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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)： | 11/36頁 |
| 文件大?。?/td> | 381K |
| 代理商： | AN-15 |
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A
6/96
AN-15
11
higher self-resonant frequency. Installing the larger choke to
attenuate the fundamental may have the effect of letting through
current components above 3 MHz.
mode choke winding. I
and I
are “common mode” currents
which may or may not be related in magnitude and phase. The
common mode choke behaves like a large inductor to common
mode currents.
Differential mode chokes are usually used in EMI filters for
both differential mode and common mode filtering only for the
lowest output power levels (under 5 Watts). At higher power
levels, a properly selected common mode choke will also have
differential mode inductance for essentially no additional cost.
Common Mode chokes
Common mode chokes are specialized inductors designed
specifically for common mode EMI filters. The common mode
choke consists of two identical windings wound such that the
magnetic fields caused by differential mode currents cancel.
Figure 19 shows a toroidal implementation which is good for
illustration purposes but (as will be seen shortly) is not the best
choice for low-cost and practical EMI filter implementations.
Figure 19 shows three current components I
, I
, and I
. I
is
a differential mode current (shown also in Figure 7) which
circulates by starting at the AC mains source, flows through one
common mode choke winding towards the power supply, flows
through one bridge rectifier diode, charges the high voltage
energy storage capacitor C
, flows back through another bridge
rectifier diode, and then flows back towards the source through
the other common mode choke winding. The magnetic fields
within the core due to the circulating differential current I
D
cancel perfectly because of dot polarity. Note that the “start” of
both windings enters the core on the same side and the “finish”
of both windings leaves the core on the other side. Common
mode chokes behave like short circuits for circulating differential
mode currents such as I
which flow in through one common
mode choke winding and flow out through the other common
103
104
105
106
107
Frequency (Hz)
I
)
IMPEDANCE vs. FREQUENCY
105
P
100
100
μ
H
104
103
102
101
1 mH
Figure 18. Typical Differential Mode Choke Impedance.
18max.
10±0.5
4
16max.
0.7
PI-1634-111695
Figure 20. U Core Common Mode Choke (All dimension in mm).
Two low-cost bobbin style common mode chokes simplify
EMI filter design. Figure 20 shows a typical “U-core” style
common mode choke in which the windings are wound on a
conventional bobbin. Two U-core halves are inserted into the
bobbin and secured with clamps. U-core common mode chokes
are widely available from several companies such as Tokin
(16)
,
Tamura
(17)
, Panasonic/Matsushita
(18)
, TDK
(19)
, and Murata
(20)
.
IN
OUT
PI-1633-111695
ID
ID
Figure 19. Ideal Common Mode Choke.
Figure 21 shows a newer common mode choke design with a
“spool” style two-piece bobbin. The two-piece bobbin is
snapped together around a one-piece ungapped core. A sprocket
on the bobbin engages a gear on a winding machine to spool the
wire onto the bobbin. Spool style common mode chokes are
available from Panasonic/Matsushita
(18)
and Tokin
(16)
.
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| AN-20 | Transient Suppression Techniques for TOPSwitch Power Supplies |
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