Transcript POWER ELECTRONICS R & D LABORATORY DESIGN OF FORWARD CONVERTER WITH LCD SNUBBER Presented by Laszlo Huber November, 1999 Chungli, Taiwan POWER ELECTRONICS R & D LABORATORY EXAMPLE DPS-200PP-76 1.

POWER ELECTRONICS
R & D LABORATORY
DESIGN OF FORWARD CONVERTER
WITH LCD SNUBBER
Presented by
Laszlo Huber
November, 1999
Chungli, Taiwan
POWER ELECTRONICS
R & D LABORATORY
EXAMPLE DPS-200PP-76
1. SPECIFICATIONS
– Line voltage: 90-265 V, 47-63 Hz
– Outputs: 5 V / 1.5-20 A
12 V / 0.2-8 A
3.3 V / 0-20 A
-5 V / 0-0.3 A
-12V / 0-0.4 A
5 Vaux / 5-720 mA
Pomax = 200 W (combined 5V/3.3V Pomax = 120 W)
Pomin = 9.9 W (~ 5%)
– Hold-up time: 10 ms
2. TOPOLOGY
Multi-output forward converter with non-dissipative
(LCD) snubber and voltage-doubler rectifier (w/o PFC)
3. MAJOR DESIGN COMPONENTS
–
–
–
–
–
Bulk capacitors CB1,2
Primary-side switch Q
Secondary-side diodes
Forward Transformer T
Forward inductors
POWER ELECTRONICS
R & D LABORATORY
4. BULK CAPACITORS CB1,2
V
B
m
a
x
V
B
V
B
r
i
p
p
V
B
m
i
n
V
V
C
1
m
a
x
C
2
m
a
x
V
V
&
C
1
C
2
V
C
2
V
C
1
V
V
C
1
m
i
n
C
2
m
i
n
T
/
2
L
VB max  2  265  375 V
VB min  ?
At low-line range VB min  VC 2 min  VC 1(at VC 2 min )
CB2 
With
Po
2
dc / dc  fL
2

V
1
a cos C 2 min

VC 2 max
VC22 max  VC22 min
CB1  680 μF (200 V)
CB1 
Po
2
1

VC 2 min  93.5 V
V
1
a cos C 2 min

VC 2 max
dc / dc  fL VC21max  VC21(at VC 2 min )
CB1  680 μF (200 V)
VB min  206 V
VC 1(at VC 2 min )  112.5 V
POWER ELECTRONICS
R & D LABORATORY
5. VBmin WITH HOLD-UP TIME
i
C
B
C
B
1
P
o
v
B
Const. power load

d
c
/
d
c
C
B
2
Po
CB1 CB2 VB2min(w/ line )  VB2min(w/o line )

TH
CB1  CB2
2
dc / dc
2062  VB2min 200
340 

 10 m
2
0.8
VB min  167 V
Forward converter should be designed for
bulk-voltage range:
VB min  167 V
VB max  375 V
POWER ELECTRONICS
R & D LABORATORY
6. FORWARD CONVERTER TRANSFER FUNCTION
Vo  VF 
VB
D
N
Dmax VB max 375


 2.25
Dmin VB min 167
Derivation of forward-converter transfer function
from forward-inductor flux-balance (CCM):
L
T
V
B
V
o
N
 VB

 VF  VO  D  (VO  VF ) (1  D)

N

VB
D  (VO  VF ) D  VO  VF  (VO  VF ) D
N
Vo  VF 
VB
D
N
POWER ELECTRONICS
R & D LABORATORY
7. COMPARISON OF DIFFERENT RESET
TECHNIQUES FOR FORWARD
TRANSFORMER
–
–
–
–
Reset winding
RCD clamp
Active clamp
LCD snubber
7.1 FORWARD CONVERTER WITH RESET WINDING
T
N
R
N
N
P
S
V
B
D
R
Q
Transformer flux balance:
VB  D 
VB
NP Dr
NR
Dr 
NR
D
NP
Maximum switch voltage:
VDS max  VB 
Usually:

VB
N 
NP  VB  1  P 
NR
NR 

NR  NP
Dr  D
D  0.5
VDSmax  2  VB max
POWER ELECTRONICS
R & D LABORATORY
Example:
Dmax  0.48
N 5V 
VB min Dmax 167  0.48

 14.6
Vo  VF
5 .5
VDS max  2  375  750 V
V
VD5max

375
 25.7 V
14.6
900 V MOSFET
40-45 V SCHOTTKY
DIODE
POWER ELECTRONICS
R & D LABORATORY
7.2 FORWARD CONVERTER WITH RCD CLAMP
T
R
c
l
C
c
l
N
N
P
S
V
B
V
c
l
D
c
l
Q
Clamp voltage Vcl  f (VB) !
v
p
r
i
m
V
B
m
a
x
V
B
m
i
n
D
D
m
a
x
m
i
n
0
1
t
/
T
S
V
c
l
VDS max  VB  Vcl
Goal: 600-V MOSFET
0.85  600  375  Vcl
Vcl  135 V
From transformer flux balance:
VBmin  Dmax  Vcl  (1  Dmax )
Dmax 
Turns ratio (@ 5V output)
N5V 
VB min  Dmax 167  0.45

 13.7
Vo  VF
5 .5
Sec-side diodes:
V
VD5max

375
 27.4V
13.7
Vcl
135

 0.45
Vcl  VB min 135  167
POWER ELECTRONICS
R & D LABORATORY
7.3 FORWARD CONVERTER WITH ACTIVE CLAMP
T
V
c
l
Q
c
l
V
B
Q
D
c
l
Clamp voltage
VCl ~
1
VB
VDS max  VB  Vcl  const.
v
p
r
i
m
V
B
m
a
x
Goal: 600- V MOSFET
V
B
m
i
n
D
1
D
m
i
n
m
a
x
0
t
/
T
S
V
c
l
m
i
n
V
c
l
m
a
x
From transformer flux balance:
VBmin  Dmax  Vcl max  (1  Dmax )
VBmax  Dmin  Vcl min  1  Dmin 
Design approach:
VBmin  Vcl max  VBmax  Vcl min
Dmax  Dmin  1
Dmax
 2.25
Dmin
1
 0.308
3.25
 0.692
Dmin 
Dmax
VB max  Dmin
1  Dmin
V
D
 B max min
Dmax
Vcl min 
 VBmin
VDS max  VBmax  VBmin  375  167  542V
POWER ELECTRONICS
R & D LABORATORY
Derivation of Dmax + Dmin = 1
VBmin  Dmax  Vcl max  (1  Dmax )
(1)
VBmax  Dmin  Vcl min  (1  Dmin )
(2)
VBmin  Vcl max  VBmax  Vcl min
(3)
Substitute Vclmax from (1) and Vclmin from (2) into (3):
VB min 
VB min  Dmax
V
D
 VB max  B max min
1  Dmax
1  Dmin
(4)
VB min
V
 B max
1  Dmax 1  Dmax
(5)
VB max 1  Dmin

VB min 1  Dmax
(6)
From the forward-converter transfer function
(Vo  VF 
VB
 D)
N
VB max Dmax

VB min Dmin
Combining (6) and (7):
Dmax 1  Dmin

Dmin 1  Dmax
2
2
Dmax  Dmax
 Dmin  Dmin
2
2
Dmax  Dmin  Dmax
 Dmin
 (Dmax  Dmin )  (Dmax  Dmin )
Dmax  Dmin  1
(7)
POWER ELECTRONICS
R & D LABORATORY
Turns ratio (@ 5V output)
N5 V 
VB min  Dmax 167  0.692

 21
Vo  VF
5 .5
Sec-side diodes:
V
VD5max

375
 17.9V
21
25 - 35 V Schottky Diode
Advantages of active-clamp reset:
— larger N
lower current stress on prim.-side
lower voltage stress on sec.-side
— transformer operates in I and III quadrant
B
largerB
B
H
From Faraday’s Law:
B 
Vo  VF
NS  AC  fS
smaller core ( Ac )
— ZVS (Zero-voltage-svitching)
efficiency
higher switching frequency
— lower EM I