Column: EMC
Filter placement in designs By Dr Min Zhang, EMC consultant at Mach One Design, and design engineers at REO UK W
hen it comes to designing EMI filters, many engineers follow the classic approach of solving
EMI problems at the noise source. Tis leads them to place a filter close to the noise, which, counterintuitively, creates further frequency problems. So, then, what is the best place for EMI filters? Placing an EMI filter close to the noise,
such as a switch-mode power supply (SMPS) on a PCB or a line filter close to a motor drive circuit, creates problems, because the strong leakage field of the noise source will couple strongly with the filter’s passive components. As a result, a carefully-designed filter, which according to the simulation/calculation is supposed to give 60-80dB attenuation, oſten ends up having only 10-20dB insertion loss – particularly true with increasing frequency. If the overall infrastructure allows, a
filter should always be placed in a quiet environment, away from any noise source.
Location Te input capacitors are part of the SMPS design, therefore capacitors should be designed so there’s always enough energy delivered most efficiently whenever the switch is turned on. Tis is oſten achieved by populating the input supply rail with several decoupling capacitor sizes (0402, 0603, 0805, etc.) so that energy is available over a wide frequency spectrum. Te decoupling capacitors should also be
connected as close as possible to the Vin pin, with the smallest capacitor (in this case C1, 0402) being first. Te electrolytic capacitor C4 serves as
the main energy storage device, but it also provides damping of the system due to its relatively larger equivalent series resistance (ESR). If the electrolytic capacitor has a metal housing, such as aluminium, due to its larger size, the housing also serves as a shield, blocking some of the electric field created by the SMPS. Te filter is multi-stage, consisting of
two inductors and several ceramic capacitors. Figure 1 shows the input stage of a typical buck converter used with an integrated switching IC. Te filter stage and the input capacitors are shown separated by a red dashed line. Note that there can never be a strict
separation between the filter and the input capacitors, since the input capacitors also provide a low impedance path to noise, so they work well with the filter; but, here, we separated the two for argument’s sake. Te red line indicates that there must be an important distance between the filter and the input capacitors. Tis is to avoid field coupling and make the input filter stage more effective. On a PCB level, this is achieved by following a few steps, explained below.
Design steps First, we need to place the input filter away from the noise source. If the noise source is an SMPS located on one side of the PCB, the best location for the filter is on the opposite side. If the filter stage must be on the same
Figure 1: The input stage of a typical integrated buck converter
side as the SMPS, maximise the distance between the two, with the distance depending on the strength of the SMPS’s leakage field. So, if the switch node of the SMPS is kept quiet by a shield, then the distance between the filter and SMPS can be shorter. Te connection between the filter stage and the input capacitors should always be a high impedance path such as an inductor – in Figure 1 this is L1. Te same principles apply to much
Figure 2: REO line filter 10 April 2023
www.electronicsworld.com
larger systems like industrial motor drive systems or power supplies. For instance, a line filter used for an industrial motor drive application like the one shown in Figure 2 is always much more effective if placed near the mains entry point of the cabinet – i.e., to keep the mains wiring and line filter far away from other wiring and harnesses inside the cabinet. Again, the goal is to avoid field coupling between the noise source and filter components.
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