SMPS - Switch Mode Power SupplyDC Power Supply
• 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 LOADAC, 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 sizeTurns-ratio provides flexibility to the designCan provide multiple outputs
INTRODUCTION
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
Detailed model: leakage inductances, winding resistances, magnetizing inductance, losses
Simplified model: no leakage and winding resistances
+V1
+V2
I1 I2
✔Lm
Ll1R1
Rc Lm
Ll2 R2
✔Ideal model,
FLY-BACK
• Derived from Buck-Boost converter
• Isolation provided by high frequency transformer
FLY-BACK
(ΔiL)closed + (ΔiL)open=0
Inductor volt-second balanced (Average inductor voltage = 0)
Derivation of output voltage , Vo
OR
FLY-BACK
Derivation of output voltage , Vo
Switch CLOSED (ON)
Switch OPEN (OFF)
FLY-BACK
Switch CLOSED (ON)
Derivation of output voltage , Vo
Switch OPEN (OFF)
(ΔiL)closed + (ΔiL)open=0 Inductor volt-second balanced (Average inductor voltage = 0)
FLY-BACK
Waveforms for Fly-back ConverterClosed
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
(iii)
(SW3, SW4) closed: (i) vp = -Vs
(ii) D1 OFF, D2 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 (C1 and C2) equally divide input voltage, therafore Vs/2 appear across primary when Sw1 closed and –Vs/2 when Sw2 closed.
Hence