A High Efficiency Isolated DC/DC Converter
Using Series Connection on Secondary Side
Satoshi Miyawaki*, Jun-ichi Itoh*, and Kazuki Iwaya*** Nagaoka University of Technology, 1603-1 Kamitomioka-cho Nagaoka City Niigata 940-2188, Japan** DENSEI-LAMBDA, Ltd., 2701 Tokawa Settaya Nagaoka City Niigata 940-1195, Japan
Abstract-- This paper proposes a new circuit topology fora high efficiency isolated buck-boost DC/DC converter. The
proposed converter consists of a high efficiency resonancehalf-bridge converter and a series converter. This proposed
circuit regulates the output voltage by the series converter,
which provides only the differential voltage between the
input voltage and the output voltage. Therefore, the circuitachieves high efficiency when the input voltage is closed to
the nominal input voltage, since only the resonanceconverter will operate. Experimental results are shown inorder to demonstrate the advantages of the proposed
converters in comparison with a conventional converter.
The experimental results confirmed that the proposed
circuit, which converts 48 V input to 12 V, achieves 94.0%
maximum efficiency point at the nominal input voltage
region.
Index Terms—DC/DC converter, Isolated converter,
Series voltage compensation, Current resonance

I. INTRODUCTION
Recently, high efficiency and high power density of
power supplies are increasingly desired for telecom
applications and microprocessor boards. In addition,these power converters have to supply larger output
current with very low output voltage for the loads. In
order to achieve high efficiency and high power density,
the power system is divided into several converters, suchas point-of-load converter, bus converters and front end
converter. In this system, the power is distributed to eachof the device by a DC voltage line [1-5].
The DC bus converter, which regulates the constantDC voltage for output side, is required at the input pointof each device, because the DC bus voltage will fluctuate
due to the bus impedance when the load condition hadchanged. The DC bus converter is also used to obtainother levels of DC voltage from the DC bus voltage.
Therefore, high efficiency DC/DC converters are required
for these bus converters.
Resonant type half-bridge converters, which use aresonance between the leakage inductance oftransformers and the DC capacitor, are one of the mosteffective circuit topologies to obtain high efficiency.
However, it is difficult to regulate the output voltage inwide range while there is a fluctuation in the input
voltage because the switching timing is constrained by
the resonance period. Therefore, a resonant-type halfbridge converter is generally connected to a voltage
control converter, such as a buck chopper [6-7]. As aresult, the converter loss increases because all the power

passes through two converters; the voltage control
converter and the resonant converter.
This paper proposes a new topology for an isolated
DC/DC converter using series voltage compensation. The
input voltage fluctuation is compensated by the auxiliary
circuit that outputs only the fluctuation of the output
voltage [8-11]. One of the advantages of the proposed
circuit is that high efficiency can be achieved when the
input voltage is closed to the nominal input voltage
because the power of the auxiliary circuit becomes verysmall in comparison to the input power. Generally, thelarge fluctuations of the DC bus voltage are not generated
for long time periods, and as a result, the converter loss
can be reduced.
Firstly, an approach uses to obtain high efficiency withthe proposed series compensation method is introduced inthis paper. Secondly, the circuit configuration and control
strategy are described. Finally, some experimental results
are given in order to demonstrate the advantages of the
proposed converter.
II. CIRCUIT CONFIGURATIONS
A. Conventional circuit
Fig. 1 shows a configuration of a conventional DC/DC
converter. The conventional circuit consists of a resonant
half-bridge converter and a voltage control converter,
such as a buck boost chopper. The fluctuation of the input
voltage is constantly controlled by the buck chopper. The
output side is isolated by the resonant half-bridge
converter to the input side. This system has two stages ofthe power flow from input to output, as shown by Fig.
Buck chopper

Resonant converter

Vout

Vin

(a) Circuit diagram.

(b) Power flow diagram.
Fig. 1. Conventional circuit using a buck chopper and a resonant
converter.

1(b). The total efficiency of the conventional circuit ηc is
obtained from (1), using both the resonant converter
efficiency η1 and the buck chopper efficiency η2.

ηc = η1η 2(1)

All the power passes through both converters despite
the relation between the input voltages; therefore, theconverter efficiency is decreased.
B. Series connection on primary side
Fig. 2 shows the configuration of the DC/DC converter
using series connection in the primary side which has
been proposed by authors in [11]. This system consists of
two power converters; main circuit and auxiliary circuit,which are connected in series to the output side using atransformer.
The main circuit is that the resonant type half-bridge
converter with isolation transformer. Zero current
switching (ZCS) is implemented by the resonance circuit
between the leakage inductance of the transformer andthe resonance capacitor in the DC part, in order toachieve high efficiency.
The auxiliary circuit is a full-bridge converter uses forregulating the output voltage. The output voltage of theauxiliary circuit is added to the output of the main circuitby the transformer. That is, the auxiliary circuit
compensates only the fluctuation voltage, against theoutput voltage commands. Therefore, the converter loss
is decreased.
The problem of this circuit is that the auxiliary circuit
is connected on the primary side. Thus, by using seriesconnection on primary side, the voltage rating of theswitching device in auxiliary circuit is dominated by the
input voltage Vin. Here, when the specification of theturns ratio in the main circuit transformer is 2N11<N12, theoutput voltage is lower than the input voltage. In thiscondition, the switching loss in the auxiliary circuit isincreased. In addition, the wire turns of the transformer in
the auxiliary circuit needs to be increased.
C. Series connection on secondary side
Fig. 3 shows the configuration of the proposed
converter using series connection on secondary side. Like
the series compensation on primary side, the proposedsystem consists of two power converters, which areconnected in series to the output side using a transformer.
However, the auxiliary circuit is connected on thesecondary side. In this circuit, the switching loss in
auxiliary circuit can be reduced in comparison to theseries compensation on primary side.
It should be noted that the voltage rating of theswitching device in the auxiliary converter is lower thanthe main circuit because the output voltage is lower than
the input voltage. That is, the conduction loss of theauxiliary circuit can be decreased.
The switching frequency of the main circuit isconstrained by the resonance frequency. From theequivalent circuit of the transformer shown in Fig. 3(a),the resonant inductance of the proposed circuit L is
obtained from (2), using the leakage inductance lxx,(a) Circuit diagram.

(b) Power flow diagram.
Fig. 2. Proposed circuit using series connection on primary side.

(a) Circuit diagram.

(b) Power flow diagram.
Fig. 3. Proposed circuit using series connection on secondary side.

mutual inductance Mxx and the number of wire turns Nxx.
L = l11 +M 1 (l12 + LT 2 )M 1 + (l12 + LT 2 )

(2)

2

⎛ N ⎞ ⎛ N ⎞⎛l M ⎞where LT 2 = ⎜⎜ 11 ⎟⎟ ⎜⎜ 22 ⎟⎟⎜⎜ l21 + 22 2 ⎟⎟lNN
22 + M 2 ⎠⎝ 12 ⎠ ⎝ 21 ⎠⎝

Vin/2

VTm

-Vin/2

The resonance frequency of the proposed circuit f0 is
obtained by
f0 =

Vout

VTa

1

The reason of high efficiency in the proposed circuit is
as follows. The power Pout is obtained by adding theauxiliary converter power P3 to the directly power P0, as
shown by Fig. 3(b). The total converter efficiency of the
proposed circuit ηp is obtained by (4), using the auxiliary
circuit efficiency η3 and the power ratio k=P3/P0.

1 + kη3
η p = η1
1+ k

0

-Vout

i ii iii iv v vi
Sm1

State

Boost mode
Sm1 Sa1 Sa3

i
ii
iii
iv
v
vi

ON OFF OFF
ON OFF ON
ON ON ONOFF ON ON
OFF ON OFF
OFF OFF OFF

Sm2 = Sm1， Sa2 = Sa1， Sa4 = Sa3

Trans.1
Vout

Vin

Sm2
Trans.2(i)(4)

Therefore, the total efficiency ηp of the proposed
circuit is higher than the conventional circuit where therelation of efficiency for each circuit is given by

Sm1

Sa1

Sa3

Sa2

Sa4

Trans.1
Vout

Vin

Sm2
Trans.2

1 + kη3
> η2
1+ k

(5)

(ii)
Sm1

III. CONTROL STRATEGY
In the proposed circuit, the main circuit is controlled
under an optimum condition as the resonant converter. In
order to obtain the maximum efficiency, the resonant
converter operates at 50% duty cycle based on resonancefrequency f0. Therefore, the main circuit can achieve zerocurrent switching (ZCS) at any time.
Fig. 4 shows the operation mode and the waveforms in
the primary side of the transformer in the boost mode.
The switching timing of the auxiliary circuit issynchronous with the main circuit. The auxiliary circuit
can output three different voltage levels; +Vout, -Vout, andzero voltage. In the boost mode; i.e. when the input
voltage is decreased, the auxiliary circuit outputs positive
phase voltage to the output voltage of the main circuit
without preventing the resonance operation in the main
circuit. Similarly, in the buck mode; i.e. when the input
voltage is increased; the auxiliary circuit outputs negative
phase voltage to the output voltage of the main circuit.
The output voltage is controlled by changing the pulse
width D of the auxiliary circuit. If the voltage drop fromthe leakage inductance and winding resistance are
negligible, the pulse width of the auxiliary circuit D is
obtained by (6), using the switching (resonance) cycle T
and the output voltage command Vout*.
D=

D

T

(3)

2π LC

0

*
T N 21 ⎛ Vout
N
V ⎞
⋅
⋅ ⎜⎜
− 12 ⋅ in ⎟⎟2 N 22 ⎝ Vout 2 N11 Vout ⎠(6)

Fig. 5 shows the control block diagram of the proposed
circuit. The duty cycle of the main circuit is set to 50%.
In the auxiliary circuit, the output pulse width D is
calculated by an automatic voltage regulator (AVR). The
phase shift gain is then calculated according to D. The

Sa1

Sa3

Sa2

Sa4

Trans.1
Vout

Vin

Sm2
Trans.2

(iii)

Sa1

Sa3

Sa2

Sa4

Fig. 4. Corresponding of the switching pattern and operation mode.

Fig. 5. Control block diagram of the proposed circuit.

phase shift is used to decrease the switching loss in theauxiliary circuit. According to the relation between the
output and input voltages, the mode selector determines
the operation mode. It should be note that the proposed
converter has the reference voltage mode, which meansthat the auxiliary circuit stops when the input voltage isalmost the same as the nominal input voltage, in addition
to the buck and boost mode operation.
IV. OPERATIONAL MODE ANALYSISThe proposed circuit is clarified in order to thestability of operation and operational mode is analyzed

using a simple equivalent circuit. Fig. 6 shows theequivalent circuit of the proposed circuit under the boost
operation. In the AC signal analysis, the switching
operation of the main circuit and the rectifier part at theoutput side is expressed as a square wave power source,which has the amplitude of the input and output voltage
at the frequency of the switching frequency respectively.
The capacitors of the half-bridge converter can be
represented as one resonance capacitor with idealtransformers. Moreover, the full-bridge converter in theauxiliary circuit is represented as three different voltage
levels; +Vout, -Vout, and zero voltage. Note that theexcitation inductance of the transformer is disregarded in
the mode analysis because it is much larger than theleakage inductance. The resonance frequency isdominated by the leakage inductance.
Fig. 7(a) and (b) show the simplified equivalent circuit
which is separated into the main circuit side and auxiliary
circuit side, according to the superposition principle. The
separation of the equivalent circuit is achieved byfocusing on the transformer input current iTm of the main
circuit. The equivalent circuit is obtained from theperspective of the main circuit, when the input of theauxiliary circuit is short-circuited. Similarly, theequivalent circuit is obtained from the perspective of theauxiliary circuit, when the input of the main circuit isshort-circuited; iTm is obtained by

iTm = iTm _ m + iTm _ a

iTm_m C±⎤⎡1 V
⎢ ⋅ in sin (ωt ) − αVout sin (ωt )⎥ωL⎥⎢ 2 ωL
1 ⎢
− t
⎧
⎫⎥
1
iTm _ m = e τ ⎢+ im (k ) ⎨cos(ωt ) −sin (ωt )⎬ ⎥
⎢
2Q
⎩
⎭⎥
⎥
⎢
⎛
1 ⎞
⎥
⎢
⎜
⎟
(
)
q
ω
sin
ω
t
−
+
1
⎥
⎢ m (k ) ⎜ 4Q 2 ⎟⎝⎠⎦⎣1 ⎛ 1⎞
ωL−⎜ 2 ⎟ , Q =LC ⎝ τ ⎠
R

2L
,ω=
R
2

2

⎛α ⎞
⎛α ⎞
R = R1 + ⎜⎜ ⎟⎟ R2 , L = L1 + ⎜⎜ ⎟⎟ L2
⎝β ⎠
⎝β ⎠

R1

2

L1

L2

2

Vin
2

R2

± Vout(a) Main circuit only.
Vout

iTm_a C

R1

2

L1

L2

2

R2

± Vout(b) Auxiliary circuit only.
Fig. 7. Control block diagram of the proposed circuit.

im(k) and ia(k) are the initial currents of the leakage
inductions, and qm(k) and qa(k) are the initial charges ofresonance capacitor for the main and auxiliary sides,
respectively. The optimization design of the proposed
circuit can be obtained from this equation.

(7)

where iTm_m is the input current on the main circuit side asshown in Fig. 7(a) and iTm_a is the input current on theauxiliary circuit side as shown in Fig. 7(b). Consequently,
the equation of the entire circuit can be obtained byderiving each equation from each of the circuit equationin Fig. 7. The input current iTm_m and iTm_a are obtained
from (8) and (9).
⎤⎡1 V
⎢ ⋅ in sin (ωt ) − αVout sin (ωt )⎥ωL⎥⎢ 2 ωL
⎥
1 ⎢
− t
⎧
⎫
1
iTm _ m = e τ ⎢+ im (k ) ⎨cos(ωt ) −(8)sin (ωt )⎬ ⎥
⎢
2Q
⎩
⎭⎥
⎥
⎢
⎛
1 ⎞
⎥
⎢
⎢− qm (k )ω ⎜⎜1 + 4Q 2 ⎟⎟ sin (ωt ) ⎥⎝⎠⎦⎣where τ =Fig. 6. Equivalent circuit of the proposed circuit.

V. EXPERIMENTAL RESULTAt first, experimental results of the main circuit arepresented to confirm the high efficiency can be achievedin the main circuit. Then, experimental results of the
proposed circuit are presented at the second. Finally, theproposed circuit with connection in secondary side iscompared with the connection in the primary side.

A. Experimental results of the main circuit
Fig. 8 shows the load characteristics of the main circuit(input voltage: 48V). The turn ratio of the transformer inthe main circuit was 2:1. Therefore, the output voltage
becomes 12V when the input voltage of the main circuit
is 48V. However, the control method is an open-loop
control, that is, the duty cycle is set to 50%. As a result,the maximum efficiency of 94.0% is obtained at load of
96

14

94

13

92

12

90

11

88

10

(9)

86

0

50

100

150

200

Fig. 8. Load characteristics of the main circuit only.

9TABLE I
EXPERIMENTAL PARAMETERS OF SECONDARY SIDE COMPENSATION

95
94

Pout = 60W

93
92
91

Pout = 100W

B. Experimental results of the proposed circuit
Fig. 9 presents the efficiency of the proposed converter
at a constant load (cf., Table 1) when the input voltage
has fluctuation of ±25%. In order to confirm the validityof the concept, the proposed converter was tested. The
maximum efficiency of 94.0% is obtained, as shown in
Fig. 9, when the input voltage is very close to thenominal input voltage. The reason for low efficiency inthe boost mode compared to the buck mode is that thecurrent in the boost mode is increased by the circulation
current between the auxiliary circuit and the rectifier.
Consequently, the conduction loss is increased by thecirculating current.
Fig. 10 shows the input current of the transformer andthe terminal voltage of Sm2. In these modes, referencemode, boost and buck mode accordingly, the half-bridge
converter maintain at zero current switching. A switchingfrequency of approximately 200 kHz was confirmed.
Fig. 11 shows the loss analysis of the proposed
converter based on the simulated and experimental results.
In order to clarify the power loss of the proposed circuit,the loss measurement and analysis for each of the part
have been examined. When the input voltage is very
close to the nominal input voltage, the maximum
efficiency of the proposed circuit is obtained. The
auxiliary FET loss increases in both boost and buckmodes, and the loss in the main transformer and main
FET are predominant. Therefore, in order to obtain higher
efficiency, the selection of FETs in the auxiliary circuit isimportant. The low on-state resistance device should bechosen because the voltage rating of the FET can be
reduced in comparison with the main circuit. Besides, theparameters design of the transformer based on theequivalent circuit analysis are required.
C. Comparison between the connection on primary sideand secondary side
Fig. 12 shows the comparison between of the seriesconnection in the primary side and the secondary side.
Table 2 shows the experimental parameters for the
proposed circuit that is using series connection in the
primary side. The same one as the proposed circuit using
series connection on secondary side is used for the main
circuit and the rectifier. However, the wire turns of the

89
88
32

36

40

44
48
52
56
60
64
Input voltage [V]Fig. 9. Characteristics of the efficiency for the input voltage
fluctuations.

(a) Reference mode (Vin = 49 V)(b) Boost mode (Vin = 40 V)
Sm2
Voltage50[V/div]

0

Trans.1
0Input current

5[A/div]
1[ s/div](c) Buck mode (Vin = 56 V)
Fig. 10. Input current of the transformers and the terminal voltage of
Sm2 (Load: 60 W).

Loss [W]60 W, as shown in Fig. 8. It is confirmed that the highefficiency is achieved in the main circuit. It should be
noted that the synchronous rectification using MOSFET
is applied to the secondary side in order to reduce theconduction losses of the rectifier.

90

Fig. 11. Loss analysis at 60 W load.

A series type isolated DC/DC converter consisting of aresonant-type half-bridge converter and an auxiliary
converter has been proposed. The concept of series
compensation is that the output voltage is regulated byadding or subtracting the auxiliary converter voltage tothe half-bridge converter voltage. The experimental
results confirmed the highest efficiency of 94.0% is
obtained when the input voltage was closed to thenominal input voltage. It was confirmed that voltage
control can be regulated while maintaining a highefficiency. The connection in the secondary side iscompared with the connection in the primary side andshow that the connection in the primary side can obtain ahigher efficiency.
In future work, the optimal design approach will beclarified.

95
94
93

Efficiency [%]VI. CONCLUSIONSTABLE II
EXPERIMENTAL PARAMETERS OF PRIMARY SIDE COMPENSATION

92
91
90
89
88
32

36

40

44

48

52

56

60

64

(a) Pout = 60 W
95
94

Connection on
secondary side

93

Efficiency [%]transformer in the auxiliary circuit is 8:1, in order toequate the compensation circuit with the both circuit.
In both proposed circuit, the maximum efficiency of
94.0% is obtained, as shown in Fig. 12. When the input
voltage is very closed to the nominal input voltage, theefficiency of the series connection in primary side ishigher than the series connection in secondary side.
During the operation of the boost mode and buck mode,the efficiency of the series connection in the primary sideis decreased, because the switching loss in the auxiliary
circuit is increased for high input voltage. In addition, thetendency of the efficiency is different since the power
flow of each of the proposed circuit is different.
In the condition of the output voltage is lower than the
input voltage, the efficiency of the connection on primaryside is higher than the connection on secondary side has
been confirmed.

92
91

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89
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