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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電源電磁干擾設計技術 |
| 文件頁數(shù)： | 19/36頁 |
| 文件大?。?/td> | 381K |
| 代理商： | AN-15 |
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6/96
AN-15
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ground, up to secondary, and couples back to
TOPSwitch
through transformer winding capacitance.
I
: Splits between C
(with ESL) and V+. V+ component
flows out the bridge rectifier, down through line and neutral
wires (and is sensed by the LISN) to earth ground, up to
secondary, and couples back to
TOPSwitch
through
transformer winding capacitance.
Superposition of all these different displacement currents will
lead to some cancellation but there will always be “l(fā)eftover”
high frequency current components measured as common
mode conduction emissions. The asymmetries in various
parasitic capacitance terms also explain how common mode
emission currents can become “unbalanced” creating net current
flowing only in the line or neutral wire. A similar analysis can
be performed using the output rectifier anode as the driving
point voltage source. For obvious reasons, common mode
emission currents are best measured because analysis is quite
difficult.
Common mode filters require relatively high values of inductance
because safety standards restrict common mode Y-capacitor
size to limit leakage current as previously discussed. Common
mode chokes between 10 mH and 33 mH are used in most
applications because inductance normally required is unaffected
by the circulating differential mode current. Discrete chokes
can also be used in some low power applications if the peak
current is taken into account and a discrete choke is placed in
each leg for balanced high frequency impedance.
Physical component layout becomes increasingly critical above
1 MHz. Improper layout can lead to increased capacitor ESL.
It is also possible for noise voltages in close proximity to the
EMI filter to couple around the filter directly into the mains.
Common mode capacitors must have extremely short traces
connecting directly to the transformer pins and to each other as
well.
Power Cord Damping
Applications with 3-wire power cords require special attention.
A six foot power cord can be modeled as a transmission line
with distributed inductance and capacitance, characteristic
impedance of approximately 100
, and little damping which
leads to a sharp, well defined resonance, typically between 15
and 25 MHz. This resonance can amplify existing common
mode emission currents to levels in excess of the desired limit.
A small, lossy ferrite bead or toroid placed over the earth ground
lead wire reduces the resonant peak by adding series damping.
The ferrite bead or toroid
(21)
should have an effective impedance
of 100
in the 15 to 25 MHz range. The bead is placed over
the safety ground wire between the enclosure power entry
connector and the internal safety ground attachment point of the
enclosure. The toroid is installed in similar fashion but can
accommodate up to 5 or 6 turns of the insulated safety ground
wire.
Transformer Construction
Flyback transformers use gapped ferrite cores which may have
fringing fields as shown in Figure 34. Gaps should be confined
to the center leg of either one or both core halves so that the
fringing field can be effectively shielded by the windings. End
gaps “l(fā)eak” magnetic flux due to the fringing field which can
produce common mode emissions.
Proper transformer construction techniques are necessary for
reducing common mode emissions. Figure 35 shows a typical
insulated wire wound transformer cross section. The transformer
primary connects between the relatively quiet high voltage DC
bus and the noisy
TOPSwitch
Drain pin (which has the high
voltage switching waveform). When the primary is wound with
two layers, the primary half with the dot mark is connected to
TOPSwitch
which is then buried or shielded under the primary
half connected to the high voltage DC bus as shown. One layer
of 2 mil tape separates the two primary halves to reduce
capacitance and high frequency ringing. Another layer of tape
separates the primary winding from the insulated wire wound
secondary. The combination of tape and insulation thickness
reduces capacitance between primary and secondary which
reduces common mode emission currents. One more layer of
tape separates the secondary from the primary referenced bias
winding.
PI-743-032392
NO
END GAP
END GAP
WITH
FRINGING
FIELD
Figure 34. End Gap Magnetic Flux Leakage.
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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