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| 型號： | AN-15 |
| 英文描述： | TOPSwitch Power Supply Design Techniques for EMI & Safety |
| 中文描述： | 的TOPSwitch電源電磁干擾設(shè)計(jì)技術(shù) |
| 文件頁數(shù)： | 20/36頁 |
| 文件大小： | 381K |
| 代理商： | AN-15 |
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AN-15
A
6/96
20
PI-1646-111695
PRIMARY
SECONDARY
(INSULATED)
BIAS
TAPE
TAPE
}
{
Figure 35. Insulated Wire Wound Transformer Cross Section.
connected to each core half (manganese zinc ferrite normally
used in 100 kHz flyback transformers is conductive) and the foil
ends are electrically connected to create a shorted turn. The flux
band can usually be left floating without making additional
bobbin changes to meet reinforced insulation requirements for
safety. The flux band (and core) may also be connected back to
primary or to secondary but bobbin construction must be
examined to ensure creepage distance is sufficient to meet
reinforced insulation requirements for safety.
In some applications, a copper foil “flux band” over the outside
of the completed transformer as shown in Figure 37 may reduce
some common mode emissions. The copper foil wraps one
complete turn over the exposed (but insulated) windings and
each core end gap. Foil width is cut to fit between bobbin
flanges while maintaining required creepage distance for
reinforced insulation. For best effect, the foil is electrically
Figure 36 shows a typical margin wound transformer cross
section. Each winding is placed between symmetric margins as
shown while the safety insulation extends beyond the margins
up to the walls of the bobbin flange. The split primary
construction shown reduces leakage inductance in higher power
applications. The primary half connected to
TOPSwitch
is the
first layer followed by one wrap of 2 mil polyester film tape for
basic insulation. The bias winding is wound next in a single
layer. The bias winding is usually just
a few turns but wound
using up to three parallel wires to cover more of the bobbin
width and effectively shield the noisy
TOPSwitch
Drain
connected primary half. Three wraps of 2 mil polyester film
tape (3M 1298 or equivalent UL recognized tape) provide the
necessary reinforced insulation for safety as well as reducing
capacitance between primary and secondary to minimize
common mode emission currents. Secondary is now wound
between margins followed again by three wraps of 2 mil
polyester film tape for reinforced insulation. The second half
of the primary is wound followed by three wraps of 2 mil
polyester film tape as final insulation.
PI-1647-111695
SECOND PRIMARY HALF
REINFORCED INSULATION
SECONDARY
MARGINS
(BOTH
SIDES)
BIAS
FIRST PRIMARY HALF
REINFORCED INSULATION
Figure 36. Margin Wound Transformer Cross Section.
In most
TOPSwitch
power supplies, a transformer shield is not
necessary because
TOPSwitch
has controlled turn on which
limits high voltage dv/dt and reduces common mode emission
currents. For those few applications where further reduction in
common mode emission currents is desired, Figure 38 shows
proper shield placement within the transformer. The shield
intercepts interwinding capacitive displacement currents and
returns them to the primary circuitry. Figure 38 shows the
shield connected to V+ but the shield can also be connected to
V- if more convenient for construction or layout reasons. Note
that safety insulation or creepage distance is required between
the primary connected shield and SELV secondary outputs.
The foil shield width is selected to fit between primary safety
margins. Length is precut for one full turn with slight overlap
at the ends. The termination lead wire is soldered to the copper
foil shield in the center (equidistant from each end). Tape
insulation is usually applied to the copper foil shield before
placing on the transformer. The ends must be insulated such
that the foil shield does not form a shorted turn inside the
transformer (compared with the external “belly band” described
earlier which has a shorted turn but is physically located outside
the transformer).
Refer to AN-18 for more information on transformer
construction.
PI-1648-111695
TRANSFORMER FLUX BAND
COPPER
FOIL
STRAP
Figure 37. Transformer Flux Band.
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| AN-16 | TOPSwitch Flyback Design Methodology |
| AN-17 | Flyback Transformer Design for TOPSwitch Power Supplies |
| AN-18 | TOPSwitch Flyback Transformer Construction Guide |
| AN-19 | TOPSwitch Flyback Power Supply Efficiency |
| AN-20 | Transient Suppression Techniques for TOPSwitch Power Supplies |
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