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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

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SMPS - Switch Mode Power Supply DC Power Supply

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  1. SMPS - Switch Mode Power Supply DC Power Supply

  2. 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: To the LOAD AC, 50hz supply 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

  3. Typical SMPS block diagram:

  4. Typical SMPS block diagram:

  5. TRANSFORMER MODEL For SEE 4433 simplified model of transformer will be used to describe the circuit operation of SMPS I1 I2 ✔ Ideal model, + V1  + V2  ✔ Simplified model: no leakage and winding resistances Lm R1 Ll1 Ll2 R2 Detailed model: leakage inductances, winding resistances, magnetizing inductance, losses Rc Lm

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

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

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

  9. 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)

  10. FLY-BACK Waveforms for Fly-back Converter Closed Open

  11. FLY-BACK Minimum Lm for continuous current Boundary condition when ILm,min= 0 It can be shown that:

  12. 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:

  13. FULL-BRIDGE DC-DC CONVERTER The switches are switched in a pair: (SW1, SW2) and (SW3,SW4) (SW1, SW2) closed: (i) vp = Vs (ii) D1ON, D2OFF (iii) (SW3, SW4) closed: (i) vp = -Vs (ii) D1OFF, D2ON (iii)

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

  15. FULL-BRIDGE DC-DC CONVERTER Minimum Lx for continuous current Minimum Lx when ILx,min = 0

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

  17. HALF-BRIDGE DC-DC CONVERTER Capacitors (C1 and C2) equally divide input voltage, theraforeVs/2 appear across primary when Sw1 closed and –Vs/2 when Sw2 closed. Hence

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