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)： 3/24頁(yè)
文件大?。?/td> 569K
代理商： AN-22
C
5/98
AN-22
3
Regulation Requirements
Specification of the regulation requirements on all outputs is
essential to successful design of the circuit configuration and
transformer. Requirements differ significantly depending on
the application.
One output usually requires tighter regulation than the others.
Usually the 5 V supply for logic circuitry requires regulation
of
±
5% or less, while other outputs have a wider tolerance of
typically
±
10%. Many applications now require both 3.3 V and
5 V outputs, with
±
5% regulation specifications. There are
several techniques which can be used to achieve this
performance, and they are discussed in more detail in Appendix
B of this application note.
While a 5 V output may have the most stringent regulation
specification, a different winding often has a higher output
load specification. Consideration must therefore be given to
the required cross regulation between these outputs, because it
will influence the transformer winding technique for an optimum
design.
Table 1 gives an outline specification for a 25 W power supply
with three outputs. Note that the 5 V output has the highest
current and the tightest regulation, but the 12 V output delivers
the highest power. The techniques presented here can be
extended to any number of outputs. Some specific
considerations for more outputs are discussed later.
The next step of the design is to determine the most appropriate
feedback technique. As a quick reference for deciding the
optimum feedback technique, Table 2 provides broad design
rules which can be used, based on the required output tolerances
of a specific application. If no tighter than
±
10% tolerance is
required on all outputs, a primary side feedback scheme may
be employed. This technique eliminates the need for an
optocoupler by using the primary bias winding of the
transformer to derive information about the regulated output
on the secondary. This type of feedback scheme is detailed in
AN-16. It is difficult, however, to achieve the output voltage
tolerance of
±
5% with this scheme alone.
If outputs requiring
±
5% are only lightly loaded, primary side
feedback may be used with a linear post regulator on these
outputs at the expense of some drop in efficiency. From the
specification in Table 1, however, the 2 A peak load on the 5
V output would lead to excessive dissipation in a linear
regulator; therefore, the remainder of this application note will
concentrate on feedback that uses an optocoupler.
There are two common techniques to generate a secondary
reference with optocoupler feedback. The first uses a simple
Zener diode as a secondary reference. This technique is
described in the supporting literature for Power Integrations
’
RD5 reference design board. The output voltage is determined
by the Zener voltage, the forward voltage of the optocoupler
’
s
LED and the series resistor that sets the loop gain. A 2%
tolerance Zener diode allows
±
5% tolerance on the regulated
output voltage. However, it is often necessary to improve cross
regulation by providing feedback from more than one output.
The second technique uses a TL431 precision shunt regulator
to offer more flexibility in such cases.
The TL431 precision shunt regulator integrates an accurate
2.5 V bandgap reference with an amplifier and driver into a
single device. It is popular as a secondary referenced error
amplifier. The TL431 also introduces the possibility of
combining feedback from two or more outputs simultaneously
to its reference pin. This can be a useful technique when it is
required to employ one output as the primary source of
feedback but also introduce a proportion of the feedback
from another output. This advanced technique is described in
more detail later.
This application note, therefore, focuses on the use of the
TL431 shunt regulator. Figure 1 shows a schematic in a
typical application with an optocoupler to provide tight
regulation on the 5 V output of a multiple output power supply.
Transformer Design
The choice of
TOPSwitch
and calculation of the primary
transformer characteristics is independent of the number of
outputs. As such, the Power Integrations standard transformer
design spreadsheets (available from your local Power
Integrations representative or on the Power Integrations Web
site at
www.powerint.com
) can be used to define the basic
transformer specification in terms of the transformer core,
primary inductance, primary turns and the output volts per
turn. This basic design can then be extended to define the turns
and wire selection on other outputs.
Two spreadsheets are available: one for discontinuous
conduction mode (DCM) designs and one for continuous
conduction mode (CCM) designs. Refer to AN-16 and AN-17
in the Power Integrations 1996-97 Data Book and Design
Guide for further explanation of converter operation and use of
the spreadsheets.
Operation in DCM results in smaller transformer core sizes for
a given output power, but the smallest size is often not the most
desirable choice in multiple output power supplies. Transformer
hardware is usually selected to allow optimum circuit board
layout. This motivation drives the selection of a transformer
bobbin with the best arrangement of the number of pins and the
pin spacing.
Designing for CCM provides the optimum utilization of the
TOPSwitch
silicon for a given output power. Therefore, this
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