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Dual Functioning Converter Utilizing Flyback

Struture Used in Solar Energy Generation

Hossein Kazemi Kargar, Zeinab Sudi, Amin Hajihosseinlu

Department of Electrical EngineeringShahid Beheshti University

Tehran, Iran

h_kazemi@sbu.ac.ir, keipour@ieee.org, hajihosseinlu@yahoo.com

AbstractIn some applications it is needed to convert a variable

DC voltage to both AC and DC desirable voltages based on

situation. In this paper a new converter has been proposed. This

converter which has ability to generate both ac and dc output

voltage is taken from the conventional Flyback converter.

Flyback change a dc input voltage to a higher or lower level.

Using a simple switching method this dc-dc converter

wonderfully produces an ac output voltage so much close to anideal sinusoidal voltage. Applying sinusoidal pulse width

modulation (SPWM) method to a unidirectional switch makes

output voltage to be sinusoidal. Inductors in transformer and the

output capacitor are utilized to filter transferring energy.

Changing the switching method from sinusoidal PWM to linear

PWM simply let user to get either ac or dc output voltage. All of

these have been proved successfully in the simulation results.

Keywordsinverter; minimizing component; single switch

inverter; DC-AC converter; SPWM; Dual Functioning; PV

application

I. INTRODUCTIONThe flyback converter is used in both AC/DC and DC/DC

conversion with galvanic isolation between the input and anyoutputs. More precisely, the flyback converter is a buck-boostconverterwith the inductor split to form a transformer, so thatthe voltage ratios are multiplied with an additional advantageof isolation. When driving for example a plasma lamp or avoltage multiplier the rectifying diode of the buck-boostconverter is left out and the device is called a flybacktransformer. Inverters are electrical devices that convert directcurrent (DC) to alternating current (AC). A DC-DC converterchanges a DC voltage level [1]. In [1] Hajihosseinlu used asimple method in switching pulses to generate an ac outputvoltage via a dc-dc converter. The converted AC can be at anyrequired voltage and frequency with the use of appropriate

transformers, switching, and control circuits. Inverters haveapplication in uninterruptible power supplies, inductionheating, HVDC power transmission, variable-frequency drives,air conditioning and electric vehicle drives. There are severalcharacteristics of inverters efficiency. These specificationswhich define the performance and efficiency of the inverter aresmall size, little number of devices, isolated circuit, simplecircuit, minimization of total harmonic distortion (THD), lowcost and output of desirable and controllable voltage. Toachieve these demands, several power electronics circuittopologies have been presented in literature [1][8]. Severalmultilevel inverters have been also considered and proposed in

several papers in recent years [9], [10]. But in the multilevelinverters the added complexity of the circuit, and the additionalcomponents, reduce both the overall efficiency and reliabilityof the system, and may raise the overall cost of the powerelectronics interface. Also, there are different attempts toreduce losses of switches with soft-switching method [11], butthis method adds extra component to circuit and makes it more

complex. In this paper a novel inverter has been proposedwhich eliminates some of these disadvantages. The novelinverter has only one switch which reduces energy losses andcosts. It has just one simple control system which can becontrolled with a simple microcontroller and this simplecontrol method sets the output voltage to exactly what isneeded: smaller or greater amplitude, AC or DC outputvoltage. It consists of a small transformer that makes the circuitisolated, a MOSFET, a diode, a capacitor and load. Therefore,circuit is simple and its size is greatly reduced. Because of asingle switch, dead time is not needed between the switches.As it is explained in section II, the output sinusoidal voltage iscontrolled easily. Finally, in AC mode, its total harmonicdistortion, as it is proved in simulation results, is greatly low.This paper is organized as follows:

In section II, the circuit topology is introduced. In sectionIII, numerical analysis is illustrated. In section IV, simulationresults and diagrams are portrayed. Finally, conclusion isexplained in section V. This papers overall discussion is aboutthe AC mode of the circuit, because the DC to DC mode isquite simple and it namely is a flyback converter.

II. THE CIRCUIT TOPOLOGYA.DC-DC converter circuitDC-DC converter circuit is very simple and it can be any kindof these converters such as Buck, Boost, Buck-boost and etc.

A simple schematic model for a DC-DC converter used inphotovoltaic application is shown in fig.1.

http://en.wikipedia.org/wiki/AC/DC_conversionhttp://en.wikipedia.org/wiki/DC-DC_conversionhttp://en.wikipedia.org/wiki/Galvanic_isolationhttp://en.wikipedia.org/wiki/Buck-boost_converterhttp://en.wikipedia.org/wiki/Buck-boost_converterhttp://en.wikipedia.org/wiki/Plasma_globehttp://en.wikipedia.org/wiki/Voltage_multiplierhttp://en.wikipedia.org/wiki/Diodehttp://en.wikipedia.org/wiki/Flyback_transformerhttp://en.wikipedia.org/wiki/Flyback_transformerhttp://en.wikipedia.org/wiki/Flyback_transformerhttp://en.wikipedia.org/wiki/Flyback_transformerhttp://en.wikipedia.org/wiki/Diodehttp://en.wikipedia.org/wiki/Voltage_multiplierhttp://en.wikipedia.org/wiki/Plasma_globehttp://en.wikipedia.org/wiki/Buck-boost_converterhttp://en.wikipedia.org/wiki/Buck-boost_converterhttp://en.wikipedia.org/wiki/Galvanic_isolationhttp://en.wikipedia.org/wiki/DC-DC_conversionhttp://en.wikipedia.org/wiki/AC/DC_conversion

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Figure1. DC generating application

B.Inverter circuitSchematic figure of system when proposed converter producesac voltage is shown as follows. It does not need to change any

element of the circuit to switch the output mode between acand dc and vice versa. By changing pulses output voltageeasily changes.

Figure 2) AC generating application

The simplest structure of inverter is shown in Fig. 3. As it isobvious, the circuit consists of four main components: atransformer, a switch, a diode and a capacitor, which worktogether to convert the DC voltage to a desirable voltage.

Figure 3) The structure of proposed inverter

C.Method of gate pulses for AC productionIn this subsection the method of switching is explained. In

simulation, gate pulses are modeled by comparing a triangularand a sinusoidal control signals as it is shown in Fig. 3. Theoutput of the comparator is 5 volts if (1) is applied and is 0volts if (2) is applied.

Switch on if control triangularV V

Switch off if control triangularV V

Figure 1. Pulse width modulation with comparison of two signals

Frequency of control voltage is controlf which can directly

control the output frequency and the frequency of carrier signal

(or triangular signal, triangularf ). The number of samples can be

defined as (3).

triangular

control

fnf

For example, in this simulation frequency of triangularsignal is 1 KHz and the frequency of control voltage(sinusoidal) is 50 Hz. So based on (3) and (4), the number ofsamples is 20 in each period and the frequency of gate pulse is1 KHz which is not a high frequency.

1

2050

KHz

Hzn

Although with increase of triangular voltage frequency thenumber of samples is increased, that causes better outputvoltage and lower THD, in this frequency the simulation resultproves minimization of THD.

For better view of performance of the pulses and how theycause sinusoidal voltage in output, Fig. 4 is applied. In thisfigure three signals and output voltage are shown at the sametime.

For controlling the amplitude of output voltage to adesirable sinusoidal voltage, m is defined as:

control

triangular

V

V

m

Form = 1, the inverter produces the sinusoidal voltage 10volts peak to peak and the output voltage changes linearly withthe value ofm. Form = 0.5 the output voltage is 5 volts peak topeak. Fig. 5a and Fig. 5b show two different situations forcontrolling of output voltage amplitude.

Figure 2. Three signals and output voltage with m = 1, n = 20,ftriangular= 1KHz andfcontrol=fVout= 50Hz

DC-DC

flybackconverter

DC-AC

INVERTER

SPWM

PWM

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(a)

(b)Figure 3. Control and triangular voltages with (a) m = 1 (b) m = 0.5

In this section and almost all of sections in this paper themethod is analyzed for AC producing since the DC-DC modeis quite simple. With an easy change in the pulse method andreplacement of the sinusoidal control signal by a DC controlsignal, pulses will be the same and this converter would changeto the conventional flyback converter.

III. NUMERICAL ANALYSISWhen switch is turned on, the voltage of DC source is

dropped on primary winding of transformer and when it isturned off the primary winding is short-circuited by diode.

Hence, the voltage of primary winding is DC gV V t . gV t

is shown in Fig. 6.

The voltage of secondary winding, sV t , can be expressedas:

2 1s DC gV t N N V V t

For instance, if N1=N2, the input voltage could be modeled asequation (7).

() { () () ()

Assuming the solar energy is in a situation that generateenergy with 24 volts. So the modeled input voltage can beillustrated in figure 4. It is notable that because modulation

ratio (m), as expressed in equation (5), is equal to 0.5, peak ofthe input voltage is about 12 volts.

Figure 4. Modeling of input voltage

It is also worth mentioning that if voltage source changes,because of any reasons, by adjusting of m output voltageremains constant.

Input voltage in frequency domain could be considered andanalyzed. It is shown n Figure.5. There are mainly three groupof harmonics. First of all, DC part which is same as the

average of modeled input voltage (). Secondly, the mainharmonic. Third, switching frequency which directly dependson frequency ratio ,was defined in equation (3).

Figure 5. FFT analysis of Vin(t)

Figure 6. Circuit of the proposed inverter

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The resistance of winding is much smaller than itsinductance and can be ignored. The transformer can bemodeled by an ideal transformer which has two inductors insecondary. It is shown in Fig. 7.

By utilizing mentioned model of transformer, the circuit ofthis inverter can be drawn as Fig. 8.

The Laplace transform of the output voltage, outV S , can

be expressed in (7), in which sV S is the Laplace transform

of sV t .

2out s

RV S V S

LCRS LS R

Therefore the output voltage, outV t , can be summarizedas (8) :

2 2

2

2 2

2 4sin

24

t

RCout s

R LCR LV t e t V t

RCLLCR L

2 2

2

2 20

2 4sin

24

tt

RCout s

R LCR LV t e t V d

RCLLCR L

In (8), the symbol ( ) denotes the convolution integral. In

this inverter as it is illustrated in section II-A, sV t is a pulsewhich is obtained with pulse width modulation.

IV. SIMULATION RESULTS AND DIAGRAMSThe new inverter has been simulated with both PSIM and

MATLAB SIMULINK and both of results are brought andverified. The circuit utilizes a DC voltage, a transformer, aswitch, a diode, a capacitor and a load. DC voltage is 10 volts,the capacitor is 1mF, resistance load is 10 and the MOSFETis switched in 1 KHz frequency. Transformer values are shownin Table I.

TABLE I. TRANSFORMER VALUES

R1 0.01

R2 0.01

L1 1 mH

L2 1 mHLm 10 mH

N1 15

N2 15

Figure 7. The modeled transformer

Figure 8. The complete circuit is simulated in PSIM software

Figure 9. Two output voltages for circuit of Fig. 10

The parameters in Table I are the values of modeled

transformer which is shown in Fig. 9.Due to the low number of windings and the small core of

the transformer, the magnetizing inductance is relatively low.Small transformer core area causes increasing in leakage flux

that is modeled as leakageL . The circuit that is evaluated in

PSIM is shown in Fig. 10.

As it is shown in Fig. 10, there are two independent circuitswhich work separately. The left circuit is the new invertercircuit and the right circuit is sinusoidal voltage source. Thesinusoidal voltage source is set on a 10 volts peak to peak withfrequency of 50 Hz. The parameters of the inverter circuit areevaluated in a way to produce a 10 volts peak to peak in 50 Hz.

These two output voltages are drawn in Fig. 11.The original sinusoidal voltage that is produced by right

circuit is drawn with blue and the output voltage of the inverteris drawn with red. As it is evident, the voltage which isproduced by this inverter is so close to a sinusoidal voltage thatit is very difficult to distinguish the two voltages from eachother. The circuit is also simulated with MATLAB SIMULINKas it is shown in Fig. 12. It works with m = 0.5, so it produces 5volts peak to peak based on (5).

The output voltage is shown in Fig. 13. The fundamentalamplitude is about 2.5 volts. The biggest harmonic, except first

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harmonic (fundamental), is the DC output voltage which isnamed harmonic number 0. It is about 9.18% of fundamentalfrequency and the second harmonic is zero, because outputvoltage is completely symmetrical. The third harmonic is0.01%. The total harmonic distortion is defined as (10).

FFT analysis in SIMULINK

2

2

1

i

i

V

THDV

In which i is the harmonic number. If THD is evaluated,approximately it is about 0.01%. The accurate value of THD isalso obtained in SIMULINK and from FFT browser. In FFTbrowser the fundamental frequency is set to 50Hz and thesoftware evaluates the value of THD for 10 cycles that reach toa steady state condition. As it is shown in Fig. 14, the THD isreduced to a desirable value of 0.01%.

V. CONCLUSIONThere are several attempts to ideally achieve a better structureof inverters with less THD. In addition of generating adesirable DC output voltage This new sturucture circuit also

produces a sinusoidal output voltage with THD lower than1%. The inverter has been reduced in size because of

minimizing of components. It utilizes only one simple switchwhich removes the problems of dead time. It does not need

several individual control systems and use only a simplecontrol method. The output voltage easily can be controlled inamplitude and frequency. Plus the output voltage easily can beswithed between ac and dc mode. This structure also canproduce an extremely high output voltage in the resonancefrequency. All of this statement is successfully simulated andverified.

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