isolated dc power supplies
Post on 13-Dec-2015
233 Views
Preview:
DESCRIPTION
TRANSCRIPT
• 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
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• Assume continuous iLm
FLY-BACK
(ΔiL)closed + (ΔiL)open=0
Inductor volt-second balanced (Average inductor voltage = 0)
Derivation of output voltage , Vo
OR
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
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:
FORWARD
• Derived from Buck converter• Isolation provided by high
frequency transformer• Assume continuous iLx
(ΔiL)closed + (ΔiL)open=0
Inductor volt-second balanced (Average inductor voltage = 0)
Derivation of output voltage , Vo
OR
FORWARD
Derivation of output voltage , Vo
Switch CLOSED (ON)
FORWARD
We know,
We can write
Therefore,
The slope of the magnetizing current is given by:
Derivation of output voltage , Vo
Switch OPEN (OFF)
FORWARD
We know,
Since
Therefore,
The slope of the magnetising current is given by:
What is vLm ? Therefore
If
Derivation of output voltage , Vo
FORWARD
(ΔiL)closed + (ΔiL)open=0 Inductor volt-second balanced (Average inductor voltage = 0)
Switch CLOSED (ON) Switch OPEN (OFF)
x
osclosedLx L
DTV
N
NVi
1
2
osLx VNNVv 12
1
2
N
NDVV so
1
2
N
NDVV so
FORWARDWaveforms for Forward Converter
It is important that iLm goes to zero before the start of a new cycle
This condition is fulfilled if
Now
1
31
3
1
3
1
111
NN
DN
ND
TDTDTN
N
TDTTx
If N3/N1 =1, then D < 0.5
v1
vLx
iLx
iLm
i1
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
x
oomax,Lx L
TD.V
R
VI
2
50
x
ooinm,Lx L
TD.V
R
VI
2
50
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
top related