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SWITCH -MODE POWER SUPPLIES AND SYSTEMS. Lecture No 6. Silesian University of Technology Faculty of Automatic Control, Electronics and Computer Sciences Ryszard Siurek Ph.D., El. Eng. Switch-Mode Power Converters. Application of the switching transformer

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switch mode power supplies and systems

SWITCH-MODE POWER SUPPLIES AND SYSTEMS

Lecture No 6

Silesian University of Technology

Faculty of Automatic Control, Electronics

and Computer Sciences

Ryszard Siurek Ph.D., El. Eng.

switch mode power converters
Switch-Mode Power Converters

Application of the switching transformer

- ensures galvanic isolationbetween output and input circuit (safety

regulations, output voltage polarity not restricted)

- small transformer dimensions according to high switching frequency

- output voltage higher or lower than the input voltage independently of

switching regulator configuration

- possibility of efficient operation (with optimum duty cycle) in presence of

high difference between input and output voltage (mains input voltage -

very low output voltage)

- several output voltages easily available

slide3

Single-ended forward converter

typical step-down regulator output filter

Ip

T

IS

D2

L

Up

UIN

Uw

t

T

D1

C

R0

Zp

ZS

U0

CIN

Za

turns ratio

Transformer model

ideal transformer

*

rp

Llp

Lls

Ip

Iw = Iw /n

IS

FM

Lp

*

Up

US

Up = USn

IM

n

Assumptions: Llp, Lls = 0

rp, rs = 0

forward converter equivalent circuit
Forward converter equivalent circuit

I cycle 0 < t <t

transistor T - ON , diode D2 - ON, diode D1 - OFF

L

T

Ip = IT

IS

IL

D2

FM

Up

US

UIN

IM

R0

U0

C

Lp

D1

IM

n

IT

IT

IMmax

IM

IMmax

t

IL

ILmax

ILmin

FM

FMmax

slide5

II cycle t < t < T

Transistor T - OFF , diode D2 - OFF, diode D1 - ON

Da

L

IS=0

IL

T

Ip = 0

Zp

dUp

D2

dUa

UT

FM

dUS

IM

R0

U0

UIN

Za

C

D1

Lp

Up

Ua=UIN

n

IM

US=Up/n

IT

Up=

IT

IMmax

When T switches off the overvoltage dUp appears across Zp and is transformed as dUS to the secondary side

Diode D2 switches off , overvoltage is transformed to the winding Za and diode Da switches on

The voltage Ua acrosss Za approaches the value of UIN and can not rise any more

Voltage Ua=UIN is transformed to the primary winding Zp and is limited on the value Up

The core of the transformer is being demagnetised during t1 (core reset)

IM

IMmax

t

T

ILmax

IL

I0

ILmin

FM

IM

FMmax

current in Za

current in Zp

IMmax

t

t1

UT

UIN

transformer core reset magnetizing energy recovery detailed analysis
Transformer core reset (magnetizing energy recovery)– detailed analysis

UT

dULL

UT

iZa(t)

iZp(t)

real overvoltage

T

UIN

dULL

LL

UIN

t1

UIN

Lp

t

T

Zs

IZp

IMmax

Zp

Za

To keep dULL low, LL should be small enough - it requires very good magnetic coupling between the windings Zp and Za

In practice Zp = Za and both windings are bifilary wound

hence UTmax = 2UIN

IZa

FM

FMmax

Full demagnetization of the transformer is possible under the following condition:

typical magnetizing energy recovery circuits
Typical magnetizing energy recovery circuits

Da

Zp=Za

D2

L

D1

C

R0

Zp

ZS

U0

CIN

Za

UIN

Cs

overvoltage dumping circuit -

snubbar circuit

T

Rs

Ds

Disadvantages:

necessity of placing two bifilar windings , difficult construction, high transformer cost, problems with insulation, duty cycle limited to g < 0,5, snubbar circuit required to avoid voltage stress across the switching transistor

Advantages:

most of transformer magnetizing energy is recovered (higher efficiency), only one switching transistor, simple transistor gate drive circuit

slide8

D2

L

Rs

Cs

D1

C

R0

Zp

ZS

U0

Up

UIN

CIN

When Uo rises (higher value of Rs), core reset time t1 decreases and may be shorter than t. That is why the duty cycle may be higher (g > 0,5)

T

UT=Up+UIN

Disadvantages:

magnetising energy is dissipated in Rs, lower efficiency, high power resistor (resistors) are required, heating of some components, high coltage stress across the tswitching transistor ( UT > 2UIN)

Advantages:

simple and cheap transformer, duty cycle not limited to 0,5, no extra voltage spikes across the transistor – no influence of leakage inductance

This configuration is not used in practice due to excesive power losses

slide9

T1

D1

D2

L

Ip

US

Up

D1

C

R0

Zp

UIN

ZS

U0

CIN

T2

Up=UIN g< 0,5

D2

Disadvantages:

two switching components, complicated drive circuits, higher cost

Advantages:

simple transformer, no excesive voltage spikes – problem of leakage inductance does not exist, transistor voltahe does not exceed UIN

This configuration often used in power supplies with higher output power – usually over 300 – 500W

magnetic core behaviour magnetising curve core saturation
Magnetic core behaviour, magnetising curve ,core saturation

B

B

Bs

Bs

H

H

Core saturation in case of improper transformer design

-Bs

-Bs

iM(t)

Ip

FM

IM

demagnetising current

FM

magnetising current

FMmax

*

iS(t)

IM

IMmax

t

t

t

t1

slide11

Core saturation as a result of incomplete core reset (transformer demagnetisation)

FM

B

Bs

t

H

Ip

t

-Bs

slide12

Output choke (inductance) saturation

B

IL

Bs

I1

DB

B0

I0

t

DH

H

H0(I0)

H1(I1)

Ip

-Bs

t