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| 型號(hào)： | 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(jì)技術(shù) |
| 文件頁數(shù)： | 4/36頁 |
| 文件大?。?/td> | 381K |
| 代理商： | AN-15 |
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AN-15
A
6/96
4
voltage when viewed on an oscilloscope may have a peak
voltage of 1 Volt and hence an RMS voltage of 0.707 Volts. The
spectrum analyzer (with 50
input) will display a value for this
100 kHz signal of 0.707 volts (or 117 dB
μ
V or 10 dBmW)
regardless of which detection method is used (peak, quasi-peak,
or average) because the signal is continuous, narrow band, and
not modulated or gated. If the signal was broadband, modulated,
gated at a duty cycle, or in some other way not continuous, the
displayed RMS value will change with the detection method.
The measured display will then be the magnitude of an equivalent
continuous sinusoidal signal with an RMS value equal to the
RMS content of the LISN signal measured at the output of the
detector stage.
Peak detection is the simplest and fastest method when measuring
conducted emissions. Resolution bandwidth is set to 200 Hz for
measurements from 10 kHz to 150 kHz and set to 9 kHz for
measurements from 150 kHz to 30 MHz. Sweep times are
relatively low. When displaying emissions in real time with no
averaging, the peaks are not constant but change in magnitude
with each measurement sweep due to the bridge conduction
gating pulse effect described above. Most spectrum analyzers
have a “maximum hold” feature which displays the highest
peak occurring over many measurement sweeps. The peak
detector measures the magnitude of the largest signal occurring
during the bridge conduction gating pulse.
The average detector is simply a low pass filter with corner
frequency sufficiently below the gating pulse repetitive
frequency or PRF. In typical spectrum analyzers, the video
filter bandwidth can be reduced to 30 Hz or below to average the
signal but the sweep time must be increased for a calibrated
measurement. For test purposes, the full conducted emissions
range starting at 10 kHz (or 150 kHz or 450 kHz, depending on
the regulation) up to 30 MHz should first be examined with a
peak detection measurement. Peak detected emissions with
insufficient margin compared to the regulation average limit
should be centered on the spectrum analyzer display with the
lowest possible frequency span per division setting before
reducing video bandwidth and performing the average
measurement sweep
(10)
. Figure 8 shows typical conducted
emissions from 10 kHz to 500 kHz with both peak detection and
average detection. Note that peak detection picked up an
envelope of high order harmonics from line frequency
rectification in addition to the fundamental and first three
harmonics of the 100 kHz switching frequency.
The quasi-peak detector is designed to indicate the subjective
annoyance level of interference. As an analogy, a soft noise that
happens every second is much more annoying than a loud noise
110
A
μ
V
100
90
80
70
60
50
40
30
20
100
200
Frequency (KHz)
Peak Data
Average Data
300
500
400
P
Figure 8. Peak Data vs Average Data.
that happens every hour. A quasi peak-detector (actually a
calibrated, intermediate bandwidth video filter) behaves as a
leaky peak detector that partially discharges between input
signal pulses. The lower the pulse repetitive frequency (PRF),
the greater the dB differential between the peak and quasi-peak
measured response
(8) (9)
.
Quasi-peak and average detection methods will always give a
lower measured value compared to peak detection. If a peak
detector measurement meets the average or mean specification
limit with sufficient margin, additional measurements using
average detection are not necessary. When no average limit is
specified, if the peak measurement meets the quasi-peak limit
with sufficient margin, additional measurements using quasi-
peak detection are not necessary. In general, when testing
TOPSwitch
power supplies to the C.I.S.P.R. Publication 22,
EN55022, or Vfg 243/91(and Vfg 46/92) limits, peak measured
data usually meets the quasi-peak limit but, in some areas, may
have insufficient margin when compared with the average
limit. In this case, further measurement is necessary using
average detection.
Safety Principles
Safety principles must be examined before proceeding further
with EMI filter concepts because safety requirements place
several constraints on EMI filter design.
Virtually all equipment including computers, printers,
televisions, television decoders, video games, battery chargers,
etc., must be safety recognized by meeting the safety standard
for the intended market and carrying the appropriate safety
mark. Safety principles are very similar among the various
standards. This application note will focus on the electric shock
hazard requirements of one popular standard, IEC950
(11)
.
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