1. SMPS - Switch Mode Power Supply
DC Power Supply

2. 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

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 + 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

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

7. 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)

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

9. 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)

10. Waveforms for Fly-back Converter
Closed
Open

11. Minimum Lm for continuous current
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) D1 ON, D2 OFF
(iii)
(SW3, SW4) closed: (i) vp = -Vs
(ii) D1 OFF, D2 ON
(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, therafore Vs/2 appear across primary when Sw1 closed and –Vs/2 when Sw2 closed.
Hence