參數(shù)資料
型號(hào)： AN-18
英文描述： TOPSwitch Flyback Transformer Construction Guide
中文描述： 回掃變壓器的TOPSwitch施工指南
文件頁(yè)數(shù)： 25/32頁(yè)
文件大小： 275K
代理商： AN-18
A
7/96
AN-18
25
Triple Insulated Wire Construction Example
In the following paragraphs, the design parameters needed to
complete the Table 6 spreadsheet input section will be described.
A triple insulated wire transformer construction example will
then be completed with the information from the spreadsheet.
Determining Transformer Construction Variables
In order to complete the input portion of the Table 6 spreadsheet,
information is needed for the transformer/core construction
variable section. Table 7 shows the transformer core/construction
variables chosen for the triple insulated transformer construction
example. The EF20 core was chosen from the transformer core
chart in Appendix A. Dimensional and electrical characteristics
for the core and a compatible bobbin are shown in Figures 6 and
7. The core electrical parameters necessary for the spreadsheet
design are A
, L
, and A
, and are loaded into spreadsheet cells
(B24), (B25), and (B26), respectively. Margin width (M) was
set to zero for the triple insulated wire design and loaded into
cell (B28). The number of primary layers (B29) is set at 2 to
optimize the core size and to reduce the transformer leakage
inductance and stray capacitance. BW (Bobbin Physical Winding
Width) is calculated as in the margin wound example, since it
is not directly available from the Figure 7 bobbin drawing.
W
is 13.4 mm and W
F(MAX)
is 0.7 mm, so BW for the EF20
bobbin is:
[
=
.
(
13 45
2
200-500 circular mils/ampere. The CMA and resulting AWG
values in the spreadsheet are dependent variables and cannot be
adjusted directly. The number of primary turns in the spreadsheet
cannot be manipulated directly, as it is a function of the number
of secondary turns. Gross adjustments can be made indirectly
to the primary CMA value by changing the number of secondary
turns (N
) or the core size (see AN-16). Adjusting the number
of secondary turns changes the number of primary turns
proportionally to maintain the reflected output voltage, V
, at
its specified value. Changing the core size changes the available
bobbin width (BW). The spreadsheet will change the primary
wire size to fill the available bobbin width using the specified
number of primary winding layers.
In some cases, changing the number of secondary turns results
either in too large a change in primary wire size or has a
deleterious effect on other parameters, such as maximum flux
density or transformer gap length. Also, it may not be desirable
to change the core size for reasons of cost, availability, or size
constraints. The following techniques are useful for fine
adjustment of the AWG and CMA of the primary winding
without changing the core/bobbin size or the number of
secondary turns:
To reduce the primary wire size to a slightly smaller value,
adjust the number of primary layers to a value less than the
default value of 2 layers in increments of 0.1 layer (for
example, try 1.9 layers, 1.8 layers, etc.).
To increase the primary wire size to a slightly larger value,
adjust the reflected voltage V
downward
in increments
of 1-2 volts. This will slightly reduce the number of
primary turns. Maximum duty cycle (D
MAX
) will be reduced
slightly, L
will become smaller and B
will rise slightly.
Do not adjust the reflected primary voltage more than 10%
below the maximum recommended value suggested in
AN-16. If more adjustment is needed, reduce the number
of secondary turns instead or use a larger core size.
Initially, the Table 6 spreadsheet used the default value of 135V
for V
in cell (B16). For N
= 9 turns, this resulted in N
= 96
turns, and a primary wire size of 33 AWG, which for this design,
is slightly too small (CMA of 196 circular mils/ampere) to
satisfy the CMA requirement of 200-500 circular mils/ampere.
Adjusting the number of primary turns by reducing the secondary
turns resulted in a transformer design with a maximum flux
density larger than the design limit of 3000 Gauss. To avoid this
condition, the number of primary turns was instead reduced
from 98 turns to 94 turns by reducing V
from 135V to 130V
in small steps. This allowed the primary wire size, AWG (D56),
to change from 33 AWG to 32 AWG and brought the primary
winding CMA (D58) up to 243 circular mils/ampere. Maximum
duty cycle and maximum flux density were only slightly
affected by this change.
BW
W
W
mm
T MIN
(
F MAX
0. )
=
×
]
.
×
)
(
)
2
12 0
This BW value is entered into cell (B27) of the Table 6
spreadsheet. The optimum number of secondary turns, N
S
(B30), is determined as 9 turns after iteration. The number of
primary layers (B29) is set at 2 to optimize the core size and to
reduce the transformer leakage inductance and stray capacitance.
Completion of Triple Insulated Wire Transformer
Example
Information necessary to specify the triple insulated wire
transformer example is shown in Table 8. The entries in
Table 8 were taken directly from the Table 6 spreadsheet or
calculated from the spreadsheet values. Unlike the Table 1
spreadsheet design for the EF25 transformer, the Table 6
spreadsheet requires one additional iteration cycle to optimize
the primary wire size. This iteration cycle and the remaining
steps in the transformer design are described below.
Adjusting Primary Wire Size and CMA
The design spreadsheet adjusts the primary wire size to the
closest AWG value that will fit within the available bobbin
width. In some cases the AWG wire size given by the spreadsheet
may result in a CMA value outside of the desired range of
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