Is input filter design too complicated?

Radiated emissions are largely determined by parasitic elements and can be reduced by optimized board layout. A highly innovative approach that can reduce radiated emissions by up to 40 dB (i.e. 10,000 times) is to use ADI’s switching regulators, which operate according to the “silent switcher” principle. Here, the pulsed input currents are very symmetrical, so that the resulting electric fields largely cancel each other out.

Conducted emissions can be reduced with the help of filters. However, there are several things that must be considered. Not only does the filter need to be optimized to reduce noise in a certain frequency range, but it also affects the stability of the entire power supply. The switching regulator has a certain input impedance Z _{IN . It must be higher than the output impedance Z OF} of the input filter . The block diagram in Figure 1 shows these two impedances.

Figure 1. Using an input filter to reduce conducted emissions.

Both impedances must be considered in the filter design. To simplify the design work, a new feature has been added to LTpowerCAD®, a switching regulator calculation tool from ADI. LTpowerCAD can be downloaded for free from the ADI official website and has been considered a practical switching regulator design tool for nearly a decade. The latest addition to the tool is "Filter Design". The critical common-mode noise that is closely related to the line voltage is not very dependent on parasitic effects, so it can be reliably calculated and the calculation results can well reflect the actual situation.

Figure 2 shows a switching regulator calculation done in LTpowerCAD with an optimized filter on the input side.

Figure 2. Conducted emissions calculation showing CISPR 25 Class B limits when using a filter.

As mentioned earlier, it is not only necessary to achieve an emission level below a certain EMI specification level, but also the stability of the overall circuit that constitutes the switching regulator and filter is important. To achieve this, the impedances shown in Figure 1 must be matched. The output impedance of the filter, Z _OF (impedance output filter), must be lower than the input impedance of the filter, Z _IN (impedance input source). To ensure this, LTpowerCAD provides this calculation and representation in the simplified diagram shown in Figure 3.

Figure 3. Check the respective impedances Z _IN and Z _OF to avoid oscillation.

Previously, there were calculation tools and simulation tools available for switching regulator design. You can also use calculation tools such as LTpowerCAD to calculate appropriate filters. This new approach is very suitable for minimizing conducted emissions in switch-mode power supplies. To download LTpowerCAD, please click "Read the original text"~

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