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 11DD 
 nn2 2
Vout 1  D   DV
DV
inin   
 n1n1
Vout
D

nn22 
 nn2 2  
 DV
DVinin DDVVoutoutVVinin    
 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
-