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| 型號： | AN-15 |
| 英文描述： | TOPSwitch Power Supply Design Techniques for EMI & Safety |
| 中文描述： | 的TOPSwitch電源電磁干擾設(shè)計技術(shù) |
| 文件頁數(shù)： | 2/36頁 |
| 文件大?。?/td> | 381K |
| 代理商： | AN-15 |
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AN-15
A
6/96
2
Safety is a vital issue which determines EMI filter component
selection, the transformer reinforced insulation system, and PC
board primary to secondary spacing. In fact, safety is an integral
part of the power supply/EMI filter design and is difficult to
discuss as a separate issue. Throughout this application note,
design guidance will also be presented for meeting safety
requirements in
TOPSwitch
power supplies.
EMI Specifications
The applicable EMI specification must be identified for the
intended product family and target market. In the United States,
the Federal Communications Commission (FCC) regulates
EMI specifications. Canadian specifications are similar to FCC
specifications. Figure 1 shows the conducted emissions limits
governed by FCC rules, Part 15, subpart J. Note that specification
limits are given only for quasi-peak detection methods. A
recent part 15 amendment allows manufacturers to use the
limits contained in C.I.S.P.R. Publication 22 as an alternative
when testing devices for compliance
(1)
.
The member countries of the European Community (EC) have
established a harmonized program for electromagnetic
compatibility. EN55022 for Information Technology
Equipment is one of the first harmonized documents. EN55022
together with companion measurement document C.I.S.P.R
Publication 22 set the conducted emission limits shown in
Figure 2 for information technology products marketed to the
European Community. In fact, EN55022 limits are the same as
C.I.S.P.R Publication 22 limits. Note that class A and class B
specification limits are given for both average and quasi-peak
detection methods
(2) (3)
.
Figure 3 shows the well-known and most stringent VDE 0871
specification (narrow band limits) for German markets which
has traditionally been the design target. German regulation Vfg
1046/1984 requires Information technology or Electronic Data
Processing Equipment to meet the VDE 0871 class B narrow
band limits from 10 kHz to 30 MHz. Note that specification
limits are given only for quasi-peak detection methods. When
marketing products only in Germany, there is a choice between
meeting the regulation requirements of Vfg 1046/1984 or the
new German regulation Vfg 243/1991 (as updated by Vfg 46/
1992) which has relaxed limits from 10 kHz to 150 kHz and is
harmonized with EN55022 from 150 kHz to 30 MHz. Vfg243/
1991 sets quasi-peak limits and Vfg 46/1992 adds mean or
average limits as shown in Figure 4. Figure 3 also shows
Vfg243/1991 class B quasi-peak limits to compare with
VDE0871
(4) (5) (6)
. The EMI filter designed to meet VDE 0871
(per Vfg 1046/1984) will generally be higher cost than the EMI
PI-1625-111695
VSL
_
VSN
+
LF
LF
CF
CF
LINE
+
_
INPUT
OUTPUT
NEUTRAL
CC
CC
RSL
RSN
Figure 5. Line Impedance Stabilization Network (LISN).
filter designed to meet Vfg/243 regulation requirements.
Measuring Conducted Emissions
Details of testing apparatus and methodology are governed by
the various EMI regulations, but share the same general concept.
Conducted emissions measurements are made with a Line
Impedance Stabilization Network (LISN). Figure 5 shows the
effective filter, represented by L
and C
, inside the LISN which
passes line frequency currents but forces higher frequency
power supply conducted emission currents to flow through
coupling capacitor C
and sense resistor R
. A spectrum
analyzer or EMI receiver reads the current emission signal
magnitude as sensed voltages V
SL
and V
SN
across R
SL
and R
SN
in dB
μ
V.
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| AN-20 | Transient Suppression Techniques for TOPSwitch Power Supplies |
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