A Bi-directional Z-source Inverter for Electric Vehicles

Makoto Yamanaka and Hirotaka Koizumi Tokyo University of Science

1-14-6 Kudankita, Chiyoda-ku Tokyo 102-0073 Japan Email: hosukenigou@ieee.org littlespring@ieee.org

AbstractIn this paper, a novel power converter for electric vehicles (EVs) is proposed. The proposed converter has a Z-source inverter which drives an ac motor, and performs as a current-fed Z-source dc-dc converter against reverse power flow. Thus, the proposed circuit does not need a bi-directional buck-boost dc-dc converter. The operation of the proposed converter was simulated by MATLAB SIMULINK and tested by circuit experiments for each operation as a Z-source inverter and a current-fed Z-source dc-dc converter.

Keywords-Z-source inverter; bi-directional; electric vehicles;

I. INTRODUCTION EVs, which dont exhaust CO2, are different from

conventional internal combustion engine powered cars. From the view point of the environmental issues, EVs are strongly expected to be put into practical use [1]. For EVs, power converters are needed to drive motors. The power converter is composed of an inverter to make a boosted ac voltage for a motor from a dc voltage of a battery, and a buck dc-dc converter to charge the battery during reverse power flowing [2]. A Z-source inverter, which is shown in Fig. 1, is a kind of inverter invented by F. Z. Peng [3]. By using shoot-through switching, the Z-source inverter can boost the output voltage without a boost converter. However a dc-dc converter is needed to accept a reverse power flow and to reduce the regenerative voltage to a battery voltage. The Z-source inverter includes a Z-network which is an X-shaped combination of two capacitors and two inductors. On the other hand, a current-fed Z-source dc-dc converter, which is shown in Fig. 2, has been also proposed [4]. The current-fed Z-source dc-dc converter performs as a buck converter when the duty ratio is over 50%, and as a polarity reversed buck-boost converter when the duty ratio is under 50%.

This paper proposes a novel Z-source inverter for EVs. The proposed circuit has one Z-network which works as a Z-source inverter in the case of driving a 3 motor and as a current-fed Z-source dc-dc converter in the case of reverse power flow. Therefore the proposed circuit does not need an external bi-directional buck-boost dc-dc converter compared with conventional power converters for EVs. The operational principle of a conventional Z-source inverter, a current-fed Z-source dc-dc converter and the proposed circuit are described in section II. Experimental circuits and results of the proposed

circuit in the case of Z-source inverter mode and current-fed Z-source dc-dc converter mode are described in section III.

II. A BI-DIRECTIONAL Z-SOURCE INVERTER FOR ELECTRIC VEHICLES

A. A conventional Z-source inverter A conventional Z-source inverter is shown in Fig. 1 and

the switching scheme of the Z-source inverter is shown in Fig. 3. Fig. 3 shows a carrier waveform es, reference waveforms eA, eB, eC, shoot-through lines e1, e2, and driving waveforms S1~S6. In the case that es is higher than e1 and es is lower than e2, all driving waveforms S1~S6 are high. In this time, inverter legs are in short circuited state called shoot-through. The Z-source inverter takes two states which are the shoot-through state and the inverter drive state. The two equivalent circuits of the Z-source inverter are shown in Fig 4. In shoot-through state, double capacitor voltage 2VC impresses the circuit. Therefore, the Z-source inverter can boost the output voltage. The voltage gain of the Z-source inverter can be expressed as:

II

DA L1

L2

LA

LB

LC

RA

RB

RC

CC

CB

CA

S1

S2

S3

S4

S5

S6

D6

D5 D3

D4

D1

D2

C2 C1

VI

vA

vB

vC

Fig. 1. Z-source inverter.

C1 C2

L2

L1

DC SC

DA

SA C0 RI VI VG

LG

Fig. 2. Current-fed Z-source dc-dc converter.

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212I

xV

MMv

= (1)

where xv is the output peak phase voltage (x = A, B, or C), M is the modulation index, and VI is the input voltage [3].

B. A current-fed Z-source dc-dc converter A current-fed Z-source dc-dc converter is shown in Fig. 2.

A current-fed Z-source dc-dc converter is also construct of a Z-network. The switch SA and SC are driven by pulse width modulation (PWM). The current-fed Z-source dc-dc converter is divided into two states which are SA:ON, SC:OFF and SA:OFF, SC:ON. The equivalent circuits of the current-fed Z-

source dc-dc converter are shown in Fig. 5. The voltage gain of the current-fed Z-source dc-dc converter can be expressed as:

GI VDDV = 12 (2)

where VG is the input voltage, D is the duty ratio of SA, and VI is the output voltage. Equation (2) implies that the current-fed Z-source converter performs as a buck converter when the duty ratio is over 0.5, and as a polarity reversed buck-boost converter when the duty ratio is under 0.5. When the duty ratio is 0.5, the output voltage unlimitedly approaches 0 [4].

C. A bi-directional Z-source inverter for electric vehicles Fig. 6 shows a bi-directional Z-source inverter for electric

vehicles. Compared to the conventional Z-source inverter shown in Fig. 1, the input diode DA in Fig. 1 is replaced to a bidirectional switch composed of two IGBTs SA, SB and two diodes DA, DB in Fig. 6. A smoothing capacitor C0 is set in parallel to the battery VI. Also a switch SC with an antiparallel diode DC is placed between the Z network and the 3 bridge. Fig. 7(a) shows an equivalent circuit when the proposed

Fig. 3. Switching scheme of a Z-source inverter.

vL

C1 VC C2 VC

L2

L1 vL

vL

vO VI C1 VC C2

L2 vL

VC vO

L1

VI

DA DA

(a) (b)

Fig. 4. Equivalent circuits of a Z-source inverter, (a) inverter drive state, (b) shoot-through state.

L1

L2

C2 C1

SB

DB

C0 VI SC DC

S1 D1

D2 S2

S3

S4

S5

S6 D6

D5

D4

D3

SA

DA 3 MOTOR

Fig. 6. A bi-directional Z-source inverter for electric vehicles.

L1

L2

C2 C1 DA

C0 VI

S1 D1

D2

S2

S3

S4

S5

S6 D6

D5

D4

D3 3 MOTOR

Power flow

DB

SA:OFF SB:ON

SC:OFF DC

(a)

VI

IL

C1 C2

L2

L1

SC

SA

RI

VI

IL

iC iC

IG

VG C1 C2

L2

L1

SC

SA

VG RI

IL

IL

iC iC

IG

(a) (b)

Fig. 5. Equivalent circuits of a current-fed Z-source dc-dc converter, (a)SA:ON, SC:OFF, (b)SA:OFF, SC:OFF.

L1

L2

C2 C1 DA

C0 VI

S1 D1

D2

S2

S3

S4

S5

S6 D6

D5

D4

D3 3 MOTOR

Power flow

DB

SA:active SB:OFF

SC: active

DC

(b) Fig. 7. Equivalent circuits when the proposed converter performs as, (a)

Z-source inverter, and (b) current-fed Z-source dc-dc converter.

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converter performs as a Z-source inverter loaded with a 3 motor. Fig. 7(b) shows an equivalent circuit when the proposed converter performs as a current-fed Z-source dc-dc converter which charges the battery by the reverse current flow from the 3 motor.

In the case of the Z-source inverter mode (Fig. 7(a)), the state of switches is SA:OFF, SB:ON, SC:OFF, and the state of diodes is DA:active, DB:OFF DC:OFF, where the active state of DA means DA works as an input diode of Z-source inverter. The 3 bridge is driven by PWM driving signals including the shoot-through switching states.

In the case of the current-fed Z-source dc-dc converter mode (Fig. 7(b)), the state of switches is SA:active, SB:OFF, SC:active, and the state of the diodes is DA:active DB:active DC:active. SA and SC are driven by PWM, and the diodes block the current according to the switching states of SA and SB. Assuming that the 3 motor performs as a current source during reverse power flow, the 3 bridge performs as a 3 full bridge rectifier composed of the diodes D1-D6. The reverse current is fed to the Z-network. In the case of the current-fed Z-source dc-dc converter mode, when the charge voltage is lower than the battery voltage, the unintended current flows from the battery to the motor. To prevent the unintended current flow, the switch SB maintains OFF state. Therefore, if the charge voltage is lower than the battery voltage, the unintended current prevented by the diode DB.

III. EXPERIMENTAL RESULTS A proposed circuit was built and tested in the cases of

the Z-source inverter mode and the current-fed Z-source dc-dc converter mode. The proposed circuit was simulated by MATLAB SIMULINK and tested by an experimental circuit. Experimental circuits are shown in Fig. 8. In both cases, the designed value of the circuit elements, inductors L1 = L2 = 1[mH], capacitors C1 = C2 = 147[F], and C0 = 100[F], switching frequency = 30[kHz] were given. The MOSFETs IRF510 were used for the switches in the circuit experiment. The internal diode forward voltage = 2.5[V] of the MOSFET was set in the simulation. The diodes SR340 were used in the circuit experiment. The forward voltage = 0.5[V] of the diode was set in the simulation.

In the Z-source inverter mode, a voltage source VI = 10[V], and the modulation index M = 0.8 were given. Maximum boost control [5] was used. The voltage gain of the maximum boost control can be expressed as:

233I

xV

MMv

=

(3)

where xv is the output peak phase voltage (x = a, b, or c). The inverter was loaded with a 3 RLC network instead of a 3 motor, where the resistances RA = RB = RC = 100[], the inductors LA = LB = LC = 1[mH], and the capacitors CA = CB = CC = 0.47[F]. In the simulation, the solver ode14x was used with the fixed time step of the sampling time 4e-9[s].

In the current-fed Z-source dc-dc converter mode, a dc voltage source VG = 15[V] with a series inductor LG = 1[mH] was used instead of the motor. An electronic dc load FK-200L2 at VI = 10[V] in the circuit experiment. In the simulation a dc voltage source VI = 10[V] was used with a series resistance 0.7[]. The capacitor C0 was used with a series resistance 0.1[]. The duty ratio D = 0.8 was given for the switch SA and the duty ratio D = 0.2 was given for the switch SC. In the simulation experiment, the solver ode14x was used with the fixed time step of the sampling time 4e-9[s].

Fig. 9 shows the driving waveforms of switch S1 and S2. The shoot-through state which S1 and S2 are ON at the same time was found in Fig. 9. Fig. 10 shows the simulated waveforms of the output voltages vA, vB, and vC and the inverter bus voltage vdc in Z-source inverter mode. The output frequency was 1.00[kHz] and the theoretical value of the output voltage was 24.75[V]. From the simulated waveforms, it was confirmed that the proposed circuit operated following the theory. Fig. 11 shows the observed waveforms in the circuit experiment. As shown in Fig. 11, the peak to peak value of the boosted output voltage is 24.0[V]. The difference of the experimental result and the theoretical value is about the same to the forward voltage drop of the diodes. The output frequency is 1.00[kHz]. From these results, the proposed circuit performs as a Z-source inverter in this mode.

II

L1

L2

LA

LB

LC

RA

RB

RC

CC

CB

CA S1

S2

S3

S4

S5

S6

C2 C1

VI SC DC

DA

C0 vB

vC

SA SB

DB

vA

vdc

(a)

vGSA L1

L2

C2 C1

SA

DA

C0 SC

DC

VG

LG

VI

SB

DB II

vGSC

(b)

Fig. 8. Experimental circuits, (a) Z-source inverter mode, and (b)current-fed Z-source dc-dc converter mode.

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Fig. 12 shows the simulated waveforms of the driving waveforms vGSA for SA and vGSC for SC, and the current II in current-fed Z-source dc-dc converter mode. From the simulated waveforms, the input current decreases when SA is ON and SC is OFF. On the other hand, the input current increases when SA is OFF, and SC is ON. The average input current II = -0.84[A] means that the proposed circuit charged the battery VI in the case of reverse power flow. Fig. 13

(a)

(b)

(c)

Fig. 12. Simulated waveforms of the driving waveforms vGSA for SA and vGSC for SC, and the current II in current-fed Z-source dc-dc converter

mode, (a)driving waveform of SA, vertical: 0.5[V/div], (b)driving waveform of SC, vertical: 0.5[V/div], and (c) charging current waveform,

vertical: 0.02[A/div], horizontal: 10[s/div]

Fig. 13. Observed the gate source voltage VGSA, VGSC and the charging current II waveforms in current-fed Z-source dc-dc converter mode,

vertical: vGSA 10[V/div], vGSC 10[V/div], II 0.1[A/div], horizontal: 5[s/div].

Fig. 9. Driving waveforms of switch S1 and S2, vertical: 10[V/div], horizontal: 10[s/div]

Fig. 10. Simulated waveforms of the output voltages vA, vB, and vC and the inverter bus voltage vdc in Z-source inverter mode, vertical: 10[V/div],

horizontal: 500[s].

Fig. 11. Observed output voltages vA ,vB, vC and the inverter bus voltage vdc waveforms in Z-source inverter mode, vertical: vA, vB, vC 20[V/div], vdc

50[V/div], horizontal: 500[s/div].

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shows the observed the gate source voltage VGSA, VGSC and the charging current II waveforms in the current-fed Z-source dc-dc converter mode. The average input current II = -0.80[A] was measured. The input current value makes a difference by the internal resistance of the battery.

IV. CONCLUSION This paper has presented a bi-directional Z-source inverter

for electric vehicles. The proposed inverter has a Z-source inverter which drives an ac motor, and performs as a current-fed Z-source dc-dc converter against reverse power flow. The operation of the proposed converter was confirmed by the simulation and circuit experiments for each operation as a Z-source inverter and a current-fed Z-source dc-dc converter.

REFERENCES [1] C. C. Chan, The state of the art of electric, hybrid, and fuel cell

vehicles, in Proc. of the IEEE, vol.95, No.4, pp. 704-718, April 2007. [2] Jih-Sheng Lai and Douglas J. Nelson, Energy management power

converters in hybrid electric and fuelcell vehicles, in Proc. of the IEEE, vol95, No.4, pp. 766-777, April 2007.

[3] F. Z. Peng, Z-source inverter, IEEE Trans. Industry Applications, vol.39, pp. 504-510, Mar/Apr 2003.

[4] Xupeng Fang, A novel Z-source DC-DC converter, in Proc. IEEE International conference on Industrial Technology 2008, pp. 1-4, April 2008.

[5] F. Z. Peng, Miaosen Shen and Zhaoming Qian, Maximum boost control of the Z-source inverter, in Proc. IEEE Power electronics specialist conference 2004, vol.1, pp. 255-260, June 2004.

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