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電源電磁干擾設計技術
文件頁數(shù)： 10/36頁
文件大?。?/td> 381K
代理商： AN-15
AN-15
A
6/96
10
account. Figure 14 shows a typical test setup for measuring
leakage current.
Inductors or chokes
Proper inductor selection for EMI filters requires attention to
three key parameters: effective impedance characteristic, current
rating, and surge current capability.
Figure 15 shows impedance characteristics for ideal and non-
ideal inductor behavior. Ideal inductors have an impedance
characteristic that increases linearly with frequency. Real
inductors have parasitic series resistance R
and parallel
interwinding capacitance (C
). C
creates a resonant frequency
as shown on the plot. Beyond the resonant frequency (f
r
), the
inductor actually behaves like a capacitor.
Single
Layer
Windings
PI-708-031992
TOROIDAL
SOLENOIDAL
Figure 16. Diffferential Mode Chokes.
Power supplies have bridge rectifier input filters which draw
line frequency currents with high peak values but relatively
narrow widths as previously shown in Figure 7. A discrete filter
choke usually has a minimal effect on the peak current but must
pass the peak current without significant saturation (which
reduces effective inductance). The discrete choke must also be
rated to safely pass the higher peak value of the first surge of
current occurring when AC power is initially applied with input
capacitor C
IN
completely discharged.
Differential mode chokes
Differential mode chokes are simply discrete inductors designed
for EMI filters that pass line frequency or DC currents while
blocking or filtering high frequency conducted emission currents.
Differential mode chokes are usually wound on low cost
solenoidal cores of either iron powder or ferrite material as
shown in Figure 16. Toroids tend to be significantly higher in
cost but can also be used. Chokes with single layer windings
have the lowest capacitance and highest resonant frequency.
Effective inductance varies with peak differential mode choke
current flow. Refer again to Figure 7 where the bridge rectifier
and filter creates a high voltage DC bus from the AC line. AC
input current flows only during a small conduction time as
shown. Peak AC input current during normal operation
is relatively high. Differential mode chokes are designed
or selected to limit saturation at peak AC input current.
Figure 17 shows how inductance for a powdered iron toroidal
core varies with number of turns and peak current. To achieve
the desired inductance under high peak AC input current, higher
numbers of turns and/or larger choke cores are normally required.
Typical impedance characteristics for two different differential
mode chokes are shown in Figure 18. Note that the larger choke
resonates at a lower frequency and becomes capacitive. The
smaller choke has a higher impedance above 3 MHz due to the
PI-709-031992
I
# of Turns
No
Bias
Heavy
Bias
Current
Figure 17. Inductance Under Current Bias.
PI-1725-121895
Z
f
CW
RS
RS
L
Z=
1
2
π
fCW
Ideal
Z= 2
π
fL
fr
Actual
Figure 15. Comparison of Real and Ideal Inductor Impedance.
相關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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