Minimum Flying Capacitor for N-level
Capacitor DC/DC Boost Converter
Asmarashid Ponniran1, Koji Orikawa1, and Jun-ichi Itoh1
1

Nagaoka University of Technology, Japan

Abstract-- This paper proposes a Flying Capacitor
DC/DC Boost Converter (FCBC) with a small capacitanceof the flying capacitor. As a result the size and weight of the
proposed converter can be reduced. The capacitance of theflying capacitor is designed based on switching device
voltage rating and regardless of the output voltage ripple
influences. Moreover, the achieved maximum efficiency ofthe designed 3-level FCBC is 98.5% at the output power of 1
kW. Finally, the effectiveness of small capacitances of theflying capacitor in the 3-level and 5-level FCBCs areinvestigated. The characteristics of the distorted voltage
across an input voltage source and an input inductor andthe distorted current to the output side which is an output
capacitor and a load are investigated with several
capacitances of the flying capacitors for 3-level and 5-level
FCBCs. As a result, it is experimentally confirmed that the
distortion of the voltage across the input voltage source andthe input inductor is drastically reduced by increasing thenumber of level. On the other hand, the distortion of thecurrent to the output side which is the output capacitor andthe load is almost same even the number of level is increased.
Therefore, the experimental results show that small
capacitance of the flying capacitor can be used in the n-level
FCBC.
Index Terms—flying capacitor, output voltage ripple,
multi-level flying capacitor boost converter.

I. INTRODUCTION
In general, a DC/DC boost converter in high-powercapacity applications typically demands high conversion
efficiency with low weight, volume, and cost of theconverter. Nevertheless, if a typical conventional DC/DC
boost converter is considered, the bulkiness of the input
inductor must be taken into consideration because thetypical conventional DC/DC boost converter requires
large inductors for energy storage.
The FCBC requires only a small input inductor inorder to suppress the inrush current from a power supply
to a flying capacitor [1-4,7]. In addition, one importantfeature of the FCBC is that the boost-up energy istransferred from the flying capacitor to the output side,which means that the input inductor can be designed witha small inductance and a reduced core volume of theinductor[2,4]. Consequently, the overall size and weightof the input inductor can be greatly reduced as compared
to a conventional two-level DC/DC boost converter[1,2,5,7].
Generally, in conventional 3-level FCBCs, thecapacitance of the flying capacitor should be large inorder to keep the flying capacitor voltage of half of theoutput voltage [1,2,4,7]. As a result, the low-voltagerating switching devices can be used. Low-voltage-rating
switching devices usually have fast switching
performance and low on-resistance features compared tohigh-voltage-rating devices. However, nowadays lowpower-loss and high-voltage-rating switching devices,such as SiC-MOSFETs have been developed [6]. Thus,
small capacitances such as from ceramics and films types
with high voltage capability can be considered together
with high-voltage-rating switching devices. As a result,the volume of the flying capacitor can be reduced
compared to conventional FCBCs. The 3-level FCBC
with a small flying capacitor is discussed in Ref [7].
However, the minimum capacitance selection and its
influence in the n-level FCBC have not been discussed[1-3,8,9].
In this paper, the FCBC considering a small
capacitance of the flying capacitor is proposed in order toreduce the size and weight of the power converter. First,
the principle of the FCBC is described. Next, a design
method for minimum capacitance of the flying capacitor
is clarified. Then, the minimum capacitance design
method for the flying capacitor is experimentally verified.
Finally, the distortion of the voltage across a input
voltage source and a input inductor and the current to
the output side which the output capacitor and the load
are investigated for the 3-level and the 5-level of the
FCBCs in terms of small capacitance of the flying
capacitor.
II. PRINCIPLE OF THE FCBCFirst, a principle of a 3-level FCBC is described. Next,
parameters of passive components in a n-level FCBC is
described.
A. 3-level FCBC
Basically, the 3-level FCBC consists of two diodes,
two switches, an input inductor L, a flying capacitor Cfc,
and an output capacitor Cout. The input inductor is used inorder to control independently the output voltage bycontrolling the duty ratio D. The relationship between the
input voltage Vin and the output voltage Vout is expressedby (1) using the boost ratio β

Vout = βVin ...................................................................... (1)β is expressed by (2).
β=1...................................................................... (2)1− D

Fig. 1 shows the circuit configuration and theoperation mode of the 3-level FCBC. During one
switching period, the operation mode is repeated. Only
the boost ratio in the range of 1 < β ≤ 2 is considered inthis study. The inductance of the input inductor L(3-level)
and inductor core volume Vol(3-level) in the 3-level FCBC
is expressed by (3) and (4) [4,7].

(a) Mode I(b) Mode IIL(3−level) = 0.25 × Lconventional .............................................. (3)Vol(3−level ) = 0.35 × Volconventional ......................................... (4)where Lconventional is the inductance of the input inductor
and Volconventional is the volume of the input inductor in theconventional 2-level DC/DC boost converter.
From (3) and (4) the inductance of the input inductor
and the inductor core volume are greatly reduced
compared to the conventional 2-level DC/DC boost
converter.

B. n-level FCBC
Fig. 2 shows the n-level FCBC circuit configuration.
In the n-level FCBC, the balancing of each flying
capacitor voltages need to considered. The unbalanced
condition is due to improper charging and discharging of
the flying capacitor [10-12]. This condition will lead anincreasing the voltage stress of switches. In the section III(C), the important expressions for the n-level FCBC are
discussed.
The inductance of the input inductor L(n-level) and
inductor core volume Vol(n-level) in the n-level FCBC are
expressed by (5) and (6), respectively.

L(n−level ) =Lconventional(n − 1)2....................................................... (5)⎛ 1 ⎞⎟
Vol(n−level) = ⎜⎜2 ⎟⎝ (n − 1) ⎠0.75

×Volconventional ........................... (6)From both of (5) and (6), it is confirmed that theinductance of the input inductor and inductor core
volume are inversely proportional to the number on leveln. Thus the inductance of the input inductor and inductor
core volume are reduced when the higher level of a
FCBC is considered.
III. MINIMUM CAPACITANCE ESTIMATION OF THE FLYINGCAPACITOR

A. 3-level FCBC
Table 1 shows the specification of the simulation andexperiment conditions. First, the detailed analysis ofminimum capacitance of the flying capacitor is based on
3-level FCBC. In this chapter, the relationship between
the flying capacitor Cfc(3-level) and the output voltage ripple
ΔVout. is considered.
Principally, the time constant RoutCout, which consistsof the output resistance Rout and output capacitance Cout,is considered in order to estimate the relationshipbetween the capacitance of the flying capacitor and theoutput voltage ripple. If the output voltage ripple isFig. 1.

(c) Mode III
Operation modes f 3-level FCBC

Vfc1(n−level) =1
x2
Vout Vfc2(n−level) =Vout Vfcx(n−level) =
Vout
n −1
n −1
n −1

Fig. 2. n-level FCBCTABLE 1
SPECIFICATIONS OF THE SIMULATION AND EXPERIMENT FOR
3-LEVEL AND 5-LEVEL FCBCSSpecification
Input Voltage Vin
Output Voltage Vout
Output power Pout
Input current Iin
Switching frequency fsw
Duty ratio D
Input inductance L
Capacitance of the flying capacitor Cfc
Output capacitance CoutMaximum inductor current ripple ΔIL-max
MOSFET
SiC Schottky DiodeValue
262.5 V
350 V
1 kW
4.24 A
100 kHz0.25
200 μH
1.1 μF
1.5 μF
1.1 A
IRFB4229PBF
IDW30G65C5TABLE 2TIME CONSTANT AND SWITCHING PERIOD
OutputCapacitanceCout
1.5 μF
6 μF
9 μF

OutputResistance
Rout
110 Ω

Time
Constant
RoutCout
165 μs660 μs990 μsSwitching
Period1/fsw = T
10 μs

always constant even the capacitance of the flying
capacitor varies, the time constant RoutCout should begreater than the switching period 1/fsw, as expressed by(7)

Rout Cout =

2
Vout
1
................................................ (7)Cout >
Pout
f sw

where Pout is the output power.
In the simulation and the experiment, threecapacitances of the output capacitor and an output resistor
are selected. The switching frequency fsw is 100 kHz andequal to the switching period of 10 μs. Table 2 shows thecalculation results which are all of the time constants
RoutCout for the numerous output capacitances are greaterthan the switching period. In the condition of the Table 2,
the capacitance of the flying capacitor and the output
voltage ripple is always independent of each other. The
output voltage ripple ΔVout can be expressed as (8).

ΔVout =

Pout (β − 1)................................................... (8)β Vout Cout f sw

From (8), it is obvious that the relationship between
Fig. 3. Simulation waveforms of the inductor current, the flying capacitor
the flying capacitor and the output voltage ripple iscurrent, the flying capacitor voltage and the output voltage
independent. Consequently, the capacitance of the flying
capacitor is designed without the consideration of theshown in Fig. 3. The flying capacitor voltage ripple in the
output capacitor and output voltage ripple.
3-level FCBC ΔVfc(3-level) is expressed by (12).
Next, the design of the small capacitance of the flying
I in − max D
Pin D
capacitor Cfc(3-level) based on the maximum switching................ (12)ΔV fc (3−level ) ==device voltage rating VDS(3-level)-max is described. If Mode I
C fc (3− level ) f sw Vin C fc (3−level ) f sw
in Fig. 1 is referred, the maximum voltages across thewhere Pin is the input power.
flying capacitor Vfc(3-level)-max and the switch S2 VS2(3-level)max are same due to the parallel connection of the flying
The average flying capacitor voltage is Vout/2.
capacitor and the switch S2. In addition, In Mode I, both
Therefore, the maximum flying capacitor voltage Vfc(3voltages are reached up to maximum. Therefore, thelevel)-max is expressed by (13). The maximum voltage
maximum voltage rating of the switch S2(3-level) is
across the switch S2 VS2(3-level)-max is also determined byaccording to the maximum flying capacitor voltage. The
the maximum of the flying capacitor voltage Vfc(3-level)-max
maximum voltage rating of other switches also can be
as shown by (13). Therefore, referring to (13), thebased on it.
minimum capacitance of the flying capacitor Cfc(3-level)-min
Theoretically, in the 3-level FCBC, the averagein the 3-level FCBC is expressed by (14) from (13).
voltage of the flying capacitor is half of the output
V
1
voltage as expressed by (9). Meanwhile, from the
V fc (3−level ) − max = VS 2 (3−level )− max = out + ΔV fc (3−level )22
operation mode of the 3-level FCBC, the flying capacitor........ (13)and drain-source terminal of MOSFETs of S1(3-level) andVout 1
Pin D=+
S2(3-level) are connected in parallel. Thus, the maximum
2
2 C fc ( 3−level )− minVin f sw
voltage stresses for both devices can be expressed by
(10).
Pin D
C fc (3−level )−min =................... (14)2VVout (average)
fc (3−level )− max − Vout Vin f sw
V fc (3−level ) =.................................................... (9)2B. 5-level FCBC
In the 5-level FCBC, three flying capacitors are
VS1(3−level )−max = VS 2(3−level )−max = V fc (3−level )−max ................... (10)required which are Cfc1(5-level)-min and Cfc2(5-level)-min, Cfc3(5Fig. 3 shows the simulation results of the waveforms
whichlevel)-min. The maximum flying capacitor voltagesof the inductor current IL(3-level), the flying capacitor
are Vfc1(5-level)-max, Vfc2(5-level)-max and Vfc3(5-level)-max in the 5voltage Ifc(3-level), the flying capacitor voltage Vfc(3-level) andlevel FCBC are expressed by (15), (16) and (17),the output voltage Vout, respectively. The total charge
respectively.
Qtotal from charging and discharging processes for the
Pin D
1
1
flying capacitor current is equal to zero. Therefore, theV fc1(5−level )−max = Vout +............... (15)42 C fc1(5−level )−minVin f sw
charge for both of the charge Qcharge and the discharge
Qdischarge are expressed by (11) as in Fig. 3, [13-15].
Pin D
1
1
Qcharge = Qdischarge ............................................................ (11)
V fc 2(5−level )−max = Vout +............... (16)22 C fc 2( 5−level)−minVin f sw
The peak-to-peak flying capacitor voltage ripple ΔVfc(3level) is determined by the flying capacitor current as()

Pin D
3
1
V fc 3(5−level )−max = Vout +.............. (17)42 C fc3( 5−level )−minVin f sw
Furthermore, capacitance of each flying capacitors in
5-level FCBC are expressed by (18), (19) and (20),respectively.

C fc1( 5−level )−min =C fc 2(5−level )−min =C fc3( 5−level)−min =2Pin D............... (18)
4V fc1(5−level) −max − Vout Vin f sw()

2 Pin D............ (19)
4V fc 2(5−level )−max − 2Vout Vin f sw((a) 3-level FCBC: Cout = 1.5 μF, Cfc = 1.1 μF)

2Pin D............. (20)
4V fc 3( 5−level )−max − 3Vout Vin f sw()

The maximum voltage stress on switching devices forthe 5-level FCBC is expressed by (21).

Pin D
1
1
Vsw(5−level )− max = Vout +............... (21)42 Vin C fc1( n−level) −min f sw
C. n-level FCBC
In this section, the capacitance of the flying capacitor
and the maximum voltage stress on switching devices in
the n-level FCBC are explained.
The flying capacitor voltage in the n-level FCBC Vfcx(nlevel) and the required number of flying capacitor m areexpressed by (22) and (23), respectively.

V fcx(n−level) =x
V .................................................. (22)(n − 1) out

m = n − 2 ..................................................................... (23)where x is the nth of flying capacitor in n-level FCBC.
The minimum capacitance of the flying capacitor in nlevel FCBC is expressed by (24).

C fcx ( n−level)−min =(2(n −1)V(n − 1)Pin D
fcx ( n−level ) − max− 2 xVout )Vin f sw(b) 3-level FCBC: Cout = 1.5 μF, Cfc = 0.35 μF

0, ( Vfc1)(25 V/div)

Flying Capacitor Voltage Ripple Vfc1 (V)

Vfc3

V

Flying Capacitor Voltage Ripple Vfc2 (V)

Vfc2

V

Flying Capacitor Voltage Ripple Vfc3 (V)

Vfc1

V

0, ( Vfc2)(25 V/div)
0, ( Vfc3)(25 V/div)
0, ( Vout)(10 V/div)

Output Voltage Ripple Vout (V)T = 10 s

VoutV
4 ( s/div)(c) 5-level FCBC: Cout = 1.5 μF, Cfc = 1.1 μF.. (24)On the other hand, the ripple voltage of the flying
capacitor for n-level FCBC is expressed by (25).
ΔV fcx (n−level ) =Pin D...................................... (25)
Vin C fcx ( n −level ) f sw

Therefore the maximum voltage stress on switching
devices for the n-level FCBC is expressed by (26).

Pin D
1
1
Vsw(n−level )−max =Vout +....... (26)(n − 1)
2 Vin C fc1( n−level)−min f sw
IV. EXPERIMENTAL RESULTS
In order to confirm the operation of the FCBC underthe minimum capacitance of the flying capacitor and the
maximum voltage stress on switching devices in the nlevel FCBC, 3-level and 5-level FCBCs prototypes are(d) 5-level FCBC: Cout = 1.5 μF, Cfc = 0.35 μF
Fig. 4. (a), (b): Experimental waveforms of the output voltage, the flying
capacitor voltage and the inductor current in 3-level FCBC; (c), (d):Experimental waveforms of the flying capacitor voltage ripples and theoutput voltage ripple in 5-level FCBC.

constructed. The specifications of the experiment andsimulation conditions are shown in Table 1.
Fig. 4 shows the experimental waveforms of the flying
capacitor voltage ripples and the output voltage ripple
when the output capacitance is 1.5 μF and thecapacitances of the flying capacitors are 1.1 μF and 0.35

μF in the 3-level and 5-level FCBCs. From Fig. 4(a) and(b), it is confirmed that the peak-to-peak output voltage
ripple is always 5.6 V at the 3-level FCBC althoughvarious capacitance of the flying capacitor are used. In
addition, the output voltage ripple is always 5.6 V at bothof the 3-level FCBC and the 5-level FCBC althoughvarious capacitances of the flying capacitor are used.
These experimental results are agreed with the theoreticalanalysis in the section III whereby the relationshipbetween capacitances of the flying capacitor and the(a) 3-level FCBC
output voltage ripple are independent.
Besides, Fig. 4 shows the flying capacitor voltage
ripples in the 3-level and 5-level FCBCs when thecapacitance of the flying capacitor of 1.1 μF and 0.35 μF
are used. From these results, the flying capacitor voltage
ripples are same when same capacitance of the flying
capacitors is used. The flying capacitor voltage ripples
are approximately 10 V and 29 V, respectively as shownin Fig. 4. Therefore, it is confirmed that the experimental
result agrees with the theoretical analysis shown in thesection III.
(b) 5-level FCBC
Fig. 5(a) shows the experimental waveforms of thevoltage of the switch S2 is 230 V when the capacitance of Fig. 5. (a) Experimental waveforms of the switch S2 voltage andthe inductor current in the 3-level FCBC; (b) Experimental
the flying capacitor which of 0.11 μF is used in the 3waveforms of the switch voltages (S1 and S4) and the output
level FCBC. Experimental value of the maximum voltage
voltage in the 5-level FCBC.
of the switch S2 agrees with the calculated value which is99
230 V by (13).
Besides, Fig. 5(b) shows the experimental waveform98of the switch voltages S1 and S4 when the capacitance ofthe flying capacitor of 0.35 μF is used in the 5-level
97
FCBC. The experimental maximum switch voltage of S4
is 110 V. It is confirmed that the experimental value
96
agrees with the calculated value based on (21).
Furthermore, the voltage across a switching device is95
determined based on the flying capacitor voltage ripple
200
400
600
800 1000 1200
and capacitance of the flying capacitor. Thus, the design
method to determine the minimum capacitance for the
flying capacitor was experimentally verified.
Fig. 6. Converter efficiency characteristic in 3-level FCBC
Fig. 6 shows the converter efficiency characteristics.
The output voltage is fixed at 350 V. Several
100
capacitances of the flying capacitor are considered for the
100% = 16.58 W
efficiency measurement of the constructed prototype
Other losses
circuit. The achieved maximum efficiency is 98.5 % at
the output power of 1 kW. In the heavy load region, theefficiency is almost same among the three conditions of
the flying capacitor. On the other hand, the efficiency
No load loss
with the largest capacitance of the flying capacitor is
ESR loss
50
higher than that of other conditions of the flying capacitor.
Inductor copper loss
This is because the switching loss under the largest
Mosfet switching loss
Mosfet conduction loss
capacitance of the flying capacitor is lower than that ofother conditions due to low voltage stress of the switches.
Fig. 7 shows the power loss distribution of the 3-level
Diode conduction loss
FCBC for several output power when a capacitance of theflying capacitor is 1.1 μF and an output capacitance is 1.5
μF. The total power loss of 100 % (16.58 W) is based on0
the total power loss when the output power is 1 kW. The
500
800
1000
total power loss at the output power of 1 kW is a
Output power Pout (W)reference in Fig. 7. The major power loss is the diodeFig. 7. Loss distribution in 3-level FCBC with various output
conduction loss when the output power is 1 kW.
powersMoreover, the ESR losses in the flying capacitor andoutput capacitor can be further decreased by considering
low ESR capacitors. 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 byconnecting the MOSFETs in parallel connection.

40

V. THE EFFECTIVENESS OF SMALL CAPACITANCE OF
FLYING CAPACITOR

B. Harmonics analysis of the voltage across the input
voltage source and the input inductor, and the current to
the output side in the 3-level and 5-level FCBCs
Fig. 9 (a) shows the experimental results of the kiout
which is the ratio between the effective values of
distorted components Infsw against DC component IDC,
which is introduced in order to evaluate the distorted
current to the output side which is the output capacitor
and the load. The results show ratio of kiout converges into
a constant value with various capacitances of the flying
capacitors. Therefore, the ratio of kiout is not affected on
the variation of capacitances of the flying capacitors
although small capacitance is used. Thus, the small
capacitances can be used for the flying capacitor in the 3level and 5-level FCBCs.
Fig. 9 (b) shows the experimental results of the kvconv
which is the ratio between the effective values of
distorted components Vnfsw against DC component VDC,
which is introduced in order to evaluate the distorted
voltage across the input voltage source and the input

ΔV fcx(n−level) =00Fig. 8.

Pin D
VinC fc f sw

0.5

1

1.5

Experimental results of the ripple voltage of flying capacitor withvarious of capacitances of the flying capacitors

0.8
Ratio of the effective value of distorted
components against DC component of kiout

A. Ripple voltage of the flying capacitor characteristic
Fig. 8 shows the relationship between flying capacitor
voltage ripples and several capacitances of the flying
capacitors in the 3-level and 5-level FCBCs. It is
confirmed that each voltage ripple of the flying capacitors
are same when same capacitance of the flying capacitor isselected in the 3-level and 5-level FCBCs although 5level FCBC has different switching patterns and threedifferent flying capacitor voltages. As a result, it isexperimentally confirmed that the ripple voltage of theflying capacitor voltage does not depend on the numberof level as shown in (25). Besides, it is confirmed that theflying capacitor voltage ripples is reduced with regard to
the increasing capacitances of the flying capacitor as
shown in (25).
The increasing of the flying capacitor voltage ripples
will contribute into switching loss of the semiconductor
switches. However the performance of high voltage
semiconductor switches is improving recently. Therefore,high flying capacitor voltage ripple is acceptable althoughsmall capacitance of the flying capacitor is used.
The flying capacitor voltage ripples are same in the 3level and 5-level FCBCs when same capacitance of theflying capacitor is considered even the number of thelevel is different. Therefore, the capacitance of the flying
capacitor can be reduced by considering the constant ratioof the flying capacitor voltage ripples against
capacitances of the flying capacitor. In order to design theminimum capacitance of the flying capacitors in the 5level FCBC, (18)-(20) are referred.

20

3-level (experiment)

0.6

5-level (experiment)

0.4

40

∑I
n =1

k i out =

0.2

2
nf sw

I dc

Idc: DC component
Infsw: Harmonic component

0
0

5

10

15

Capacitance of flying capacitor Cfc ( F)(a) In case of iout

0.4

0.3
40

kvconv =

0.2

∑V
n=1

2

nfsw

Vdc

0.1

0
0

5
(b)

10

15

In case of vconv.

Fig. 9. Ratio of the effective value of distorted components against DC
component

inductor. It is confirmed that the ratio kvconv is converges
into a constant value when the capacitance of the flying
capacitor is increased.

300

300

200

200

100

100

0

0
3-level FCBC: vconv components, Cfc = 1.1 μF(a)3-level FCBC
Cfc = 0.35 F
fsw = 100 kHz

300
200

300
Voltage of vconv (V)

2 fsw

200

0, (vconv)

T = 10 s

100

0

100

2 ( s/div)

vconv (100 V/div)

05-level FCBC: vconv components, Cfc = 1.1 μF

(b)

2

0

4

Frequency f (MHz)(c)

3-level FCBC: vconv components, Cfc = 0.35 μF(d) 5-level FCBC: vconv components, Cfc = 0.35 μF
Fig .10. Harmonics components of vconv

4

3

3

Amplitude iout (A)

4

2
1
0

1
0

(a)

Amplitude iout (A)2

3-level FCBC: iout components, Cfc = 1.1 μF

5-level FCBC: iout components, Cfc = 1.1 μF

(b)

4

4

3

3

2

2

1

1

0

0

5-level FCBC
Cfc = 0.35 F
fsw = 100 kHz
Current of iout (A)

fsw

0,

iout
T = 10 s
iout (2 A/div)0

2

2 ( s/div)

4

Frequency f (MHz)(c)

3-level FCBC: iout components, Cfc = 0.35 μF
Fig .11.

(d)

5-level FCBC: iout components, Cfc = 0.35 μF

Harmonics components of iout

From Fig. 9, the small capacitance can be used for the
flying capacitor in both 3-level and 5-level FCBCs. Fromthese experimental results, 5-level FCBC has obviously
lower kvout compared to the 3-level FCBC. Thus the kvout
has significant difference when the level is increased.
Meanwhile, 5-level FCBC also has lower kIx compared tothe 3-level FCBC. However the different is small.
C. Harmonics components of vconv and iout in the 3-leveland 5-level FCBCs
Fig. 10 shows the harmonics spectrum of vconv and iout
with various capacitances of the flying capacitors in the
3-level and 5-level FCBCs. For these experimental results,
only two capacitances of the flying capacitors are
selected which are 1.1 μF and 0.35 μF.
Fig. 10 (a), (b), (c) and (d) show the harmonics
spectrum of vconv in the 3-level and 5-level FCBCs. Fromthese experimental results, 5-level FCBC has obviously
lower harmonic components compared to the 3-level
FCBC when capacitances of 1.1 μF and 0.35 μF for the
flying capacitor are used.
Meanwhile, Fig. 11 (a), (b), (c) and (d) show theharmonic components of iout in the 3-level and 5-level
FCBCs. From these results harmonics spectrum, thepatterns between 3-level and 5-level FCBCs are almost
same. The difference of harmonic components is not
obvious because the ripple current of iout is not different
from between two FCBCs due to their operation modes.
VI. CONCLUSIONSIn this paper, the authors revealed the minimum
capacitance of the flying capacitor consideration in multilevel FCBC based on the switching device voltage rating.
Moreover, the relationship between the capacitance of theflying capacitor and the output voltage ripple isindependent of each other and it was confirmed bysimulation and experimental results. Meanwhile, theminimum capacitance based on the maximum voltage
rating of switching devices was confirmed by theexperimental results in the 3-level and 5-level FCBCs.
Besides the expression clarification in order to design
minimum capacitance of the flying capacitor in the nlevel FCBC is also shown. The maximum efficiency ofthe 3-level FCBC prototypes with a small capacitance ofthe flying capacitor is 98.5 % at the output power of 1
kW. Then, the distortion of the voltage across the input
voltage source and the input inductor is measured. In
addition, the distortion of the current to the output sidewhich is the output capacitor and the load is alsomeasured. As a result, it is experimentally confirmed thata small capacitance of the flying capacitors can be used inthe 3-level, 5-level and n-level FCBCs.
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