ee552 power electronics project report

8/9/2019 EE552 Power Electronics Project Report

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EE 552 Power Electronics

Project Report

Project Title

Design and Implementation of a Flyback Converter

for DC Micro grid Applications

Group members:

Mashood Nasir 14060018

Umer Irfan 14060003


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Contents

Abstract: .......................................................................................................................................... 3

Introduction ..................................................................................................................................... 3

Flyback Converter:.......................................................................................................................... 5

Flyback Operation during DTs: ................................................................................................... 5

Flyback Operation during (1-D)Ts: ............................................................................................. 6

Project Design and Specifications: ................................................................................................. 8

Calculations for Snubber circuit: .............................................................................................. 11

Practical Results: ........................................................................................................................... 16

High to low conversion ............................................................................................................. 16

High to low conversion ............................................................................................................. 16

Conclusion: ................................................................................................................................... 16

Figure 1: Equivalent circuit of a flyback converter ........................................................................ 5

Figure 2 : Flyback operation during DTs ........................................................................................ 6

Figure 3: Flyback operation during (1-D)Ts ................................................................................... 6

Figure 4: Bidirectional Flyback converter switch realization ......................................................... 8

Figure A: RCD Snubber .11Figure 5 : PSIM implementation of 12V to 120V Flyback converter .......................................... 12

Figure 6: Output voltage of PSIM Implementation of 12 to 120V flyback converter .................. 12

Figure 7: Voltage across Switch 1 ................................................................................................ 13

Figure 8: Voltage Spike across Switch 2 without Snubber Circuit .............................................. 13

Figure 9: Peak Stress Reduction on Switch 2 ............................................................................... 14

Figure 10: PSIM Implementation of 120V to 12V Flyback Converter ........................................ 14

Figure 11 : Output Voltage of PSIM Implementation of 120 to 12V Flyback Converter ............ 15

Figure 12 : Voltage Spike across Switch 2 Without Snubber Circuit.......................................... 15

Figure 13 : Peak Stress Reduction on Switch 2 using RCD Snubber ........................................... 16


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Abstract:

In this project the design and implementation of a high gain bidirectional flyback converter forDC microgrid applications is presented. Flyback converter is highly suitable for low power

applications up to the range of 100W. Flyback converter has inherent advantages of simple

design and less component usage over other types of buck- boost converters, therefore, highly

suitable for DC microgrid applications. A flyback converter is explicitly modeled for its

operation in continuous conduction mode (CCM). A snubber circuit is designed to reduce the

peak stress on the switching device. The switch realization for bidirectional flow of power

between DC microgrid and battery storage system is delineated. Flyback transformer is designed

to achieve the intended level of voltages. The proposed bidirectional model is implemented on

PSIM and simulation results verify that the proposed model can be efficiently used for the

bidirectional flow of power between DC microgrid and its associated battery storage system.

Introduction:

The power industry has been facing the tremendous challenges of generation diversification,

efficient deployment of the costly power equipment, supply demand management, consumer

empowerment and reduction in carbon contents. The conventional grid topologies are not

capable enough to cope up with these multiple challenges. In order to address these critical

issues, the need of the revolutionized smart grid is inevitable. Smart grids are becoming more

and more popular these days and it seems that a large number of them will be installed in future.

A microgrid is considered as the central part of future smart grids and it provides an efficient

way of controlling the system with multiple renewable energy resources distributed generation

(DG) units without re-designing the actual system [1, 4].

Considering the large scale renewable energy resources integration, the evolution of future smartgrids are essentially considered as plug and play integration of micro grids. A microgrid is a

distribution network that contains multiple loads and generation sources that can be modeled as a

single load or a source. Microgrids can provide a very high local reliability and useful heat by

connecting DG units and critical loads in close proximity. Microgrids can employ various DG

technologies such as fuel cells, photovoltaic systems, small diesel generators, wind turbines, and


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micro-turbines together with energy storage devices such as batteries, flywheels and condensers

[1, 2, 4].

The major advantages of a microgrid are:

It provides power to critical loads during an emergency condition, e.g. a fault in the grid.

It can operate independently in feeding power to critical loads and shedding off non-

critical loads. It provides a higher local reliability than the reliability of the grid. This

increased reliability is very important for loads like chemical processes and

communication devices where failure can cause large financial loss.

It allows integration of renewable sources in the power system without major

modifications or redesign. It is an excellent topology for the integration of more and more

green sources in an existing power system.

Plug and play functionality allows switching between grid connected and isolated modes

of operation. It allows a constant frequency and voltage during the islanding mode and

resynchronization once the fault is removed.

Modular structure allows insertion of new microgrids without redesign of the system.

Any number of distributors can be converted into microgrids.

As the DG sources are placed close to the loads, the waste heat from these units can be re

utilized. The expenditure on installing of new transmission lines and transmission line

losses are greatly reduced by the production of electricity close to load centers.

Microgrids solve power-quality problems, e.g. voltage sags, load imbalance, harmonics

and transient stability.

Microgrids have energy storage devices such as batteries, condensers, and flywheels to store

energy for a small duration as a back-up power. This back-up energy is needed to minimize the

effect of source dynamics. In order to ensure the optimum transfer of energy between the

microgrid and batteries storage system, an isolated bidirectional converter is required that can

efficiently transfer energy between DC microgrid and DC batteries [3, 4]. The flyback converter

has multiple advantages over other type of isolated buck boost converters and are listed below

Relatively simple design

High conversion ratio using flyback transformer turn ratio


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Non inverted output with the help of inverting transformer

The magnetizing inductance of flyback transformer may be utilized for the storage and

transfer of energy from source to load, therefore eliminating of the need of the external

inductor.

The bidirectional flyback converter is proposed for the flow of power between 120V DC

microgrid and 12V battery storage system.

Flyback Converter:

Flyback converter is the modified isolated form of buck boost converter as shown in figure 1 and

its operation in CCM is presented [6]. The mosfet operation is divided in two time intervals. Out

of the total switching time Ts, it conducts for DTsand remains off for (1-D)Ts, where, D is the

duty cycle for CCM operation.

Figure 1: Equivalent circuit of a flyback converter

Flyback Operation during DTs:

During DTs the mosfet Q1 conducts and the diode D1 remains off. The equivalent circuit is

shown in figure 2.


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Figure 2 : Flyback operation during DTs

The energy from the source is stored in the magnetizing inductance L M of the flyback

transformer. Using KCL and KVL the voltage on inductor VLand current on capacitor Icmay be

found and are given in equations 1 and 2.

gl VV (1)

R

VIc (2)

Where, Vg= input voltage, V= output voltage and R = load resistance.

Flyback Operation during (1-D)Ts:

During subinterval 2 i.e. for (1-D)Ts the mosfet remains off while the diode conducts. So, the

energy stored in the magnetizing inductance of the primary is transferred to the load. The

equivalent circuit operation during sub interval 2 is shown in figure 3 and its associated

equations are given by (3) and (4).

Figure 3: Flyback operation during (1-D)Ts

n

VVL (3)


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R

V

n

IIc (4)

Where, n = turn ratio of flyback transformer and I is the dc component through the magnetizing

inductance of the flyback transformer.

By the application of volt- sec balance across the inductor and amp sec balance across the

capacitor the conversion ratio of voltages and currents are found and are given by equations

below

D

Dn

V

VDM

g

1 (5)

RD

nVI

'

(6)

Where, I is the dc component of magnetizing current.

Bidirectional Flyback Switch Realization:

In order to utilize the flyback converter for DC microgrid applications its bidirectional switch

realization is performed and it has been shown that by replacing the diode of the figure 1 with

another controlled switch and then changing its orientation such that without effecting the

continuity of the circuit the source of the newly added mosfet is grounded. In this way the gate

driver circuit design will be easier as there will be no requirement of complex bootstrapping

circuit [5].


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Figure 4: Bidirectional Flyback converter switch realization

Project Design and Specifications:

The proposed project is to design and implement an isolated bi-directional flyback converter that

may efficiently transfer the energy between 120V DC microgrid and 12/24V DC batteries. The

proposed power rating for the isolated bidirectional flyback converter is 60W.

Using (5) for D = 0.34 the transformer turn ratio may be calculated and therefore, the flyback

transformer is designed for n=6. The reason for the selection of D= 0.34 is that the maximum

switch utilization in flyback converter and the minimum stress occurs at this duty cycle [6, 7].

After the calculation of turn ratio the next step is the design of magnetizing inductance such that

the converter operates in CCM and is independent of the load resistance for a fair range of

operation.

From (6) it may be written as

RD

V

n

nIm

)(1

2 (7)

Where, n2/n1is the turn ratio. For the 120 to 12 V volt conversion at 60 W and output resistance

of 2.4 and D

4.2

12

625.

1

6

1mI = 0.365


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The maximum allowable ripple current Imin order to keep the converter in CCM is set to be

20% of the magnetizing current Imand is given by (8)

2667.0%20 mm II (8)

Thus the maximum current carried by the magnetizing inductance Lm is the sum of dc

component and ripple component and is given by (9)

AIm 6.1max (9)

Thus the value of magnetizing inductance Lm can be calculated by (10)

mHKi

DTVLm

sgm 101043.8

667.210

1375.120

25

(10)

The current on the high voltage side is given by (11)

2

1 )(3

11

m

mm

I

iDII

A822.0375.334.1 (11)

In the same way the current through low voltage side is given by (12)

AI 37.6)334.1(3

)2667.0(11625.0334.162 (12)

The total Current Itot referred to high voltage side carried by the flyback transformer is given by

the summation of (11) and (12)

AmpIn

nIItot 88.12

1

21 (13)

The geometrical constant Kgfor the core of transformer is calculated and is given by (14)

ucu

mtotm

KPB

IILKg

2

max

82

max

2210

93.3)3.0)(15()25.0(

10)66.1()68.1()1043.8(10724.12

822236

(14)


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Where Bmax = maximum flux density, Pcu= allowable limit of copper losses and Ku is the

utilization constant of the core and is generally taken as 0.3 [6].

The nearest available core in the standard EE core table is EE70/68 with the following

parameters [6]

0.18

0.14

75.6

24.3

06.5

m

A

c

g

l

MLT

W

A

K

Where, Ac= area of the core, WA= window area, MLT= mean length per turn and l m= total

length of core.

The required air gap length is given by (15)

mmAB

IIl

c

MMg 33.1

102

max

42

max0

(15)

The number of turns for high and low voltage sides is calculated by (16) and (17)

TurnsAB

ILn

c

Mm 16751.16610

max

4

max1

(16)

Turnsn 286

1672 (17)

The fraction of window area allocated to high voltage and low voltage sides are given by 1and

2

43.088.1

822.011

totI

I (18)

57.0

88.1

37.6

6

12

1

12

totIn

In (19)

The respective thickness for the high and low voltage sides winding thus may be calculated as

201021.5167

75.63.043.0 3

1

11 guage

n

WKA Auw

(20)


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121022.4128

75.63.057.0 3

2

22 guage

n

WKA Auw

(21)

Snubber Circuit

Transformer leakage reactance Llcomes effectively in series with the mosfet. Therefore, during

switch off cycle drain of the mosfet experiences a peak voltage due to the energy stored in the

leakage inductance. An RCD snubber circuit as shown in figure A is designed to clamp this

voltage.

Figure A: RCD Snubber

Calculations for Snubber circuit:

For the designed transformer at the mentioned ratings, the calculations for snubber circuit arelisted below

Lm = 38.4*10-3

HLl = 0.03* 38.4*10

-3= 1.152*10

-3H

WILm32

1leakage 10*47.12

1E

Pleakage = E leakage* f = 14.7W

Rclamp = Vclamp KR

VR

clamp

clamp 721.22

FRf

Css

clamp 221

Simulation Results and Discussions:


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The proposed circuit is implemented on PSIM for as shown in figure 5 for 12 V to 120 V

conversions i.e. the flow of power from battery to the microgrid:

Figure 5 : PSIM implementation of 12V to 120V Flyback converter

The above circuit has power supply of 12 volts connected at the input side and transformers turn

ratio is 6. During first interval energy is stored in primary side and is passed onto the secondary

side in second interval. Output voltage is 120volts.

Figure 6: Output voltage of PSIM Implementation of 12 to 120V flyback converter

The leakage Inductance LL of the flyback transformer comes effectively in series with the

switches; therefore, high di/dt causes voltage spikes the switch as shown in figure 7 and 8. At


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switch 2 (high tension side), we have to block a voltage of 192 volts. It is necessary to block

these peaks using a snubber circuit otherwise it can damage the mosfet.

Figure 7: Voltage across Switch 1

Figure 8: Voltage Spike across Switch 2 without Snubber Circuit

Without snubber snubber circuit a voltage peak of 250+ volts is shown on switch 2. The

employed mosfets have a blocking voltage threshold of 200 volts. 250 volts peak can damagemosfets instantly. Therefore, RCD snubber as shown in the figure 5 reduces the peak stress on

the MOSFET 2 and results are shown in figure 9. It is evident from the results that the snubber

circuit has limited the voltage peak to 200 volts.


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Figure 9: Peak Stress Reduction on Switch 2

The proposed circuit is implemented on PSIM for as shown in figure 10 for 120 V to 12 V

conversions i.e. the flow of power from microgrid to the battery:

Figure 10: PSIM Implementation of 120V to 12V Flyback Converter

The above circuit has power supply of 120V connected at the input side and transformer turn

ratio is 6. During first interval energy is stored in primary side and is passed onto the secondary

side in second interval. Output voltage is 12volts. The output voltage is shown in figure 11.


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Figure 11 : Output Voltage of PSIM Implementation of 120 to 12V Flyback Converter

The peak stress on the switch 2 without snubber circuit is shown in figure 12.

Figure 12 : Voltage Spike across Switch 2 Without Snubber Circuit

Using RCD snubber the peak is limited to 200 V and is shown in the results of figure 13.


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Figure 13 : Peak Stress Reduction on Switch 2 using RCD Snubber

Practical Results:

High to low conversion

D = 0.42 RL= 6.6 10Vin

Vo

High to low conversion

D = 0.3 RL= 940

10

1

Vin

Vo

Conclusion:

In this project a methodology for the bidirectional transfers of energy between Microgrid and

energy storage system is presented. The Design of flyback transformer to interconvert the

voltages from 12 to 120 V, 120 to 12V and associated flyback transformer to operate in CCM

is delineated. Design of Snubber Circuit for reducing the switching stresses in either

direction of power flow is also presented. The proposed scheme may be efficiently used to

supply the backup power to the loads attached with isolated and grid connected microgrids.

References:

[1].T. S. Ustun, C. Ozansoy, A. Zayegh, Recent developments in microgrids and example

cases around the worldA review, Renewable and Sustainable Energy Reviews 15

(2011) 40304041.


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[2].H. Farhangi, The path of the smart grid. IEEE Power and Energy Magazine, 8(1):18

28, 2010.

[3].G. J. Zhang, X. Tang and Z. P. Qi, Application of Hybrid Energy Storage System of

Super-Capacitors and Bat- teries in a Micro-Grid, Automation of Electric Power

Systems, Vol. 34, No. 12, 2010, pp.85-87

[4].G.M. Masters, Renewable and efficient electric power systems, John Wiley & Sons,

Inc. Publications, 2004

[5].L.Huang, et al., Battery Powered High Output Voltage Bidirectional Flyback Converter

for Cylindrical DEAP Actuator Power Modulator and High Voltage Conference

(IPMHVC), 2012 IEEE International,2012 pp. 454457

[6].R. W. Erickson and D. Maksimovic, Fundamentals of Power Electronics. Norwell, MA:

Kluwer, Mar. 1, 1997.

ee552 power electronics project report

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