EET426 Power Electronics II Isolated Forward Converter 1 EET 426 – Power Electronis II Prepared by...

EET426 Power Electronics II

Isolated Forward Converter

1EET 426 – Power Electronis II

Prepared by : Mohd Azrik Roslan

2

DC Transformer conceptIsolated buck converter circuitIsolated converter advantages and disadvantagesIdeal and real transformer reviewLeakage inductanceOverlap loss

What you should know after this lecture

EET 426 – Power Electronis II

EET 426 – Power Electronis II 3

DC Transformer Concept


Buck Converter

Ei n

Vout C

R

L

Basic converter have several limitations:• Single input Single output• No isolation Can cause prob during fault• Output voltage relative to the input (based on duty cycle only)


ISOLATED BUCK CONVERTER

Ei n

Vout C

R

L

TRANSFORMERINSERTION

POINT



Ei n

Vout C

R

L

NOT REQUIRED:

DC TRANSFORMER CONCEPT



Ei n

Vout C

R

L

ON

winding polarity (dot notation) ensures D1 forward biased when mosfet is onDfwd reverse biased when mosfet is on

resulting in forward transfer of energy

D1

Dfw

d



Ei n

Vout C

R

L

LOW-SIDE mosfet: easier drive requirements

D1

Dfw

d



Ei n

Vout C

R

L

D1

Dfw

d


ProblemWhen the transistor switch is off the

transformer core must be fully reset by the end of the switching cycle.

This is to avoid core saturation and the resulting increase in switch current the next time it turns on. The time available for reset reduces as the

switch duty cycle increases hence reset must be possible during the minimum switch off -time.



Ei n

Vout C

R

L

D1

Dfw

d

Drese

t



Ei n

Vout C

R

L

D1

Dfw

d

Drese

t

ON

+ +

+

OFF

OFF

ON

MOSFET is ON



L

sw

D

on

on

off D V V V

p s r

E in

D r D off

n n

n r p s

p inV E

MOSFET is ON

s ss p in

p p

n nV V E

n n

r rr p in

p p

n nV V E

n n

,sw sw onV V

Dr in rV E V

14


EET 426 – Power Electronis II

Ei n

Vout C

R

L

D1

Dfw

d

Drese

t OFF

winding polarity (dot notation) ensures Dreset forward biased when mosfet is offD1 reverse biased when mosfet is off

ON



Ei n

Vout C

R

L

D1

Dfw

d

Drese

t

OFF

+

+

+

ON

ON

OFF

MOSFET is OFF



L

sw

D

off

off

on

on

D

D

V V V p s r

E in

D r

I p,swon

n n n r p s

pp in

r

nV E

n

MOSFET is OFF

s ss r in

r r

n nV V E

n n

r inV E

1 psw in p in

r

nV E V E

n

,Dr DrV V on


nr < np nr = np nr > np

Dsw < 0.5

Dsw = 0.5

Dsw > 0.5

Dsw >>0.5 requires reduced nr

→ increased Vp → Vsw ↑ → limit

nr = np OK for Dsw ≤ 0.5

limit

p

rsw

n

nD

1

1

o

V p t

D<0.5

E in

mag.

de mag

o

V p t

D<0.5

E

-E

in

in

mag.

demag. o

V p t

D<0.5

E in

mag.

demag.

o

V p

t D=0.5

E in

mag.

de mag

o

V p t

D=0.5

E

-E

in

in

mag.

demag o

V p

t D=0.5

E in

mag. demag ex

o

V p t

D>0.5

E in

mag.

demag. o

V p

t D>0.5

E

-E

in

in

mag.

demag. excess o

V p t

D>0.5

E in

mag.

demag. excess

inr

pE

n

n

inr

pE

n

n

inr

pE

n

n

inr

pE

n

n

inr

pE

n

n

inr

pE

n

n


Possible to have multiple outputs

No common input-output connection (increase safety) Indirect converter

Output voltage polarity choiceDepending upon winding polarity

Output voltage can be varied dependent uponTurns ratioDuty cycle

Isolated converter advantages


ExtraCostSizeWeight

Increase lossesWinding resistanceCore loss

Leakage inductance overlapReduce output voltageProduce transient voltages

Possible to have core saturationNeed core reset

Isolated converter disadvantages


Leakage Inductance Overlap


Ideal Transformer

VPRIM VSEC

IPRIM ISEC NP NS

secP

Sprim V

N

NV

secsecprimprim IVIV

POWER BALANCE

=100%

P

S

prim

secN

N

V

V

secS

Pprim I

N

NI


FunctionTransfer energyScale current and voltage

Magnetic device

Provide electrical isolation

No energy storage

Ideal Transformer


Have some energy storage

Magnetizing inductance (within core)Can be minimized by usinggapless coreHigh permeability core

Leakage inductance (external to core)

Low frequency core saturation

SMPS frequency core loss

REAL TRANSFORMER


REAL TRANSFORMERNP NS

RC LM

IMAG

practical magnetic cores have finite permeability

magnetising current IMAG required to establish core flux

effect represented by magnetising inductance LM

CORE LOSS represented by RC


REAL TRANSFORMER

NP NS

RC LM

RP RS

winding copper loss

represented by PRIMARY & SECONDARY winding resistance


Leakage InductanceParasitic Element


NP NS

RC LM

RP RSLLP LLS

TRANSFORMER LEAKAGE INDUCTANCEinductive parameter of transformers (& inductors )

due to imperfect magnetic linking between windings

magnetic flux that does not link primary-secondary windings

represented as primary & secondary series inductive impedances

P

leak,PPleak,P I

NL

S

leak,SSleak,S I

NL


Leakage Inductance Effects

ISOLATED FORWARD CONVERTER

Vout Ei n

C

R

L

SCH1

SCH2

VS,2 VS

VP

LL.P LL.s

NP: NS

NORMALLY STEP DOWN


LEAKAGE INDUCTANCE MEASUREMENT

LCR meter primary : LL,P +LPRIM

NP NS

LPRIM

LL,P

LCR meter

OPENCIRCUIT

LEAKAGE INDUCTANCE is an INTEGRAL PROPERTY of the

TRANSFORMER

IMPOSSIBLE to measure DIRECTLY


LEAKAGE INDUCTANCE MEASUREMENT

LCR meter primary : LL,P

NPNS

LPRIM

LL,P

LCR meter

PERFECT

SHORTCIRCUIT



Vout Ei n

C

R

L

SCH1

SCH2

VS,2 VS

VP

LL.P LL.s

NP: NS

NORMALLY STEP DOWN

VS < VP IS > IP


IMPEDANCE TRANSFORMATION

NP NS

VSECVPRIM

ISECIPRIM

ZSEC

ZPRIM

2

prim

secprimsec N

NZZ

STEP DOWN CONVERTERS

2

sec

primsecprim N

NZZ


IMPEDANCE TRANSFORMATION

STEP DOWN CONVERTERS

LL(reflected)< LL,P

LL,P

VSECVPRIM

NSNP

VSECVPRIM

LL(reflected)

2

p

sP,L)reflected(leak N

NLL



VS < VP IS > IP

Vout Ei n

C

R

L

SCH1

SCH2

VS,2 VS

VP

NP: NS

NORMALLY STEP DOWN

LL.P LL.r


ISOLATED FORWARD CONVERTER: OVERLAP

0Vgs

Ids0

ISCH1 0

ISCH20

Vout

Ei n

C

R

L SCH1

SCH2

VS,2 VS

VP

LL,

r

NP

NS

2 overlap intervals

SCH1 & SCH2 ON

LEAKAGE INDUCTANCE prevents

instantaneous commutation between SCH1 SCH2

SCH2 ON VS,2 = 0



0Vgs

Ids0

ISCH1 0

ISCH20

0VSEC

Vs,2

VL,r

0

0

Vout

Ei n

C

R

L SCH1

SCH2

VS,2 VS

VP

LL,

r

NP

NS

OVERLAP INTERVAL 2NO EFFECT on Vout

DUTY CYCLE REDUCTION

OUTPUT VOLTAGE LOSS DUE to OVERLAP

OVERLAP INTERVAL 1

DswTsw

0 tovlapTsw



0Vgs

0VSEC

Vs,2

VL,r

0

0

Vout

Ei n

C

R

L SCH1

SCH2

VS,2 VS

VP

LL,

r

NP

NS

DswTsw

0 tovlapTsw

sec

swoutr,L

sw

ovlapoverlap,sw V

fIL

T

tD

p

sdsin

swoutr.Loverlap,sw

n

nVE

fILD

ovlap

outr,Lr,Lr,Lsec t

IL

dt

dILVV

during overlap

sec

outr,Lovlap V

ILt

Effective Duty Cycle LOSS



0Vgs

0VSEC

Vs,2

VL,r

0

0

Vout

Ei n

C

R

L SCH1

SCH2

VS,2 VS

VP

LL,

r

NP

NS

DswTsw

0 tovlapTsw

swoutr,Lsw

ovlap

ovlap

outr,L

sw

ovlapovlapave.out fIL

T

t

t

IL

T

tVV

ovlap

outr,Lovlap t

ILV

during overlap

Vout LOSS NON EFFICIENCY RELATED Vout LOSS


Isolated Forward Converter

Analysis


input voltage Ein

transformer primary turns np

transformer secondary turns ns

secondary current maximum Is(max)

primary current max Ip(max)

transistor switch duty cycle Dsw

transistor on-state resistance rdson

transistor power loss Pmosfet

output current Iout

inductor ‘resistance’ rind

inductor ‘rind’ power loss Pr,ind

rectifier D1 and D2 voltage drops VF DATA SHEETrectifier D1 and D2 current max Iak,max

combined D1 and D2 duty cycle Drects 1

combined D1 and D2 loss Prects

transformer primary loss Pprim

transformer secondary loss

Psec

pnsn

(max)sI

dsonpsw rID 2

max

indoutindrmsind rIrI 22,

outak II max,

Fakrects VID max,

primpsw rID 2

max

sec2

max rID ssw


total power loss Ploss,total

input powerPin

output powerPout

efficiency

‘ideal’ output voltage

Vout(ideal)

‘real’ output voltage

Vout real

sec, PPPPP primrectsindrmosfet

maxpswininavin IDEIE

lossin PP

in

out

P

P

pnsn

swin DE

)(idealoutV


txer leakage inductance (ref prim)

Lleak(prim)

txer leakage inductance (ref sec)

Lleak(sec)

average output voltage ‘overlap’ loss

output voltage Vout

2

)(

p

sprimleak n

nL

overlapV swoutleak fIL (sec)

)( )( overlapidealout VV


‘effective’ duty cycle loss

output voltage Vout

PNsN

inE

swfoutIleakL

(sec)

pnsn

swswin DDE )(

overlapswD ,


Table shows component parameter and operational information for the isolated forward converter shown in the figure. Determine

i. the isolated forward converter output voltageii. the isolated forward converter efficiencyiii. the mosfet voltage requirement

Example 1

Ein

200 V

100 : 100 : 10

L

r ind

esr

C Vout

f SW = 50 kHz

R

Iout =10A D1

D2

200 V

Dsw = 0.3

L

rind Rds(on) Lleakage

(ref to secondary) VF :

D1 D2

2.0 mH

0.04 0.3 2.0 H 0.9 V


Example 2

Ein

200 V

200 : 200 : 20

L

r ind

esr

C Vout

f SW = 40 kHz

R

Iout =10A D1

D2

rind VF : D1 D2

0.04 0.9 V

200 V

Dsw = 0.4

Table shows component parameter and operational information for the isolated forward converter shown in the figure. Determine

i. the isolated forward converter output voltageii. the isolated forward converter efficiency

EET426 Power Electronics II Isolated Forward Converter 1 EET 426 – Power Electronis II Prepared by...

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