[ieee 2009 twenty-fourth annual ieee applied power electronics conference and exposition (apec) -...

A Family of Z-source and Quasi-Z-source DC-DC Converters

Dong Cao, Fang Z. Peng

The Department of ECE at Michigan State University 2120 Engineering Building

East Lansing, MI 48824 USA

Abstract-This paper presents a family of four-quadrant dc-dc

converters with minimal number of switches and passive devices. These converters employ a Z-source or a quasi-Z-source network with two active switches to provide four quadrant operation which means bipolar output voltage and bidirectional current operation. Bipolar output operation can also be realized by using one active switch and a diode. They also own buck and boost characteristics when the duty cycle is changed from zero to one. At 0.5 duty cycle, these converters can output either zero or infinity voltage. Experimental results are given to demonstrate the validity and features of these circuits.

I. INTRODUCTION

At present, many renewable DC sources (photovoltaic arrays, fuel cells, thermoelectric modules) have been used for DC motor drives. A four quadrant dc-dc converter is needed for the four-quadrant operation of motor (forward, backward motoring and breaking).[1, 2] H-bridge has been used to operate in four quadrant operation for motor drives for many years, but it utilizes four active switches which are costly and bulky. By utilizing Z-source network, Z-source and quasi-Z-source inverter has been proposed recently with buck, boost and voltage bipolar characteristics.[3-6] Other topologies that make use of the Z-source network as a basic cell to achieve four quadrant operation dc-dc converters have been investigated.[7] Some topologies using coupled inductor or hybrid Z-network to achieve four quadrant operation have been investigated.[4, 8] However, all existing topologies have to use additional active switches, extra inductor and capacitor or coupled inductor to attain four-quadrant operation thus increasing costs, circuit complexity and reducing the reliability.

This paper presents a family of dc-dc converters based on Z-source or Z-network structure that is able to achieve the four quadrant operation without increasing the cost significantly. This Z-source dc-dc converter family utilizes a basic Z-source structure and a quasi-Z-source structure to generalize a group of dc-dc converters. By employing two active switches (MOSFETs with their anti-parallel diodes) four of these converters can realize bidirectional and bipolar operation. This family owns both buck and boost characteristics depending on control of the duty cycle. At 0.5 duty cycle four of these converters can output zero voltage while others can output the infinity voltage theoretically. Duty cycle 0.5 is the boundary of the positive output and negative output. The number of the

active switches, inductors and capacitors has been minimized, which makes the circuit more reliable and cost efficient. One of the active switches of some topologies can also be replaced by a diode, making it a single switch bi-polar output circuit. A 40W Z-source dc-dc converter was built to confirm the operation. Experiment results are given to demonstrate the specific features of proposed circuits.

II. PROPOSED CIRCUIT AND OPERATING PRINCIPLE

A. Topology derivation Fig.1 shows the basic structures of Z-source and quasi-Z-

source dc-dc converter. Fig.1a shows the basic structure of Z-source dc-dc converter, with a Z-source network and two switches. Fig.1b shows the basic structure of quasi-Z-source dc-dc converter with a quasi-Z-source network and two switches.

Each structure is formed by two inductors, two capacitors, two switches (active or passive) and an input voltage source. Load can be added to either of the capacitor and the position of voltage source and the capacitors position can be exchanged. In this way eight different circuits can be derived as shown in Fig.2.

(a). Z-source dc-dc converter basic structure

(b). quasi-Z-source dc-dc converter basic structure Fig. 1. Z-source and quasi-Z-source dc-dc converter basic

structure

978-1-422-2812-0/09/$25.00 ©2009 IEEE 1097

(a) (b)

(c) (d)

(e) (f)

(g) (h)

Fig. 2 Z-source and quasi-Z-source dc-dc converter family.

Fig.2a-Fig.2d are the derived dc-dc converters based on the Z-source structure, Fig.2e-Fig.2h are the derived dc-dc converters based on quasi-Z-source structure. When duty cycle changes from (0~0.5) Topology a, b, e, f have the boost characteristics and positive voltage output, when duty cycle changes from (0.5~1) they have buck-boost characteristics and negative output. When duty cycle changes from (0~0.5) topology c, d, g, h have the buck characteristics and positive output, when duty cycle changes from (0.5~1) they have buck-boost characteristics and negative voltage output. If MOSFET and its anti-parallel diode were used as the switches, topology c, d, g, h can realize four-quadrant operation. If MOSFET and a four-quadrant switch were used, topology a, b, e, f can also achieve four-quadrant operation. Fig.3 shows some four quadrant switches. If either switch of topology c, d, g, h is replaced with a diode, bipolar output characteristics can be achieved too.

(a) (b) (c)

Fig. 3 Some four quadrant switches

B. The DC operation mode of proposed converter The topology shown in Fig. 2(c) is used as an example to

demonstrate the circuit DC working stages: in the first stage S1 is on, S2 is off as shown in Fig. 4(a, c) and in the second stage S2 is on, S1 is off as shown in Fig. 4(b, d). D is the duty cycle of S1. Fig. 4(a, b) shows the case D<0.5 while Fig. 4(c, d) shows the case D>0.5. The direction of inductor current is different in two cases as shown in Fig. 3. Assume the voltage across the capacitor C1 is VC1, the current through L1 is IL1, the current through L2 is IL2. The voltage across C2 is Vo. According to the inductor voltage-second balance equation on L1, one has

1 2( ) (1 ) 0C gV D V V D⋅ + − ⋅ − = (1)

According to the voltage-second balance equation on L2, one has

1( ) (1 ) 0g o CV D V V D⋅ + − ⋅ − = (1)

From (1) and (2), one has 1C gV V=

(3) (1 21

)go

VV

DD⋅ −

=−

⋅

(4) From equation (4), one can notice that for D<0.5, Vo>0

and for D>0.5, Vo<0. For D<0.66 one can get a step-down characteristic. For D>0.66 one can get a step-up characteristic. Therefore when D<0.5 it has the positive output buck characteristic, when D>0.5 it has the negative output buck-boost characteristic.

1 2( )1

o

g

V DM DV D

− ⋅= =

− (5)

Equation (5) demonstrates the voltage gain of the circuit. And this was also shown as the dotted line in Fig. 5. Fig. 5 also shows the voltage gain curves of other topologies shown in Fig. 2. Topology c, d, g, h have the same voltage gain curve as shown in dotted line. By using the same idea shown above, one can get the voltage gain equation of topology a, b, e, f.

1( )1 2

o

g

V DM DV D

−= =

− ⋅

This equation was shown as solid line in Fig. 5.

(a)

978-1-422-2812-0/09/$25.00 ©2009 IEEE 1098

(b)

(c)

(d)

Fig. 4 Topological stages for the converter of Fig. 2(c). (a,b) D<0.5 (c,d) D>0.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 15−

4−

3−

2−

1−

01234

5

Duty cycle d

c,d,g,h

a,b,e,f

Fig. 5 Voltage gain curve for proposed family

Fig. 6 shows the Key wave forms of the proposed converter. Fig. 6(a) shows the case when D<0.5 while Fig. 6(b) show the case D>0.5. S1, S2 are the control signal of switch. Is1, Is2 are the current wave form through the switch.

(a)

(b)

Fig. 6 Key waveforms of the proposed converter. (a) D<0.5, (b) D>0.5

978-1-422-2812-0/09/$25.00 ©2009 IEEE 1099

III. FOUR-QUADRANT OPERATION OF THE PROPOSED CONVERTER

The proposed converter has both capability of providing bi-polar output voltage and a bi-directional character. By using an active-device (MOSFET) with its anti-parallel diode topology c, d, g, h is able to achieve four quadrant operation. Fig.7 shows the four quadrant operation of topology shown in Fig. 2(c) as an example. In the plane Vo-Io, First and second quadrant Vo is positive while D<0.5. Third and fourth quadrant Vo is negative while D>0.5. E can be considered as the motor or other renewable source with internal resistance R. In the first and third quadrant the energy was transferred from the input voltage source Vg to load, while in the second and fourth quadrant the energy was transferred from load to the input voltage source Vg.

IV. EXPERIMENTAL RESULT

The proposed converter bi-polar characteristic and four quadrant operation with only an active switch and a diode was confirmed by a Vg=24V, 40W converter at the fs=50KHz switching frequency. Fig. 8 shows the circuit. E is able to operate at positive and negative voltage. Fig.9 shows the waveforms of quadrant 1 while D<0.5. Vgs and Vds are the switching waveforms of S2, Vin and Vo are the input and output voltage while Iin and Io are the input and output voltage, which are all positive. Fig. 10 shows the waveforms of quadrant 4 while D>0.5. Vgs and Vds are the switching waveforms of S2, Vin and Vo are the input and output voltage while Iin and Io are the input and output current. Vo is negative output as shown in Fig. 10(b).

V. CONCLUSION

A family of Z-source and quasi-Z-source dc-dc converter has been proposed in this paper. This family has bi-polar output voltage characteristic and four-quadrant operation characteristic with minimized number of switching devices, inductor and capacitor. It meets the needs of DC drives and other renewable energy sources. With one polar input voltage source this converter can output reversible load voltage only by controlling duty cycle, which makes the converter simpler and more economical than other converters.

Fig. 8 Quadrant 1 and 4 operation verification experiment circuit with a diode and an Active switch

Vgs(10V/div)Vds(20V/div)

Io (0.5A/div)

Iin (0.5A/div)

(a)

Vin (10V/div) Vo (10V/div)

Io (0.5A/div)

Iin (0.5A/div)

(b)

Fig. 9 Experiment waveforms of the proposed converter (c) D<0.5

Vgs(10V/div)Vds(50V/div)

Io (0.5A/div)

Iin (0.5A/div)

(a)

Iin (0.5A/div)

Io (0.5A/div)

Vo (10V/div)

Vin (10V/div)

(b)

Fig. 10 Experiment waveforms of the proposed converter (c) D>0.5

978-1-422-2812-0/09/$25.00 ©2009 IEEE 1100

Fig. 7 Four quadrant operation of proposed converter

REFERENCES

[1] N. Mohan, T. M. Underland, and W. P. Robbins, Power Electronics Converter, Applications, And Design: John Wiley & Sons, Inc, 2003.

[2] M. H. Rashid, Power electronics Handbook. Englewood Cliffs,NJ: Prentice Hall, 1993.

[3] F. Z. Peng, "Z-source inverter," in Industry Applications Conference, 2002. 37th IAS Annual Meeting. Conference Record of the, 2002, pp. 775-781 vol.2.

[4] F. Z. Peng, "Z-source networks for power conversion," in Applied Power Electronics Conference and Exposition, 2008. APEC 2008. Twenty-Third Annual IEEE, 2008, pp. 1258-1265.

[5] J. Anderson and F. Z. Peng, "A Class of Quasi-Z-Source Inverters," in Industry Applications Society Annual Meeting, 2008. IAS '08. IEEE, 2008, pp. 1-7.

[6] J. Anderson and F. Z. Peng, "Four quasi-Z-Source inverters," in

Power Electronics Specialists Conference, 2008. PESC 2008. IEEE, 2008, pp. 2743-2749.

[7] Y. Berkovich, B. Axelrod, S. Tapuchi, and A. A. I. A. Ioinovici, "A Family Of Four-Quadrant, PWM DC-DC Converters," in Power Electronics Specialists Conference, 2007. PESC 2007. IEEE, 2007, pp. 1878-1883.

[8] J. Wang, W. G. Dunford, and K. Mauch, "Some novel four-quadrant DC-DC converters," in Power Electronics Specialists Conference, 1998. PESC 98 Record. 29th Annual IEEE, 1998, pp. 1775-1782 vol.2.

978-1-422-2812-0/09/$25.00 ©2009 IEEE 1101