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Download Isolated DC-DC converter - SMPS
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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, 50hz supply Controls DC-DC Converters (non-isolated) To the LOAD 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 I1 I2 + V1 + V2 V1 I 2 N1 = = V2 I1 N 2 Simplified model: no leakage and winding resistances Lm R1 Ideal model, Ll1 Ll2 Rc Lm ✔ ✔ R2 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 (ΔiL)closed + (ΔiL)open=0 OR Inductor volt-second balanced (Average inductor voltage = 0) FLY-BACK Derivation of output voltage , Vo Switch CLOSED (ON) ( DiLm )closed = Vs DT Lm vLm = Vs Switch OPEN (OFF) -Vo (1- D)T æ N1 ö ç ÷ ( DiLm )open = Lm è N2 ø æN ö vLm = -Vo ç 1 ÷ è N2 ø FLY-BACK Derivation of output voltage , Vo Switch CLOSED (ON) ( DiLm )closed V DT = s Lm vLm = Vs (ΔiL)closed + (ΔiL)open=0 Vs DT -Vo (1- D)T æ N1 ö + ç ÷=0 Lm Lm è N2 ø æ D öæ N 2 ö Vo = Vs ç ÷ç ÷ è 1- D øè N1 ø Switch OPEN (OFF) -Vo (1- D)T æ N1 ö ç ÷ ( DiLm )open = Lm è N2 ø æN ö vLm = -Vo ç 1 ÷ è N2 ø Inductor volt-second balanced (Average inductor voltage = 0) æN ö (DT )Vs -Vo ç 1 ÷ (1- D)T = 0 è N2 ø æ D öæ N 2 ö Vo = Vs ç ÷ç ÷ è 1- D øè N1 ø 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: (SW1, SW2) and (SW3,SW4) (SW1, SW2) closed: (i) vp = Vs (ii) D1 ON, D2 OFF æN ö (iii) vx = Vs ç s ÷ çN ÷ è pø (SW3, SW4) closed: (i) vp = -Vs (ii) D1 OFF, D2 ON æN ö (iii) vx = Vs ç s ÷ çN ÷ è pø FULL-BRIDGE DC-DC CONVERTER Derivation of output voltage , Vo Inductor volt-second balanced (Average inductor voltage = 0) æ æN ö ö çVs çç s ÷÷ -Vo ÷ DT -Vo ( 0.5 - D) T = 0 ç N ÷ è è pø ø æN ö Vo = 2Vs çç s ÷÷ D è Np ø æN ö Vs çç s ÷÷ -Vo è Np ø -Vo vLx iLx FULL-BRIDGE DC-DC CONVERTER Minimum Lx for continuous current I Lx,max = I Lx + I Lx,min = I Lx - DiLx 2 DiLx I Lx 2 æ Vo ö æ Vo ( 0.5 - D) T ö I Lx,max = ç ÷ + ç ÷ èRø è 2Lx ø æ V ö æ V ( 0.5 - D) T ö I Lx,max = ç o ÷ + ç o ÷ èRø è 2Lx ø Minimum Lx when ILx,min = 0 \Lx,min = ( 0.5 - D) R 2f FULL-BRIDGE DC-DC CONVERTER Output voltage ripple iC DQ = CDVo DiLx 2 From the figure æ T öæ 1 ö Vo ( 0.5 - D) T Vo ( 0.5 - D) = è 4 øè 2 ø 2Lx 16Lx f 2 DQ = ç ÷ç ÷ \ DVo Vo = (0.5 - D) 16LxCf 2 = Vo ( 0.5 - D) T 2Lx 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 æN ö Vo = Vs çç s ÷÷ D è Np ø