S2PC 2017, Changhua, Taiwan

Dead-time Compensation Method for Dual Active Bridge
Converter with Three-level Operation
Jun-ichi Itoh , Kengo Kawauchi , Hayato HigaNagaoka University of Technology
This paper proposes a compensation method of a transferred power error for a dual active bridge (DAB) converter. At light load, theerror of the transferred power occurs due to dead time, which is conventionally compensated by using feedforward control. However, thedead-time error due to neutral point clamping cannot be compensated by the conventional method because the error is varied depending onthe width of the neutral point clamping. In the proposed method, the neutral point clamping can be avoided by using three-level operation.
Thus, the feedforward control compensation can still be applied to correct the transferred power error. The validity of the proposed method
is confirmed by a 2.5 kW prototype. As experimental results, the error of the transferred power is reduced from 101% to 2.84%.
Keywords Dual active bridge converter, Dead-time compensation, Three-level operation

1. Introduction
In recent years, energy storage systems for electric vehicles andsmart grid system have been actively researched [1]. In the energy
storage system, a bidirectional operation and a galvanic isolation
are required. In order to satisfy these requirements, a bi-directional
isolated DC-DC converter is necessary in the energy storage
system. In general, dual active bridge (DAB) converters aregenerally employed. The DAB converters can achieve zerovoltage switching (ZVS) without any additional component.
However, at light load, the error of the transferred power occurs
due to neutral point clamping which is caused by dead time [2].
In order to reduce the error of the transferred power, thedead-time compensation has been proposed by using afeedforward control [3]. However, the dead-time error due to theneutral point clamping cannot be compensated by the conventional
method because the transferred power error is varied depending on
the width of the neutral point clamping.
This paper proposes a transferred-power error-compensation
method at light load for the DAB converter by applying thethree-level voltage including a zero-voltage period, i.e. three-level
operation. By applying the three-level operation, the neutral pointclamping is avoided because the zero-voltage period andphase-shift angle can be controlled. In order to confirm thevalidity of the proposed method, experiments with a 2.5-kW
prototype are conducted.

2.

Circuit structure and operation principle

Fig. 1 shows circuit configuration of the DAB converter. The
primary and secondary inverters output the square-wave voltage or
the three-level voltage including the zero-voltage period, which iscategorized as two-level operation and three-level operation. Notethat the three-level operation is then divided into two suboperation mode.
Fig. 2 shows the operation waveform of the two-level operation
with/without the dead time. In Fig. 2(a) the transferred power iscontrolled by the phase-shift angle  between the output voltagesof two inverters [3]. When the dead time is introduced, thetransferred power error occurs because the neutral point isclamped due to the dead time.
Fig. 3 shows the operation waveform of two modes ofthree-level operation with/without the dead time. Fig. 3 (a) and (b)show two modes of the three-level operation without the dead time.
In particular, both operation modes of the three-level operationequalize the length of both inverters output zero voltage period and . By using two modes of three-level operation, the transferredCorrespondence to: Jun-ichi Itoh. E-mail: itoh@vos.nagaokaut.ac.jp,Nagaoka University of Technology
1603-1, Kamitomioka-machi, Nagaoka, Niigata, Japan, 940-2188

NL
Vin

vse

vpr

ipr

Vout

Fig. 1. Dual active bridge converter.
dt Neutral point clampVin,Vout0

vpr-Vin,-Voutvse

vse

ipr

vprq

ipr

0q
(a) Without dead-time.

(b) With dead-time.

Fig. 2. Operation waveform with 2-level operation.
Vin,Vout0

vpr2-Vin,-Vout0dt

vse

ipr

vpr2

0q

vpr

vse

2

(c) Three-level operation
mode1with dead-time.

dt

dt

vpr

vse

q2

-Vin,-Vout0q

ipr(a) Three-level operation
mode1 without dead-time.
Vin,Vout

dt

vse

ipr
(b) Three-level operation
mode2 without dead-time.

iprq(d) Three-level operation
mode2 with dead-time.

Fig. 3. Operation waveform with 3-level operation.
power error compensation can be applied over wide load range.
The neutral point clamp can be avoided because the three-level
operation can adjust the zero voltage period and the phase-shift.
Fig.3 (c) and (d) show two modes of the three-level operation withthe dead time. The transferred power error of the two-level
operation is caused by the neutral point clamping. In contrast, thetransferred power error of the three-level operation is not caused
the neutral point clamping, but simply by the dead time voltageerror. Hence, the transferred power error of three-level operation
can be compensated by the conventional dead-time compensation.
Fig. 4 shows a flowchart to determine operation modes. It
should be noted that the flowchart is based on the condition ofVin=Vout. Firstly, the phase-shift angle  is calculated by therelationship between the phase-shift angle  and the transferredpower Pref [1]. The operation mode is decided by the phase-shift
angle and the dead time dt. Under the condition of ref >2dt, theS2PC 2017, Changhua, Taiwan
Table 1. Experimental parameters.

two-level operation is applied. In contrast, the three-level
operation is used in the condition of ref ≤2dt. The three-level
operation with mode 1 is applied when the zero-voltage periodsatisfies (1).
ε  π / 4  1 / 2δdt
(1)



100 V

Leakage inductance

Output voltage

100 VTransformer turn ratio

1Rated power

2.3 kW

Dead time

3.2sSwitching frequency

Then, the zero-voltage period  is given by (2) in order tooutput the reference transferred power Pref.
12ε   π  2πωLPref / Vin (2)2The transferred power Pref is determined only by the zero-voltage
period  because the phase-shift  does not affect the transferredpower P.
At the condition that (1) is not established, the DAB is operatedby the three-level operation with mode 2. In this mode, the zerovoltage period  is calculated by (3) in order to the output
reference transferred power Pref.
12ε  2π  δ  2πωLPref / δVin(3)4Note that the phase-shift in (3) is defined as a constant inorder to reduce a degree of freedom. By using two types of thethree-level modes, the transferred power error still occurs but it iscaused by only the dead time voltage error. Equations in (4)indicate the reference values considering the dead-time voltage
error compensation.

24 H

Input voltage

20 kHzStart

INPUT
Pref calculation

VinVout

No2-level mode2dt>
3-level mode

fsw
L

Yes

Cal. of ,, : Eq.(2)dtNo≥/41/2dt
3-levelmode 1

CircuitSpecifications

3-levelmode 2

Cal. of ,, : Eq.(3)Yes

Cal. of ref, ref, refDead-time comp:Eq(4)ref=

FinishFig. 4. Flowchart of determination of operation mode.
Output voltage of primary inverter 250V/div

δref  δco n st  δd t / 2

Output voltage of secondary inverter 250V/div

εref  ε  δd t / 2(4)γref  ε

P=0.61 kW, Pref =1.45kW, Error=58.2 %Note that when mode 1 is applied,const is /2 rad. Otherwise,
when the mode 2 is applied,const is /4 rad.

Inductor current of primary inverter 50A/div

3.

Experimental results

20s/div(a) With two-level operation.

Table 1 shows the experimental parameters. A 2.5 kW prototypeof the DAB converter is tested in order to confirm validity of thetransferred power compensation using the three-level operation.
Fig. 5 shows the operation waveforms with the two-level
operation and the three-level operation. By applying thethree-level operation, the neutral point clamping is avoided. In Fig.
5(a) and (b), the error of the transferred power is reduced from
58.2% to 6.18%.
Fig. 6 shows the characteristics of the transferred power against
the reference transferred power. When using only two-level
operation, the transferred power at light load becomes 0 W
because the phase reference  is smaller than the dead time dt. By
applying the three-level operation, the transferred power error atlight load is reduced compared with that of two-level operation.
However, the maximum error of 11.3% of the three-level
operation in the boundary between the two-level operation and thethree-level operation because the period of the dead time is longfor mode 2. The operation mode at the changing point of twomodes is derived in future work.

Conclusion and future work

This paper proposed the transferred power error compensation
method for the DAB converter with three-level operation. In theproposed method, the error of the transferred power is reduced
from 101% to 2.84%.
In future work, the operation mode at the changing point
between the two-level and the three-level operation will beconsidered.
References[1] F. Krismer, IEEE Trans. PELS., Vol. 27, No.1, (2012)[2] B. Zhao, et al: IEEE Trans. PELS, Vol.29, No.4, (2014)[3] D. Segaran, et al: IEEE Trans. PELS, Vol.28, No.1, (2012)

Output voltage of secondary inverter 250V/div

P=1.36 kW, Pref =1.45 kW,Error=6.18 %Inductor current of primary inverter 50A/div
20s/div(b) With three-level operation.
Fig. 5. Experimental waveform of two-level operation andthree-level operation.
Three-level mode 1

1

Transferred power P [p.u.]

4.

Output voltage of primary inverter 250V/div

1p.u.=2.3kW

0.8
0.6

Three-level mode 2
Pref = P

Proposed method
error=2.84%

0.4

Two-level mode withdead-time effect

0.20
Two-level mode max.error=101%
-0.2

00.2
0.4
0.6
Reference transferred power Pref [p.u.]

Fig. 6.

0.8

Characteristics of transferred power.

1