Flyback Converters

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Flyback Converters

RESOSO, Alvin C.

TOLENTINO, Ellis Zaldy M.


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Report Outline

1. Review: Transformers

2. Introduction to Flyback Converters

3. Difference with the Buck-Boost

4. Isolated SMPS

a. Non-Isolated SMPSb. Isolated SMPS

c. Advantages vs. Disadvantages

5. Topology

6. Assumptions for Analysis

7. CCM Operation

a. Switch Closed

b. Switch Open

8. DCM Operation

9. Applications


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Review: Transformers [1]

Basic function is for I/O isolation

Power supply applications work at a high frequency so transformershould be as small as possible

Ideal Model:

[1] DC to DC Converter (Chopper). Dr. Zainal Salam, UTM-JB, Power Electronics and Drives (Version 3-2003)


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Basic I/O relationship:

Voltage:

Current:



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For most power electronics application, we use the model with

magnetizing inductance at the primary side



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Introduction

Simplest and most implemented SMPS topology

Commonly used in low-power applications (a few watts to ~100W)[1]

Used when output needs to be isolated from the input (more on

this later) Can offer multiple isolated output voltages

Energy efficiency is lower than other SMPS circuits, but itssimplicity and low cost is favored

Has controllable switch (usually MOSFET) with operatingfrequency of around 100 kHz [1]

[1] Kharagpur. Version 2 EE IIT. Lesson 22: Fly-Back Type Switched Mode Power Supply. p3


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Introduction

Almost same as the Buck-Boost, except that the inductor isreplaced by a Transformer.

DC to DC Converter (Chopper). Dr. Zainal Salam, UTM-JB, Power Electronics and Drives (Version 3-2003)


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Introduction [1]

Almost same as the Buck-Boost, except that the inductor isreplaced by a Transformer.



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Introduction [1]

Model with magnetizing inductance at the primary side



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Difference with the Buck - Boost [1]

Flyback:

1. Uses Transformer to store

energy (typically step-up or

isolation Transformer - more

on this later)2. Can't operate on 2 modes.

3. Used for high power

applications (CRT screens,

etc.)

Buck - Boost:

1. Uses Inductor to store energy

2. Can operate in 2 modes:

a. Buck mode

b. Boost mode3. Used generally for low power

operations.

[1] What are differences between buck-boost and flyback smps.

Available: http://www.edaboard.com/thread141689.html
http://www.edaboard.com/thread141689.htmlhttp://www.edaboard.com/thread141689.htmlhttp://www.edaboard.com/thread141689.html


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Isolated SMPS

Non-Isolated SMPS:

These topologies have their inputs and output not isolatedfrom each other (electrically connected)

Examples:

Buck Converter [Step Down] Boost Converter [Step Up]

Buck-Boost Converter [Cascaded Buck and Boost]


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Isolated SMPS [1]

Isolation is the principle of coupling one circuit to another withoutconnecting them directly (no direct current flow between input

and output)

In isolated SMPS topologies (generally, in power electronics), the

most commonly used form of isolation is the TRANSFORMER Isolated SMPS Topologies:

Flyback

Foward

Bridge (Half, Full)

Push-Pull


Available: http://www.allaboutcircuits.com/vol_2/chpt_9/3.html
http://www.allaboutcircuits.com/vol_2/chpt_9/3.htmlhttp://www.allaboutcircuits.com/vol_2/chpt_9/3.html


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Isolated SMPS - Advantages [1][2]

Isolation between the ground of the input supply and the output load is

often desired to isolate the common returns from different parts of the

electronic system in order to eliminate ground loops between circuitries.

Serves as protection for both the user and the equipment against

dangerous transient voltages induce Improved noise immunity

Ease of output connections without conflict with the primary ground

The introduction of a transformer provides necessary voltage scaling thus

enabling a higher output voltage (Vo/Vin does not depend solely on the duty

cycle)

Offers the advantage of having multiple outputs at different voltage levels

[1] Flyback Converter. National Semiconductor, 2010

[2] Power Switching Converters. Simon S. Ang, 1995, pp. 119-120


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Isolated SMPS - Advantages

Flyback converter with multiple output[1]

[1] Multi-Output Flyback Offline Power Supply, ON Semiconductor


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Isolated SMPS - Disadvantages [1]

Complexity (key design issue is the power transformer)

Electromagnetic Interference or EMI

disturbance that affects an electrical circuit due toelectromagnetic induction


Available: http://en.wikipedia.org/wiki/Electromagnetic_interference
http://en.wikipedia.org/wiki/Electromagnetic_interferencehttp://en.wikipedia.org/wiki/Electromagnetic_interference


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Topology

Flyback converter:

Elements:

Switching Device [usually MOSFET] Diode Capacitor Flyback Transformer [different from ordinary transformer]

DC to DC Converter (Chopper). Dr. Zainal Salam, UTM-JB, Power Electronics and Drives (Version 3-2003)


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Topology - MOSFET

Used to switch the primary of the transformer ON or OFF

Has switching frequency in the 100 kHz range

Used with a dynamic control (feedback system) to adjust dutycycle (which in turn dictates the desired regulated output voltage)


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Topology - Flyback Transformer [1]

Used for voltage isolation

Primary and secondary windings so that they are linked by nearlythe same Magnetic Flux

Doesn't work the same way as a normal transformer

Primary and Secondary windings of a flyback transformerdoesn't conduct simultaneously.

The magnetic design of the flyback transformer is more like ofan inductor.

The flyback transformer may be more appropriately called aninductor-transformer

Can have multiple output windings for generating multiple isolatedvoltages (shown earlier).

[1] Kharagpur. Version 2 EE IIT. Lesson 22: Fly-Back Type Switched Mode Power Supply. pp. 3-4


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Assumptions for Analysis [1][2]

Magnetic circuit - > ideal

Coupling between Primary & Secondary -> ideal

No winding leakage inductances.

Voltage drops of switch (MOSFET) and diode are neglected.

Windings, transformer core, capacitor are lossless

CCM Additional Assumptions:

Very large output capacitance -> constant output voltage

Circuit operating in steady-state. All voltages and currents areperiodic, with period T

Duty ratio of the switch is D: closed for DT, and open for (1-D)T

Switch (MOSFET) and diode are ideal.

[1] Hart, Daniel W. Power Electronics. pp. 267-268

[2] Kharagpur. Version 2 EE IIT. Lesson 22: Fly-Back Type Switched Mode Power Supply. p4


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CCM Mode of Operation [1]

Switch closed (Assuming all components are ideal)



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Switch opened



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At steady-state,



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Output voltage

Similar to the I/O relationship of the buck-boost converter (exceptfor the transformer ratio factor)



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CCM Waveforms [1]




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CCM Waveforms [1]




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DCM Mode of Operation

For DCM, the output voltage is given by

R is the load resistance

T is the switching period


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CCM-DCM Boundary [1]

Ave. Inductor current



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CCM-DCM Boundary [1]

Max. inductor current

Min. inductor current



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CCM-DCM Boundary [1][2]

This is the minimum required inductance to operate at CCM.


[2] Power Electronics. Marvin J. Fisher, 1991. pp.311-315


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Applications

Low Power Switch Mode Power Supplies (i.e. Cell phone chargers,etc)

Multiple outputs PS (i.e. Computer [Desktop] Power Supply)

High Voltage supply for CRT in TV sets.

High Voltage generation


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All References

Rashid. Power Electronics Handbook. pp. 217-218.

Dr. Salam, Zainal. Power Electronics and Drives. version 3-2003.Chapter 3: DC to DC Converter (Chopper).

Kharagpur. Module 3: DC to DC Converters. version 2 EE IIT.

Lesson 22: Fly-back Type Switched Mode Power Supply. Fisher, Marvin J. Power Electronics. pp. 310-315

Ang, Simon S. Power Switching Converters. pp. 119-146

Wikipedia. Flyback Converter. Available: http://en.wikipedia.org/wiki/Flyback_converter

Wikipedia. Flyback Transformer. Available: http://en.wikipedia.org/wiki/Flyback_transformer
http://en.wikipedia.org/wiki/Flyback_transformerhttp://en.wikipedia.org/wiki/Flyback_converterhttp://en.wikipedia.org/wiki/Flyback_transformerhttp://en.wikipedia.org/wiki/Flyback_transformerhttp://en.wikipedia.org/wiki/Flyback_converterhttp://en.wikipedia.org/wiki/Flyback_converter


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Thank you for listening!


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Quiz [Total 10 points]

Flyback Converters

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