實驗室 flyback converters -...

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Ying-Yu TzouFlyback ConvertersYing-Yu TzouYing-Yu Tzou Ying-Yu Tzou Ying-Yu Tzou2008215

Flyback Converters

, Sept. 2006. 808-PowerLab 1

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Flyback Converters

Power Electronics Systems & Chips Lab.

Power Electronics Systems & Chips Lab., NCTU, Taiwan

2/57

Flyback Converters

Derivation of the Flyback ConverterFeatures of Flyback ConverterWhy Choose the Flyback Converter?Properties of Flyback ConvertersDisadvantages of the Flyback ConvertersOperating Principle of the Flyback ConverterTypical Waveforms of the Flyback ConverterDesign EquationsFlyback Converter Transformer-ChokeCross Regulation in Multiple Outputs Flyback Converters

Flyback Converters


3/57

Invention of the Flyback Converter

How to design an isolated buck-boost converter with minimum number of components?

vo

vg

?

4/57

Derivation of the Flyback Converter

(a)

Vi Vo

+

L+

(b)

Vi Vo

+

L1+

L2

(c)

Vi Vo

+

L2L1

(d)

Vo

_

+

L2Vi

L1

Flyback Converters


5/57

Features of Flyback Converter

The isolated flyback converter is basically a buck-boost derived converter with an isolation winding, so that the input circuit is isolated from the output circuit, and the output voltage can be either positive or negative, depending on the winding and diode connected polarities.Application: Lowest cost, multiple output supplies in the 5-150W range. E.g. mains input T.V. supplies, small computer supplies.

DC-DC ConverterInput Stage

AC LineInput

Vout

+

+

PWMVSENSE

Isolati on

6/57

Why Choose the Flyback Converter?

Flyback power supplies use the least number of components. At power levels below 75 watts, total flyback component cost is lower when compared to other techniques. Between 75 and 100 Watts, increasing voltage and current stresses cause flyback component cost to increase significantly. At power levels higher than 100 Watts, topologies with lower voltage and current stress levels (such as the forward converter) may bemore cost effective even with higher component counts.

Flyback Converters


7/57

Properties of Flyback Converters

In design of a flyback power supplies, transformer design is usually the biggest stumbling block. Flyback transformers are not designed or used like normal transformers. Energy must be stored in the core, the core must be gapped, and the transferred energy is stored in the air gap. The air gap inductance represents most of the magnetization inductance. However, this magnetization inductance is relative small.

n : 1 +

+

oI

VoVin

Co RD

Q

8/57

The Flyback Transformer

n : 1 +

+

oI

VoVin

Co RD

Q

n : 1 +

+

oI

VoVin

Co RD

Q

LM

Transformer Model

The air gap inductance represents most of the magnetization inductance.

LM

Ideal Transformer

Flyback Converters


9/57

The Flyback Transformer

A two-winding inductor Symbol is same as transformer, but function differs significantly from ideal transformerEnergy is stored in magnetizing inductanceMagnetizing inductance is relatively small

Current does not simultaneously flow in primary and secondary windingsInstantaneous winding voltages follow turns ratioInstantaneous (and rms) winding currents do not follow turns ratioModel as (small) magnetizing inductance in parallel with ideal transformer

n : 1 +

+

oI

VoVinCo RD

Q

LM

Transformer Model

10/57

Current Flow in Flyback Converter

Current effectively flows in either the primary or secondary winding but never in both windings at the same time.

n : 1 +

+

oI

VoVin

Co RLM

Q ON and D OFF

I outT

T

inV2

inV

on off onvgs

VDS

Ip

Is

Io

Q OFF and D ONn : 1

+

+

oI

VoVin

Co RD

LM

Flyback Converters


11/57

Current Flow in Flyback Converter

Q OFF and D OFFn : 1

+

+

oI

VoVin

Co RD

LM

I outDT

T

inV2

inV

on off onvgs

VDS

Ip

Is

Io

12/57

Flyback Converter in Discontinuous Conduction Mode

I outDT

T

inV2

inV

on off onvgs

VDS

Ip

Is

n : 1

+

+

IoVin

Co R

Ip

Q

D

vgs

Vo

Io

Is

p

satCEDC

LVV

dtdI )(=

The Primary Current rising slope:

Peak Primary Current:p

satCEDCp L

DTVVI

)( )(=

Flyback Converters


13/57

Flyback Converter in DCM

A1

A2

A1 = A2

(a)

(b)

(c)

(d)

Ip

IsTon

Tr Tdtdcpmdc )V/N(NV +

dcV

C1

Vdc

D1T1

Nsl

Vat

D2

Nsm

Vom

C0 R0

R1

R2

EAVref

DC voltage-controlledv ariable width

pulse generator

Q1

Lp

Np

s

pps N

NII =

14/57

When the Primary Switch Is ON

+

+

VoVin

Co R

Q

Ip

Ton

Ip

Lp Primary Magnetizing Inductance

L1 Lg L2

g

2g1p

L//L//LLL

=Lp

21g //LLL >>Q

Io

The secondary diode is reverse biasedThe load power is supplied by the output capacitor The primary current store energy into the primary magnetizing inductance!

Lg

Flyback Converters


15/57

Energy Stored in The Magnetization Inductance

+

+

VoVin

Co R

Q

Ip

Ton

Ip


Io

2)(IL

E2

pp=The Maximum Stored Energy is:

Most energy is stored in the air gap!

CoLg

The Gap Effect

16/57

Release The Stored Energy to the Output Rectifier

+

+

VoVin

Co R

D

Q

Lg

Ip

Ip Is


Io

s

pps NN

II =

Ip(max) 2)(ILE

2ss=

The released energy is equal to the stored energy when operating in steady state.

When the Primary Switch Is OFF

Ton

Toff

Flyback Converters


17/57

Flyback Converter: Typical Waveforms

nuu 1

2

10

=

s

u eb

u 1

i

n : 1

C RL

ioD

+

-

+

-u ce

21 u O

I

T

T

S

u ce

n

1

T2

Ni

T

T

u1

s

s

s s

s

2

1

S

u ce

n

1

i

ii

2

Ni

DCM CCM

i

18/57

Typical Characteristics n : 1 +

+

IL

VoVin

Co R

Suitable for Off-line SPS under 75 Watts

Typical converter efficiency: = 80%

Max. duty ratio: Dmax = 45%

Max. transistor voltage: Vds = 2Vin(max) + leakage spike

DC voltage gain (CCM):

DC voltage gain (DCM):

D-1Dn

VV

in

o =

P

L

in

o

LT

2RnD

VV

=

Note: DCM buck-boost characteristic is linear.

Flyback Converters


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Flyback Converters


21/57

Buck-Boost Converter: CCM and DCM Static Characteristics

CCM DCM

K1

Buck-

boos

t

0 0.2 0.4 0.6 0.8 1.0

1.0

D

CCMM(D, K)

Buck

Boost

0 0.2 0.4 0.6 0.8 1.0

1.0

D

CCMM(D, K)

22/57

Output Power and Voltage as a Function of PWM Duty

+

+

VoVin

Co R

Q

Ip

Ton

Ip Io

2)(IL

E2

pp=The Maximum Stored Energy is:

CoLg

The Average Power Pass Through The Choke-Transformer

[Watts] 2T

)(ILP

2pp

avg =

p

ONsatinp L

)TV(VI =Q [Watts] 2TL)T(V

2TL)])TV-[(VP

p

2ONin

p

2ONsatin

avg ==

Flyback Converters


23/57

Voltage Conversion Ratio in DCM Operation

+

+

VoVin

Co R

Q

Ip Io

CoLg

Pi Input Power Po Output Power

+

=Tt

t pini0

0(t)dt(t)iv

T1P=Power Input Average

+

=Tt

t o

Flyback Converters


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Turn Ratio Maximum Switch Voltage

n : 1 +

+

oI

VoVinCo RD

Q

LM

Transformer Model

Maximum OFF-Voltage Stress?

age)spike(leakdiode(on)os

pdc(max)Q(max) V)V(VN

NVV +++=

Max. transistor voltage: Vds = 2Vin(max) + leakage spike

Worst Case Condition:

26/57

Ensure Core Does Not Saturate Remains in DCM Mode

Ip

Is

+

+

VoVin

Co R

D

Q

Lg

Ip IoIp(max)

Ton

ToffTo guarantee the core does not saturate, the stored energy must be completely released!

TR

Reset Time

A

B

A = B

Rs

pdiode(on)oon(max)Q(on)in(min) TN

N)V(VTV(V += )

Tdt

Dead Time

A 20%T dead time margin will limit the maximum output voltage with a specified turn ratio!

Flyback Converters


27/57

Why Flyback Converters Are Usually Designed in DCM Mode?

To Ensure Core Does Not Saturate

Saturated! Not Saturated!

Ip

DIV21

T

TI21

V P pinONp

ini ==

To get larger input power , we need higher input current! To ensure the inductor does not saturate, we need to reduce the inductance! This also implies a lower volt-second product. For a same DC input, it means a lower duty ratio!

28/57

Why Flyback Converters Are Usually Designed in DCM Mode?

To Avoid Sub-Harmonic Oscillations Due to Duty Divergence

For peak current mode control, when the current falling slope is greater than the rising slope, i.e. |m2|>|m1|, this occurs when the converter reach into the CCM region, the current will diverge under a constant current reference. To avoid this phenomena, we can restrict the converter operating in DCM mode. If the converter needs to be operated in CCM mode, then a negative slope compensation is needed to stabilize this sub-harmonic oscillations.

I. Zafrany and S. Ben-Yaakov, A chaos model of subharmonic oscillations in current mode PWM boost converters, IEEE PESC Conf. Rec., pp. 1111-1117, 1995.

Flyback Converters


29/57

Determination of Turn Ratio

Np : Ns+

+

oI

VoVinCo RD

Q

LM

Transformer Model How to determine turn ratio?

Rs

pdiode(on)oon(max)Q(on)in(min) TN

N)V(VTV(V += )

TTTT dtRon(max) =++

If a 20% dead time is specified as the safe margin, then 0.8TTT Ron(max) =+

)/N)(NV(V)V(V)0.8T/N)(NV(V

Tspdiode(on)oQ(on)in(min)

spdiode(on)oon(max) ++

+=

30/57

Determination of Turn Ratio ..

s

p

NN

n =Define

)/N)(NV(V)V(V)0.8T/N)(NV(V

Tspdiode(on)oQ(on)in(min)

spdiode(on)oon(max) ++

+=

nVVn0.8TVT

21

2on(max) +

=

diode(on)o2 VVV +=Q(on)in(min)1 VVV =

TTT Ron(max) +=Define

max

max

diode(on)o

Q(on)in(min)

D-D

VVVV

n+

=

The proper turn ratio must be determine to ensure the DCM operation under a minimum input voltage, maximum specified duty, and a safety dead time ratio.

For the DCM flyback transformer design

Flyback Converters


31/57

Determination of Primary Inductance


Np : Ns+

+

oI

VoVinCo RD

Q

LM

Transformer Model

To ensure the flyback converter operating in DCM mode:

IpIp(max)

Ton

T

TR

Reset Time

Tdt

2

=

o

on(max)in(min)op V

TV2TR L

T

)I(L21

RV

1P

1 P

2pp

o

2o

oi ===

32/57

Design Equations: Maximum Switch Current (DCM)

Max. transistor current:

n : 1 +

+

IL

VoVin

Co R

p

on(max)dc(min)p(max) L

TVI =

Ip(max)

Ton(max)

maxE

Flyback Converters


33/57

Primary RMS Current and Wire Size

Ton(max)

p


TVI =

TT

3I

I on(max)p(max)p(rms) =

T

At 500 circular mils per rms ampere, the required number of circular mils is:

TT

3I

500I (primarty) required mils Circular

on(max)p(max)

p(rms)

=

=

34/57

Definition of Circular mils

mils = thousandths-of-an-inch

1 circular mil = define the Area of a circular wire with a diameter of 1 mil1 square mil = define the Area of a square wire with width of a mil

Current Notes:The current shown per wire size listed above is based on 1 amp/700 Circular mils, other tables provide different current per wire size, and different current for open air ~ check your local electrical code for the correct current capacity [Ampacity]. The 1 amp/ 700 Circular mils seems to be the most conservative, other sites provide/allow for 1 amp per 200 or 300 Circular mil. For shot wire lengths use 1A/200 Circular mil, for longer wire runs use 300 Circular mil, and for very long wire runs use the table above, 1 amp / 700 Circular mil.

817.7.57726.1740420.124

648.4.72820.7651122.623

514.12.91816.4664025.322

407.81.1613.0581228.521

323.41.4610.35102432.020

256.51.848.210128935.919

203.42.326.510162440.318

161.32.935.163205245.317

127.93.694.094258150.816

101.44.653.247326057.115

80.445.872.575410964.114

63.807.402.042518472.013

50.599.331.619652980.812

Feet per Pound

Current Carrying

Ohms/1000ft

Circular mils

Diam. (mils)AWG

Flyback Converters


35/57

RMS Values of Typical Waveforms

A period sinusoidal waveform with amplitude of A, its half period average value is 2A/ (0.636A) and RMS value is .2/A

A

AA

F RMS 707.02(sin) ==

AsqrF RMS =)(

AAtriF RMS 577.03)( ==

AF AVG 2(sin) =

AsqrF AVG =)(

2)( AtriF AVG =

Triangular wave

SIN wave

Square wave

1.151.732

1.111.414

11

Form FactorCrest Factor

36/57

RMS Values of Typical Waveforms ..

2

311

+=

IIII RMS

pkRMS II =

II RMS =i(t)I

t

pkI

pkI

i(t)

t

i(t)

t

II

Ts

Ts

3II RMS

=

321 DDII pkRMS

+=

3D

II pkRMS =

i(t)

0t

I

i(t)

0 t

i(t)

t00

0

Ts

Ts

DTs

D1Ts D2Ts

pkI

pkI

Flyback Converters


37/57

Primary RMS Current and Wire Size

Ip

Ton

T

TR

Reset Time

Tdt

p


TVI =

s

pp(max)s(max) N

NII =

TT

3)/N(NI

I rspp(max)s(rms) =

At 500 circular mils per rms ampere, the required number of circular mils is:

s(rms)500I )(secondary required mils Circular =

38/57

Flyback Transformer

The ideal flyback transformer operating as an ideal transformer parallel with a magnetization inductance. This magnetization inductance is the value required for a buck-boost converter. Therefore, the flyback transformer performs the function of an inductor as well as a transformer. To let the flyback transformer carry large primary current without saturating, the core is selected as

Gapped Ferrite CoreMPP (Molybdenum Permalloy Powder) Core

In practical realization of the flyback transformer, it is difficult to keep the leakage inductance small, especially when the rated power is increased.

Is

+

+

VoVin

Co R

D

Q

Lg

Ip Io

Flyback Converters


39/57

Energy Stored in an Inductor

Mean path length l Cross-sectional area A

Permeability

I

N: number of turns

lANL 2=

The energy stored in the core:

===tt

L LIdiLiPdtE 02

0 21''

The energy density (energy/volume) is:

2

211

2

22

222221

B

NlB

lAN

AlAlLI

B

=

==

The energy stored in the core:

coreBL VLIE ==2

21

Vcore: volume of the core

2

2r0

2

2c

core BLI

BLIV ==

40/57

Flyback Converter Transformer-Choke

Since the transformer-choke of the flyback converter is driven in one direction of the B-H characteristic curve, it has to be designed so that it will not saturate.

The effective transformer-choke volume is:

2max

2o(max)sr0

BIL

Volume =

0 = permeability of vacuum spacer = relative permeability of the chosen core material

Io(max) = maximum load currentL = output (secondary) inductance

Bmax = maximum flux density of the core

Flyback Converters


41/57

Effect of the Leakage Inductance

Np : Ns+

+

oI

VoVin

Co RD

Q

LM

Transformer Model

Llk

Leakage Inductance

+ vlk

VinD1

n:1

TR1

Vo

Co

Vin

ton toff0

Primarycurrent

Seccurrent

Switchingvoltage

T

0

0

Ip

ISW

IS

ID

VceorVds

leakageinductance

spike

ISec =Idiode

Vin+Vo Vin

t

t

t

Ip = Vin.ton / Lp(discontinuous)

discontinuous

21

nn

Q(off)

p(max)lklk(max) T

ILv =

The induced leakage spike voltage is:

TQ(off) is the primary switch turn-off time.

42/57

Clamp Circuits for Flyback Transformer

DrainDrain

D

C R

D

Dz

Flyback Converters


43/57

Flyback Converters


45/57

Transient Oscillation Due to Mode Transition

(a)

(b)

(c)

(d)

CA E F

DB

KH

G J I L

N

M P S

RO

T X

YU

ZV

W

Tdt

Ip2Ip1Ip discontinuous

Ip2Ip1

Ip discontinuous

Ip continuous

Is continuous

46/57

C o m p o n e n t S e l e c t i o n : M a i n P o w e r M O S F E T

Flyback Converters


47/57

Design Considerations for Flyback Converters

Poor Dynamics: In flyback converters, the gapped transformer inductance results a zero in the right-half-plane (RHP), which makes the closed-loop compensation in CCM (continuous conduction mode) very difficult. Typically, the closed-loop bandwidth in CCM is very narrow and the transient response is slow. Larger Output Capacitor: In flyback converters, a large output capacitor is required due to the lack of a second-order low-pass inductor/capacitor filter at the output.

Difficulties in Flyback Transformer Design: Processing power is limited by the flyback transformer. It is hard to reduce the leakage inductance for when the processing power is increased.

High Blocking Voltage: Main power transistor blocking voltage must sustain 2Vin plus leakage voltage spike.

48/57

Multiple Outputs of Isolated Flyback Converter

An advantage of the flyback converter is it is easy to provide multiple outputs.

This is because the isolation element acts as a common choke to all outputs, thus only a diode and a capacitor are needed for an extra output voltage.

+

Vin

Vout,1

Vout,2

Q

Flyback Converters


49/57

Cross Regulation in Multiple Outputs Flyback Converters

The cross regulation of a multi-output flyback converter can be significantly improved by lowering the clamp voltage, especially to slightly above the reflected output voltage. However, it causes more loss in a traditional RC clamp. Some solutions for improving thecross regulation with low loss are provided and discussed in future publications. Larger leakage inductance in the secondary windings leads to better cross regulation of that output when it is lightly loaded.Larger leakage inductance in primary side will be beneficial to improve the cross regulation of multiple output flyback converters. However, it results more loss in a traditional RC clamp. Reducing core gap to achieve larger magnetizing inductance in flyback converter design will improve cross regulation.

50/57

Interleaved Flyback Converter

Flyback Converters


51/57

Flyback Converters


53/57

Control of a Flyback Converter

IsolatedFeedback

Vin

Vac

Vcc

Lp

Ls

Ip

IsVout

Drainn:1

Max. Duty cycle

Driv erOSCILLATOR Clock

Clock

COMP GND

S

R Q1

1/100

PWM OCP

LEB

Rsense

L6590L6590DL6590A

2.5V

VFB E/A+

+

0.5V

FrequencyCompensation

54/57

1.3 W, 24 Vin, 3.3 Vout Flyback DC-DC Converter

C1 C2

24V

GEN

R1

R2

C4

C5

CSC3

GND

16T 7TC9

C77T

C6

D1 D3

COMP

BYPASS

33F 0.1F

0.1F

0.1F

1F 1F

220F10MQ100N

10MQ100N

D2

3V3 400 mA

Feedback23TBAS21

180pF

27k

0.25

VNEG VOUT

VCC

LX

Si9121DY

Flyback Converters


55/57

A 5W Flyback Converter with 170 Vin and Multiple Outputs

56/57

50 Watt, 48Vin, 5Vout, Isolated Flyback ConverterUsing a Primary Side PWM Control UCC3809 and the UC3965 Precision Reference and Error Amplifier

Flyback Converters


57/57

100VIN to 300VIN, 5VOUT at 1A Isolated Flyback Power Supply

Isolated Flyback Converter Regulates Without an Optocoupler

808 ()

Flyback Converters

808 ()

n : 1 +

+

IL

VoVin

Co R
Ying-Yu Tzou

808 ()
Ying-Yu TzouFlyback Converter: Selected Reading [1] G. Chryssis, High-Frequency Switching Power Supplies: Theory and Design, Chap. 3: Types of Power Converters, McGraw-Hill Book Company, 1984.[2] Christophe Basso, Average simulations of FLYBACK converters with SPICE3, Technical Report, May 1996. [3] Sanjaya Maniktala, Chap. 6: Isolated topologies for off-line applications of Switching Power Supply Design & Optimization, 2004. [4] Bob Bell, On Semiconductor, "Two-switch topology benefits forward and flyback power converters," EDN pp. 107-111, September 1, 2000. [5] Design Guidelines for Off-line Flyback Converters Using Fairchild Power Switch (FPS), Application Note AN4137, Fairchild. [6] Ravindra Ambatipudi, Design of Isolated Converters Using Simple Switchers Using the LM2587, National Semiconductor. [7] MathCAD Design Example: Switching Power Supply Design: Discontinuous Flyback Converter, Michele Sclocchi, Application Engineer, National Semiconductor. [8] MathCAD Design Example: Switching Power Supply Design: Continuous Flyback Converter, Michele Sclocchi, Application Engineer, National Semiconductor. [9] Lisa Dinwoodie, Design Review: Isolated 50 Watt Flyback Converter Using the UCC3809 Primary Side Controller and the UC3965 Precision Reference and Error Amplifier, TI Application Note U-165, 1999. [10] Sanjaya Maniktala, Chap. 10: Flyback Transformer Design of Switching Power Supply Design & Optimization, 2004. [11] Technical Bulletin CG-03, For Flyback Transformers . . .Selecting a Distributed Air-Gap Powder Core, Magnetics. [12] S.-K. Chung, "Transient characteristics of high-voltage flyback transformer operating in discontinuous conduction mode," IEE Proc.-Electr. Power Appl., vol. 151, no. 5, pp. 628-634, Sept. 2004.slua086.Isolated 50 Watt Flyback Converter.pdfData SheetsUCC3809UC3965

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