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Published byNathen Chatelain Modified about 1 year ago

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SMPS - Switch Mode Power Supply DC Power Supply

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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 LOAD AC, 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 size Turns-ratio provides flexibility to the design Can provide multiple outputs INTRODUCTION

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Typical SMPS block diagram:

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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+V1 +V2+V2 I1I1 I2I2 ✔ LmLm L l1 R1R1 RcRc LmLm L l2 R2R2 ✔ Ideal model,

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

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

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

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

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FLY-BACK Waveforms for Fly-back Converter Closed Open

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FLY-BACK Minimum L m for continuous current Boundary condition when I Lm,min = 0It can be shown that:

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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:

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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) v p = V s (ii) D 1 ON, D 2 OFF (iii) (SW 3, SW 4 ) closed:(i) v p = -V s (ii) D 1 OFF, D 2 ON (iii)

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FULL-BRIDGE DC-DC CONVERTER Derivation of output voltage, V o Inductor volt-second balanced (Average inductor voltage = 0)

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FULL-BRIDGE DC-DC CONVERTER Minimum L x for continuous current Minimum L x when I Lx,min = 0

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FULL-BRIDGE DC-DC CONVERTER Output voltage ripple From the figure

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HALF-BRIDGE DC-DC CONVERTER Capacitors (C 1 and C 2 ) equally divide input voltage, therafore V s /2 appear across primary when Sw 1 closed and –V s /2 when Sw 2 closed. Hence

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