Power Electronics and Drives (Version 3-2003) Dr. Zainal Salam, UTM-

JB

1

High frequency transformer

: Models

;

iprelationshoutput -input Basic voltage varying- meup/down ti step 2)

isolation electricaloutput -Input 1):function Basic

1

2

2

1

2

1

2

1

NN

ii

NN

vv

V1V2

+

+

i1 i2N1 N2

Ideal model

V1V2

+

+

i1 i2N1 N2

Model used formost PE application

Lm

Power Electronics and Drives (Version 3-2003) Dr. Zainal Salam, UTM-

JB

2

Flyback Converter

Vd

+

Vo

Vd

N1 N2i1

i2

+

v2v1

+

iLM

iD

+ vDiC

iR

C R

vSW+

+

iS

LM

Vo

Flyback converter circuit

Model with magnetisinginductance

Power Electronics and Drives (Version 3-2003) Dr. Zainal Salam, UTM-

JB

3

Operation: switch closed

Vd

Vo

N2

+

0

0

v1

v1=Vs

iLM

is=iLM +

N1

+

v2

0 and 0Therefore,

off turneddiode i.e. ,0

er, transform theof side load On the

12

1

2

1

2

1

212

1

ii

NNVVv

NNV

NNvv

LDTVi

LV

DTiL

dtiL

dtdiL

dtdiLVv

doD

d

m

dclosedL

m

dmmm

Lmmd

m

Power Electronics and Drives (Version 3-2003) Dr. Zainal Salam, UTM-

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4

Switch opened

Vs

+

v1

+

iLM

N1 N2

v2= VS

+

+

Vo

iD

vSW

2

1

2

1

2

1

2

11

2

1

2

121

2

2

11

:switch theacross Voltage

)1(

1

But

NNVVv

NN

LTDVi

NN

LV

TDi

dti

dtdi

NNV

dtdi

Lv

NNV

NNvv

VvNNVv

odSW

m

oopenmL

m

omLmLmL

omL

m

o

o

o

Power Electronics and Drives (Version 3-2003) Dr. Zainal Salam, UTM-

JB

5

Output voltage

1

2

2

1

1

01

0operation, state-steadyFor

NN

DDVV

NN

LTDV

LDTV

ii

do

m

o

m

d

openedLclosedL mm

• Input output relationship is similar to buck-boost converter.

• Output can be greater of less than input,depending upon D.

• Additional term, i.e. transformer ratio is present.

Power Electronics and Drives (Version 3-2003) Dr. Zainal Salam, UTM-

JB

6

Flyback waveforms

iLm

is

t

t

iD

iC

v1

-V(N1 /N2)

Vo/ R

Vs

DT T

iLM

1

20

2

1

22

20

20

20

0

)1(

)1(

:asn can writte

for Solving

:as torelated is

NN

RDV

NN

RDDVI

IDRV

VI

RVDIV

I

DITDTI

I

IIR

VIV

PP

dL

L

dL

Ld

L

LL

s

Ls

sd

s

m

m

m

m

m

mm

m

Power Electronics and Drives (Version 3-2003) Dr. Zainal Salam, UTM-

JB

7

Max, Min inductor current

RCfD

VVr

NN

fRDVL

fLDV

LDTV

NN

RDDV

I

LDTV

NN

RDDV

iII

LDTV

NN

RDDV

iII

dm

m

d

m

dd

L

m

dd

LLL

m

dd

LLL

m

mmm

mmm

0

0

2

2

12

min

2

1

22

min

2

1

22

min,

2

1

22

max,

boost, similar to isn calculatio Ripple2

)1(

22)1(

0, CCM,For

2)1(

2

2)1(

2

Power Electronics and Drives (Version 3-2003) Dr. Zainal Salam, UTM-

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8

ExampleThe Flyback converter has these specifications:DC input voltage: 40VOutput voltage: 25VDuty cycle: 0.5Rated load: 62.5WMax peak-peak inductor current ripple:25% of the average inductor current.Maximum peak-peak output voltage: 0.1VSwitching frequency: 75kHz

Based on the abovementioned specifications, determine• Transformer turns ratio• Value of magnetizing inductor Lm. • Maximum and minimum inductor current.• Value of capacitor C.

Power Electronics and Drives (Version 3-2003) Dr. Zainal Salam, UTM-

JB

9

Full-bridge converter

SW2SW4

SW3

VS

NS

NS

+

vx C

Lx

R+

Vo

+

vp

SW1

SW1,SW2

SW3,SW4 DT T

2T DTT

2VP

VS

-VS

Vx

P

SS N

NV

DT2T

DTT 2

T

Power Electronics and Drives (Version 3-2003) Dr. Zainal Salam, UTM-

JB

10

Full bridge: basic operation

• Switch “pair”: [S1 & S2];[S3 & S4].

• Each switch pair turn on at a time as shown. The other pair is off.

• “AC voltage” is developed across the primary. Then transferred to secondary via high frequency transformers.

• On secondary side, diode pair is “high frequency full wave rectification”.

• The choke (L) and (C ) acts like the “buck converter” circuit.

• Output Voltage

DNNVV

p

sso

2