Guided by ; Presented by

Mrs. DEEPA M U Ajmal khan N

Asst. Pro., EEE Dept. Roll. No: 03

EEE, S7

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CONTENTS INTRODUCTION

FLYBACK CONVERTER

CHALLENGES OF FLYBACK CONVERTER

DISCONTINUOUS CURRENT MODE

BLOCK DIAGRAM

CONVERTER DESCRIPTION AND OPERATING PRINCIPLE

CONVERTER ANALYSIS

FLYBACK TRANSFORMER DESIGN

CONTROL SYSTEM DESIGN

SIMULATION RESULTS

EXPERIMENTAL RESULTS

CONCLUSION

REFERENCES

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INTRODUCTION

Electricity is the most versatile and widely used form of

energy.

It’s global demand is increasing.

The solar energy is considered as the most renewable

and freely available source of energy.

The research and development in the solar field is in rise.

The low cost is greatly important.

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FLYBACK CONVERTER

It’s the lowest cost converter among the isolated

topologies-it uses least number of components.

It combines the inductor with the transformer.

In other type of isolated topologies the inductor and

the transformer are separate elements.

Inductor is responsible for energy storage , while the

transformer is responsible for energy transfer.

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CHALLENGES OF FLYBACK

CONVERTER TO HIGH POWER Transformer with relatively large energy storage is

always a challenge.

For large energy storage it needs large air gap.

Large air gap

Magnetizing inductance

Leakage inductance

Poor coupling

Poor energy transfer efficiencyLeads to

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Discontinuous Current Mode

Advantages Very fast dynamic

response-better stability.

No reverse recovery problem.

No turn on losses

Easy control.

Small size of the transformer.

Disadvantages Higher form factor.

More power loss.

Current pulses-large peak ,high discontinuity.

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What is the solution?????

INTERLEAVING OF CELLS

Interleaving of high power flyback stages-increases the

ripple component at the waveform-proportion to the no.

of interleaved cells.

Which leads to easy filtering and using smaller sized

filtering elements.

current in each cells –less peak but same amount of

discontinuity.

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BLOCK DIAGRAM

Figure 1. Block diagram of the proposed grid connected PV inverter system based on interleaved DCM flyback converter topology 8

CONVERTER DESCRIPTION AND

OPERATING PRINCIPLES

Figure 2. Circuit schematic of the pr0posed PV inverter system based on three cell interleaved flyback converter topology .

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OPERATION

When flyback switches are turned on-current flows from

PV to magnetizing inductance of flyback transformer-

energy is stored.

During on time no current flows to the output

Therefore energy is supplied by the capacitor Cf &

inductor Lf.

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When switches are off ,energy stored is transferred

into the grid in the form of current.

To reduce the variations at the terminal volatge a

decoupling capacitor is placed at the flyback converter

output.

The full bridge inverter used for unfolding the

sinusoidally modulated dc current back to ac at the

right moment of the grid voltage.

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CONVERTER ANALYSIS

A . Flyback switch is turned on

Figure 3.Flyback switch control signal ,flybacktransformer primary v/g and magnetizing current over switching period when grid v/g is at its peak.

𝑖1 =𝑉𝑝𝑣

𝐿𝑚𝑡 = 𝑖𝑚…………………………………………..(1)

Lm=flyback transformer magnetizing inductance

𝑖1𝑝𝑒𝑎𝑘 = 𝑖𝑚𝑝𝑒𝑎𝑘 =𝑉𝑝𝑣 𝐷𝑝𝑒𝑎𝑘

𝐿𝑚𝑓𝑠.....................(2)

Fs=switching freq.; Dpeak=duty ratio

𝐼1 =𝐼𝑝𝑣

𝑛𝑐𝑒𝑙𝑙=

𝑉𝑝𝑣 𝐷²𝑝𝑒𝑎𝑘

4𝐿𝑚 𝑓𝑠………………………………..(3)

I1=average dc current

Ppv=Vpv Ip=𝑛𝑐𝑒𝑙𝑙 𝑉²𝑝𝑣 𝐷²𝑝𝑒𝑎𝑘

4𝐿𝑚 𝑓𝑠………………………(4)

ncell= no. of interleaved cells

Ppv= PV source o/p power.

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B.Flyback switch is turned off

ni2=im=𝑉𝑔𝑟𝑖𝑑

𝑛 𝐿𝑚𝑡………………..(5)

Vgrid=peak of the grid voltage

n= flyback transformer turns ratio

I2=𝐼𝑔𝑟𝑖𝑑

𝑛𝑐𝑒𝑙𝑙=

𝑉²𝑝𝑣 𝐷²𝑝𝑒𝑎𝑘

2 𝐿𝑚 𝑓𝑠 𝑉𝑔𝑟𝑖𝑑………(6)

I2=max. value of the grid current.

oComparing eqa.(4) &(6)-the average power from PV panels equal to

the active power transferred to the grid assuming an ideal converter.

Ppv=Vpv Ipv=𝑛𝑐𝑒𝑙𝑙 𝑉²𝑝𝑣 𝐷²𝑝𝑒𝑎𝑘

4 𝐿𝑚 𝑓𝑠=

𝑣𝑔𝑟𝑖𝑑 𝐼𝑔𝑟𝑖𝑑

2= 𝑃𝑔𝑟𝑖𝑑……….(7)

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c. ANALYSIS FOR SIZING OF DECOUPLING CAPACITOR

The control system has no feedback loop for the

regulation of o/p current.

Since the PV source is not an ideal v/g source it’s o/p

voltage is fluctuating-we provide a decoupling

capacitor –i/p of the flyback converter.

Major sizing criterion is the effectiveness in diverting

double line freq. away from PV source.

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Peak to peak voltage ripple across the decoupling capacitor, ΔVpv=ΔVc= Xc ΔIc………..(8) ΔIc= current ripple

CONVERTER ANALYSIS

Switching freq. is 40khz-higher efficiency with smaller sized

magnetics.

A clamp or a snubber is provided to keep switching transients

within safe operating area .

Flyback transformer will use the most optimum winding

strategy for the lowest leakage inductance practically

possible.

𝑐 ≥2𝐼𝑝𝑣

2𝜋100 𝛥𝑉𝑝𝑣…………….(9)

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Flyback Transformer Design Air gap length of the flyback transformer can be found

using….

Lowest leakage inductance can be achieved by……

Making coil & core heights longer

Reducing the number of winding layers –less space b/t layers.

Using sandwiched windings –magnetic field inside the

window area is reduced-reduces leakage inductance.

𝑙𝑔 =𝑁²µ˳ 𝐴𝑐𝑜𝑟𝑒

𝐿𝑚

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Figure 4. PLECS model of the proposed PV inverter system including the power stage and the controller.

oThe PLECS software comes with a 65w PV model developed by plexim engineers based on the commercial BP365 part numbered PV panel. 17

CONTROL SYSTEM DESIGN

The control system is designed for two functions

simultaneously without feedback loop.

Harvesting the max. power & pump the power to utility

grid with high quality.

Because of implementation simplicity, the perturb and

observe (P&O) method is selected .

Figure 6 shows the flow chart of the P&O algorithm

implemented in the DSP controller.

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Figure 6. flow chart of P&O algorithm 19

Besides the magnitude regulation for max. power transfer, the controller should achieve synch. Of current with grid v/g

For this purpose the o/p of MPPT block is multiplied by the PLL o/p.

T=fundamental period of the grid signal.

Figure 7. PLL structure based on T/4 transport delay technique.

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Another control signal that is also synch. With PLL o/p is used to control H-bridge IGBT inverter for unfolding purpose.

The whole control system is implemented in TMS320F2335 Texas Instrument’s DSP controller.

Figure 8. Flowchart of the DSP firmware 21

SIMULATION RESULTS

Figure 10. simulated wave form of the grid v/g and current.

Figure 11. simulated waveforms of the PV module terminal v/g & the grid current.

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EXPERIMENTAL SETUP

Figure 12. Experimental Setup

Figure 13. Exp.wave form of grid v/g(purple) & grid current(green).

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EXPERIMENTAL RESULTS

The energy harvesting effi. Of the MPPT algorithm at

the nominal power is 98.5%.

The power delivered to the load-grid interface is

measured as 1732.4 W.

The THD of the grid current & v/g is measured as

4.42% and 2.49% respectively.

The pf is measured as .9975.

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CONCLUSION

The 2 KW power level is achieved by interleaving of

three flyback cells each rated at 700w.

The power harvesting effi. of the MPPT controller is

measured as 98.5%.

The THD of the grid current is 4.42% & pf =.998.

Interleaved flyback topology is practical at high power

as central type PV inverter.

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REFERENCES

[1] Solar energy (2013, July 23). [Online]. Available: http://www.conserveenerg

future.com/SolarEnergy.php.

[2] Europe Photovoltaic Industry Association (EPIA) (2013, July 23) Global market

outlook for photovoltaics 2013–2017.

[3] Y. Xue, L. Chang, S. B. Kjaer, J. Bordonau, and T. Shimizu, “Topologies of

single-phase inverters for small distributed power generators: An overview,”

IEEE Trans. Power Electron., vol. 19, no. 5, pp. 1305–1314, Sep. 2004.

[4] S. B. Kjaer, J. K. Pedersen, and F. Blaabjerg, “A review of singlephase grid-

connected inverters for photovoltaic modules,” IEEE Trans. Ind. Appl., vol. 41,

no. 5, pp. 1292–1306, Sep. 2005.

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