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| 型號(hào)： | 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)： | 28/36頁 |
| 文件大?。?/td> | 381K |
| 代理商： | AN-15 |
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EMI Filter Layout Issues
Filter layout is extremely important to obtain the desired
attenuation. Poor layout practice can cause conducted emissions
to actually couple around the filter components directly into the
AC mains conductors or cause radiated emissions.
Keep power stage and output components away from the EMI
filter to prevent coupling around the filter. The best approach
is to place the EMI filter at one end of a rectangular power
supply shape and place the output at the other end as shown in
Figure 47. Square power supply shapes should be avoided if
possible since the power stage and the output components will
be in close proximity to the EMI filter, allowing noise to couple
directly into the mains.
PI-275-081090
(a)
WRONG
(b)
RIGHT
Figure 48. Bending the Bus to Minimize Resistive Effects.
PI-745-032392
EMI
Output
Power
Stage
(b)
WRONG
(a)
RIGHT
EMI
Power
StageOutput
Figure 47. Power Supply Layout to Minimize Noise Coupling.
Capacitor lead length must be minimized as much as possible
to reduce ESL. This includes the traces on the PC board leading
up to the capacitor pads. Y-capacitor lead lengths and trace
lengths are the most critical because the Y-capacitors couple
high frequency currents (10 MHz to 200 MHz) back to primary
circuitry. Figure 48 shows the right and wrong way to route PC
traces to capacitors.
Locate the differential mode filter capacitor across the AC input
conductors as close as possible to the power entry point.
Practical Considerations
Successful EMI filter design begins with knowledge of the
switching power supply noise sources generating differential
mode and common mode conducted emissions which have
been described in detail. The EMI filter must attenuate the
emissions below the specification limit. Implementing a
successful EMI filter is an iterative process. The basic steps
include:
Identify and attenuate the differential mode fundamental
Identify and attenuate the common mode fundamental
Identify other emissions over the spec limit.
Determine whether each emission is differential mode or
common mode.
Use average or quasi-peak measurements on peak emissions
to verify that the emission actually has insufficient margin
compared to the EMI standard.
Determine whether each emission is coupling around or
passing through the EMI filter.
Change the filter design or control the circuit source to
attenuate each emission below the specification limit.
Go back and check the earlier emission levels to make sure
a change did not cause a different problem to occur.
Differential mode Versus Common mode
The first frequency sweep for EMI conducted emissions on a
power supply with no EMI filter will usually produce a spectrum
as shown in Figure 49. The fundamental is outside the
specification limit as well as some of the harmonics. Each
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