single phase z source pwm ac ac converters 11h

IEEE POWER ELECTRONICS LETTERS, VOL. 3, NO. 4, DECEMBER 2005 121

Single-Phase Z-Source PWM AC-AC ConvertersXu Peng Fang, Zhao Ming Qian, Senior Member, IEEE, and Fang Zheng Peng, Fellow, IEEE

Abstract—The letter proposes a new family of simple topologiesof single-phase PWM ac-ac converters with a minimal number ofswitches: voltage-fed Z-source converter and current-fed Z-sourceconverter. By PWM duty-ratio control, they become “solid-statetransformers” with a continuously variable turns ratio. All theproposed ac-ac converters in this paper employ only two switches.Compared to the existing PWM ac-ac converter circuits, they haveunique features: providing a larger range of output ac voltagewith buck-boost, reversing or maintaining phase angle, reducingin-rush and harmonic current, and improving reliability. Theoperating principle and control method of the proposed topologiesare presented. Analysis, simulation, and experimental resultsare given using the voltage-fed Z-source ac-ac converter as anexample. The analysis can be easily extended to other convertersof the proposed family. The proposed converters could be used involtage regulation, power regulation, and so on.

Index Terms—AC–AC converter, power-line conditioning,PWM converter, solid-state transformer.

I. INTRODUCTION

FOR ac–ac power conversion that normally requires vari-

able output voltage and variable frequency, the most pop-

ular topology is the voltage-source inverter with a dc link, i.e.,

a pulse width modulation (PWM) inverter with a diode-recti-

fier front end and dc capacitor link. However, for applications

where only voltage regulation is needed, a direct PWM ac–ac

converter is a better choice to achieve smaller size and lower

cost. AC–AC converters, or ac–ac line conditioners, can also

perform conditioning, isolating, and filtering of the incoming

power in addition to voltage regulation [1]. The use of self-com-

mutated switches with PWM control can significantly improve

the performance of ac–ac converters. This has been presented

in a number of technical publications [1]–[9], where different

ac-ac converters were proposed. Some simulation results were

given to illustrate performance in the presence of voltage sags,

surges, and load fluctuations.

The Z-source inverter is a novel topology [10], [11] that over-

comes the conceptual and theoretical barriers and limitations

of the traditional voltage-source converter and current-source

converter. Its operating principle and applications for fuel cell

inverters and ASD (adjustable speed drive) systems have been

Manuscript received Janaury 3, 2005; revised June 22, 2005. This workwas supported by the National Science Foundation of China under Contract50377038 and by the National Science Foundation under NSF Award ECS0424039. Recommended by Associate Editor L. M. Tolbert.

X. P. Fang is with the Zhejiang University, Hangzhou 310027, China and alsowith the Shandong University of Science and Technology, Qingdao 266510,China (email: [email protected]).

Z. M. Qian is with the Zhejiang University, Hangzhou 310027, China.F. Z. Peng is with the Department of Electrical and Computer Engineering,

Michigan State University, East Lansing, MI 48824 USA.Digital Object Identifier 10.1109/LPEL.2005.860453

Fig. 1. Single-phase Z-source ac–ac converter: (a) voltage-fed and (b)current-fed.

presented in [10]–[13]. This paper extends the Z-source concept

to ac–ac conversion. Although the Z-source ac–ac converters

given here are quite similar to those published in [1], some

unique features revealed in this paper, such as phase reversing

and buck-boosting are interesting for certain applications.

II. PROPOSED Z-SOURCE AC–AC CONVERTER TOPOLOGIES

Fig. 1(a) and (b) show the proposed single-phase Z-source,

PWM voltage-fed, buck-boost converter and current-fed buck-

boost converters, respectively. Both converters utilize only two

active devices (S1 and S2), each combined with a full diode

bridge for bidirectional voltage blocking and bidirectional cur-

rent paths. All the inductors and capacitors are small and used

to filter switching ripples. The symmetrical Z-source network,

which is a combination of two inductors and two capacitors, is

the energy storage/filtering element for the Z-source ac–ac con-

verter. Since the switching frequency is much higher than the ac

source (or line) frequency, the inductor and capacitor require-

ments should be low.

The proposed ac–ac converters can operate with PWM duty-

ratio control in exactly the same way as for conventional dc-dc

converters. Fig. 2 shows the switching functions common to

both proposed ac-ac converters. As shown in Fig. 2, S1 and S2

are turned on and off in complement. A small snubber circuit

may be needed for each switch to suppress switching surges and

to provide commutation paths. Table I shows the steady-state

input-output voltage gains of these converters as a function of

duty ratio D. By controlling the duty ratio, the output voltage

can be regulated as desired.

1540-7985/$20.00 © 2005 IEEE

Authorized licensed use limited to: IEEE Xplore. Downloaded on May 10,2010 at 19:03:42 UTC from IEEE Xplore. Restrictions apply.

122 IEEE POWER ELECTRONICS LETTERS, VOL. 3, NO. 4, DECEMBER 2005

Fig. 2. Duty-ratio control of Z-source ac–ac converters.

TABLE IVOLTAGE TRANSFER RATIO OF Z-SOURCE AC–AC CONVERTERS

III. ANALYSIS, SIMULATION, AND EXPERIMENTAL RESULTS

OF THE PROPOSED AC–AC CONVERTER

For the Z-source PWM ac–ac converters, the control scheme

described in Fig. 2 is simple and easy to implement. As an

example, the voltage-fed, Z-source ac-ac converter shown in

Fig. 1(a) is analyzed. A similar analysis can be extended to the

current-fed, Z-source ac-ac converter. The switches S1 and S2

are gated on and off in complement as shown in Fig. 2. Two

states exist in this circuit; Fig. 3(a) and (b) show their equiva-

lent circuits. Since the inductors and capacitors of the Z-network

have the same inductances (L) and capacitances (C) in Figs. 3(a)

and (b) respectively, the Z-source networks become symmet-

rical. Then we have

(1)

and the input and output voltages are, and

(2)

where , , , are phase angles of the Z-network inductor

current, Z-network capacitor voltage, and output voltage, re-

spectively.

In State 1, the bidirectional switch S1 is turned off and S2

turned on. The ac source charges the Z-network capacitors,

while the inductors discharge and transfer energy to the load.

The interval of the converter operating in this state is ,

where D is the duty ratio of switch S1, and T is the switching

cycle, as shown in Fig. 3(a). As a result, one has,

(3)

Fig. 3. (a) State 1: S2 is on and S1 is off. (b) State 2: S2 is off and S1 is on.

In State 2, the bidirectional switch S2 is turned off and S1

turned on. The Z-network capacitors discharge, while the in-

ductors charge and store energy. The interval of the converter

operating in this state is DT, as shown in Fig. 3(b). Thus

(4)

The average voltage of the inductors over one ac line period

in steady state should be zero, ignoring the fundamental voltage

drop. Thus, from (3) and (4) we have

.

(5)

Assuming that the filter inductor and the inductor in the Z-net-

work are very small and there is no line frequency voltage drop

across the inductor, the voltage across the load should equal Vc,

the voltage across the capacitor of the Z-network, that is

and

(6)

In summary, we have

(7)

Evidently, by controlling the duty ratio D, the output voltage

of the proposed ac-ac converter can be bucked or boosted. In

addition, the output voltage can be in-phase or out-of-phase

with the input voltage depending on operating regions of the

duty cycle. This is a unique feature of the Z-source converter.

Fig. 4(a) shows the voltage gain versus the duty cycle. It clearly

shows that there are two operating regions. When the duty

cycle is greater than 0.5, the converter enters negative gain

region, i.e., the output voltage is 180 out-of-phase with the

input voltage. When the duty cycle is less than 0.5, the output


FANG et al.: SINGLE-PHASE Z-SOURCE PWM AC-AC CONVERTERS 123

Fig. 4. Voltage gain of the Z-source ac–ac converters: (a) voltage-fed and (b)current-fed.

voltage is in-phase with the input voltage. Similar analysis

can be applied to the current-fed, Z-source converter. Fig. 4(b)

shows the voltage gain curve. Again, it is noticeable that the

unique phase-reserving feature happens at the duty cycle of 0.5.

This unique phase-reversing feature suggests that an inverter

can be implemented for a dc source using two unidirectional

voltage and bidirectional current switches.

Simulation results for the voltage-fed, Z-source ac–ac con-

verter are shown in Fig. 5. The parameters used were

, and ,

. When the line input voltage has the nominal

value of 220 V rms and has 110 V rms at 50% voltage sag, by

PWM duty-ratio control we can keep the output voltage constant

at 165 V rms. The converter operates in buck mode when the

input voltage is normal at its nominal voltage. The output and

input voltages are inverted. The converter then operates in boost

mode during the voltage sag. In the simulation, the switching

frequency is 10 kHz and the output power is 3 kW.

Fig. 5. Simulation results.

Experiments were done and the same circuit parameters were

used: , , and

, . Experimental results at input voltage

, and , are

shown in Fig. 6. The experimental results verify the rationality

of the Z-source ac–ac converter.

IV. CONCLUSION

A new family of simple topologies of single-phase, Z-source,

ac–ac converters was proposed in this paper. By duty-ratio

control, the Z-source ac–ac converters become “solid-state

transformers” with a continuously variable turns ratio. The

ac–ac converters can be used for ac–ac line conditioning to

overcome voltage sags, surges, and load fluctuations. Because

the proposed converters employ only two active devices, they


124 IEEE POWER ELECTRONICS LETTERS, VOL. 3, NO. 4, DECEMBER 2005

Fig. 6. Experimental results of the proposed converter. (a) Input voltage: 22Vrms, D = 0:7; (b) input voltage: 110 Vrms, D = 0:25.

can reduce cost and improve reliability. Steady-state analysis,

simulation, and experimental results were illustrated using

the buck-boost converter as an example. In the same manner,

other Z-source, ac–ac converter topologies can be derived. The

unique phase-inversing feature teaches us that inverter circuits

can be easily derived, according to both circuits shown in

Fig. 1, by replacing both switch-diode bridges with a traditional

voltage-source inverter phase-leg switch (i.e., a combination of

switch and antiparallel diode).

REFERENCES

[1] F. Z. Peng, L. Chen, and F. Zhang, “Simple topologies of PWM AC-ACconverters,” IEEE Power Electron. Lett., vol. 1, no. 1, pp. 10–13, Mar.2003.

[2] B. R. Lin, T. Y. Yang, and T. C. Wei, “Single-phase ac ac converter basedon half-bridge NPC topology,” in Rec. Circuits and Systems, Int. Symp.,vol. 3, 2003, pp. III-340–III-343.

[3] E. C. Aeloiza, P. N. Enjeti, L. A. Moran, and I. Pitel, “Next generationdistribution transformer: to address power quality for critical loads,”in Rec. IEEE Power Electronics Specialist Conf., vol. 3, 2003, pp.1266–1271.

[4] J. Chang, T. Sun, and A. Wang, “High energy-efficient and compactAC-AC converters,” in Rec. IEEE Power Electronics and Drive Systems

Conf., vol. 2, 2001, pp. 543–548.[5] C. B. Jacobina, M. B. R. de Correa, R. L. A. Ribeiro, T. M. Oliveira, E.

R. C. da Silva, and A. M. N. Lima, “AC AC converters with a reducednumber of switches,” in Rec. IEEE Industry Applications Conf., vol. 3,2001, pp. 1755–1762.

[6] M. Kazerani, “A direct AC AC converter based on current-sourceconverter modules,” IEEE Trans. Power Electron., vol. 18, no. 5, pp.1168–1175, Sep. 2003.

[7] B.-R. Lin and T.-C. Wei, “Implementation of a single-phase AC–ACconverter based on neutral-point-clamped topology,” IEEE Trans.

Aerosp. Electron. Syst., vol. 39, pp. 625–634, Apr. 2003.[8] A. Shenkman, B. Axelrod, and Y. Berkovich, “Improved modification of

the single-switch AC–AC converter for induction heating applications,”Proc. Inst. Elect. Eng., Electr. Power Applicat., vol. 151, pp. 1–4, Jan. 9,2004.

[9] B.-R. Lin and D.-J. Chen, “Implementation of a single-phase three-legAC AC converter with neutral-point diode-clamped scheme,” Proc. Inst.

Elect. Eng., Electr. Power Applicat., vol. 149, pp. 423–432, Nov. 2002.[10] F. Z. Peng, “Z-source inverter,” in Rec. IEEE Industry Applications

Conf., 2002, vol. 2, 2002, pp. 775–781.[11] , “Z-source inverter,” IEEE Trans. Ind. Applicat., vol. 39, pp.

504–510, Mar./Apr. 2003.[12] X.-P. Fang, Z.-M. Qian, Z.-M. Qi-Gao, Z.-M. Bin-Gu, F.-Z. Peng, and

X.-M. Yuan, “Current mode Z-source inverter-fed ASD system,” in Rec.

IEEE Power Electronics Specialist Conf., 2004, pp. 2805–2809.[13] F. Z. Peng, X.-M. Yuan, X.-P. Fang, and Z.-M. Qian, “Z-source inverter

for adjustable speed drives,” in Rec. IEEE Power Electronics Specialist

Conf., 2003, pp. 33–35.


single phase z source pwm ac ac converters 11h

Documents