SMPS Switch Mode Power Supply DC Power Supply

SMPS - Switch Mode Power Supply DC Power Supply

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 One possible solution: AC, 50 hz supply Controls DC-DC Converters (non-isolated) To the LOAD PROBLEMS: Transformer operated at 50 Hz 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 I 1 I 2 + V 1 + V 2 Simplified model: no leakage and winding resistances Lm R 1 ✔ Ideal model, Ll 1 Ll 2 Rc Lm ✔ R 2 Detailed model: leakage inductances, winding resistances, magnetizing inductance, losses

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

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

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

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

FLY-BACK Waveforms for Fly-back Converter Closed 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: (SW 1, SW 2) and (SW 3, SW 4) (SW 1, SW 2) closed: (i) vp = Vs (SW 3, SW 4) closed: (i) vp = -Vs (ii) D 1 ON, D 2 OFF (ii) D 1 OFF, D 2 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 (C 1 and C 2) equally divide input voltage, therafore Vs/2 appear across primary when Sw 1 closed and –Vs/2 when Sw 2 closed. Hence