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| 型號(hào)： | AN-19 |
| 英文描述： | TOPSwitch Flyback Power Supply Efficiency |
| 中文描述： | TOPSwitch的反激式電源供應(yīng)器效率 |
| 文件頁數(shù)： | 10/20頁 |
| 文件大小： | 177K |
| 代理商： | AN-19 |
AN-19
A
6/96
10
below 1% are difficult to attain and still meet safety regulations.
Designing transformers for low leakage inductance involves
several considerations:
Minimize number of turns
Keep winding build (ratio of winding height to width) small
Increase width of windings
Minimize insulation between windings
Increase coupling between windings
These considerations are also discussed in AN-17 and AN-18.
Figure 15. Cross Section of a Transformer Construction Using
Magnet Wire and Margins.
Minimizing Winding Turns and Build
The first step in building a low-leakage transformer is choosing
the proper core geometry. Since leakage inductance varies as
the number of turns squared, it is important to choose a core that
will not require too large of a number of turns to attain the
desired primary inductance. The core should be sufficiently
large so that the required primary turns can be wound using 2
layers or less. This minimizes leakage inductance and winding
capacitance. Figure 13 shows a comparison between a high
leakage and a low leakage core geometry. Cores with short, fat
window areas should be avoided, as the winding build will be
large, resulting in high leakage inductance. Such cores include:
pot cores, RM cores, PQ cores (especially the short versions),
and some E cores. More suitable cores are ETD cores, EER
cores, EC cores, and many E cores.
In many cases, the optimum core size and shape for a transformer
of a given rating will depend on whether a triple insulated wire
or magnet wire is used in the transformer construction. Triple
insulated wire has three separable layers of insulation, any two
of which can withstand the full UL/IEC hipot test voltage.
Triple insulated wire thus satisfies the requirements for a
reinforced insulation per UL/IEC regulations, and can be used
to construct a transformer without the creepage margins required
in a design using conventional magnet wire. A cross -sectional
comparison between a triple insulated wire transformer design
and a conventional magnet wire design is shown in Figures 14
and 15. The triple insulated wire design uses a magnet wire
primary and a triple insulated secondary. This is generally the
most cost effective and space-efficient way to utilize the benefits
of triple insulated wire, as it is larger in diameter and more
costly than magnet wire. The secondary winding will require
fewer turns of larger diameter wire than the primary, so the cost
and space impact of the triple insulated wire is minimized. Note
that in the triple insulated wire design , the full width of the
transformer bobbin is usable, due to the reinforced insulation
provided by the triple insulated wire. A transformer using a
triple insulated wire design will generally be 1/2 to 2/3 of the
size of a transformer of the same power capability using a
magnet wire design due to the elimination of safety margins.
Leakage inductance will also be less for the triple insulated
design due to the improved utilization of space on the transformer
bobbin.
PI-1687-112795
PRIMARY
SECONDARY
BIAS
MAGNET WIRE
MAGNET WIRE
TRIPLE INSULATED
TAPE
TAPE
}
{
Figure 14. Cross Section of Transformer Using Triple Insulated
Wire.
Figure 13. Leakage Characteristics of Different Core Shapes.
PI-968-032293
HIGH
LEAKAGE
CORE
GEOMETRY
LOW
LEAKAGE
CORE
GEOMETRY
PI-1647-111695
SECONDARY
MARGINS
BIAS
PRIMARY
REINFORCED INSULATION
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