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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(shè)計技術(shù) |
| 文件頁數(shù)： | 13/36頁 |
| 文件大?。?/td> | 381K |
| 代理商： | AN-15 |
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A
6/96
AN-15
13
together in parallel and wound as a pair for typically 3 to 5 turns.
The toroidal core should be ferrite and “l(fā)ossy” at high frequency
such as Fair-Rite 75 material. Fair-Rite toroid part number
5975001101 (with 0.5 inch OD x 0.32 inch ID x 0.25 inch
thickness) is suitable for most applications
(21)
. This high
frequency common mode choke is usually located between
power entry and the rest of the power supply EMI filter. This
common mode choke technique can also be used on power
supply output wires.
Figure 25. U-Core Wound Common Mode Choke (Differential
Mode Impedance).
I
(
)
Frequency (Hz)
1K
500K
100K
10K
10
1K
100
1
0.1
10K
100K
1M
10M
40M
33 mH
18 mH
10 mH
Toroid
1mH
P
I
(
)
Frequency (Hz)
1K
500K
100K
10K
10
1K
100
1
0.1
10K
100K
1M
10M
40M
33 mH
2.2 mH
0.82 mH
Toroid
1mH
P
Figure 26. Spool Wound Common Mode Choke (Differential Mode
Impedance).
Bobbin style common mode chokes can have either one or two
sections in each winding. One section per winding is lowest
cost but two sections per winding splits the winding capacitance
in half to increase resonant frequency and effective bandwidth.
The U-core common mode choke shown in Figure 20 has one
section per winding while the spool style common mode choke
shown in Figure 21 has two sections per winding. Figure 23
shows that the single section U-core style common mode
impedance is lower and resonant frequency is lower with
sharper peaking compared with the two section spool style
common mode impedance shown in Figure 24. Two sections
per winding reduce capacitance to improve common mode
impedance at high frequency.
The common mode choke must also survive the surge current
occurring when voltage is first applied to the power supply as
described earlier, as well as operate at the steady-state RMS
input current.
For reducing high frequency common mode conducted emissions
in the 10 MHz to 200 MHz range, a simple common mode choke
using a small ferrite toroid
(21)
and insulated wire can be wound
as shown in Figure 27 and used in addition to one of the bobbin
style common mode chokes. Both wires have thick, safety
insulated wires with different colors. The wires are held
Flyback Power Supply EMI Signature
Flyback power supplies have a distinctive EMI signature caused
by superposition of several waveforms shown in Figure 28. The
transformer primary current I
,
TOPSwitch
Drain voltage
V
Drain
, diode voltage V
, and transformer secondary current
I
waveforms each generate emission currents which may
exceed the desired EMI specification limits without proper
EMI design technique.
Primary Current Waveform
Primary current I
begins to flow when
TOPSwitch
turns on.
Transformer primary current ramps to a peak value determined
by input voltage, primary inductance, switching frequency, and
duty cycle. This trapezoidal (or triangular) current waveform
is characterized in the frequency domain by a spectrum with a
fundamental at the switching frequency and harmonics
determined by the relative squareness of the waveform and
causes primarily differential mode emission currents to circulate
between the AC mains and the power supply input. This current
waveform can also create common mode emissions due to
radiated magnetic fields if the current path defined by the PC
board layout encircles a large physical area.
TOPSwitch
Drain-Source Voltage Waveform
The Drain-Source voltage waveform V
is characterized by
high dv/dt transitions. Parasitic circuit elements (leakage
inductance,
TOPSwitch
output capacitance, and transformer
INSULATED WIRES
FERRITE
TOROID
INPUT
OUTPUT
HIGH FREQUENCY COMMON MODE CHOKE
PI-1641-111695
Figure 27. High Frequency Common Mode Choke.
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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