Transcript Document
Switch Mode Power Supplies
Use of Transformers for Electrical (DC) Isolation
• Safety – Isolate output voltages from
Electrical Grid
• Multiple Reference ( “ground”) Potentials
• Voltage Matching – Large difference
between voltage levels of power source
and converter outputs.
• Feedback must traverse isolation boundary
1. Separate winding for feedback
2. Optical isolation
Vdd
Gate
Drive
Transformer Converter Types:
• Flyback Converter – Derived from Buck-Boost
• Forward Converter - Derived from Buck
• Full Bridge – Derived from Buck
Buck-Boost
Flyback Converter
+
np
iLp(t)
Vin
is(t)
Vout
F(t)
-
Df
+
ns
np DI p
R
1
np
f (t)
-
ip(t)
Vin DTs
DT vp t dt np
Vin DTs
R Df R
DI p
2 Vin DTs
np
Lp
np
is(t)
CCM)
(Assume CMM)
Df
Flyback Converter (cont)
Vin DTs Vout 1 D Ts
Df
np
ns
Vout
ns
Vin
n
p
D
1 D
Same as Buck-Boost
n p Iˆp
ˆ
n
I
fˆ
s s
R
R
ns
Iˆp Iˆs
n
p
ns
DI p D I s
n
p
Flyback Converter (cont)
DI p
I out
Vp DTs
Lp
Vp D
Lp f s
ˆ DI p
I in I p
D
2
n1 ˆ DI p
ˆ DI s
Is
1 D I p 1 D
2
2
n2
n1 I in
I out
1 D n2 D
D n2
Iin I out
1 D n1
Same as Buck-Boost
+
n1
n2
+
Vin
Vout
-
-
n2n2 DD
V
V
in in
n1n1 11DD
nn2 2
Vout 1 D DV
DV
inin
n1n1
Vout
D
nn22
nn2 2
DV
DVinin DDVVoutoutVVinin
DVout
out
nn11
n1n1
Vout
n2
Vin
Vout
n1
Multiple Secondaries
dF
vi ni
dt
np
ns1
ns 2
I p I s1
I
n s 2 n
p
p
ˆI Iˆ ns1 Iˆ ns 2
p
s1
s2 n
n
p
p
ns1
ns 2
DI p DI s1 DI s 2
n
n
p
p
Feedback
ns1
+
Vout,1 >
0
ns 2
+
Vout,2 < 0
-