參數(shù)資料
型號(hào)： AN-22
英文描述： OBSOLETE when inventory is depleted. 10% tolerance no l
中文描述： 設(shè)計(jì)多路輸出電源的的TOPSwitch
文件頁(yè)數(shù)： 7/24頁(yè)
文件大?。?/td> 569K
代理商： AN-22
C
5/98
AN-22
7
lower voltage outputs where the requirement for integer
numbers of turns can introduce a significant deviation from the
desired value.
1. If the output in question requires a high degree of
accuracy, then a higher output voltage can be defined in
equation (2) and a linear post regulator employed to
achieve the output voltage.
2. If the tolerance is less critical, a series resistor and a
Zener diode of appropriate value can be used as a shunt
regulator for low power outputs.
3. The fundamental transformer design could be modified
such that the main 5 V output uses a number of turns
which yields an integer number of turns on the other
windings when calculated using equations (1) and (2).
4. The choice of rectifier on the main regulated output can
be used to influence the volts per turn. If a Schottky
diode with a forward voltage of typically 0.4 V were
employed on the 5 V output, the V
from (1) would be
1.35. A standard PN diode on the 30 V output would
from equation (2) yield 22.7 turns, which is closer to the
integer number 23.
Use of the Schottky diode with 4 turns on the 5 V output,
however, would decrease the accuracy of the 12 V output. The
required number of turns would move farther away from an
integer value, from 8.9 to 9.4 turns.
The designer can investigate alternative integer turns ratios
with both Schottky and PN diodes by repeating the spreadsheet
design for other values of secondary turns. If a need for higher
efficiency calls for a Schottky diode on the 5 V output, then 3
turns on the 5 V output with 7 and 17 turns for the 12 V and
30 V outputs respectively may give acceptable results.
Designers often use the "golden ratios" of 3:7:9 with a
Schottky diode for the 5 V output and a PN diode for the 12 V
output, or 4:9:11 with all PN diodes to achieve outputs of 5, 12
and 15 V. Another useful ratio is 2:3 for outputs of 3.3 and
5 V with Schottky diodes on each. The turns could be in the
ratio of 3:4 if the 3.3 V output uses a PN diode and the 5 V uses
a Schottky diode. All designs need to be tested thoroughly to
verify acceptability.
In practice, if tight tolerance is required on windings other
than the main feedback output, some form of post regulation
or combined feedback circuitry is often necessary. These
issues of cross regulation are discussed later in the section on
circuit performance.
In this case, as mentioned above, the choice of 22 turns for the
30 V output will not introduce a significant inaccuracy. The
final choice of turns on each output is therefore shown in
Figure 3(a), and summarized as follows:
5 V
—
4 turns
12 V
—
9 turns
30 V
—
22 turns
Figure 3 illustrates two winding diagrams: one with separate
windings for each output and one with stacked output windings.
These two configurations are discussed in detail later in the
section on transformer construction.
Choice of Output Wire Gauge
Appropriate wire gauge for the outputs is determined on the
basis of the maximum continuous RMS current rating for each
winding. The analysis of the distribution of current in the
various outputs can be very complex, but a few reasonable
assumptions make the task easy.
The waveshapes of the currents in the individual output
windings are determined by the impedances in each circuit.
Leakage inductance, rectifier characteristics and capacitor
values are some of the parameters that affect the magnitude and
duration of the currents. The average currents are always equal
to the DC load current, while the RMS values are functions of
peak magnitudes and conduction times. The RMS values
determine the power dissipation in the windings. For ordinary
multiple output designs it is valid to make the reasonable
simplifying assumption that all output currents have the same
shape as for the single output case. This is the case of greatest
dissipation.
Ultimately the final design of the transformer has to be decided
on the basis of tests and consultation with transformer suppliers.
However, the first order analysis that assumes the same
waveshape for all output currents provides a start point for the
choice of wire gauge.
The single output design of the spreadsheet calculates the RMS
current in the secondary as if the 5 V winding supplied all the
power. However, from the specification of Table 1, the 5 V
output supplies a maximum of 10 W. The actual currents in the
multiple output application are computed from quantities on
the single output spreadsheet.
Since we assume the currents in the output windings have the
same shape, each will have the same ratio of RMS to average
as the single output case. If K
RA
is the ratio of RMS to average
current, then
K
I
I
RA
SRMS
O
=
=
=
7 62
5
1 524
.
.
A
A
(3)
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