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)： 17/36頁(yè)
文件大小： 381K
代理商： AN-15
A
6/96
AN-15
17
I n
V
n
R
n
f
C
SN
s
s
D
( )
(
)
=
×
+
×
×
×
×
1
4
2
2
π
RMS differential current I
(1) is 638
μ
A and I
(2) is 119
μ
A.
The target differential inductance L
D
can now be calculated.
L
V
4
n
I n
n
f
D
PRI
×
s
=
×
×
×
( )
π
The ST202A power supply operating from 115 VAC and
delivering 15 Watts is found to operate in the discontinuous
mode with a triangular drain current waveform. Peak Drain
current I
is 0.8 A and duty cycle is 0.3. C6 (0.1
μ
F) is
differential capacitor C
. ESR of input capacitor C1 is 0.375
.
From simulation, calculation, or measurement with the power
supply connected to the LISN but without an EMI filter, the
equivalent source voltage fundamental V
(1) is 59.3 mVRMS
and second harmonic V
(2) is 43.0 mVRMS. Differential
inductance L
is found to be 74
μ
H in each leg for attenuation of
the fundamental but the second harmonic requires a higher
inductance value of 144
μ
H in each leg to achieve the desired
attenuation because the EN55022 specification is more stringent
at 200 kHz. The higher inductance value is used in the design.
Note also that different combinations of L and C are possible but
the LC product will remain the same. Note also that, in common
mode chokes, total measured differential inductance is twice
the value calculated for each leg (288
μ
H in this example).
Peak load current normally limits the size of discrete chokes
to between 100
μ
H and 1 mH (especially in mains applications
with peak-charging capacitive input filters). Practical discrete
chokes are cost effective only at the lower output power levels
(5 Watts and below). Single discrete chokes attenuate the
differential mode but have little effect on common mode
emission currents. These limitations for discrete, differential
mode chokes can be overcome by selecting a common mode
choke with parasitic leakage or differential inductance equal to
or greater than the differential choke inductance value calculated
above. (Note: with a common mode choke, measure inductance
of one winding with the other winding shorted for total leakage
or differential inductance. The effective differential inductance
in each leg is half the measured value.)
Filter effectiveness decreases as parasitic elements of the filter
components themselves become significant. The effective
circuit model above 1 MHz is shown in Figure 32. Note the
additional ESL terms in both energy storage capacitor C
and
differential capacitor C
. Note also the shunt winding
capacitance C
across each differential mode filter choke L
D
.
As the frequency increases, the parasitic components begin to
dominate, reducing filter effectiveness. Fortunately, the
Figure 32. High Frequency Model of the Differential Mode Filter.
RS
RS
CD
LD
LD
ESR
PI-1860-050796
ESL
ESL
CW
CW
CIN
harmonics of the trapezoidal (or triangular)
TOPSwitch
Drain
current waveform are also decreasing above 1 MHz, which
tends to offset the degradation in filter performance. Above
1 MHz, current emissions which exceed the desired specification
are usually common mode emissions caused by either ringing
waveforms identified earlier or resonances caused by parasitic
components themselves.
Physical component layout becomes increasingly critical above
1 MHz. Improper layout can lead to increased capacitor ESL.
It is also possible for noise voltages or currents to couple around
the EMI filter directly into the mains.
Common Mode Filter Analysis
Common mode conducted emissions are caused by common
mode currents that do not circulate between the AC mains and
power supply input. Balanced common mode currents flow
simultaneously in power supply line and neutral input wires
such that common mode line current is equal in magnitude and
in phase with common mode neutral current. Unbalanced
common mode currents flow in either power supply line or
neutral input wires separately. Common mode conducted
emissions are caused by
TOPSwitch
Drain Voltage V
DRAIN
and
output Diode Voltage V
DIODE
as shown in Figure 33.
TOPSwitch
Drain voltage V
drives displacement current
through various stray parasitic capacitance terms. C
is stray
TOPSwitch
Drain capacitance to earth ground. C
is
TOPSwitch
output capacitance. C
through C
are the
effective capacitance terms across each bridge diode. C
is the
capacitive coupling across the AC mains input (which is very
low when testing with LISNs). Note that secondary is shown
connected directly to earth ground. Transformer capacitance is
distributed but can be modeled with the following six discrete
capacitance terms:
C
: Winding capacitance from “noisy” or switching side
of the transformer primary to “noisy” side of the secondary.
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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