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SWITCH -MODE POWER SUPPLIES AND SYSTEMS

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SWITCH-MODE POWER SUPPLIES AND SYSTEMS

Lecture No 7

Silesian University of Technology

Faculty of Automatic Control, Electronics

and Computer Sciences

Ryszard Siurek Ph.D., El. Eng.

Flyback converter

D1

Ip

ID

I0

IC

C

R0

UIN

Zp

ZS

U0

IT

CIN

T

t

T

transfer ratio

- exceptional topology comprising transformer and output choke in one magnetic
- component
- topology with lowest compenet count – cheapest solution

L

T

IT

ID

D1

I0

IC

C

R0

UIN

Zp

ZS

U0

CIN

t

T

Compare to flyback switching regulator

Flyback converter basic relations analysis

Cycle I - transistor T is ON

IT

I0

D1

Ipmax

IC

UIN

R0

C

Zp

ZS

U0

B

t

IT

BS

T

Magnetic energy stored in the core

By the end of cycle I

H

Cycle II - transistor T is OFF

ID

ID

D1

I0

IDmax

IC

dUp

T

R0

C

Zp

ZS

t

t’

B

UIN

nU0

U0

BS

IT=0

T

H

Magnetic energy recovered

from the core by the end of cycle II

From energy balance :

IDmax

I0

(1)

iD(t)

~

UC

LS

Ro

(2)

U0

From equation (1) :

Al - core constant

hence:

U0 may be calculated also from energy balance:

valid only in case of discontinuous

flux (current) flow, it means t’ < T - t

(3)

At the point of t’ - ID(t’) = 0, hence:

(4)

From equation (4) :

ID

IDmax

U0(R0)

U’0(R’0 < R0)

t

t’

T

Compare to flyback regulator

for R0 < R0cr (I0 > I0cr) the flux in the core does not decay to 0 – so called „continuous flux flow” starts

U0

IT

g > 0,5

ITmax

g =0,5

t

T

g < 0,5

ID

IDmax = nITmax

IDmax

IDmin = nITmin

I0kr

I0

IDmin

recovery of energy stored in the leakage inductance

Dd

Zp=Za

D

C

R0

Zp

ZS

U0

CIN

Za

UIN

Cs

snubbar circuit for dumping overvoltage spikes and reducing transistor power losses

T

Rs

Ds

Advantages:

Energy stored in leakage inductace is recovered, transitor voltage does not exceed 2UIN

Disadvatages:

Complicated and expensive transformer

Leakage inductance measurement method

LL

LL

D

LL

Rs

Cs

C

R0

Zp

ZS

U0

Up

UIN

CIN

T

UT=Up+UIN

Disadvatages:

Energy stored in the leakage inductance is dissipated in resistor Rs, lower efficiency, necessity of power resistor utilisation, component heating, possibility of transitor voltage higher than 2UIN

Advantages:

Cheaper transformer, lack of extra overvoltage spikes due to residual leakage inductance

This topology often used in low power converters up to 100W

Flyback topology

D1

ID1

I01

R01

US1

C1

ZS1

U01

D2

ID2

I02

Zp

UIN

T

CIN

R02

US2

C2

ZS2

U02

Feedback loop

In II cycle U01 = US1

U02 = US2

In this topology output voltages are dependent only on the secondary numbers of turns. In case of perfect magnetic coupling only one output voltage may be regulated to obtain the regulation of other outputs.

Valid for discontinuous as well as continuous current flow

One of the cheapest and simple solution delivering several regulated output voltages.

Forward converter

D1

L1

Da

D2

C

R01

ZS1

U01

Zp

CIN

D3

L2

Za

UIN

D4

C

R02

ZS2

U02

T

This relation only valid in case of cotinuous magnetic flux (current) flow in L1 & L2

Coupled output inductors

D1

L1

Dd

R01

D2

C1

ZS1

U01

Zp

CIN

D3

L2

Za

UIN

R02

D4

C2

ZS2

U02

T

Equivalent output circuit valid for cycle I :

L1

U01

L2

U02

Equivalent output circuit valid for cycle II:

L1

U01

L2

U02

To achieve proper relation between output voltages the following condition must be satisfied :

- In real circuit:
- diode voltage drop and nonlinear diode characteristics have significant influence on output voltages
- influece of winding resistances
- significant influence of leakage (poor winding coupling)
Detailed relations of turns number for particular windings are usually set by the way of experiment in practice – equations presented above give only the rough approximation.

The other way of obtaining auxilliary regulated output voltages with low load requirements :

D2

C2

R2

U2

Da

L2

D2

Zp=Za

L

D1

C

R0

U0

Zp

Zw

CIN

Zd

UIN

T

C2

R2

U2

C2

R2

U2

z2

L2

L2

zL

U0

L

U0

L