A Novel Buck Boost Inverter

Canadian Solar Buildings Conference Montreal, August 20-24, 2004

Refereed Paper

A NOVEL BUCK-BOOST INVERTER FOR PHOTOVOLTAIC SYSTEMS

Liuchen Chang, Zhumin Liu, Yaosuo Xue and Zhenhong Guo Dept. of Elec. & Comp. Eng., University of New Brunswick, Fredericton, NB, Canada

Phone: (506) 447-3145, Fax: (506) 453-3589, Email: [email protected]

ABSTRACT For inverter-based PV systems in grid-connected applications as distributed generators (DG), variable sources often cause wide changes in the inverter input voltage above and below the output ac voltage, thus demanding a buck-boost operation of inverters. Many traditional full-bridge buck inverters, two-stage inverters, single-stage buck-boost inverters either have complex structure or have limited range of input dc voltage. The Authors have proposed and developed an innovative single-phase, single-stage, flyback-based, buck-boost inverter for renewable energy systems, especially for photovoltaic (PV) systems in grid-connected applications. This paper focuses on the analysis of the working principles, computer simulation of the operation, and design consideration of the inverter for grid-connected applications.

I. INTRODUCTION For inverter-based PV systems in grid-connected applications as distributed generators (DG), resources often cause wide variations in the input voltage to inverters above and below the output ac voltage. This is particularly true for PV and wind systems. This then demands the buck-boost (i.e., step-down and step-up) operation of inverters. A general structure of the grid-connected PV systems is shown in Figure 1.

Traditional full-bridge buck inverters as shown in Figure 2 do not have the flexibility of handling a wide range of input dc voltage, and require heavy line-frequency step-up transformers [Xue et al., June 2004]. Although this topology currently has the largest market share of the commercial PV system

market due mainly to its simplicity and electrical isolation, it is gradually replaced by advanced topologies using “more silicon and less iron”. This leads to the pursuance of compact designs with wide input voltage ranges and improved efficiency.

Figure 2. Buck inverter with a low frequency transformer. Two-stage inverters normally accomplish dc voltage boost in the first stage, and achieve buck dc-ac conversion in the second stage, with a typical high-frequency transformer to accomplish the voltage boost as shown in Figure 3 [Xue, et al., June 2004]. Although they can accommodate a wide range of input voltage, the complicated structure makes them costly, particularly for small PV systems.

A single-stage inverter is an inverter with only one stage of conversion for both stepping-up and stepping-down the dc voltage from PV sources and modulating the sinusoidal output current or voltage. Single-stage buck-boost inverters, as presented in Figure 4, have a simple circuit topology and low component count, leading to low cost and high efficiency. Previously available single-stage buck-

Figure 1 Representation of a single-phase grid-connected PV system.

Figure 3. Two-stage inverter with a high-frequency transformer.


Refereed Paper boost inverters either need more than 4 power switching devices or have a limited range of input dc voltage. Most of them have two symmetrical dc-dc converters operating in the opposite phase angle in order to generate a sinusoidal current waveform feeding to a single-phase grid.

For small grid-connected PV systems, inverters should be small, inexpensive and reliable. Further efforts have been directed to innovative inverters and controls.

II. A NEW BUCK-BOOST INVERTER WITH 4 SWITCHING DEVICES The Authors have proposed and developed an innovative single-phase, single-stage, flyback-based, buck-boost inverter for renewable energy conversion systems, especially for photovoltaic (PV) systems in both grid-connected and standalone applications. As shown in Figure 5, this buck-boost inverter has 4 switching devices [Xue, Jan. 2004].

The operating principles of this buck-boost inverter can be described by three operation modes.

Mode 1: Charge mode In this mode, switches T1 and T4 are turned on and switches T2 and T3 are turned off. The equivalent circuit is shown in Figure 6 without the consideration of inductor copper loss and semiconductor conduction losses. From an energy point of view, during Mode 1, inductor 1L is charged to store energy and the output current is provided by the discharge of capacitor C, i.e.

cii −= .Obviously, 02 =i , 0=pi , and 1iis =

with parameters defined in Figure 6. The circuit equations of this mode can be described by:

svdtdiL =1 (1)

vvdtdiL cf −= (2)

idtdvC c −= (3)

Figure 7 sketches the waveforms of these state variables.

Mode 2: Positive half cycle (PHC) discharge mode

In this mode, switch 4T is turned off and 3T is

turned on, while 1T is keeping on and 2T off. Figure 8 gives the equivalent circuit in this mode.

Figure 4. A typical single-stage inverter with 4 power switching devices.

Figure 5. A new buck-boost single-stage inverter with 4 switching devices.

Figure 6. Equivalent circuit of the charge mode.

Figure 7. Waveforms in Modes 1 and 2.


Refereed Paper

In this mode, the dc source is disconnected temporarily from the output, i.e. 0=si , and we have

cvdtdiL −=1 (4)

vvdtdiL cf −= (5)

iidtdvC c −= 1 (6)

Also, 02 =i , and 1iip = .

After inductor 1L is charged in Mode 1, its current,

1i , reaches a peak value '0I . During the course of

Mode 2, the energy stored in the inductor is transferred to the single-phase grid through a low pass filter Lf-C, which intends to smooth the output current during the charge mode and discharge mode.

Two current conduction modes can be defined here. If the time of Mode 2 is so short that the inductor current is not decreasing to zero when the next charge cycle Mode 1 starts, the current of energy-storage inductor is continuous, and we define this operation the continuous conduction mode (CCM). On the contrary, if the inductor current drops zero in Mode 2 and probably sustains zero for certain time, the operation is defined the discontinuous conduction mode (DCM), as illustrated in Figure 7.

So far, in the PHC of ac output, the energy is transferred from dc source (i.e. PV) to ac grid through the alternations of Mode 1 and Mode 2.

Mode 3: Negative half cycle (NHC) discharge mode

This mode is combined with Mode 1 to provide ac NHC output when switch 1T is tuned off and 2T is turned on. The equivalent circuit of Mode 3 is shown in Figure 9.

Through a flyback operation, the current of primary side of the coupled inductor L drops to zero suddenly and the current of secondary side reaches to the initial current of primary side, if the inductances and turns of both sides of the coupled inductor are identical and there is no magnetic leakage. From Figure 9, we have:

0=si , 01 =i , 2iip −= and

cvdtdiL =2 (7)

vvdtdiL cf −= (8)

iidtdvC c −−= 2 (9)

The only differences between Mode 3 and Mode 2 are that in Mode 3, the grid is in the negative half cycle and the discharging current has an opposite direction. Then similar arguments regarding energy exchange and transfer in Mode 2 can be also applied to Mode 3. As a result, in the NHC of ac output, the energy is transferred from dc source to ac grid through 1L ,

2L and C by the alternations of Mode 1 and Mode 3.

In summary, during each switching interval, the energy-storage inductor is charged from a dc source (i.e. PV) and discharged to a grid through a low pass filter. The inductor current can be discontinuous as shown in Figure 10, and continuous as shown in Figure 11.

Figure 8. Equivalent circuit of PHC discharge - Mode 2.

Figure 9. Equivalent circuit of NHC discharge - Mode 3.


Refereed Paper

The simulation waveforms for the buck-boost inverter subject to a variable dc voltage sources, controlled by an open loop feedforward compensation [Xue et al., 2004], are shown in Figure 12. The current total harmonic distortion (THD) of the 120V grid side is 2% for a switching frequency of 9.6 kHz.

The simulation waveforms for the buck-boost inverter subject to a variable dc voltage sources, controlled by a closed-loop sinusoidal PWM modulation [Xue et al., 2004], are shown in Figure 13. The current total harmonic distortion (THD) of the 120V grid side is 3.4% for a switching frequency of 9.6 kHz. It is noted that the grid voltage has been assumed containing significant harmonic contents.

III. A NEW BUCK-BOOST INVERTER WITH 3 SWITCHING DEVICES Based on the buck-boost inverter with 4 switching devices as developed by the Authors for small distributed generators, further improvements have been proposed, which leads to a new buck-boost inverter with 3 switching devices [Liu, 2004]. This inverter is shown in Figure 14. The simple circuit topology of this invention provides the possibility for a low cost and high efficiency dc-ac converter appropriate for small PV applications. The inverter has a low component count with only 3 power semiconductor switches to accomplish dc-ac conversion.

Figure 10. Buck-boost inverter operation in the discontinuous current mode.

Figure 12. Simulated waveform of the buck-boost inverter under the open loop feedforward compensation control.

Figure 11. Buck-boost inverter operation in the continuous current mode.

Figure 13. Simulated waveform of the buck-boost inverter under the closed loop sinusoidal PWM control.


Refereed Paper

The inverters can accommodate a wide range of input dc voltage for an improved energy output from variable PV resources. The input source and the output grid are separated based on flyback operation principles. As compared to traditional buck inverters with line-frequency transformers, two-stage buck-boost inverters, and other single-stage buck-boost inverters, both the component count, cost and size of the newly proposed buck-boost inverter are reduced, thereby presenting a more reliable and economical design for PV systems and other distributed generators.

The two coupled inductors L1 and L2 have the same inductance L. Since only one switch is turned on in each operation mode and an inductor is always connected in the charge/discharge circuit, the dead time for preventing two switches from shoot-through can be eliminated. The inverter operation can be divided into charge and discharge operation working in the positive half cycle and in negative half cycle, similar to the buck-boost inverter with 4 switching devices, as presented in the previous section.

Mode 1: Charge mode

During Mode 1, switches 2Q and 3Q are turned off,

and switch 1Q is turned on to charge inductor 1L

from the dc source through diode 1D . Capacitor C provides the continuous current for the grid in Mode 1. The governing equations are the same as in Mode 1 of the 4-switch buck-boost inverter.

Mode 2: Positive half cycle (PHC) discharge mode

Mode 2 is the discharge mode in positive half cycle. During Mode 2, switches 1Q and 3Q are turned off,

and switch 2Q is turned on to discharge the energy,

which was stored in inductor 1L , to the grid through

diode 2D . Figure 15 is the operation waveforms in a positive half cycle.

I(Q1)

I(Q2)

Iout1

Ich+

Idisch+

Ich+

Idisch+

Idisch+

Q2

Q1

D1

D2

ON

ON

ON

ON ON

ON ON

ON

t1t0

I(L1)

Figure 15 Operation waveforms in a positive half

cycle.

Mode 3: Negative half cycle (NHC) discharge mode

Mode 3 follows Mode 1 in a negative half cycle of the grid voltage. During Mode 3, switches 1Q and 2Q

are turned off, and switch 3Q is turned on. The

energy which is stored in the coupled inductor 1L

will transfer to the coupled inductor 2L and then

discharges to the load through switch 3Q and

diode 3D . Figure 16 is the operation waveforms in a negative half cycle.

Assume that the resistance of the switches, diodes, and coupled inductors are negligible; two coupled inductors are perfectly coupled; the inverter works in discontinue current mode (DCM); the averaged current of Mode 2 is the average output current of the inverter and can be expressed as,

Figure 16. Operation waveforms on a negative half

cycle.

∫ ===− 2

0 grids

21

grids

21

2dc

11 221 T

s VTLI

VLTTVdti

Ti (10)

where, grid

12 V

LIT = , and TS is the switching period.

Figure 14. Newly proposed buck-boost inverter with 3 switching devices.


Refereed Paper It is assumed that the utility line voltage Vgrid is expressed as a sinusoidal waveform:

)sin(2grid tVV ω= (11)

One of the algorithms for sinusoidal PWM is to control the turn-on time of switch Q1 in proportion to the utility voltage Vgrid.

)sin(s1 tkTT ω= (12)

where k is the coefficient factor. Substituting (11) and (12) into (10), the ac output current iac is expressed as,

)sin(22

2s

2dc

ac tLkTVi ω= (13)

In practical implementation of an inverter control, a sinusoidal reference wave, serving as the modulating signal, is compared with a triangular wave, serving as the carrier signal. The intersection points determine the switching angles and pulse widths as in Figure 17 [Xue, Jan. 2004].

Figure 17 Sinusoidal pulse-width modulation. The current ratings of the power semiconductor switches of the 3-switch buck-boost inverter are the same as those of the 4-switch buck-boost inverter presented in the previous section. The voltage stresses of the power semiconductor devices in the charging control circuits are the same for the 4-switch buck-boost inverter (T1 and T4) and for the 3-switch buck-boost inverter (Q1), and are equal to the Vdc+vc, where vc is the capacitor voltage and is in the same order as the grid voltage. The voltage stresses of the power semiconductor devices in the discharging circuits of the 3-switch buck-boost inverter and the 4-switch buck-boost inverter are somewhat different. The blocking diodes in the discharging circuits of the two inverters have the same reverse voltage of Vdc+vc. However, the switching devices (IGBTs or MOSFETs) in the discharging circuits of the 3-switch inverter have a reverse voltage of 2vc, which is twice the reverse voltage of the 4-switch inverter of vc. For a 120V/60Hz single-phase grid, the peak value of vc is in the level of 200V. In summary, the voltage stress of some switching devices of the 3-switch buck-boost inverter is twice that of the 4-swich inverter, but still in the range readily available in commercial IGBT or MOSFET devices.

IV. SIMULATION RESULTS OF THE 3-SWITCH BUCK-BOOST INVERTER In designing the parameters of the inverter, a consideration for overall inverter operation under various input voltage is required. Through simulation studies, the required modulation index and operation region under variable dc input voltages are presented in Figure 18.

Based on SPWM strategy of Figure 17, the inverter gating signals are shown in Figure 19. The operation of the inverter is simulated for different dc source voltages from 50V to 300V, as if it were from a PV panel. The inverter is designed for a rated power of 1 kW. The grid voltage is fixed at 120V/60Hz. The switching frequency is set at 5 kHz, considering a compromise between reducing switching losses and ensuring output current quality. Figures 20-23 present the simulated output current waveforms. Table 3.1 summarizes the simulation parameters and output current performance.

Table 1 Summary of simulation results

From the figures, it has been seen that the newly proposed flyback single-stage single-phase buck-boost inverter can accomplish both buck and boost

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

2

4

6

8

10

12

14

16

18

20

Modulation Index, M

Out

put C

urre

nt, I

o (A

mpe

res)

Control-to-Output Curves of SPWM Control

Vs=50V

Vs=100V

Vs=137.3V

Vs=150V

Vs=190V

Vs=250V

Vs=300V Maximum Modulation IndexCurve

1 kW Curve

x ---- Simulation Data

Figure 18. Control-to-output curves of SPWM control. (L = 150 × 10-6H, V = 120V)

DC voltage

Power factor

Output current

Output power

THD (%)

50 V 0.30 1.24 A 44.6W 2.05

100 V 0.50 1.52 A 91.2W 2.00

200 V 0.95 5.66 A 645.2W 4.50

300 V 0.98 8.50 A 999.6W 4.90


Refereed Paper operation, feeding power to a grid with a reasonable power quality from a widely variable dc source.

Figure 19 Gating signals of the 3-device buck-boost

inverter.

Figure 20. Output current waveform when the dc voltage is 50V.




The implementation of the 3-switch buck-boost inverter is still yet to be done. The output current waveforms are to be improved, possibly using a close-loop sinusoidal PWM as presented in [Xue, et al, June 2004].

V. CONCLUSION

The Authors have proposed an innovative single-phase, single-stage, flyback-based, buck-boost inverter for renewable energy conversion systems, based on a previously developed 4-switch buck-boost inverter. The simple circuit topology of this inverter provides the possibility for a low cost and high efficiency dc-ac converter. The inverters have a low component count with only 3 power semiconductor switches to accomplish dc-ac conversion with a high output power quality. The inverter can accommodate a wide range of input dc voltage for an improved energy output from variable PV resources. The inverter separates the input source from the output grid through a flyback operation. As compared to traditional buck inverters with line-frequency transformers, two-stage buck-boost inverters, and previous single-stage buck-boost inverters, both the cost and size of the newly proposed inverter are reduced, thereby presenting a more reliable and economical design for PV systems. The analysis of the working principles, and computer simulation of the operation for this inverter have proved its feasibility for dc-ac conversion in PV applications. The implementation and tests of the inverter is yet to be done in the future.

VI. ACKNOWLEDGMENT The authors wish to thank Natural Sciences and Engineering Research Council of Canada (NSERC) for the financial support to this research project.

VII. REFERENCES Liu, Z., Study Of Single-Phase Single-Stage Buck-Boost Inverters, University of New Brunswick M.Sc. Thesis, Aug. 2004.


Refereed Paper Xue, Y., Study Of Single-Phase Single-Stage Buck-Boost Inverters, University of New Brunswick M.Sc. Thesis, Jan. 2004.

Xue, Y., Chang, L., "Closed-Loop SPWM Control for Grid-Connected Buck-Boost Inverters,” IEEE Power Electronics Specialists Conference 2004, Aachen, Germany, Vol. 5, pp.3366-3371, June 2004.

Xue, Y., Chang, L., Baekhj Kjaer, S., Bordonau, J. and Shimizu, T., “Topologies of single-phase inverters for small distributed power generators: an overview,” IEEE Trans. Power Electronics, vol. 19, pp. 1305-1314, Sept. 2004.

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A Novel Buck Boost Inverter