Reduction Method of Recovery Current in a Bi-directional Chopper
Yusuke Fujita*

Satoshi Miyawaki*

Jun-ichi Itoh**Nagaoka University of Technology
Abstract:- This paper demonstrate a circuit to suppress the loss of recovery that occur in switching devices. The test circuit has structurethat the body diode of a switching device does not flow a freewheeling current. Recovery current is not generated by a freewheeling current
that flow through SiC-SBD. The two small inductors in the test circuit are coupled and compared. The experimental results confirmed thatthe test circuit achieves to suppress the loss of recovery in compared with the conventional circuit.
Keywords : Bi-directional chopper , Non-isolated , Recovery current , Coupled inductor

1.

Introduction

Recently, non-isolated bi-directional DC/DC converters areactively researched. Compact size and high efficiency of DC/DC
converters are desired for interconnecting with battery which ismounted in the EV and HEV [1-3]. The chopper circuit is usefulfor bi-directional power conversion due to simple structure of thecircuit. However, the losses incurred in the body diode of theswitching devices are matters [4].
This paper demonstrates a structure of a chopper circuit, which
consists of two choppers. SiC - Schottky Barrier diode (SBD) andsmall inductors is added to the conventional chopper circuit. The
test circuit can suppress the recovery loss due to by using
SiC-SBD with superior characteristic freewheeling current.
Therefore, the recovery current does not flow through the body
diode of switching devices. As a result, it is possible to suppress
the conduction loss of the body diode. In addition, the current
paths can be varied by using the coupled inductors.
This paper discusses that which the structure of the suppression
inductor is the best, a positive coupled, negative coupled or
non-coupled .

2.

Fig. 1. Test circuit using two choppers.

Structure of circuit and operating principle

Fig. 1 shows the bi-directional test circuit which consist ofstep-down chopper and a boost-chopper. The test circuit has twocharacteristic structures; 1) The diodes are connected in series toeach switching devices of the bi-directional chopper and 2) The
inductor is connected to each of the connecting point of theswitching device and the diode [5]. This structure can prevent a
freewheeling current that flows through the body diode of theswitching devices. A freewheeling current flows into the SiC-SBD
with superior characteristic of switching time. The body diode of
the switching device is not superior characteristic of switchingtime. Therefore, the conduction losses and the recovery losses ofthe diodes can be suppressed in comparison with the conventional
circuit.
Fig. 2 shows the typical waveforms of the test circuit. In thebuck chopper operation, there are five operating modes in one
switching period of the test circuit.
Fig. 3 shows the current-flow path of test circuit for each mode.
Iload is current that flows through L. The operating modes aredescribed as follows.
1) Mode 1 : Fig. 3 (a) shows the current-flow path in the mode 1.
In this mode, S1 is turned on. S2, D1 and D2 are turned off.
Power of VDC is transferred to the output. Further, L1 and L
store the magnetic energy from the input side.
© 2011 The Institute of Electrical Engineers of Japan.

Fig. 2. Typical waveforms of the test circuit in a buck chopper
operation.

2) Mode 2 : Fig. 3 (b) shows the current-flow path in the mode 2.
In this mode, S1 is turned off. S2 and D2 are turned off. A backelectromotive force is induced in L1 and L. As a result, acurrent keeps to flow in the same direction. Thus, a current
flows through L1, L, R, and D1.
3) Mode 3 : Fig. 3(c) shows the current-flow path in the mode 3.
In this mode, S2 is turned on. S1 and D2 are turned off. D1 isstill on because a current keeps to flow through L1. When S2 isturned on, some of the current that has flew through L1 flows
through L2. The current of L2 is smaller compared with currentof L1. It should be noted that if the on-state voltage of S2 islower than that of D1, the current flows in S2. As a result, theconduction loss can be reduced in this circuit.
4) Mode 4 : Fig. 3(d) shows the current-flow path in the mode 4.
In this mode, both of S1 and S2 are turned off. The current ofL2 flows through D2, VDC and D1.
5) Mode 5 : Fig. 3(e) shows the current-flow path in the mode 5.
1Fig. 5. Typical waveforms of the test circuit with the positive coupledinductor at buck chopper operation.

Fig. 3. Current flowing paths of test circuit at buck chopper operation.

(a)Non coupling(b)Positive coupling(c)Negative coupling

Fig. 4. The coupling direction of the inductors.

In this mode, S1, S2 and L2 are turned off. This mode is thesame as the mode2.
When the condition is changed from the mode 5 to the mode 1,the current flow D1 in the test circuit. As a result, in case of D isSBD, the recovery current is not induced in the test circuit. In theconventional circuit, D is the body diode of S2. Therefore,
recovery current occurs in the conventional circuit

3.

Coupled small inductors

The two inductors, L1, L2 can be magnetically coupled. In caseof L1 and L2 are magnetically coupled, current flowing paths aredifferent from the Fig. 3.
Fig. 4 shows the three kinds of the coupled inductor. Fig.4 (a)shows non-coupled inductors. Fig. 4 (b) shows positive coupledinductor which a mutual inductance M is positive Fig. 4 (c) shows
negative coupled inductor which M is negative.
Fig. 5 shows the typical waveforms and Fig.6 shows thecurrent-flow path of test circuit when the positive coupledinductor.
Fig. 7 shows the typical waveforms and Fig. 8 shows thecurrent-flow path of test circuit when the negative coupledinductor.
In the buck chopper operation when the positive coupledinductor, there are five operating modes in one switching period.
The operating modes are described as follows.
1) Mode 1 : Fig. 6(a) shows the current-flow path in the mode 1.
In this mode, S1 is turned on. S2 and D1 are turned off. The
power of VDC is transferred to the output. Further, both of L1and L store the magnetic energy. Furthermore, a backelectromotive force is generated in L2 because L1 and L2 aremagnetically coupled. As a result, the current flows throughD2 to S1.
2) Mode 2 : Fig. 6(b) shows the current-flow path in the mode 2.

Fig. 6. Current flowing paths of test circuit with positive coupledinductor at buck chopper operation.

In this mode, S1 keeps turn on. The current of L1 is the steadystate. Therefore, the back electromotive force is not generatedin L2. As a result, the current does not flow through L2.
3) Mode 3 : Fig. 6(c) shows the current-flow path in the mode 3.
In this mode, S1 is turned off. The current of L1 keeps to flowin the same direction due to the back electromotive force.
Therefore, D1 is turned on. Then the current of L1 decreases.
Therefore the back electromotive force is generated in L1 andL2. As a result, the current flows from the L1 and L2 to the L.
Therefore, the current flows through the body diode of S 2.
4) Mode 4 : Fig. 6(d) shows the current-flow path in the mode 4.
In this mode, S2 is turned on. The current flows through thesame paths of the mode 3. However, the current flow throughon resistance instead of the body diode of S2. The current of
L2 gradually decreases.
5) Mode 5 : Fig. 6(e) shows the current-flow path in the mode 5.
In this mode, S2 is turned off. The current flows through thesame paths of the mode 3. The current of L2 becomes almostzero.
In the buck chopper operation when the negative coupled2Fig. 7. Typical waveforms of the test circuit with negative coupledinductor at buck chopper operation.

inductors, there are six operating mode in one switching period.
The operating modes are described as follows.
1) Mode 1 : Fig. 8(a) shows the current-flow path in the mode 1.
In this mode, S1 is turned on. S2 and D1 are turned off. The
power of VDC is transferred to the output. Further, L1 and L
store the magnetic energy. Furthermore the back electromotive
force is generated in L2 because L1 and L2 are magneticallycoupled. As a result, the current flows through D2 to S1. Thisbehavior is the same as the mode when the positive coupledinductors. However, larger current is generated. This cause isthat the inductance of L2 is increased by the mutualinductance.
2) Mode 2 : Fig. 8(b) shows the current-flow path in mode 2. In
this mode, S1 keeps turn on. The current of L1 is the steadystate. Therefore, the back electromotive force does not occur
to the L2. As a result, the current does not flow through L2.
3) Mode 3 : Fig. 8(c) shows the current-flow path in the mode 3.
In this mode, S1 is turned off. The current of L1 keeps to flowin the same direction due to the back electromotive force.
Therefore, D1 is turned on. In addition, the current of L1
decreases. Therefore the back electromotive force is generatedin L1 and L2. As a result, the current flows through the L1 andL2 to the load. Therefore, the current flows through the body
diode of S2.
4) Mode 4 : Fig. 8(d) shows the current-flow path in the mode 4.
In this mode, S2 is turned on. The current flows through thesame paths of the mode 3. However, the current flow throughon-resistance instead of the body diode of S2. The current ofL2 gradually increases.
5) Mode 5 : Fig. 8(e) shows the current-flow path in the mode 5.
In this mode, all of the stored energy of L1 is discharged. As aresult, the current of D1 becomes zero. However, the current
keeps to flow through S2 to L2.
6) Mode 6 : Fig. 8(f) shows the current-flow path in the mode 6.
In this mode, S2 is turned off. The current keeps to flow in thesame direction. Therefore, the current flows through the body
diode of S2.
In the case that these coupled inductor is used, the advantage ofthe test circuit that ”the current does not flow thorough the body
diode of switching device” diminish due to couple with the smallinductors.

Fig. 8. Current flowing paths of test circuit with negative coupledinductor at buck chopper operation.

Input voltage

Table 1. Experimental parameters.
50 (V)
Duty

50 (%)

Switching frequency

300 (kHz)

Dead time120 (ns)

MOSFET

H5N2508DS

Diodes

IDH06SG60C

L

270 (H)

C100 (F)R

33.6 ()Table 2. Parameters of inductors.
L1[H]
L2[H]
Non coupling
12.4
12.6
(as shown in Fig. 4(a))
Positive coupling
12.2
12.2
(as shown in Fig. 4(b))
Negative coupling
11.5
12.0
(as shown in Fig. 4(c))

M[H]
11.1
-10.6

4. Experimental results
The validity of the test circuit was confirmed by experiments.
The waveforms are compared between the conventional circuitand the test circuit. The conventional buck chopper is operated byconnecting the midpoint of the switching device and the diode in
the test circuit. Table 1 shows the experimental parameters. Table
2 shows the parameters of small inductors and the coupledinductor.
Fig. 9 shows the experimental result of the conventional buckchopper circuit when S1 is turned on. In this condition, therecovery current is generated in the body diode of S2 because of areverse voltage. The theoretical value of the reverse recovery time
dif/dt is 100A/s. This value is given from the data sheet of theMOSFET which is used in the experiments. From Fig. 9, it isconfirmed a forward current of S2 is 0.8A and the reverse recovery
time is 16ns. Therefore, measured reverse recovery time is 50
A/s that is twice the theoretical value. This reason is that anincreasing of the current is slowly because the switching device isclosely connected to the inductor.
Fig. 10 shows the experimental result of the test circuit at thebuck chopper operation. From Fig. 10, it is confirmed the recovery
current does not occur. However, the high frequency oscillation
occurs when S1 is turned on. The cause of this oscillation is a3

Fig. 9. Experimental result of conventional circuit.

Fig. 11. Experimental result of the test circuit at buck chopper operation.

Fig. 10. Experimental result of test circuit at buck chopper operation.

Fig. 12. Experimental result of the test circuit with positive coupledinductor at buck chopper operation.

resonance with the parasitic capacitance of the switch and wiring
inductance of the circuit. This cause is proved by calculating thefrequency with parameters of the circuit.
Fig. 11 shows the experimental result of the current of S1 and D1
in the condition that is the same as Fig. 10. The resonance occurs
in the current of D1 when the current path is changed from D1 to
S1. However, the peak value of the resonance current of the test
circuit is small compared with the conventional circuit. Therefore,
the loss can be reduced. The experimental results confirmed thatthe test circuit could suppress the recovery current.
Fig. 12 and Fig. 13 shows the experimental results of the test
circuit with a positive coupled inductor at the buck chopper
operation. In use of a positive coupled inductor, the experimental
result is similar to the result that the non-coupled inductor is used.
In addition, the current that flows through the body diode of S2 islower than the conventional circuit. Therefore, the recovery
current does not occur as shown in Fig. 13. From theseexperimental results, in the positive coupled inductor, the test
circuit can suppress the recovery current as with the case that thenon-coupled inductor is used. Therefore, the volume of theinductor can be reduced compared with the case that thenon-coupled inductor is used.
Fig. 14 shows the experimental result of the test circuit with thenegative coupled inductor at the buck chopper operation. In thenegative coupled inductor, the current of S1 is so large. Therefore,
the current could achieve the maximum rated current of the device.
The experimental result confirmed that the test circuit with thenegative coupled inductor is not suitable.

Fig. 13. Experimental result of test circuit with positive coupledinductor at buck chopper operation.

Fig. 14. Experimental result of the test circuit with negative coupledinductor at buck chopper operation.

References
[1]

[2]

5.

Conclusion

In order to suppress the recovery current the body diode ofswitching devices, the test circuit is demonstrated at thebi-directional chopper by experiments. The experimental result
confirmed the basic operation of the test circuit. In addition, it
confirmed the test circuit can suppress the recovery current
compared with the conventional circuit. Further, the volume of theinductor can be reduced by using the positive coupled inductor.
In future work, the optimum design of inductors will bediscussed.

[3]

[4]

[5]

4

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