S2PC 2017, Changhua, Taiwan

Achievement of Zero Voltage Switching at Light Load for ParallellyConnected Dual Active Bridge Converter using Power-Circulating Operation
Hayato Higa1, Jun-ichi Itoh 1
1/Nagaoka University of Technology,This paper proposes a power-circulating operation at light load for a parallelly-connected Dual-Active-Bridge (DAB) converter in order toachieve zero voltage switching (ZVS) over entire load range. At the light load, a power flow of each DAB converter is controlled in such away that the transferred power of each DAB converter satisfies the condition for ZVS, i.e. the power-circulating operation. Consequently,
MOSFET with low voltage rating can be used due to no surge voltage in ZVS. At the heavy load, the power-circulating operation is no longer
required due to the high power flow of each DAB converter. The heavy load efficiency is not only improved but also the light load is operated
without the recovery surge. From experimental results, the converter efficiency is over 96% from the rated power of 14.4% to the rated powerat the nominal voltage. At 75% of the nominal voltage, the maximum efficiency of 97.6% is obtained.
Keywords Dual active bridge converter, Zero voltage switching, Parallel operation

1. Introduction
Recently, energy storage systems have been actively installed toDC micro-grid systems [1]. a dual active bridge (DAB) converter isgenerally applied in such systems [2]. The DAB converter canobtain the high efficiency because zero voltage switching (ZVS) isachieved without additional components. ZVS not only reduces theturn-on loss but also reduces the surge voltage which occurs in therecovery mode of the body diode into MOSFET. However, the ZVS
range is limited when the voltage fluctuates widely [2]. Due to thehard switching operation, the high voltage rating of MOSFET
which has higher on-state resistance has to be selected.
In order to extend ZVS range, Pulse width modulation isemployed with the phase-shift control [3]. However, it is not always
possible to achieve ZVS over entire load range.
In this paper, the power-circulating operation of a parallellyconnected DAB converter is proposed in order to achieve ZVS
range over entire load range with the power-circulating operation
the power flow of each DAB converter is changed in such a waythat ZVS for all DAB converters is achieved. In addition, the low
voltage rating of MOSFET can be selected because no surge voltage
occurs in ZVS. In addition, the power flow of each converter ischanged depending on the output power and the voltage condition.
Finally, the experimental results is conducued by using A 1.6-kW
prototype in order to validity of the proposed method.

2.

HV side 380 V
Cds_HighCorrespondence to: Hayato Higa, e-mail: hhiga@stn.nagaokaut.ac.jp

Vout

L N1:N2Vin

i1_HV
v1_HV

Cds_LV

i1_LV v1_LV

Lm
N=N1
N2

Cell No.2
Cell No.m

Fig. 1. Circuit configuration of parrallelly-connected DAB converter.

Pin

P1

Cell No.1 DAB
Charge operation

P2

Cell No.2 DAB
Charge operation

Pout(a) Parallel operation

Pin

Cell No.1 DAB
Charge operation

P1

Pout

Cell No.2 DAB
Not operate(b) Single operation

Pin

P1

Cell No.1 DAB
Charge operation
Cell No.2 DAB
Discharge operation

Proposed power flow control

Fig. 1 shows the circuit configuration of the parallellyconnected DAB converter. By employing the parallel connection tothe DAB converter, the conduction loss and the copper loss are
reduced in compared to those when only one DAB converter isapplied. However, the ZVS range is limited when the battery
voltage fluctuates from the nominal voltage [3].
Fig. 2 shows the power flow diagrams of the parallellyconnected DAB converter corresponding to the output power. Fig.
2(a) shows the parallel operation at heavy load where ZVS is simply
achieved due to the high power flowing through each converter. Fig.
2(b) shows the single operation in which, only one DAB converter
is operated under the ZVS condition. Fig. 2(c) shows the power-LV side(Battery) 36 V~48 V

Cell No.1

PoutP2(c) Power-circulating operationFig. 2.

Power flow diagrams of parrallelly-connected DAB converter.

circulating operation applied at light load. In the power-circulating
operation, the power flow of each DAB converter is changed. The
transferred power of each DAB converter P1 and P2 are designed tosatisfy the condition for the ZVS achievement. If the parallel
number of the converter is changed with the power-circulating
operation, only two DAB converters are operated at light load
because the transferred power P1 will have to be increased if thenumber of the active converters increases. In other words, at lightload, the No. 3~No. m DAB converter is inactive. In the power-flow
control, the operation mode is changed depending on therelationship between the reference transferred power Pref and theS2PC 2017, Changhua, Taiwan

lower limit of the transferred power for ZVS PZVS. The conditions
based on Pref and PZVS. At the condition of PZVS>Pref, ZVS is notachieved in each individual DAB converter, i.e. the parallel
operation or the single operation. Therefore, the power-circulating
operation is used. The transferred power of each DAB converter P1and P2 are adjusted to be larger than PZVS. As a result, the transferred
power of each DAB converter P1 and P2 are calculated by (1).

P2   PZVSP2  0(2)In the condition of 2PZVS ≤ Pref <Prated, ZVS is achieved in eachindividual DAB converter. Therefore, the parallel operation isactive. In the parallel operation, the transferred power of each DABconverter P1 and P2 are calculated by (3).

P1  P2  Pref / 200i1_HV 5 A/div(1)

In the condition of PZVS ≤ Pref <2PZVS, ZVS is achieved if only one
DAB converter is active, i.e. the single operation. Therefore, onlyone DAB converter is operated. The other is operated that zerovoltage is output in each inverter. In the single operation, thetransferred power of each DAB converter P1 and P2 are calculatedby (2).

P1  Prefv1_LV 100 V/div00i2_HV 5 A/div

4 ms/div(a) Single operation (b) Parallel operation (c) Power-circulating operation
Fig. 3 Operation waveforms with each mode.
100

All load operation is achieved

95

Efficiency [%]P1  Pref  PZVS

v1_HV 500 V/div

Parallel operation
Maximum eff. :97.5%

90Single operation

85

Power-circulating operation

80
75(3)

00.4

0.8
1.2
Output power [kW]1.6(a) Vin=380 V, Vout=48 V (Nominal voltage)

Experimental Results

Fig. 3 shows the operation waveforms with three operations at
the input voltage of 380 V and the output voltage of 36 V. Fig. 3 (a)shows the operation waveforms of the parallel operation at theoutput power of 1048 W, Fig. 3 (b) shows the operation waveformsof the single operation at the output power of 542 W, and Fig. 3 (c)shows the operation waveforms of the power-circulating operation
at 268 W, i.e. the achievement of ZVS in each individual DAB
converter. As shown in Fig. 3 (a), the parallel operation is confirmed
at the same power flow. In Fig. 3(b), the single operation is
confirmed because only one DAB converter is operated. Besides,the power-circulating operation is obtained because each individualDAB converter has difference power flow direction. By using thepower-circulating operation, the low power at the light load is notflown in each individual DAB converter. As a result, ZVS isachieved at light load.
Fig. 4 shows the efficiency characteristics of two parallellyconnected DAB converter. Fig. 4(a) shows the efficiency
characteristics when the output voltage is 48 V and the input voltage
is 380 V, i.e. the nominal voltage. Fig. 4 (b) shows the efficiency
characteristics when the output voltage is 36 V and the input voltage
is 380 V. In Fig. 4(a), the converter efficiency at medium load isimproved by using the single operation because the iron loss
becomes zero with the halting the operation of the other DAB
converter. By using the power-flow control corresponding to theoutput power, the converter efficiency is over 96.0% from the lightload of 14.4% to the rated load. In Fig. 4(b), the maximum
efficiency of 97.6% is achieved. However, the converter efficiency
at the light load becomes lower by the power-circulating operation
because the conduction loss and the copper loss are increased.

4.

Conclusion

This paper proposed the power-circulating operation for theAll load operation is achieved
Single operation

10095

Efficiency [%]3.

Parallel operationSurge voltage is over 90 VMaximum eff. :97.6%9085

Power-circulating operation

80
75

00.4

0.8
1.2
Output power [kW]1.6(b) Vin=380 V, Vout=36 V (75% nominal voltage)
Fig. 4. Efficiency characteristics of two parallelly-connected DAB
converter with fluctuation of output voltage.

parallelly-connected DAB converter with the aiming of achieving
ZVS over entire load range and wide battery-voltage variation. In
the proposed method, the power flow and the transferred power
were changed depending on the output power in order to operate all
DAB converters at the ZVS condition. In experimental results, theconverter efficiency is over 96.0% from 14.4% of the light load tothe rated load by changing the power flow of each individual DAB
converter.
References[1] Meiqin Mao, etc. "Accurate Output Power Control of Converters for
Microgrids Based on Local Measurement and Unified Control", IEEJ Journal ofIndustry Applications, Vol. 4, No. 4, pp.331-338, 2015.
[2]R. W. D. Doncker, etc. "A three-phase soft-switched high-power-density dc/dc
converter for high-power applications", IEEE Trans on IAS, Vol, 27, No. 1, pp. 6373, 1991.
[3] Yoichi Ishizuka, etc, "Static Characteristic Analysis of Proposed Bi-Directional
Dual Active Bridge DC-DC Converter", IEEJ Journal of Industry Applications,Vol.4,No. 5, pp.602-610, 2015.