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)： 13/24頁(yè)
文件大?。?/td> 569K
代理商： AN-22
C
5/98
AN-22
13
For reverse voltage ratings less than 100 V, Schottky diodes
can be used to minimize power losses. As discussed earlier,
Schottkys can also be used to improve the relative accuracy of
output voltages when calculating the number of turns. Schottky
diodes are more expensive than PN junction diodes. The
circuit of Figure 1 uses ultra fast recovery PN diodes for the
lowest cost, while the circuit in Figure 4 uses a Schottky diode
on the 5 V output with the same transformer design. Circuit
performance may be improved with a transformer designed
specifically for a Schottky diode on the 5 V output.
In this example many possible diodes are available to achieve
the required characteristics. The devices in the example of
Figure 4 are:
5 V output:
MBR745
7.8 A, 45 V
Motorola
12 V output:
MUR420
4.0 A, 200 V
Motorola
30 V output:
UF4004
1.0 A, 400 V
General Semiconductor
Other suitable diodes are available from different
manufacturers. Tests with a number of diodes are
recommended to verify the optimum devices in each
application.
Circuit Performance
The volts per turn defined in Equation (1) is an approximation
based on the forward voltage of the output diode. This value
changes with load current and temperature. As the outputs
have varying loads, the output diodes will exhibit different
forward voltages depending on the load conditions on the
particular output. Changing load conditions on the 5 V output,
for example, will inherently influence the voltages on the other
outputs.
In addition, secondary effects such as voltage spikes from
leakage inductance and quality of coupling between output
windings, lead to reduced voltage accuracy on outputs which
do not provide feedback through the optocoupler.
The basic circuit of Figure 4 derives feedback only from the
5 V output. As a consequence, the other output voltages vary
as the 5 V output current changes. The influence on the 12 V
output is shown in Figure 6(a). Use of a Schottky diode in a
circuit designed for a PN diode emphasizes the effect of a
change in voltage drop, as illustrated in this example.
The 5 V output voltage is well controlled since it exclusively
provides the feedback signal. The 12 V output, however, is
seen to vary by
±
2% as the 5 V load is varied between 25% and
100% (0.5 amps to 2.0 amps). For this test the 12 V output load
was held constant at 0.6 amps. The 12 V and 30 V outputs are
also below their nominal values because of the lower drop of
the Schottky diode.
Transformer construction techniques to optimize output cross
regulation were discussed earlier. However, it is often necessary
to further enhance cross regulation using external circuit
techniques. For example, if improved regulation is required
on the 12 V output, a simple technique is to derive the feedback
from both 5 V and 12 V outputs. In this example, as in most
applications, higher accuracy is required on one of the outputs.
Here it is assumed that the main output is still the 5 V, but some
feedback may be drawn from the 12 V output to improve its
Figure 8(a). Cross Regulation with Feedback from 5 V Only.
Response to Variation of 12 V Load
95
0.00
0.20
0.40
(a) 12 V Load (A)
0.60
0.80
O
P
105
100
1.00
1.20
5 V
12 V
30 V
95
0.00
0.20
0.40
(b) 12 V Load (A)
0.60
0.80
O
P
105
100
1.00
1.20
5 V
12 V
30 V
Figure 8(b). Cross Regulation with Feedback from 5 V and 12 V.
Response to Variation of 12 V Load
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