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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)： | 15/36頁 |
| 文件大小： | 381K |
| 代理商： | AN-15 |
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A
6/96
AN-15
15
ISENSE
VIN
CIN
IPRI
IPRI
IPRI
LD
ESR
LD
CD
PI-1642-111695
ACTUAL
+
+
-
+
-
+
-
-
MODEL
RSL
VSL
VSN
RSN
CD
LD
LD
ESR
IPRI
I2
ISENSE
LISN
RESISTORS
I1
I4
I4
I1
I2
IPRI
Figure 29. Circuit Origin for Differential Mode Emissions.
Suppression Techniques
Controlling EMI requires attention to the following areas.
Differential mode filtering
Common mode filtering
Power cord damping
Transformer construction
Differential mode Filter Analysis
Differential mode conducted emissions are caused by currents
circulating between the power supply and AC mains input
which means that a differential current which flows into the
power supply through the Line input wire will flow out of the
power supply through the Neutral input wire.
Most differential mode conducted emissions are caused by the
fundamental and harmonics of the triangular or trapezoidal
TOPSwitch
Drain current waveform. During EMI testing,
differential mode currents generate test voltages equal in
magnitude and opposite in phase across Line LISN sense
resistor R
SL
and Neutral LISN sense resistor R
SN
.
Differential mode analysis starts by replacing the actual circuitry
with an equivalent model as shown in Figure 29. The
primary
current is modeled by current source I
. The effective
impedance of energy storage capacitor C1 over the frequency
range of 100 kHz to 1 MHz is modeled by the Equivalent Series
Resistance or ESR. The bridge rectifier is assumed to be
conducting current and is replaced with a short circuit. The AC
source impedance is modeled by the effective series combination
of the 50
LISN sense resistors R
and R
. Differential mode
filtering is performed by the LC filter consisting of differential
mode capacitor C
and two identical differential mode chokes
L
D
. This model is valid up to roughly 1 MHz.
The primary current switching frequency fundamental and
harmonic components I
(n) must be estimated, measured, or
derived by simulation. Note that measured harmonic components
are given in RMS but calculated or simulated components are
given in peak values and must be converted to RMS. A typical
harmonics envelope is shown in Figure 30 as a function of
frequency.
Figure 30. Envelope of Typical Primary Current Fourier Spectrum.
1
2 3
Harmonic Number
F
P
5 7 . . .
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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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