship between the capacitance of the flying capacitor C f c and the output voltage ripple ΔVout
Table 7. Error ratio between the simulation and experimental results for the output voltage ripple

Fig. 9. Relationship between the flying capacitor voltage ripple and the inductor current ripple with the capacitances of the flying capacitor

ripple and capacitance variation of the flying capacitor are independent of each other. The reason for this condition is thatthe inductor current ripple is dominated by the diﬀerence in452

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Flying Capacitor Topology with Small Capacitance（Asmarashid Bin Ponniran et al.）The principal in designing the input inductor in terms of
the inductance and inductor core volume was explained. As
a result, the inductance and inductor core volume of the input inductor were reduced by approximately 25% and 35%,
respectively. Moreover, the relationship between the capacitance of the flying capacitor and the output voltage ripple
was clearly established mathematically and was confirmedby simulation and experimental results. The experimental results confirmed that the capacitance of the flying capacitor
and the output voltage ripple are independent of each other.
Moreover, the maximum eﬃciency of the prototype converter
was 98.5% at an output power of 1 kW.
In the future, balancing of the flying capacitor voltage of
the FCBC will be considered by applying appropriate control
strategies.

References
(1)

(2)

Fig. 11. Converter loss distribution when the capacitance of the flying capacitor is 1.1 μF(3)was dominated by the ‘other losses’ for a converter power
level of 500 W. The inductor copper loss was the lowest component of the total power loss. Moreover, at output power levels of 1 kW and 800 W, ‘other losses’ were the second largest
power losses, followed by the diode conduction loss for aconverter power level of 500 W.
Based on this power loss analysis, the inductor copper loss
is small due to smaller number of inductor windings. Moreover, MOSFET conduction loss is considered to be small dueto the low input current with regard to the small boost ratio.
If the boost ratio is high, the input current becomes high. As
a result, the conduction loss of the MOSFET becomes high.
Since one of the major power losses is associated with thediodes, low-on-voltage diodes must be considered in order toreduce the conduction loss of the diodes. Moreover, the conduction loss in the flying capacitor is considered to be highdue to the high value of the ESR. Therefore, a low ESR for
the flying capacitors must be selected in order to reduce theflying capacitor conduction loss. In order to reduce the power
loss in the MOSFETs when a high boost ratio is considered,
low-on-resistance MOSFETs must be selected. As an option,
the on-resistance can be further reduced by connecting the
MOSFETs in parallel connection. Therefore, by considering
these available options, the overall converter eﬃciency canbe improved further.
6.

(4)

(5)

(6)

(7)

(8)

(9)

( 10 )( 11 )( 12 )( 13 )

Conclusion( 14 )In the present paper, the authors examined an FCBC witha small-capacitance flying capacitor considering the output
voltage ripple. Moreover, the authors discussed the analysis of the input inductor design. In the present paper, theauthors have discussed (i) the fundamental circuit operation
confirmation of the FCBC, (ii) the design method of an input
inductor, and (iii) the relationship between the capacitance ofthe flying capacitor and the output voltage ripple.

( 15 )( 16 )( 17 )( 18 )

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IEEJ Journal IA, Vol.3, No.6, 2014

Flying Capacitor Topology with Small Capacitance（Asmarashid Bin Ponniran et al.）( 19 )( 20 )

Koji Orikawa (Member) received the M.S. and Ph.D. degrees in electrical, electronics and information engineering from
Nagaoka University of Technology, Niigata, Japan,
in 2010 and 2013 respectively. Since 2013, he has
been working at Nagaoka University of Technology
as a postdoctoral fellowship. He is the member ofIEEE and IEEJ. His research interests include power
conversion system especially DC-DC converters andhigh frequency techniques for power converters.

voltages in multicell converters”, in 6th International Conference on Electrical Engineering/Electronics, Computer, Telecommunications and Information Technology, 2009. ECTI-CON 2009 (2009)
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and observation of flying-capacitor voltages”, IEEE Transactions on Industrial Electronics, Vol.49, No.5, pp.998–1008 (2002)

Asmarashid Bin Ponniran (Student Member) received his Bachelor Degree in Electrical Engineering and Master Degree in Electrical Engineering (Power) from Universiti Tun Hussein Onn Malaysia in 2002 and Universiti
Teknologi Malaysia in 2005, respectively. Presently
he is a Ph.D. candidate at Nagaoka University ofTechnology, Japan. At the same time, he is attachedat Universiti Tun Hussein Onn Malaysia, Malaysia as
a lecturer. His research interest includes dc-dc converter and its application.

Jun-ichi Itoh (Member) received his M.S. and Ph.D. degrees in electrical and electronic systems engineering from Nagaoka University of Technology, Niigata, Japan in
1996 and 2000, respectively. From 1996 to 2004, he
was with Fuji Electric Corporate Research and Development Ltd., Tokyo, Japan. Since 2004, He has beenwith Nagaoka University of Technology as an associate professor. He received the IEEJ Academic Promotion Award (IEEJ Technical Development Award)
in 2007 and the Isao Takahashi Power Electronics
Award in 2010. His research interests include matrix converters, DC/DC
converters, power factor correction techniques and motor drives. He is amember of the Institute of Electrical Engineers of Japan and the Society ofAutomotive Engineers of Japan.

Koichi Matsuura (Non-member) received the M.S. degree in electrical, electronics and information engineering from Nagaoka University of Technology, Nagaoka, Japan, in
2011. Since 2011, he has been an employee of Toyota
Industries Corporation. His research interests include
DC-DC converters.

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IEEJ Journal IA, Vol.3, No.6, 2014