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參數(shù)資料
| 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)： | 29/36頁 |
| 文件大小： | 381K |
| 代理商： | AN-15 |
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AN-15
29
will have to be employed to reduce this harmonic any further.
Using Splitters
Splitters combine the output signals of the two LISNs to
determine if a specific emission is common mode or differential
mode. Two splitters are required: an in-phase splitter (Mini
Circuits Lab ZSC-2-2) where V
is the sum of the two LISN
signals and a 180 degree out-of-phase splitter (ZSCJ-2-2)
where V
is the differential between the two LISN signals
(22)
.
The splitter setup is shown in Figure 51.
Splitter units are also available which allow switching between
differential mode and common mode tests.
harmonic is composed of both differential mode and common
mode emissions.
Figure 49. Typical Conducted Emissions Data without EMI Filter.
80
40
0
1
2
Frequency (MHz)
C
μ
V
P
0
60
20
Differential Mode Splitter Measurement
Differential mode emission current (I
) circulates from the
power supply through the first LISN sense resistor (producing
an in phase sense voltage), through the second LISN sense
resistor (producing an out of phase sense voltage), and back to
the power supply. The output voltage of the in-phase splitter
will have no differential mode component because the opposite
phased sense voltages effectively cancel. The output voltage of
the 180 degree out-of-phase splitter will have differential mode
components 6 dB higher than those measured directly at the
LISN as the sense voltages are now effectively in phase and
sum.
Balanced Common Mode Splitter Measurement
Balanced common mode currents (I
) are defined as currents
with similar amplitude and phase that flow from ground through
In Figure 50, a differential mode component with magnitude
70 dB
μ
V is shown relative to a common mode component with
magnitude 50 dB
μ
V. 20 dB differential between the two
components is actually an order of magnitude between the
absolute values of the two components. The signals will add
and superimpose if the phasing is correct but the overall effect
on the measured signal level is slight (10% increase, or less than
1 dB).
Figure 50. Superimposed Common Mode and Differential Mode
Harmonics.
PI-1727-121895
70 dB(
μ
V)
30 dB
DM Filter
40 dB(
μ
V)
Differential-mode
Common-mode
A differential mode filter with 30 dB attenuation at the harmonic
frequency of interest will not attenuate the measured peak by
30 dB. The differential mode component will be attenuated
from 70 dB
μ
V to 40 dB
μ
V, but the 50 dB
μ
V common mode
peak will now dominate the measurement. Further differential
mode attenuation will have no effect on the measured harmonic
because the signal is common mode. Common mode filtering
Figure 51. Separating Differential Mode from Common Mode
Using a Splitter.
PI-1728-121895
In
ID
L
Spectrum
Analyzer
f
d
μ
V
N
G
Icb
To
Power
Supply
To
Power
Supply
Icu
Icb
+Vn
+Vcb
-Vn
+Vcb
+Vcu
Splitter
ID
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|---|---|
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| AN-18 | TOPSwitch Flyback Transformer Construction Guide |
| AN-19 | TOPSwitch Flyback Power Supply Efficiency |
| AN-20 | Transient Suppression Techniques for TOPSwitch Power Supplies |
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