Column: EMC
Common challenges in filter design and how to avoid them
By Dr Min Zhang, EMC consultant at Mach One Design, and design engineers at REO UK F 5Vout
ilters are networks of passive components, such as capacitors, inductors and resistors, that provide attenuation to signals within a certain bandwidth.
When designing a filter, engineers can come across different challenges, from nonlinearity to wire misplacement. One common mistake that can lead to
ineffective filters are ‘flying’ wires over the filter; see Figure 1. Here, both input and output wires are airborne over the PCB, rendering an ineffective filter due to strong coupling. In Figure 1, on the right-hand side there’s an open-frame power supply, with wires ‘flying’ over it. Depending on the flying wires’ length, radiated emissions can peak at a certain frequency. Tis can be counteracted with a well-designed switched-mode power supply (SMPS). In the example of Figure 2, a 24Vin
, SMPS is shown, with a coaxial cable
running over it. Te frame of the SMPS has been deliberately leſt open, and the cable is about a meter long. An RF probe was used to measure the RF current in the cable. It was found that at 100MHz, the cable
radiates efficiently. Te current measured with the RF probe peaks at the frequency at which the wire is a half-wavelength dipole. Te emission from the power supply by itself is not large enough to radiate efficiently at 100MHz. In fact, this SMPS passed all the relevant EMC tests; however, once a wire is placed closely enough, the wire does radiate efficiently.
Good implementation methods Te general rule is to keep input side wiring and filter far away from other wiring – especially output wiring. If wires must be crossed, they should be perpendicular to one another. In the case of wires flying over an inductor, these should be crossed in a way that couples the least magnetic flux. On the board level, similar mistakes
are not uncommon. In Figure 3, the filter was placed on the PCB, only to lay out the
10 May 2023
www.electronicsworld.com
Figure 1: ‘Flying’ wires over the PCB
Figure 2: Demonstration of a ‘flying’ wire over a power supply unit
traces before and aſter the filter in parallel. Tis yields strong coupling between the two traces, degrading filter performance. Similarly, when working with a big cabinet, the filter must be placed in a location and position where the noise can’t find a coupling path to escape. Another challenge with filters is the
cost associated with high-voltage and high-current filter components. When the current rating is in the tens of
amperes, the magnetic components become costly. One way of implementing a cost- effective filter is to utilise magnetic cores, which can be nanocrystalline or other material, depending on the application; see Figure 4. Here, a ferrite core combined with Y-capacitors form an R-L-C filter for common-mode noise. Te great virtue of this configuration is that the core is not subjected to saturation, so it is suitable for high-current applications.
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