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| 型號(hào)： | AN-17 |
| 英文描述： | Flyback Transformer Design for TOPSwitch Power Supplies |
| 中文描述： | 回掃變壓器設(shè)計(jì)的TOPSwitch電源 |
| 文件頁(yè)數(shù)： | 4/12頁(yè) |
| 文件大?。?/td> | 133K |
| 代理商： | AN-17 |
AN-17
C
6/96
4
currents and a ripple to peak current ratio K
of less than one
but typically greater than 0.4. K
is inversely proportional to
primary inductance so a continuous design with lower K
will
have a higher inductance. Continuous transformer designs
have a practical primary inductance upper limit approximately
four times that of a discontinuous design at the same input
voltage and output power due to the difference in peak currents
and value of K
RP
.
The primary current waveforms shown in Figures 2 and 3
deliver the same output power and therefore (assuming equal
efficiency) must have equal I
. The discontinuous current
waveform has a higher peak value and therefore must have a
higher RMS current value. Discontinuous mode requires less
inductance and reduces transformer size but operates with
higher losses and lower efficiency due to higher RMS currents.
Continuous mode requires higher inductance and larger
transformer size but offers improved efficiency and lower
power losses. The trade-off between transformer size and
power supply efficiency depends on the packaging and thermal
environment in each application.
Some control loop comments regarding continuous mode are in
order here. Most designers tend to avoid the continuous mode
whenever possible because the feedback control loop is more
difficult to analyze. Discontinuous mode power supplies are
modeled with a single pole response and are simple to stabilize.
Continuous mode offers improved efficiency, reduced losses,
lower component temperatures, or higher output power but
analysis is more difficult because a right half plane zero and
complex pole pair all shift with duty cycle. However, stabilizing
a continuous mode
TOPSwitch
power supply is quite
straightforward. Adequate phase margins are achievable over
all line and load combinations because the 70% maximum
TOPSwitch
duty cycle DC
(from the data sheet) limits right
half plane zero and complex pole pair migration. Phase margin
is generally higher than expected once the damping effect of
effective series power path resistance and output capacitor ESR
is taken into account. Crossover bandwidths of 1 KHz (or
wider) are easily achievable with phase margins of at least 45
degrees. Refer to AN-14 for circuit techniques to use in
continuous mode designs.
Transformer core, winding, and safety issues must also be
discussed before beginning design.
Transformer core and construction parameters depend on the
selected core and winding techniques used in assembly. Physical
height and cost are usually most important when selecting
cores. This is especially true in AC mains adapter power
supplies normally packaged in sealed plastic boxes. Applications
allowing at least 0.75 inches of component height can use low
cost EE or EI cores from Magnetics, Inc., Japanese vendors
TDK and Tokin, or European vendors Philips, Siemens, and
Thomson. Applications requiring lower profile can benefit
from EFD cores available from the European vendors. EER
cores offer a large window area, require few turns, and have
bobbins available with high pin counts for those applications
requiring multiple outputs. ETD cores are useful in the higher
power designs when space is not a problem. PQ cores are more
expensive but take up slightly less PC board space and require
less turns than E cores. Safety isolation requirements make pot
cores, RM cores, and toroids generally not suitable for flyback
power supplies operating from the AC mains.
Flyback transformers must provide isolation between primary
and secondary in accordance with the regulatory agencies of
the intended market. For example, information technology
equipment must meet the requirements of IEC950 in Europe
and UL1950 in the U.S. These documents specify creepage
and clearance distances as well as insulation systems used in
transformer construction. 5 to 6 mm creepage distance is
usually sufficient between primary and secondary (check with
the appropriate agency and specification). Isolation is usually
specified by electric strength and is tested with a voltage of
typically 3000 VAC applied for 60 seconds. Two layers of
insulation (Basic and Supplementary) can be used between
primary and secondary if each layer exceeds the electric
strength requirement. Three layers of insulation (reinforced)
can also be used if all combinations of two layers (out of total
three layers) meets the electric strength requirement.
Figure 4a shows the margin winding technique used in most
flyback transformers. The margin is usually constructed with
layers of tape slit to the width of the desired margin and
wrapped in sufficient layers to match the winding height. The
margin is generally half the required primary to secondary
creepage distance (2.5 mm in this example). Cores and
bobbins should be selected large enough that the actual winding
width is at least twice the total creepage distance to maintain
transformer coupling and reduce leakage inductance. The
primary is wound between the margins. To reduce the risk of
interlayer voltage breakdown due to insulation abrasion,
improve layer to layer insulation, and decrease capacitance,
the primary layers should be separated by at least one layer of
UL listed polyester film tape (3M 1298) cut to fit between the
margins. Impregnation with varnish or epoxy can also improve
the layer to layer insulation and electric strength but does not
reduce capacitance. The bias winding may then be wound over
the primary. Supplementary or reinforced insulation consisting
of two or three layers of UL listed polyester film tape cut to the
full width of the bobbin may then be wrapped over the primary
and bias windings. Margins are again wound. The secondary
winding is wound between the margins. Another two or three
layers of tape is added to secure the windings. Insulation
sleeving may be needed over the leads of one or all windings
to meet creepage distance requirements at lead exits. Nylon or
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| AN-18 | TOPSwitch Flyback Transformer Construction Guide |
| AN-19 | TOPSwitch Flyback Power Supply Efficiency |
| AN-20 | Transient Suppression Techniques for TOPSwitch Power Supplies |
| AN-22 | OBSOLETE when inventory is depleted. 10% tolerance no l |
| AN-23 | TinySwitch Flyback Design Methodology |
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