Isolated Dc Power Supplies

SMPS - Switch Mode Power SupplyDC Power Supply

• Previous DC-DC converters (Buck, Boost, Buck-Boost) do not provide electrical isolation between input and output - these are non-isolated DC-DC converters

• In most applications, isolation is required and this can be provided by transformers

DC-DC Converters

(non-isolated)

To the LOADAC, 50hz supply

One possible solution:

PROBLEMS:

Transformer operated at 50Hz frequency require large magnetic core – bulky, heavy and expensive !

Controls

SOLUTIONS:

Use transformer at switching frequency – smaller core sizeTurns-ratio provides flexibility to the designCan provide multiple outputs

INTRODUCTION

Typical SMPS block diagram:

Typical SMPS block diagram:

TRANSFORMER MODEL

For SEE 4433 simplified model of transformer will be used to describe the circuit operation of SMPS

Detailed model: leakage inductances, winding resistances, magnetizing inductance, losses

Simplified model: no leakage and winding resistances

+V1

+V2

I1 I2

✔Lm

Ll1R1

Rc Lm

Ll2 R2

✔Ideal model,

FLY-BACK

• Derived from Buck-Boost converter• Isolation provided by high frequency

transformer• Assume continuous iLm

FLY-BACK

(ΔiL)closed + (ΔiL)open=0

Inductor volt-second balanced (Average inductor voltage = 0)

Derivation of output voltage , Vo

OR

FLY-BACK


Switch CLOSED (ON)

Switch OPEN (OFF)

FLY-BACK

Switch CLOSED (ON)


Switch OPEN (OFF)

(ΔiL)closed + (ΔiL)open=0 Inductor volt-second balanced (Average inductor voltage = 0)

FLY-BACKWaveforms for Fly-back Converter

Closed

Open

v1 vLm

iLm

is

iD

ic

sV

2

1

N

NVo

R

Vo

FLY-BACK

Minimum Lm for continuous current

Boundary condition when ILm,min = 0 It can be shown that:

FLY-BACK

Output voltage ripple

Derivation of output voltage ripple is similar to Buck-Boost converter

It can be shown that the ration of the ripple to the output voltage is given by:

FORWARD

• Derived from Buck converter• Isolation provided by high

frequency transformer• Assume continuous iLx

(ΔiL)closed + (ΔiL)open=0



OR

FORWARD

FORWARD

ON OFF


Switch CLOSED (ON)

FORWARD

We know,

We can write

Therefore,

The slope of the magnetizing current is given by:


Switch OPEN (OFF)

FORWARD

We know,

Since

Therefore,

The slope of the magnetising current is given by:

What is vLm ? Therefore

If


FORWARD

osLx VNNVv 12

Switch CLOSED (ON)

Switch OPEN (OFF)


FORWARD

(ΔiL)closed + (ΔiL)open=0 Inductor volt-second balanced (Average inductor voltage = 0)

Switch CLOSED (ON) Switch OPEN (OFF)

x

osclosedLx L

DTV

N

NVi

1

2

osLx VNNVv 12

1

2

N

NDVV so

1

2

N

NDVV so

FORWARD

ON

OFF

Waveforms for Forward ConvertervLx

iLx

iLm

i1

v1

FORWARDWaveforms for Forward Converter

It is important that iLm goes to zero before the start of a new cycle

This condition is fulfilled if

Now

1

31

3

1

3

1

111

NN

DN

ND

TDTDTN

N

TDTTx

If N3/N1 =1, then D < 0.5

v1

vLx

iLx

iLm

i1

FULL-BRIDGE DC-DC CONVERTER

The switches are switched in a pair: (SW1, SW2) and (SW3,SW4)

(SW1, SW2) closed: (i) vp = Vs

(ii) D1 ON, D2 OFF

(iii)

(SW3, SW4) closed: (i) vp = -Vs

(ii) D1 OFF, D2 ON

(iii)





Minimum Lx for continuous current

Minimum Lx when ILx,min = 0

x

oomax,Lx L

TD.V

R

VI

2

50

x

ooinm,Lx L

TD.V

R

VI

2

50


Output voltage ripple

From the figure

HALF-BRIDGE DC-DC CONVERTER

Capacitors (C1 and C2) equally divide input voltage, therafore Vs/2 appear across primary when Sw1 closed and –Vs/2 when Sw2 closed.

Hence

HALF-BRIDGE DC-DC CONVERTER

DN

NVV

p

sso

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Isolated Dc Power Supplies