A New Approach for High Efficiency Buck-Boost
DC/DC Converters Using Series Compensation
Jun-ichi Itoh

Takashi Fujii

Nagaoka University of Technology
1603-1 Kamitomioka-cho Nagaoka City Niigata, Japan
itoh@vos.nagaokaut.ac.jp
Abstract - This paper proposes a new buck-boost DC/DC
converters, which is connected in series to the power supply. The
proposed circuit provides only differential voltage between the
input voltage and the output voltage command. The power ratingof a conventional circuit is dominated by the input voltage or the
output voltage. In contrast, the voltage rating of the proposed
circuit requires only the differential voltage between the input
and output voltage. In addition, the series converter generates a
positive and negative voltage to realize boost mode and buckmode, respectively. As a result, the power rating of the DC/DC
converter can be drastically reduced. A new approach for series
compensation converters is introduced in order to realize highefficiency and a reduction of power rating. Two new buck-boost
converters and their control methods are proposed based on a
new concept in which the proposed circuits consist of an Hbridge circuit and a power assist circuit. The H-bridge circuit isused in order to determine the polarity of the differential voltage.
The power assist circuit controls the DC voltage of the H-bridge
circuit depending on the output voltage command. In the
proposed circuit, a flyback converter and inverting chopper areused as the power assist circuit. Simulation and experimental
results are shown in order to demonstrate the advantages of the
proposed converters in comparison with a conventional buckboost converter. A maximum efficiency of 98% was obtainedwith the proposed circuit. The proposed circuits can decrease
losses by 2/3 in comparison with a conventional buck-boost
converter. Therefore, the proposed converter can realize highefficiency and down-sizing in applications that require the output
voltage to be closed to the input voltage.
Index DC/DC converters, High efficiency, battery applications,flyback converter, inverting chopperI.

INTRODUCTION

Recently, most mobile equipments used battery as powersource. The efficiency of DC/DC converters becomes animportant issue, in order to maintain long working times forbatteries in mobile devices such as mobile phones, laptop
computers and so on.
A general conventional buck-boost DC/DC converter uses
an inverting chopper or a combination chopper, which consistsof a buck copper and a boost chopper. The inverting chopper
stores output energy in storage device, such as reactor orcapacitors. Therefore, the converter efficiency is decreased
since the power loss occurs in the storage devices. On theother hands, because the combination chopper has two stages
for conversion process, the converter efficiency decreases.
Many circuit topologies of DC/DC converters have beenstudied in order to obtain high efficiency [1-3]. Resonant typeconverters, which use zero current switching or zero voltage

switching, are a good solution to obtain high efficiency [5-6].
Especially, the resonant converter is suitable for the DCDC
converter because the DC/DC converter requires highswitching frequency in order to realize downsizing and highspeed output voltage response. However, the number of partsin the circuit increases, because resonant converters require an
additional inductor or capacitor. Moreover, the voltage andcurrent rating of the DC/DC converter are dominated by theoutput voltage rating and the output current rating inconventional DC/DC converters.
From the viewpoint of the battery application, the input
voltage i.e. battery voltage, is almost constants under normal
operation. The battery voltage becomes markedly higher thannormal voltage in the initial condition or overcharge operation,and lower than the nominal voltage in the overdischarge.
Therefore the efficiency for voltage at normal condition isvery important.
In some battery applications, the output voltage is regulatedby the DC/DC converter as the output voltage is close to the
input voltage. In this case, the conventional DC/DC converter
has to convert all power regardless of the output voltage
because the conventional converter is connected in parallel toa power supply and a load.
In contrast, there have been some types of series convertersthat have been proposed for boost up converters [7]. The
output voltage of a series converter is obtained by adding theconverter voltage to the input voltage. Therefore, the converter
power rating can be suppressed because the converter voltage
becomes low. However, proposed types of converters can notwork for step-down operation. Many battery applications
require buck-boost operation of the power converter.
This paper proposes a new buck-boost converter that isconnected in series to the power supply. The proposed circuitprovides only differential voltage between the input voltage
and the output voltage command. As a result, the power ratingof the DC/DC converter is reduced drastically. Firstly, thispaper introduces an approach that uses series compensation
converters to obtain high efficiency and a reduction of power
rating. Two new types of buck-boost converters and theircontrol methods are proposed based on this new concept.
Finally, simulation and experimental results are shown inorder to demonstrate the advantages of the proposed
converters in comparison with a conventional buck-boost
converter.

II. PROPOSED BUCK BOOST CONVERTERS
A. Series compensation concept
Fig. 1 shows power flow diagram of the conventional
DC/DC converter and proposed series compensation DC/DC
converter. Fig. 2 shows the configuration of a conventional
one and the proposed one. The power rating of theconventional circuit is dominated by the input voltage or the
output voltage. In contrast, the voltage rating of the proposed
circuit only requires the differential voltage between the input
and output voltage. In addition, the series converter generates
a positive and negative voltage to realize boost mode and buckmode, respectively. The output voltage Vout is obtained by (1),using series converter output voltage Vconv and input voltage
Vin.
Vout = Vin ± Vconv(1)
For example, if the input voltage variation is 2.6 V to 4.0 V
and the output rating is 3.3 V, 1 A, then the power rating of
the conventional converter is 3.3 W. In contrast, for the
proposed converter, it requires only 0.7 W.
The series compensation method can obtain high efficiency.
All output power is supplied through the converter in aconventional configuration, as shown by the power flow of theconventional converter given in Fig. 1. The output power Poutis then obtained by (2), using the efficiency ηc, and the input
power Pin.
(2)
Pout = Pin ⋅η cHowever, not all the output power is directly supplied in the
proposed converter, because the converter can adjust the small
differential voltage. As a result, the total efficiency of the
proposed converter is obtained by (3) if the loss of the directmode power P1 can be neglected.
Therefore, the total efficiency obtained by using the
proposed concept is improved, as shown by (4). It should be
noted that the proposed method is so effective that thedifferential voltage is small.
P + P ⋅η(3)
ηt = 1 2 c
P1 + P2ηc >P1 + P2
P
η− 1
P2
P2

(4)

Fig. 3 shows the theoretical total efficiency of the proposedbuck boost converter. The efficiency is calculated based on (3)for each series converter efficiency. Even though the series
converter efficiency is no so high, high total efficiency is
obtained at the low output power ratio to series converter
power in the output power. As the output power ratio increase,
the total efficiency is decreasing because the converter loss
increases.
The point subjects to increasing the efficiency are theswitching loss and choice of the power device. The conduction
loss is dominated by the current. The loss is not reducing eventhough the power rating of converter becomes small. However,the voltage rating of the power device in the series converter isonly differential voltage, therefore the switching loss issmaller than the conventional circuit. Moreover, the series(a) Conventional concept.

(b) Proposed concept.
Fig. 1. power flow diagrams.

(a) Conventional topology.
(b) Proposed topology.
Fig. 2. Construction of conventional
and proposed circuit.
100

95
Series converter
efficiency c[%]
95
90
85
80
75

90

85

0

10

20
30
40
Ratio of series converter output power[%]50

Fig. 3. Relation between output voltage and efficiency of the proposedseries converter.

converter is applied the low conduction loss switching devices
since the voltage rating of the power devices is lower than theconventional one. As a result, the loss of the series converter
become small then, the total efficiency of the system isimproved.
B. Circuit configurations based on the proposed concept
Fig. 4 and Fig. 6 show two types of proposed circuit using
the proposed concept. The series converter needs a power
assist circuit in order to charge or discharge the voltage of thecapacitor CC2. A flyback converter and inverting converter areused as the power assist circuit, as shown in Fig. 4 and Fig. 6,
respectively. The proposed circuits have to isolate the series
voltage to the input voltage in order to avoid a short circuit
between the input voltage and series voltage. The flybacktransformer is used for the isolation in the proposed circuit I.
On the other hands, the short circuit is avoided by the switches
SC2 and SC4 in the proposed circuit II.
The H-bridge circuit is used to determine the polarity of thedifferential voltage. Thus, the H-bridge circuit selects only the
boost mode at positive differential voltage or buck mode at

negative differential voltage. The power assist circuit control
VCC2 depends on the output voltage command. Therefore, low
conduction loss and low voltage rating switching devices canbe used in the H-bridge circuit. Also, a switching device withlow current rating and low switching loss characteristics canbe selected for the power assist circuit.
Fig. 5 and Fig. 7 are shown the simulation results for the
proposed circuits. Since the proposed circuits operate as thebuck-boost converter, the reactor current iLc in the proposed
circuit II is similar to the flyback transformer current in the
proposed circuit I. In addition, the output voltage is controlled
to reference voltage 12V by power assist circuit. Therefore,
the inverting chopper using the switch Sc2 and Sc4 can be usedas the series converter.
These simulation results can confirm that the proposedseries compensation method has validity.

D. Design of the proposed circuit.
There are two operation modes in series converter, the boost
mode and the buck mode. The proposed circuit is designed

Sc2

Lf

SH1
Vcc2

Cc1
LTp

Cc2

SH3
SH4

LTs

Vout

Vconv
Cf

SH2
Np:Ns
Vin

Fig. 4. Proposed circuit I (Flyback converter).
15
Output voltage Vout
Input voltage Vin

10
5Converter voltage Vconv

0
1.5

Secondarycurrent

Primary
current

1
0.5
0

0

5

10
Time [ s]15

20

Fig. 5. Simulation results of the proposed circuit I.

Voltage[V]Fig. 6. Proposed circuit II(Inverting converter).

Reactor current[A]C. Control method for the proposed circuits
Fig. 8 shows control block diagrams for the proposed circuit
II, the inverting chopper type. Basically, the output voltage isadjusted by the DC capacitor voltage VCC2 in Fig. 4 and 5.
However, it is difficult to control the output voltage when the
differential voltage is close to zero, because non linear
components, such as the effects of dead time period anddevice characteristics disturb the output voltage control.
Therefore, when the input voltage is near to the output voltage,
the output voltage will be controlled by PWM using the Hbridge circuit. In the small differential voltage region, the DC
capacitor voltage VCC2 is controlled to a constant value and Hbridge is controlled as a quadrant chopper, which can generate
and regenerate the power. The threshold voltage Vcng means
the voltage starting switching operation in H-bridge part.
When the differential voltage is less than Vcng, VCC2 is held toVcng, and the series converter output voltage Vconv is controlled
by PWM modulation of H-bridge part. The threshold voltage
Vcng has to be set larger than non-linear error voltage in theseries converters. The switching loss of the H-bridge circuit
will be small, because the DC voltage VCC2 of the H-bridge
circuit is low.
Fig. 9 and Fig. 10 show the comparison of the output
voltage waveform with and without PWM modulation at low
differential voltage respectively. The output voltage is kept at
a constant voltage of 12 V, although the input voltage
increased from 10 V to 14 V and decreases from 14 V to 10 V.
In particular, when the input voltage is closed to 12 V, an
output voltage oscillation occurs when PWM is not used, as
shown in Fig. 9. The maximum error of the output voltage is
1.2 V. However, the oscillation can be suppressed to less than
0.1 V by the proposed PWM control, using an H-bridge circuit
as shown in Fig. 10. It should be noted that the control of the
proposed circuit I could apply the same strategy as shown in
Fig. 8.

Sc1

Fig. 7. Simulation results of the proposed circuit II.

individually for boost mode and buck mode. Then, dominantparameter values are adopted. It is noted that the design of the
proposed circuit II is only explained in this paper because thedesign of two proposed circuits is almost same. Table 2 shows
the specifications of the experimental circuit.

1) Boost mode
To improve efficiency, the reactor current is needed to keep
continuous. Therefore, the largest calculated inductance under
the conditions is adopted. The inductance becomes maximum
value when the storage energy is at maximum. In boost mode,
the reactor storage energy becomes maximum value when the
input voltage is minimum value. In case of Table 2, theminimum input voltage is 6V, and the output voltage isconstant 12V, then, the series converter output voltage isdifferential voltage 6V. Therefore, the series converter input
and output voltage is 6V. The duty ratio D is obtained by (5),using switch on time ton, and off time toff.

(Vin − Vconv ) = 0.5
ton=D=
ton + toff Vin + (Vin − Vconv )(5)

Table 1. Condition of simulation.
On resistance of FET
[mΩ]

12

12

Forward Voltage drop of
diode[V]0.5

Output power Pout[W]14

Lf[μH]

22

Switching frequency fsw[kHz]100

Cf[μF]

2200

Control changeover voltage
Vcng[V]1

Lc[μH]

22

ACR integration time[ms]0.375

Cc1, Cc2[μF]2200

AVR integration time[ms]3.75

H-bridge propotional
gain[pu]

2.0

ACR proportional gain[pu]0.22H-bridge integrationtime[ms]

0.72

AVR proportional gain[pu]

4.03

10 to
14

Input voltage Vin[V]Output voltage Vout[V]In this condition, the reactor storage power is obtained by(6), using the reactor current peak value i1P, and switchingfrequency fsw.
2 2

PL =

V t f
1
2
L ci1P f sw = in on sw
2 Lc
2

(6)

Next, the series converter output voltage is obtained by (7),using maximum output current Iout. Then the series converter
output power is controlled by storage energy of reactor.
Pconv = Vconv I out =Vin2 t on2 f sw
2 Lc

(7)

+

-

From (6) and (7), the inductance of the series converter
reactor is obtained by (8).
(8)

III. EXPERIMENTAL RESULTS
To confirm the validity of the proposed converter concept,
the proposed converter was tested under the experimental
conditions shown in Table 2. It should be noted that the circuit
parameters chosen is to confirm basic operation, andoptimization was not considered. In addition, the conventional
combination buck boost converter, which uses a step-down
converter and a boost-up converter, as shown in Fig. 11 istested in order to compare the efficiency. It is noted that thereactor in the conventional circuit is designed as follows.
In conventional combination chopper, the reactor has thefunctions of smoothing the output current and energy storage.
The inductance value is designed in allowance. The rector
current becomes discontinuous in light load region. The-

Fig.8. Proposed control diagram.
14

14
Input voltage Vin

1212Outputvoltage Vout

11
10

Input voltage Vin

13

Voltage[V]

13

0

5

10
15
Time[ms]20

Outputvoltage Vout

11

25

10

0

5

10
15
Time[ms]

2025(a) Boost to step down.
(b) Step down to boost.
Fig. 9. Voltage waveforms with PI control only
14

14
Input voltage Vin

13
12

12

Outputvoltage Vout

11
10

Input voltage Vin

13

Voltage[V]2) Buck mode
The reactor storage energy becomes the maximum values
when the input voltage is at maximum value. In condition of
Table 2, the maximum input voltage is 18V, the series
converter output voltage is 6V. Similarly to boost mode, theinductance of the buck mode is calculated as 40μH by (8). The
buck mode inductance is larger than the boost mode one.
Therefore the buck mode value is adopted.

+

Voltage[V]Vin2ton2 f sw
= 9[ μH ]2Vconv I out

Voltage[V]

Lc =

0

5

10
15
Time[ms]20

Outputvoltage Vout

11

25

10

0

5

10
15
Time[ms]

20(a) Boost to step down.
(b) Step down to boost.
Fig. 10. Voltage waveforms
with PI control and proposed method.

25

Table 2. Specifications of experimental circuit.

discontinuous current causes increasing loss and unstable
output voltage due to nonlinear control.
The ripple current is needed at minimum value when the
output current is minimum at output power 5W, output current
417mA.The peak to peak value of the ripple current of areactor is limited to 40% of output current, then the ripple
current is 166mA because the output current is 417mA at
output power 5W. In addition, the reactor voltage becomesmaximum value when the input voltage is at maximum valueof 18V. In this condition, the inductance of the reactor is
calculated by (9).
L=

(Vin − Vout )ton = (18 − 12) ⋅ (0.667 × 10 × 10−6 ) = 240[ μH ]ΔI out ( P − P )0.166

Input voltage Vin[V]Output voltage Vout[V]12Output power Pout[W]5～30

Switching frequency fsw[kHz]100

Table 3. Circuit parameter.

Proposedcircuit I(9)

A. Experimental results
Figures 12 and 13 show the current waveforms of proposed
cir Fig. 12 and 13 show the current waveforms of proposed
circuits I and II, respectively. The current polarity depends on
the operation mode; buck or boost. The transformer andreactor were designed so that the power assist circuit operates
in current continuous mode. These results confirmed that the
proposed assist circuit accepts bidirectional power flow,
without unnecessary surges or oscillations in the reactor ortransformer current. In addition, the short current of the power
supply does not appear in the waveforms. Therefore, theflyback converter and the inverting chopper with the isolation
switch can be used in a series converter.
Fig. 14 presents a comparison between the efficiency of aconventional buck boost converter and the proposed
converters at constant load. A maximum efficiency ofapproximately 98% was obtained for both of the proposed
circuits. For the conventional converter is approximately 94%.
In other words, the converter loss decrease from 6% to 2 %,the proposed circuit can improve the converter loss to 1/3. The
input voltage is so close to the output voltage, so thatconverter efficiency is even more improved, as shown in Fig.
14.
Fig. 15 shows the efficiency with load variation for theconventional converter and the proposed converters. In bothboost mode and buck mode, efficiency improvement ispossible. In Fig. 15, the efficiency of the buck mode is higher
than the boost mode one because the input power in the boost
mode is larger than the buck mode. The power loss in theseries converter is provided from power source in the boost
mode. In contrast, the regeneration power in the buck mode isdisappeared by the circuit power loss. Therefore, the current
flowing to the series converter in the boost mode is decreased
to buck mode one.
Fig. 16 shows the voltage waveform of Vout for load step
response. The load is changed from 5W to 14W. When theload condition is rapidly changed, the output voltage
oscillation can be suppressed to less than 0.2 V. The output
voltage oscillation can be improved by refinement of thecontrol strategy of the series converter.

6～18

LT1

40μH

LT2

40μH

Np

5

Lc
Proposedcircuit II

22μH

Cc1

470μF

Cc2

2200μF

Ns

5

Lf

22μH

Cc1

2200μF

Cf

1000μF

Cc2

220μF

L

220μH

C

2200μF

Lf

2μH

Cf

470μF

Conventional
circuitFig. 11. Configuration of a conventional combination chopper.

(a)Step down mode.
(b)Boost mode.
Fig. 12. Current waveforms of the flyback transformer in the proposed
circuit 1.
iLc:1[A/div]

iLc:1[A/div]002[ s/div]

2[ s/div](a)Step down mode.
(b)Boost mode.
Fig. 13. Current waveforms of the reactor in the proposed circuit II.

Fig. 17 shows the output voltage waveform at low
differential voltage in PWM control region of the H bridge
part. The output voltage is kept at 12 V of a constant voltage,
although the input voltage increases from 10 V to 14 V ordecreases from 14 V to 10 V. In particular, when the input
voltage is close to 12 V, the control output voltage is disturbed
by the non linear components, such as dead-time and powerdevice characteristics. However, the oscillation can besuppressed to less than 0.2 V by proposed PWM control of Hbridge as shown in Fig. 17.

Fig. 16 . Output voltage waveform for step load response.
Fig. 14. Input voltage characteristics of efficiency ηt.

(a) Boost to step down.

(a) Vin=9V(b) Step down to boost.
Fig. 17. Output voltage waveform at low differential voltage.

REFERENCES[1](b) Vin=15V

Fig. 15. Load characteristics of total efficiency ηt.

[2]

IV. CONCLUSION[3]

In this paper, a series type buck-boost converter is proposedthat uses an H-bridge circuit and a power assist circuit withsmall power rating. A flyback converter and an inverting
converter were used as power assist circuits. The experimental
results confirmed that the proposed circuits could decrease
losses by 2/3 at the maximum efficiency point. Therefore, theproposed converter can realize high efficiency and downsizing for use in applications that require the output voltage to
be close to the input voltage.

[4]
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[6]
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