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| 型號： | AN-30 |
| 英文描述： | TOPSwitch-GX Forward Design Methodology |
| 中文描述： | 的TOPSwitch - GX系列正向設(shè)計方法 |
| 文件頁數(shù)： | 11/40頁 |
| 文件大小： | 311K |
| 代理商： | AN-30 |
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AN-30
11
B
12/02
Remember to check the maximum and minimum tolerance on
I
from the data sheet for the selected device. Allow margin
to guarantee that the peak primary current I
is less than the
minimum value of I
at high temperature. With minimum
device I
LIMIT
, check that
I
I
PP
LIMIT
≤
0 96
.
(17)
Adjust the system specifications if the peak current is too high
for the largest device. While some specifications are fixed,
others are adjustable at the discretion of the designer. Raising
the minimum input voltage will give lower peak current.
Step 6. Design the transformer.
The transformer design can be either completed in-house or
delegated to a qualified supplier of custom magnetics. An
outside supplier needs to know the turns ratios and the
recommended restrictions on flux density to start a design. Even
if the ultimate design will be done outside, it is beneficial to do
a rough design in-house. A proposed design with actual
numbers of turns on each winding will reduce the time required
to obtain a satisfactory transformer.
The maximum recommended flux density for this application is
(18)
and the recommended maximum change in flux density per
switching period (AC flux density) is
(19)
The constraint on B
sets the minimum number of turns for a
particular core, while the limit on B
restricts the maximum
transient duty ratio. Although peak flux density under steady-
state conditions can be calculated, the designer should allow
sufficient margin to avoid saturation under transient conditions.
To start the design, select a core that is likely to meet the size and
efficiency requirements of the application. Since the voltages
and turns ratios are determined, all that remains is to find the
actual number of turns and the size of wire for each winding.
Compute the minimum turns for the main output.
(20)
where A
is the effective area of the core. Units in the above
expression are volts, tesla, meter
2
and hertz. Round N
MAIN
upward to the next integer value.
Compute the turns for the other power windings.
(21)
Round N
downward to the next integer. Round N
AUX
and N
IND
to the nearest integer.
Compute the turns for the bias winding.
(22)
Round N
B
upward to the nearest integer value.
Designers should use copper foil instead of wire for windings
of few turns that carry high current. It is very important to the
success of the design to minimize leakage inductance.
Compute an estimate of the peak magnetizing current.
The primary inductance in henries is
(23)
where
μ
is the permeability of free space, A
is the effective
area,
l
is the effective path length in the core and
l
is the length
of the air gap (see Zero Gap Transformer section). The
dimensionless relative permeability
μ
r
is given by
(24)
Units in the above two expressions are the SI basic units with the
exception of inductance coefficient A
L
, which has the
conventional units of nH/turn
2
.
With no gap, the primary inductance in henries is simply
(25)
Now the peak magnetizing current is given by
(26)
Units in the above expression are amperes, volts, henries and
hertz. The magnetizing current should be less than 10% of the
primary current for reasonable power dissipation in the clamp
circuit.
N
n N
P
MAIN
=
N
n
N
AUX
AUX
MAIN
=
N
n
N
IND
IND
MAIN
=
L
A N
P
P
e
r
g
=
+
μ
μ
0
2
l
μ
π
r
L e
e
A
A
=
l
400
L
A N
P NO GAP
P
=
×
2
9
10
K
I
=
1
for
I
I
K
PP
XLIMIT
I
≤
<
0 86
.
1
for
B
PEAK
≤
0 3
3000
.
(
)
tesla
gauss
I
V
D
L f
MP
MIN
MAX
=
B
M
≤
0 2
2000
.
(
)
tesla
gauss
N
N
V
V
B
P
DB
DROPOUT
=
+
8 volts
N
V
V
B A f
MAIN
MAIN
DMAINF
≥
+
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