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| 型號： | 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)： | 11/20頁 |
| 文件大小： | 177K |
| 代理商： | AN-19 |
A
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AN-19
11
using foil windings is also a good way of increasing coupling,
although this method is practical only for low voltage, high
current secondary windings. A more complete discussion of
transformer leakage inductance can be found in several of the
references listed at the conclusion of this document.
Transformer Capacitance
A high-efficiency transformer should have low interwinding
capacitance. Energy stored in the parasitic capacitance of the
transformer is absorbed by the
TOPSwitch
each cycle during
the turn-on transition, reducing efficiency. Excess capacitance
also will ring with stray inductance during switch transitions,
causing noise problems. Capacitance effects are usually the
most important in the primary winding, where the operating
voltage (and consequent energy storage) is high. The primary
winding should be the first winding on the transformer. This
allows the primary winding to have a low mean length per turn,
reducing the internal capacitance. The driven end of the
primary winding (the end connected to the
TOPSwitch
Drain
pin) should be the start of the winding rather than the finish.
This takes advantage of the shielding effect of the second half
of the primary winding and reduces capacitive coupling to
adjacent windings. A layer of insulation between adjacent
primary windings can cut the internal capacitance of the primary
winding by as much as a factor of four, with consequent
reduction of CV
2
f losses for the
TOPSwitch
.
Primary Components
Inrush Limiters
There are several options that can be used to avoid excessive
inrush current into the input filter capacitor of a power supply
during initial turn on. In very low power supplies or supplies
where there is no stringent limit on inrush current, it is possible
to use a certain amount of fixed impedance, either in the form
of a fixed resistor, or resistance built into the RFI filter inductors.
The loss in efficiency can usually be tolerated, especially as
these methods offer space and cost savings which can be vital.
The ST204A utilizes the impedance of the input common mode
choke (L2) in order to reduce inrush current. If tighter control
of the inrush current is needed along with higher efficiency, a
negative temperature coefficient thermistor can be used as an
The transformer in the ST204A uses triple insulated wire with
an EI28 core for 30 W output. A magnet wire design would
require a EI30 core to accommodate the creepage margins,
resulting in a 33% size increase.
PI-1688-112795
SECONDARY
BIAS
PRIMARY
BIAS
PRIMARY
MARGIN
MARGIN
SECONDARY
OFFSET WINDING CONSTRUCTION
(NOT RECOMMENDED)
SPLIT BOBBIN CONSTRUCTION
(NOT RECOMMENDED)
Figure 16. Offset and Split Bobbin Construction Techniques (Not Recommended).
Winding Arrangement for Minimum Leakage Inductance
The arrangement of windings in a transformer will have a large
effect on the leakage inductance. Transformer windings should
be arranged in concentric fashion for minimum leakage
inductance, as shown in Figures 14 and 15. Offset or split
bobbin construction (shown in Figure 16) should be avoided, as
this technique will result in high leakage inductance and
unacceptable losses in the primary clamp circuit.
In a multiple output transformer, the secondary with the highest
output power should be placed closest to the primary for the best
coupling and lowest leakage. For higher power applications (40
watts and above), it is recommended to use a split primary
“sandwich” construction as shown in Figure 17 to minimize
leakage inductance. Using a split primary will usually cut the
leakage inductance to half that of a transformer with a single
primary winding.
High power secondary windings consisting of only a few turns
should be spaced across the width of the bobbin window instead
of being bunched together, in order to maximize coupling with
the primary. Using multiple parallel strands of wire is also a
good technique of increasing the fill factor and coupling of a
winding with few turns. In such cases, the wire size may be
determined more by the requirement for a good fill factor rather
than the RMS current rating of the wire. Where cost permits,
PI-1689-111695
SECOND PRIMARY HALF
REINFORCED INSULATION
SECONDARY
MARGINS
BIAS
FIRST PRIMARY HALF
REINFORCED INSULATION
Figure 17. Split Primary Construction to Reduce Leakage Inductance.
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