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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(jì)技術(shù) |
| 文件頁數(shù)： | 31/36頁 |
| 文件大?。?/td> | 381K |
| 代理商： | AN-15 |
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at the source. Heat sinks can also be used as shields.
Another type of shield is a conductive plate approximately the
same size as the printed circuit board. This plate can be
connected to earth ground, primary reference, or secondary
reference (depending on the safety insulation system chosen).
This is attractive for applications connecting to the third wire
ground but without a conductive enclosure.
Lossy Beads
Small beads can be used in circuit leads to damp or eliminate
high frequency ringing. Ferrite beads from Fair-Rite
(21)
are
available in a variety of shapes. These beads feature low
impedance at low frequency for minimal effect on the current
waveform but have high impedance at high frequency with
significant parallel resistance to damp and reduce ringing
voltage waveforms.
Grounding
Some applications connect the output voltage to earth ground.
Others have no connection at all to earth ground. It is important
to identify the earth ground connection expected for each
application and to test in that configuration. EMI testing should
be performed in both grounded and ungrounded configurations.
Power Cord
The power cord resonance previously described can interfere
with conducted emissions testing. Switch between two power
cords of different lengths to separate power cord resonances
from other conducted emissions.
Miscellaneous Test Tricks
Terminate opposite LISN with 50 ohm terminator. The LISN
sense impedance is actually determined by the termination and
will change if not properly terminated.
Warm up equipment including Device Under Test (DUT) for at
least 1 hour before testing so results will be repeatable.
Make sure analyzer sweep speed is low enough to capture the
peaks of each harmonic. The bridge rectifier conducts current
(both power and emission currents) for a short time compared
with the full line cycle which effectively “pulse width modulates”
the emission currents. Slower sweep speeds will collect enough
data to accurately measure the peak of each current emission. A
peak hold test can also be used to fill in the peaks in a few
sweeps.
Peak measurements take the least amount of time but
specifications are given in quasi-peak or average limits. Both
quasi-peak and average measurement techniques give lower
readings when compared to the peak value. If the peak value
meets the average or quasi peak limit specification, there is no
need to take further data with the average or quasi peak
methods. To save test time, use the quasi-peak and average test
methods only when the measured peak value is close to or
exceeds the target specification.
Recommended Step-by-Step Procedure
1) Determine differential mode fundamental (and low frequency
harmonics).
2) Calculate and select filter X-capacitance and target differential
inductance. Select bobbin style common mode choke with
sufficient differential mode inductance and AC current rating
(use discrete chokes only for low power, under 5 Watts).
3) Measure impedance versus frequency for each component.
Select components with resonant frequencies that do not coincide
with waveform ringing frequencies in the power supply.
4) Use a nominal value AC source through the LISNs to
provide power to the power supply. Use the 180 degree splitter
to extract the differential mode fundamental current component.
Measure the fundamental with slow sweep speed and measure
peak value using maximum hold. Compare the measured
differential mode fundamental with the calculated value. Use
average or quasi-peak detection as required to properly compare
measured reading with limits of the chosen standard. Increase
X2-capacitance or select common mode choke with higher
differential inductance if necessary.
5) Examine entire frequency range for differential mode
components close to or in excess of the specification limit.
Make measurements on both Line and Neutral LISN. Pay
special attention to frequency ranges around measured
component resonances and identified circuit ringing frequencies.
Use average or quasi-peak detection as required to properly
compare measured reading with limits of the chosen standard.
Modify differential mode filter design if necessary.
6) Use in-phase splitter to extract the balanced common mode
fundamental and low frequency harmonic current components.
For 2-wire applications, place the largest value Y1-safety
capacitor (subject to leakage current limitations but typically
1 nF) to output return. Two series connected Y2-safety capacitors
(typically each with 2.2 nF value) can also be used. For 3-wire
applications, place the largest value Y2-safety capacitor (subject
to leakage current limitations but typically 1 nF up to 33 nF)
from the power supply high voltage return to earth ground and
then connect a 0.1 uF low voltage ceramic capacitor from earth
ground to secondary return.
Y-capacitor leads must be very
short to attenuate high frequency current emissions.
Measure
balanced common mode fundamental and low frequency
harmonics. Increase size of common mode choke if necessary.
Select the smallest, widest bandwidth common mode choke
(with sufficient RMS current ratings) that attenuates the balanced
common mode fundamental to the desired level. Measure
impedance versus frequency for each component. Select
components with resonant frequencies that do not coincide with
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