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# SMPS - Switch Mode Power Supply

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SMPS - Switch Mode Power Supply
DC Power Supply

One possible solution:
INTRODUCTION 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 Controls One possible solution: AC, 50hz supply To the LOAD DC-DC Converters (non-isolated) PROBLEMS: Transformer operated at 50Hz frequency require large magnetic core – bulky, heavy and expensive ! SOLUTIONS: Use transformer at switching frequency – smaller core size Turns-ratio provides flexibility to the design Can provide multiple outputs

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 + V1 V2 I1 I2 Ideal model, Lm Simplified model: no leakage and winding resistances Ll1 R1 Rc Lm Ll2 R2 Detailed model: leakage inductances, winding resistances, magnetizing inductance, losses

FLY-BACK Derived from Buck-Boost converter
Isolation provided by high frequency transformer

Derivation of output voltage , Vo
FLY-BACK Derivation of output voltage , Vo (ΔiL)closed + (ΔiL)open=0 OR Inductor volt-second balanced (Average inductor voltage = 0)

Derivation of output voltage , Vo
FLY-BACK Derivation of output voltage , Vo Switch CLOSED (ON) Switch OPEN (OFF)

Derivation of output voltage , Vo
FLY-BACK Derivation of output voltage , Vo Switch CLOSED (ON) Switch OPEN (OFF) (ΔiL)closed + (ΔiL)open=0 Inductor volt-second balanced (Average inductor voltage = 0)

Waveforms for Fly-back Converter
Closed Open

Minimum Lm for continuous current
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

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