Comparing investigation fora Bi-directional Isolated DC/DC Converter
using Series Voltage Compensation
Satoshi Miyawaki

Jun-ichi Itoh

Kazuki Iwaya

Nagaoka University of Technology
Niigata, Japan
miyawaki@stn.nagaokaut.ac.jp

Nagaoka University of Technology
Niigata, Japan
itoh@vos.nagaokaut.ac.jp

TDK-LAMBDA, Ltd
Niigata, Japank.iwaya@jp.tdk-lambda.com

Abstract -- This paper discusses a bi-directional isolated DC/DC
converter using series compensation method. The proposed
converter consists of a high efficiency resonance full-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, which is closed to the nominal voltage. In this paper,
four types of the auxiliary circuits are investigated in term ofloss. The relationship between the loss element and efficiency
characteristics is clarified. The validity of the proposed circuit isconfirmed by the loss calculation, with a maximum efficiency ofover 94% at 2kW. Moreover, the experimental results
confirmed that the proposed circuit, which converts 48-V to 380V at 1kW, achieves the maximum efficiency of 93.0% at thenominal input voltage region.

I.

INTRODUCTION

the input voltage is closed to the nominal input voltage
because the power of the auxiliary circuit becomes very smallin comparison to the input power. Normally, the large
fluctuations occur in the input or output voltage is for certain
short periods, and as a result, the converter loss can be reduced.
This paper discusses and evaluates the losses of the
proposed circuit. In order to achieve the high efficiency, fourtypes of the auxiliary circuits are investigated and it isnecessary to clarify the optimization design procedure in termsof the efficiency characteristics. The mathematical expression
is first established to estimate the power loss. The point of themathematical expression is to observe the losses of a converter
based on the study of device conditions. The first section ofthis paper introduces the principle of the proposed series
compensation method. Secondary, mathematical expression isestablished in order to estimate the power loss. Thirdly, theproposed calculation method to evaluate the power loss isconfirmed with the experimental results. Finally, someexperimental results are given in order to demonstrate theadvantages of the proposed converter.

Recently, high efficiency and high power density for thebi-directional isolated DC/DC converters are increasingly
desired for smart grid system. The applications of the DC/DC
converters are such as the secondary battery conversion andpower conditioner for photovoltaic cell or fuel cell. In addition,these power converters have to control a wide range of output
voltage [1-9].
A.
Current resonant type full-bridge converters, which use theresonance between the leakage inductance of transformers andthe resonant capacitor, are one of the most effective circuit
topologies to obtain high efficiency. However, it is difficult toregulate the output voltage in a wide range while there is afluctuation in the input voltage because the switching timing isconstrained by the resonance period. Therefore, a current
resonant type full-bridge converter is generally connected to a
voltage control converter, such as a buck chopper [10-11]. As
a result, the converter loss will increase because all the power
passes through two converters.
This paper proposes a new topology for a bi-directional
isolated DC/DC converter using the series voltage
compensation [12]. The input voltage fluctuation iscompensated by the auxiliary circuit that outputs only thefluctuation of the output voltage. One of the advantages of the
proposed circuit is that high efficiency can be achieved whenII.

CIRCUIT CONFIGURATIONSConventional Circuit
Fig. 1 shows the power flow diagram of the conventional
converter. The conventional converter consists of a resonant
converter and the voltage control converter, such as a chopper
circuit. The fluctuation of the input voltage is constantly
controlled by the chopper. The output side is isolated by theresonant converter. This system has two stages of power flowfrom the input to the output. The total efficiency of theconventional circuit c is obtained from (1), using both theresonant converter efficiency r and the buck chopper
efficiency chop.
c  rchop As a result, all the power passes through both converters
despites of the relation with the input voltages; therefore, theconverter efficiency is reduced.

B. Proposed Circuit
Fig. 2 shows the power flow diagram of the proposed
converter. In the proposed circuit, the resonant converter isused in the main circuit for the voltage isolation. In order toobtain the maximum efficiency, the resonant converter
operates at a constant duty cycle based on the resonancefrequency f0. Then, the auxiliary circuit is used to regulate theoutput voltage. The output voltage of the auxiliary circuit isadded to the output of the main circuit by a transformer. Thatis, the auxiliary circuit compensates only the fluctuation
voltage, against the output voltage commands.

Fig. 1. Power flow diagram of the conventional circuit using a chopper
circuit and a resonant converter.

The proposed system consists of two power converters,
which are connected in series to the output side. The following
explains the proposed circuit can achieve high efficiency. The
power Pout is obtained by adding the auxiliary converter power
Pa to the directly power Pr. The total converter efficiency ofthe proposed circuit p is obtained by (2), using the auxiliary
circuit efficiency a and the power ratio k=Pa/Pr.

p 



 r  k a
1 k





Therefore, the total efficiency p of the proposed circuit ishigher than the conventional circuit where the efficiency canbe expressed by (3).

 r  k a



1 k

  r chop Fig. 3 shows the proposed circuit using the series voltage
compensation. The main circuit is controlled under an
optimum condition similar to the resonant converter. In orderto obtain the maximum efficiency, the resonant converter
operates at 50% duty cycle based on the resonance frequency
f0. Therefore, the main circuit can achieve zero current
switching (ZCS) at any time. The resonance frequency of theMain Circuit

I1
Sm1

Sm3

V1

C

Sm4

Auxiliary
Circuitproposed circuit f0 is obtained by (4) using the resonantcapacitor C and the resonant inductance L that is containing
the leakage inductance of the transformers.

f0 



L

Nm1 : Nm2

vm2

V2

vm1

2 LC

Main Circuit

I1
Sm3

V1

C

L N



I2

m1 : Nm2

vm2
Sm4

V2

vm1

va

va



Fig. 4 shows the circuit configuration of the auxiliary
circuit. Fig. 5 shows the voltage waveforms when the

Sm2(a) Primary compensation.

1

In addition, the auxiliary circuit can be connected to either
primary side or secondary side of the main circuit as shown in
Fig. 3. Fig. 3(a) shows the circuit configuration that theauxiliary circuit connects to the primary side. On the otherhands, Fig. 3(b) shows a circuit configuration that theauxiliary circuit connects to the secondary side.

Sm1

vm
Sm2

I2

Fig. 2. Power flow diagram of the isolated DC/DC converter using theseries voltage compensation.

Auxiliary
Circuit(b) Secondary compensation.
Fig. 3. Circuit diagrams of the proposed circuit.

(a) Step down chopper + Full bridge.
(b) Full bridge.
Fig. 4. Circuit diagrams of the auxiliary circuit.

(a) Step down chopper + Full bridge.
(b) Full bridge.
Fig. 5. Voltage waveforms of the proposed circuit.



auxiliary circuit is connected with the primary side.
Fig. 4(a) shows the circuit configuration which is the seriesconnection of the step down chopper and full-bridge converter.
In this circuit, the switching timing of the auxiliary circuit issynchronous with the main circuit. Then, the output voltage iscontrolled by changing the DC link voltage A between thestep down chopper and full bridge converter. The highefficiency can be achieved by using the switching device with
fast switching time for the step down chopper and low onresistance for the full-bridge converter. However, the numberof the switching device is increased.
Fig. 4(b) shows the full-bridge converter is used for theauxiliary circuit. In this circuit, the auxiliary circuit can output
three different voltage levels; +V, -V, and zero voltage. The
output voltage is controlled by changing the pulse width D of
the auxiliary circuit. The number of the switching device isdecreased. However, the switching loss of the MOSFET isincreased, and the control becomes complicated.
III.

This chapter explains the power loss expression of the
proposed circuit. The power loss of the proposed circuit isseparated into the main circuit side and auxiliary circuit. In themain circuit, the switching loss is ideally zero, because themain circuit FET can achieve ZCS periodically. Therefore, thepower loss of the main circuit consists of the conduction loss
and constant loss. The conduction loss is generated from theon-resistance of the FET, wire resistance of the transformer,
and the equivalent series resistance of the capacitor. Likewise,
the constant loss is generated within the iron-loss of thetransformer, and no-load loss. On the other hand, the power
loss of the auxiliary circuit consists of the conduction loss,
constant loss, and switching loss, which are generated at theswitching period, by the forward voltage drop of a switching
device, respectively. It should be noted that the following
mathematical expression is for the case of Fig. 3(a).
The resonant current of the proposed circuit is a sinusoidal
waveform, provided that the resonant parameter of the
proposed circuit is optimally designed. Therefore, the power
loss per primary FET in the main circuit Ploss_FET_main1 can beobtained by (5) using the on-resistance of the FET Ron_FET_main1.
Ploss _ FET _ main1 



2

8

Ron _ FET _ main1 I 1 
2The power loss of the auxiliary circuit FET consists of the
conduction loss and switching loss. In the auxiliary circuit ofFig. 4(a), the conduction loss of the chopper FET
Ploss_FET_chop_con can be obtained by (6) using the on-resistanceof the chopper FET Ron_chop and the turn ratio of thetransformer in the auxiliary circuit, which is =Na1/Na2.
Ploss _ FET _ chop _ con  R on _ chop  I 1  
2In addition, the switching loss of the FET is proportional
to the applied voltage and current. Therefore, the switchingloss of the chopper Ploss_FET_chop_sw is obtained by (7).



where eon[J] and eoff[J] are the turn-on and turn-off energy
at per switching, fsw is the switching frequency. In the fullbridge converter, the switching loss is ideally zero, because
the FET achieves ZCS. Therefore, the power loss per FET in
the full bridge Ploss_FET_FB can be obtained by (8) using the onresistance of the FET Ron_FET_FB.
Ploss _ FET _ chop _ sw  V1  I 1 eon  e off  f sw In the auxiliary circuit of Fig. 4(b), the conduction loss canbe requested by expression (7) as well as the case of Fig. 4(a).
The switching loss of the full-bridge converter is obtained by(9).
 Ploss _ FET _ FB _ sw 

MATHMATICAL EXPLESSIONS OF POWER LOSS

Ploss _ FET _ chop _ sw  V1  I 1 eon  e off  f sw 2

I1 V1 eon  eoff  f sw sin 1  f swton  
2






where ton is the compensation time per one cycle of theauxiliary circuit.
The power loss of transformer consists of the iron loss andcopper loss. The iron loss occurs because of the flux changes
in the core. The copper loss is occurring in the winding
resistance of transformer. At first, when the square wave isinputted to the transformer, the flux density Bac is obtained by
(10).


Bac 

VN 1  t on
2 N1 Ac

where VN1 is the primary voltage of the transformer, N1 is
the primary wire turns, Ac[m2] is the effective cross-section of
the core. Therefore, the iron loss of the transformer Ptrans_fe is
obtained by (11) using the characteristics of the flux density core loss value Pcv[W/m3] and the effective volume of the coreVe[m3].


Ptrans _ fe  Pcv V e 

The copper loss of the transformer is obtained from thewinding resistance with skin effect. The primary winding
copper loss Ploss_Nm1 in the main circuit is obtained by (12)using the primary winding resistance of the main circuit withskin effect RN1_main. The transformer loss of the auxiliary
circuit is obtained similarly.


Ploss _ Nm1 

2
8

RN 1 _ main I 12 The capacitor loss is obtained by the charge or discharge
current of the capacitor and the equivalent series resistance ofthe capacitor. The resonant capacitor loss Ploss_C_reso isobtained by (13) using the equivalent series resistance RC_reso.



Ploss _ C _ reso 28

RC _ reso I 12 



98
96

The current based on the mean value of the sine wave ischarged or discharged to the output capacitor. Therefore, theoutput capacitor loss Ploss_C_out is obtained by (14) using theequivalent series resistance RC_out.
 2Ploss _ C _ out   1 RC _ out I 22 
8



92
90
88



IV. POWER LOSS CALICULATION
At first, the validity of the proposed calculation results is
confirmed with the experimental results from the small typep
capacity prototype. The circuit configuration is a combinationof the full-bridge converter with the secondary sidecompensation. (Fig. 3(b) and Fig. 4(b)).
Fig. 6 presents the efficiency of the proposed converter at
a constant load when the primary voltage has a fluctuation of±25%. The secondary voltage is 12 V, the switching(resonant) frequency is 210 kHz, and the turn ratio of the main
circuit transformer is N1:N2=4:2. Based on the input voltage
characteristics, a maximum efficiency of 96.0% is obtained,
when the input voltage is closed to the nominal input voltage.
It should be noted that the synchronous rectification is applied
to the secondary side in order to reduce the conduction lossesof the rectifier. In addition, the experimental results and thecalculation results are almost equivalent, especially, when thecompensate voltage is small. On the other hand, when thecompensate voltage is larger, the error margin of the loss
becomes approximately 15% larger.
Fig. 7 shows the loss partition at Pout =100W andcontrolled by three different input voltage. From the result, theauxiliary FET loss increases in both the boost and buck modes,
and the loss in the main circuit transformer are predominated.

86
84

36

40

44

48

52

56

60

Fig. 6. Comparison between the experimental results and thecalculation results by the prototype with small capacity.
The circuit configuration is a combination of the full-bridge converter(Fig. 3(b)) with the output side compensation (Fig. 4(b)).

Loss [%]



94

Fig. 7. Loss partition results of the calculation by the prototype
with small capacity (Pout=100W)

Fig. 8 shows the primary current of the main transformer
and the terminal voltage of Sm2. In both boost and step down
mode accordingly, the main circuit is confirmed achieves zerocurrent switching.
In the next section, four types of the auxiliary circuits
shown in Fig. 3 and Fig. 4 are investigated and evaluated The
circuit parameters shown in Table 1 are used for calculation.
The rating load is 2kW. The primary voltage has a fluctuationof 48V±25%, and the secondary voltage has a fluctuation of
380V±25%. Each of the primary side and secondary sidecompensate voltage by using the auxiliary circuit.

(a) Boost mode (V1 = 40V).

It should be noted that the voltage and the current value
used for the calculation are ideal. Additionally, the resonant
inductance, interconnection loss, and no-load loss are notincluded in the calculation loss.
(b) Step down mode (V1 = 56 V).
Fig. 8. Waveforms of the main transformer primary current and theterminal voltage of Sm2 by the prototype with small capacity (Pout=100W).

98

96

96

96

96

94
92
90

94
92
90

36 40 44 48 52 56 60

94
92
90

36 40 44 48 52 56 60

Efficiency [%]98

Efficiency [%]98

Efficiency [%]Efficiency [%]

98

94
92
90

36 40 44 48 52 56 60

36 40 44 48 52 56 60

98

98

96

96

96

96

94
92
90

280 320 360 400 440 480

94
92
90

280 320 360 400 440 480

Efficiency [%]98

Efficiency [%]98

Efficiency [%]Efficiency [%](a) Primary compensation.
(b) Secondary compensation.
(c) Primary compensation(d) Secondary compensation.
Step down chopper + Full bridge. Step down chopper + Full bridge.
Full bridge.
Full bridge.
Fig. 9. Calculation results of the primary voltage compensation (V1=48V±25%, V2=380V).

94
92
90

280 320 360 400 440 480

94
92
90

280 320 360 400 440 480(a) Primary compensation.
(b) Secondary compensation.
(c) Primary compensation(d) Secondary compensation.
Step down chopper + Full bridge. Step down chopper + Full bridge.
Full bridge.
Full bridge.
Fig. 10. Calculation results of the secondary voltage compensation (V1=48V, V2=380V±25%).

250

250

250

250

200

200

200

200

150

150

150

150

100

100

100

100

50

50

50

50

0

0

0

0

(a) Primary compensation. (b) Secondary compensation. (c) Primary compensation (d) Secondary compensation.
Step down chopper + FB.
Step down chopper + FB.
Full bridge.
Full bridge.
Fig. 11. Power loss partition of the primary voltage compensation (V1=48V±25%, V2=380V, P=2000W).

250
250

250

250

250

200
200

200

200

200

150
150

150

150

150

100
100

100

100

100

50
50

50

50

50

00

0

0

0

(a) Primary compensation. (b) Secondary compensation. (c) Primary compensation (d) Secondary compensation.
Step down chopper + FB.
Step down chopper + FB.
Full bridge.
Full bridge.
Fig. 12. Power loss partition of the secondary voltage compensation (V1=48V, V2=380V±25%, P=2000W).

Table 1. Specification parameters of the power loss calculation
for bi-directional isolated DC/DC converter.

The experimental circuit configuration is a combination of
the full-bridge converter with the primary side compensation.
(Fig. 3(a) and Fig. 4(b)). In this case, the battery voltage haslarge fluctuation. In addition, to obtain high efficiency, theauxiliary circuit is connected to the low voltage side.
At first, the experimental results of the main circuit arepresented in order to confirm the high efficiency can beachieved in the main circuit. Then, experimental results of the
proposed circuit are presented at the second.

From the loss calculation results, a maximum efficiency of
94.0 % is obtained in all the cases. Moreover, the highefficiency in a wide range can be achieved when the voltage
fluctuation is compensated from the voltage changing side (ex.
From the primary side when the primary side voltage changes(Fig. 9(a) and (c)), from the secondary side when thesecondary side voltage changes (Fig. 9(b) and (d))). This isbecause, the efficiency reduces due to the difference in the
power flow at the boost mode and buck mode. When theefficiency is decreased, the circulation current between theauxiliary circuit and the main circuit is increased. The
efficiency is decreased when the voltage fluctuation iscompensated from the voltage unchanging side (Fig. 9(b), (d)and Fig. 10(a), (c)). Therefore, the loss of the power circuit
can be decreased when it connects the auxiliary circuit withthe side of large voltage fluctuation side.
For example, the primary side is consisting of the large
fluctuation voltage like the battery and the secondary side is
low fluctuation range like the DC bus voltage, then, the power
loss can be reduced by connecting the auxiliary circuit to theprimary side.
Moreover, when the auxiliary circuit configuration is
chopper + full-bridge, the conduction loss is increased.
Therefore, to obtain the high efficiency, the auxiliary circuit isconnected with a high voltage side. On the other hands, when
the auxiliary circuit configuration is only full-bridge, theswitching loss is increased. Therefore, to obtain highefficiency, the auxiliary circuit is connected at the low voltage
side.

96
54
95

52
50

94

48

93

46
92

Primary voltage [V]In addition, Fig. 11 shows the power loss partition of theconditions in Fig. 8 (V1= 48V ± 25%, V2 = 380V). Similarly,
Fig. 12 shows the power loss partition on the conditions of Fig.
9 (V1 =48V, V2= 380V± 25%) .

A. Experimental results of the main circuit
Fig. 12 shows the load characteristics of the main circuit.
In this experiment, the auxiliary circuit is not operated. The
power flow is from a primary side to a secondary side. The
turn ratio of the transformer in the main circuit is 1:8.
Therefore, the secondary voltage becomes 380V when the
primary voltage of the main circuit is nearby 48V as shown in
Fig. 12. However, the control method is an open-loop control,

Efficiency [%]Fig. 9 presents the efficiency of the proposed converter at
a constant load and a constant secondary voltage 48 V when
the primary voltage has a fluctuation. Fig. 10 presents theefficiency of the proposed converter at a constant load and aconstant primary voltage 380 V when the secondary voltage
has a fluctuation.

44
91
90

42
0

200

400

600

800

1000

40
1200

Fig. 12. Experimental results of the load characteristics only the main
circuit (The auxiliary circuit is not operated).

96
95
94
93
92

V.

EXPRIMENTAL RESULTS
The proposed converter was tested in the prototype toconfirm the validity. It is considered that the proposed
converter transfers energy between from battery to the DC bus.
It is assumed that the primary side is connected to the battery.
The battery voltage is 48 V, the one has fluctuation of ±25%.
And the secondary side is connected to the DC bus. The DC
bus voltage is constant 380V.

91
90

36

40

44

48

52

56

60

Fig. 13. Experimental results of the primary voltage compensation of
the proposed circuit.

(a) Maximum efficiency condition (V1 = 48V).
(a) Maximum efficiency condition (V1 = 48V).

(b) Boost mode (V1 = 36V).

(b) Boost mode (V1 = 36V).

(c) Step down mode (V1 = 60V).
Fig. 14. Waveforms of the main transformer primary current and theterminal voltage of Sm2 (Pout=960W).

that is, the duty cycle is set to 50%.
As a result, the maximum efficiency of 94.5% is obtained
at load of 650 W, as shown in Fig. 12. It is confirmed that thehigh efficiency is achieved in the main circuit.
B. Experimental results of the proposed circuit
Fig. 13 presents the efficiency of the proposed converter
at a constant load (380 V) when the primary voltage has
fluctuation of ±25%. In order to confirm the validity of theconcept, the proposed converter was tested. The maximum
efficiency of 94.5% is obtained, as shown in Fig. 13, when the
primary voltage is closed to the nominal primary voltage (48V). In both boost and step down modes, it was confirmed thatthe voltage control can be regulated while maintaining highefficiency.
Fig. 14 presents the primary current of the maintransformer and the terminal voltage of Sm2. In the following
orders, the maximum efficiency condition (The auxiliary
circuit is not operated), and the boost and buck modes Also,
the full-bridge converter is controlled to achieve ZCS. A
switching frequency is approximately 100 kHz was confirmed.

(c) Step down mode (V1 = 60V).
Fig. 15. AC voltage waveforms of the each circuit (Pout=960W).

Fig. 15 presents the AC voltage waveforms of the eachcircuit. In both boost and step down modes, the auxiliary
circuit outputs three different voltage levels; +V1, -V2, and zerovoltage. When the input voltage is decreased (Fig. 14(b)), theauxiliary circuit outputs a positive phase voltage to the main
circuit without preventing the resonance operation of the main
converter. Similarly, when the input voltage is increased (Fig.
14(c)), the auxiliary circuit outputs a negative phase voltage.
C. Comparing of the calculation and experimental results
Since the experimental efficiency is lower than the
expected efficiency, which from the calculation, as shown in
Fig. 13 and Fig. 9(c). Therefore, in order to consider thecauses of the difference between experimental results andexpected results, the power loss of the experimental results
was separated in detail.

From the loss partition, the iron loss in the maintransformer is predominant. This is because, the maintransformer is evenly divided, and it is constructed by using
two cores to increase the high boost up ratio. In the power loss
calculation, only one core is considered for the maintransformer. Additionally, the power loss of the resonant
inductance is comprised 15% of the total loss. The resonant
inductance loss is not included in the power loss calculation. If
converter designs so that these losses become small, theefficiency of the proposed converter can be improved.
VI.

CONCLUSION

A series type bi-directional isolated DC/DC converter
consisting of a current resonant type full-bridge converter andan auxiliary converter has been proposed. The concept of theseries compensation is that the changing side voltage isregulated by adding or subtracting the auxiliary converter
voltage to the current resonant type full-bridge voltage.
Four types of the auxiliary circuits are investigated andevaluated in term of losses. The relationship between the loss
element and efficiency characteristics is clarified. The
mathematical expression is established. The validity of the
proposed circuit was confirmed by loss calculation, with amaximum efficiency of over 94% at 2kW.
The experimental results confirmed the maximum
efficiency of 94.5% at 1kW is obtained when the primaryvoltage was closed to the nominal primary voltage. It was
confirmed that the voltage control can be regulated while
maintaining high efficiency. In addition, the experimental
efficiency was lower than the expected efficiency wasclarified by the loss separation.
In future work, the converter efficiency will be improvedby the optimizing the design, the converter efficiency will be
evaluated with a higher conversion capacity, and other circuit
configuration patterns will be evaluated.
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change, the error margin of the power loss becomesapproximately 10%.

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