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

FLY-BACK

(ΔiL)closed + (ΔiL)open=0

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

Derivation of output voltage , Vo

OR

FLY-BACK

Derivation of output voltage , Vo

Switch CLOSED (ON)

Switch OPEN (OFF)

FLY-BACK

Switch CLOSED (ON)

Derivation of output voltage , Vo

Switch OPEN (OFF)

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

FLY-BACK

Waveforms for Fly-back ConverterClosed

Open

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:

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)

FULL-BRIDGE DC-DC CONVERTER

Derivation of output voltage , Vo

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

FULL-BRIDGE DC-DC CONVERTER

Minimum Lx for continuous current

Minimum Lx when ILx,min = 0

FULL-BRIDGE DC-DC CONVERTER

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