參數(shù)資料
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(yè)數(shù)： 16/36頁(yè)
文件大?。?/td> 381K
代理商： AN-15
AN-15
A
6/96
16
H s
V
V
s
s
( )
L
C
s
L
R
s
SN
PRI
D
D
D
S
( )
(
=
=
×
×
×
×
+
×
+
1
2
1
2
1
2
)
At the fundamental and harmonics of switching frequency f
S
,
equivalent series resistance (ESR) of bulk input capacitor C
is
much lower impedance compared with the L
differential mode
chokes. Primary current I
flows almost completely through
bulk energy storage capacitor C
which creates an effective
trapezoidal (or triangular) differential mode voltage source
proportional to ESR. Differential mode chokes and the
differential mode capacitor form a simple low pass filter to
attenuate the effective voltage source to a level below the
desired specification. Figure 31 shows the final simplified
model where the RMS source voltage for each n
th
current
harmonic (given in peak value) is given by:
V
n
ESR
I
n
PRI
(RMS)
PRI
(Peak)
( )
( )
=
×
×
1
2
Attenuation is determined by the differential between the
magnitude of the effective voltage source in dB
μ
V and the
desired conducted emissions specification. The voltage transfer
function H(s) is given in terms of L
D
, C
D
, and R
S
.
At high levels of attenuation normally required at the switching
frequency, the denominator of H(s) is dominated by the frequency
dependent terms and can be simplified as shown. Simple
algebra reveals a very useful frequency domain formula
consisting of the product of three separate terms. The first term
converts the effective ESR voltage source V
(s) back into
differential inductor current I
(s), the second term splits the
current between differential mode capacitor C
and LISN sense
resistors, and the third term senses the LISN current component
to create a voltage to be measured with a detector or receiver to
compare with limits in dB
μ
V. This is a general result with
equivalent ESR voltage source V
(n) of each n
th
harmonic
shown (temporarily) in the frequency domain as V
PRI
(s) which
is a function of the complex frequency variable s.
IL(
n
)
LD
RS
+
_
RS
CD
LD
PI-1643-111695
VPRI(
n
)
Figure 31. Simplified Differential Mode Model.
≈
×
×
×
×
+
×
1
2
1
2
2
(
)
L
C
s
L
R
s
D
D
D
S
V
s
V
s
L
s
C
s
R
C
s
R
SN
PRI
D
D
)
S
D
S
( )
( )
(
=
×
×
×
×
×
+
×
×
×
1
2
1
2
1
For EMI filter design, only the magnitude of the most important
frequency components are examined which allows simple
magnitude expressions in terms of the harmonic integer n to be
used (rather than the complex variable s). Filter design begins
by identifying a target sense voltage V
(n) below the
specification limits at the appropriate n
th
harmonic frequency.
For FCC testing, the specification begins at 450 kHz with the fifth
harmonic (n = 5) while excluding
TOPSwitch
100 kHz
fundamental (n = 1) and second through fourth harmonic
frequencies (n = 2, 3, 4). For European test limits, the 100 kHz
fundamental (n = 1) and the second harmonic at 200 kHz
(n = 2) should be examined because the limit changes
significantly at 150 kHz. As an example and referring to
European EN55022 average limit for class B (Figure 2), the
average limit value is 74 dB
μ
V at 100 kHz (n=1) and 53.5 dB
μ
V
at 200 khz (n=2) while the quasi-peak limit values are 10 dB
higher. In most low frequency conducted emission
measurements, the measured quasi-peak value is slightly less
(1 dB to 3 dB) than the peak value. The average value, however,
can be 12 dB below the peak value which means that if the filter
is designed to meet the average limit, the quasi-peak limit will
also be met and with greater margin. In this example and for
12 dB margin overall, the peak value should be designed to touch
the average limit and average detection will provide the
remaining 12 dB attenuation. The target sense voltages are
therefore equal to the average limit or 74 dB
μ
V at 100 kHz
(V
(1)) and 53.5 dB
μ
V at 200 kHz (V
(2)). V
(n)
is converted from dB
μ
V to an absolute value sense voltage
V
SN
(n).
V
n
e
V
n
SN
SNdB V
20
( )
.
( )
=
×
1
10
6
V
SN
(1) is 5.01 mV
and V
(2) is 473
μ
V
. Sense voltage
V
(n) is then converted into an RMS current magnitude I
L
(n)
flowing through each differential mode inductor L
D
.
相關(guān)PDF資料
PDF描述
AN-16 TOPSwitch Flyback Design Methodology
AN-17 Flyback Transformer Design for TOPSwitch Power Supplies
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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