Experimental Investigation and Loss Calculation 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 for a bi-directional isolated
DC/DC converter using series voltage compensation. The
proposed converter consists of a high efficiency resonance fullbridge converter and a series converter. The proposed circuit
regulates the output voltage by using the series converter, which
provides only the differential voltage between the input voltage
and the output voltage that is closed to the nominal voltage,
since only the resonance converter will operate. In this paper,
four types of the auxiliary circuits are investigated using the loss
calculation. The relationship between the loss element andefficiency characteristics is clarified. The validity of the
proposed circuit was confirmed by experimental results, whichconverts 48-V to 380-V at 1kW, with a maximum efficiency of
95% at 1 kW at the nominal input voltage region.

In the power grid system, series voltage compensation isused in order to compensate the grid voltage[18][19].Furthermore, the non-isolated DC-DC converters
using series voltage compensation have also proposed anddiscussed on the voltage fluctuation [20][21]. In this system,
the auxiliary converter is connected in series to the power
supply. As a result, the power rating of the converter is low
and high efficiency is achievable.

Recently, high efficiency and high power density bidirectional isolated DC/DC converters are increasingly desiredin the application of power grid system that found in the smart
grid system and electric vehicles. Examples of the applicationsof the DC/DC converters are such as the secondary battery
conversion for photovoltaic cell and also power conditioner
for fuel cell. In addition, these power converters have tocontrol a wide range of output voltage [1-10].

The authors previously have proposed the isolated DC/DC
converter using the series voltage compensation [22][23]. The
input voltage fluctuation is compensated by the auxiliary
circuit that outputs only the fluctuation of the output voltage.
One of the advantages of the proposed circuit is that highefficiency can be achieved when the input voltage is closed to
the nominal input voltage because the power of the auxiliary
circuit becomes very small in comparison to the input power.
Normally, the large fluctuations do not occur in the input or
the output voltage for long periods, and as a result, theconverter loss can be reduced. However, the previous works
have only discussed on single direction of s energy flow.
Bidirectional flow of energy in the proposed
DC/DC
converter, including the effectiveness of the auxiliary circuit
has not been evaluated.

The series resonant type full-bridge converters, which use
a resonance between the leakage inductance of transformers
and the series resonant capacitor, are one of the most effective
circuit topologies to achieve high efficiency. However, it isdifficult to regulate the output voltage in a wide range while
there is a fluctuation in the input voltage because theswitching timing is constrained by the resonance period.
Although the voltage is controllable by using the series
resonant converter, If it has a wide voltage control range, thisconverter has some problem, such as increasing reflux current
and difficult design of a transformer [11-14].Therefore, aseries resonant type full-bridge converter is connected to a
voltage control converter, such as a step down chopper [1517]. As a result, the converter loss will increase because all thepower passes through two converters.

This paper discusses and evaluates the losses of the bidirectional isolated DC/DC converter using series voltage
compensation. In order to achieve the high efficiency, fourtypes of the auxiliary circuits are investigated and it isnecessary to clarify the procedure of optimize design insubjects to the efficiency characteristics. The first section ofthis paper introduces the principle of the proposed series
compensation method. Secondary, expression for theconverter loss is established in order to estimate the power
loss. The point of the expression for the converter loss is toobserve the losses of a converter based on the device
conditions. Thirdly, the proposed calculation method that isused to evaluate the power loss is confirmed with theexperimental results. Finally, experimental results demonstrate
the advantages of the proposed converter.

I.

INTRODUCTIONII.

CIRCUIT CONFIGURATIONS

A. Conventional Circuit
Fig. 1 shows the power flow diagram of the conventional
converter, which is composed form a series connection ofseries resonant converter and voltage control converter, suchas a step down chopper [15-17]. The series resonant converter
operates at constant duty cycle based on the resonancefrequency f0, and does not control the output voltage. The
fluctuation of the input voltage is constantly controlled by the
chopper circuit. The output side is isolated by the resonant
converter to the input side. This system has two stages ofpower flow from the input to the output. The total efficiencyof the conventional circuit ηc is obtained from Eq.(1), using
both the resonant converter efficiency ηr and the buck chopper
efficiency ηchop.

η c = η rη chopFig. 1. Power flow diagram of the conventional circuit using a chopper
circuit and a resonant converter.

(1)

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 voltage isolation. In order toachieve the high efficiency, the resonant converter operates atconstant duty cycle based on the resonance frequency f0. In
addition, the auxiliary circuit is used to regulate the output
voltage. The output voltage of the auxiliary circuit is added tothe output of the main circuit by a transformer. Therefore, theauxiliary circuit compensates only the fluctuation voltage,against the output voltage commands.
The proposed system consists of two power converters,
which are connected in series to the output side. The reason
for high efficiency is as follows. The power Pout is obtained by(a) Primary compensation.

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

adding the auxiliary converter power Pa to the directly power
Pr. The total converter efficiency of the proposed circuit ηp is
obtained by Eq.(2), using the auxiliary circuit efficiency ηaand the power ratio k=Pa/Pr.

ηp =η r + kη a
1+ k

Therefore, the total efficiency ηp of the proposed circuit ishigher than the conventional circuit where the efficiency canbe expressed by Eq. (3). It should be noted that the series
efficiency of the resonant converter in the conventional circuit
is similar to the proposed 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.

(2)(a) Step down chopper + Full-bridge.

(b) Full-bridge.

Fig. 5. Voltage waveforms of the proposed circuit.

(3)from the forward voltage drop of a switching device,respectively. It should be noted that the following expression
is referred to the case of Fig. 3(a).

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

A. FET Loss in Main Circuit
The resonant current of the proposed circuit is a sinusoidal
waveform, provided that the resonant parameter of the
proposed circuit is optimally designed [18][19]. Therefore, thepower loss per primary FET in the main circuit Ploss_FET_main1
can be obtained by Eq. (5) using the on-resistance of the FET
Ron_FET_main1. Thus, the power loss per secondary FET in themain circuit Ploss_FET_main2 can be obtained by Eq. (6) using theon-resistance of the FET Ron_FET_main2.

η r + kη a
1+ k

f0 =

> η rη chop

1
2π LC

(4)

Moreover, 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 other
hand, Fig. 3(b) shows a circuit configuration that the auxiliary
circuit connects to the secondary side.
Fig. 4 shows the circuit configuration of the auxiliary
circuit. Fig. 5 shows the voltage waveforms when theauxiliary circuit is connected with the primary side.

Ploss _ FET _ main1 =Ploss _ FET _ main2 =

π2
8

π2
8

Ron _ FET _ main1 I 1

2

(5)

2

(6)

Ron _ FET _ main 2 I 2

B. FET Loss in Auxiliary Circuit I
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 Eq. (7) using the onresistance of the chopper FET Ron_chop and the turn ratio of thetransformer in the auxiliary circuit, which is β=Na1/Na2.

Fig. 4(a) shows the circuit configuration of the series
2connection of the step down chopper and full-bridge converter.
(7)
Ploss _ FET _ chop _ con = R on _ chop (β I 1 )In this circuit, the switching timing of the auxiliary circuit issynchronous with the main circuit. Then, the output voltage is
In addition, the switching loss of the FET is proportional
controlled by changing the DC link voltage between the steptotheapplied voltage and current. Therefore, the switching
down chopper and full bridge converter. The high efficiency
lossof
the chopper Ploss_FET_chop_sw can be obtained from Eq.
can be achieved by using the high speed switching devices for(8).
the step down chopper and low on-resistance switching
devices for the full-bridge converter. However, the number of(8)
Ploss _ FET _ chop _ sw = V1 (β I 1 )(e on + eoff ) f sw
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. EXPLESSIONS FOR POWER LOSS
This section explains the power loss expression of the
proposed circuit. The power loss of the proposed circuit isseparated into the main circuit loss and auxiliary circuit loss.
In the main circuit, the switching loss is ideally zero, because
the switching devices can achieve ZCS. Therefore, the power
loss of the main circuit consists of the conduction loss andconstant loss. The conduction loss is generated in the onresistance of the FET, wire resistance of the transformer, andthe equivalent series resistance of the capacitor. The constantloss is generated within the iron-loss of the transformer, andno-load loss. On the other hand, the power loss of the auxiliary
circuit consists of the conduction loss, constant loss, andswitching loss which are generated at the switching period,

where eon[J] and eoff[J] are the turn-on and turn-off energy
at per switching, and fsw is the switching frequency. In the fullbridge converter, the switching loss is ideally zero since thethe FET achieves ZCS. Therefore, the power loss per FET in
the full bridge Ploss_FET_FB can be obtained by Eq. (9) using theon-resistance of the FET Ron_FET_FB.
Ploss _ FET _ FB =π2
8

Ron _ FB (β I 1 )2(9)

C. FET Loss in Auxiliary Circuit II
In the auxiliary circuit of Fig. 4(b), the conduction loss canbe expressed as Eq. (9), similarly to the case of Fig. 4(a). The
switching loss of the full-bridge converter is obtained by Eq.
(10) due to the sinusoidal waveform.

Ploss _ FET _ FB _ sw =π2

(βI1 )V1 (eon + eoff ) f sw sin⎧⎨π (1 − f swton )⎫⎬
⎩2

⎭
(10)

where ton is the compensation time per one cycle of theauxiliary circuit.

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

D. Capacitor Loss
The capacitor loss is calculated based on the capacitor
current during the charge or discharge, equivalent series
resistance of the capacitor. The resonant capacitor loss
Ploss_C_reso is obtained by Eq. (11) using the equivalent series
resistance RC_reso.

π2

Ploss _ C _ reso =8

RC _ reso I 12

(11)

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 Eq. (12) using
the equivalent series resistance RC_out.
⎧π 2⎫− 1⎬ R C _ out I 22
Ploss _ C _ out = ⎨
8
⎩
⎭

(12)

E. Transformer Loss
The power loss of transformer consists of the iron loss andcopper loss. The iron loss is occurring because of the fluxchange 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
Eq. (13).
Bac =VN 1 ⋅ t on
2 N1 Ac(13)

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 Eq. (14) using the characteristics of the flux
density - core loss value Pcv[W/m3] and the effective volumeof the core Ve[m3].
Ptrans _ fe = Pcv V e(14)

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 Eq. (15)using the primary winding resistance of the main circuit withskin effect RN1_main. The secondary winding copper loss
Ploss_Nm2 in the main circuit is obtained by Eq. (16) using the
primary winding resistance of the main circuit with skin effect
RN2_main. The transformer loss of the auxiliary circuit isobtained similarly.

Ploss _ Nm1 =Ploss _ Nm 2 =

π2
8

π

2

8

RN 1 _ main I 12

RN 2 _ main I 22

(15)

(16)

F. Series inductor loss
The power loss of series resonant inductor consists of theiron loss and copper loss as well as the transformer loss. The
copper loss is obtained from the winding resistance similar to
the Eq. (15) and Eq. (16).
In the iron loss, the series inductor voltage is obtained at
first. Then, the flux density is obtained by Eq. (13) as well as
the transformer, and iron loss is calculated from a core loss
value. The peak value of the series inductor voltage is
obtained by Eq. (17).

VL _ peak = 2πf 0 L ⋅
IV.

π
2

I1

(17)

EFFICIENCY ESTIMATION BY LOSS CALCULATION

A. Efficiency Estimation of proposed converter
Four types of the auxiliary circuits shown in Fig. 3
and Fig. 4 are investigated using the loss calculation. The
circuit parameters are shown in Table 1. The rating power is 1
kW. The primary voltage has a fluctuation of 48 V ± 25%, andthe secondary voltage has a fluctuation of 380 V ± 25%. Fig.
6 demonstrates the efficiency of the proposed converter at aconstant load and a constant secondary voltage 380 V when
the primary voltage has a fluctuation. On the other hands, Fig.
7 presents the efficiency at a constant load and a constantprimary voltage 48 V when the secondary voltage isfluctuating.
From the calculation results, a maximum efficiency ofover 94.0% is obtained in all cases. In addition, the highefficiency in a wide range can be achieved when the voltage
fluctuation is compensated from the voltage changing side (InFig. 6(a) and (c), the primary side voltage changes while thesecondary side voltage is constant. In Fig. 7(b) and (d), thesecondary side voltage changes while the primary side voltage
is constant.). The efficiency is decreased when the voltage
fluctuation is compensated from the side that the voltage isunchanged side (Fig. 6(b), (d) and Fig. 7(a), (c)) the efficieny
is decreased due to the difference in the power flow at the
boost and buck mode. When the efficiency is decreased, thecirculation current between the auxiliary circuit and the main
circuit is increased. Therefore, the loss of the power circuit isfurther decreased when it connects the auxiliary circuit that
regulates with larger voltage fluctuation.
Moreover, when the configuration of the auxiliary circuitcomposed from the chopper and full-bridge, the conduction
loss will be increased. Therefore, to obtain the high efficiency,
the auxiliary circuit is connected with a high voltage side. On

96

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Efficiency [%]Efficiency [%]

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36 40 44 48 52 56 60

86

36 40 44 48 52 56 60

86

36 40 44 48 52 56 60

86

36 40 44 48 52 56 60(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. 6. Calculation results of the primary voltage compensation (V1=48V±25%, V2=380V).

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280 320 360 400 440 480

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280 320 360 400 440 480

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280 320 360 400 440 480

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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. 7. Calculation results of the secondary voltage compensation (V1=48V, V2=380V±25%).

the other hands, when the auxiliary circuit configuration is
only composed by a full-bridge, the switching loss will be
increased. Therefore, the auxiliary circuit is connected at the
low voltage side in order to obtain high efficiency.
Therefore, in order to achieve the high efficiency, thecircuit configuration must fulfill the following conditions.
(1) It connects the auxiliary circuit to the side that has thelarge voltage fluctuation side(2) The auxiliary circuit which connects chopper and fullbridge is connected to the high voltage side, and the auxiliary
circuit which only composed by a full-bridge is connected to
the low voltage side.
B. Determination of Circuit Configuration
Assuming that the primary side is connected to a side thathas large fluctuation voltage such as the battery (48 V) and thesecondary side is low fluctuation range like the DC bus
voltage (380 V).
Then, the power loss can be reduced by connecting theauxiliary circuit to the primary side, because the primary side
has large fluctuation voltage. In addition, the auxiliary circuit
configuration is a full-bridge circuit only since the primaryside has low voltage.
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 has alarge fluctuation. In addition, to obtain high efficiency, theauxiliary circuit is connected to the low voltage side.

V. EXPRIMENTAL RESULTS
The proposed converter was tested for validation. It is
assumed that the primary side is connected to the battery (48V ± 25%), and the secondary side is connected to the DC bus(constant 380 V).
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.
A. Experimental results of the main circuit
Fig. 8 shows the load characteristics of the main circuit. In
this experiment, the auxiliary circuit is not operated. The
power flows from a primary side to the 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 closed to 48V as shownin Fig. 8. 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 95.5% is obtained
at load of 650 W, as shown in Fig. 8. It is confirmed that thehigh efficiency is achieved in the main circuit.
B. Experimental results of the proposed circuit
Fig. 9 presents the efficiency of the proposed converter at
a constant load and constant secondary voltage 380 V when
the primary voltage has fluctuation of ±25%. The maximum
efficiency of 95.5% is obtained, when the primary voltage isclosed to the nominal primary voltage (48 V). In both boost

and step down modes, it was confirmed that the voltage
control can be regulated while maintaining high efficiency.

97

54

Fig. 10 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). Also, the full-bridge converter iscontrolled to achieve ZCS.In the boost and buck modes Also,
the full-bridge converter is controlled to achieve ZCS. The
switching frequency is approximately 100 kHz was confirmed.

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95

50

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48

Fig. 11 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.

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44

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C. Converter Loss breakdown
Fig. 12 presents the breakdown of the prototype losses.
The iron loss in the main transformer is predominant. In orderto clarify the power loss of the proposed circuit, the loss
measurement and analysis for each of the part have been
examined using expression in chapter 3. Additionally, thepower loss of the main transformer is predominant, it accountfor 15% of the total loss. The auxiliary FET loss increases inboth boost and buck modes. Therefore, in order to obtainhigher efficiency, it is necessary to optimize the transformer
design, and 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 reducedin comparison with the main circuit.
VI.

90

0

200

400

600

800

1000

40
1200

Fig. 8. Experimental results of the load characteristics
only the main circuit.
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CONCLUSION

This paper proposed a bi-directional isolated DC/DC
converter using series voltage compensation which consistingof a series resonant type full-bridge converter and an auxiliary
converter. The concept of the series compensation is that thechanging side voltage is regulated by adding or subtracting theauxiliary converter voltage to the series resonant type fullbridge voltage.
Four types of the proposed circuit are investigated andevaluated in term of losses. The relationship between the loss
elements and efficiency characteristics are clarified. The
expressions for the power loss are established and evaluated.
The validity of the loss calculation was confirmed
theoretically with a efficiency of over 94% with wide voltage
control range at 1 kW.
The experimental results confirmed that the maximum
efficiency of 94.5% at 1 kW 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. From the loss analysis, it can
notice that, the power loss of the main transformer ispredominant.
In future work, the converter efficiency will be improvedby the optimizing the design, the converter efficiency will be

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36

40

44

48

52

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60

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60

(a) Primary to Secondary.

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36

40

44

48

52

(b) Secondary to Primary.

Fig. 9. Experimental results of the primary voltage compensation of the
proposed circuit.

evaluated with a higher conversion capacity, also evaluatedwith other circuit configuration patterns.

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

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

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

Primary circuit AC voltage : vm1(c) Step down mode (V1 = 60V).

Fig. 10. Waveforms of the main transformer primary current and theterminal voltage of Sm2 (Pout=960W).

50[V/div]0

Auxiliary circuit AC voltage : (Na1/Na2)va
050[V/div]
Secondary circuit AC voltage : vm2
500[V/div]0

2[ s/div](b) Boost mode (V1 = 36V).

Fig. 12. Loss partition results of a prototype.

(c) Step down mode (V1 = 60V).

Fig. 11. AC voltage waveforms of the each circuit (Pout=960W).

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