What Types of EMI Filters are Best for Passing EMC Certification?
When you need to pass EMC certification and your new product is being crippled by a mysterious source of EMI, you’ll probably start considering a complete product redesign. Your stackup, trace geometry, and component arrangement are good places to start, but there might be more you can do to suppress specific sources of EMI.
There are many different types of EMI filters that you can easily place in your design, and that will help suppress EMI in a variety of frequency ranges. These circuits can be of the passive or active variety and provide different levels of suppression in different bandwidths. The best choice of EMI filter for your design depends on a variety of factors, ranging from space on your board to the required attenuation.
Types of EMI Filters
All EMI filters can be classified as passive and active filters, where each type is constructed with passive or active components, respectively. Let’s run through the common types of EMI filters falling into each category:
Passive EMI Filters
Perhaps the most common passive EMI filter is a ferrite choke. This is basically an inductor that provides low-pass filtration up to several tens of MHz. These components can provide filtration of common mode or differential mode conducted EMI. If you’re reading this on a laptop, then your power cord probably uses one of these chokes to remove high frequency noise on the input power line.
The other types of passive EMI filters are LC circuits. The simplest of these filters are C filters (connected as shunt capacitors) and L filters (connected as series inductors). These can be placed in a critical circuit or on the input of a critical component in order to remove noise at a broad range of frequencies. More complex configurations are shown in the image below. Regarding the Pi and T filters, these are best used with low and high-source/load impedances, respectively.
If you need to pass your desired signal into a component while suppressing all other frequencies, then you need to build a bandpass filter. Similarly, you may want to suppress a strong signal at a single frequency (such as stray emission from an antenna), which would require a bandstop filter. Note that the number of L/C elements in the circuit determines the filter number; building a higher order (i.e., cascaded) filter will provide steeper roll-off outside the passband.
When building a passive filter as an LC circuit, watch out for self-resonance frequencies in these components. Parasitic inductance in a capacitor (inductor) will cause it to exhibit inductive (capacitive) impedance at sufficiently high frequencies. If you need to work at extremely high frequencies, then you need to select components with high self-resonance frequencies. In addition, if you are using cascaded filter sections, be careful with assuming the two passbands/stopbands in your filter circuit will not overlap. The two filter circuits can mutually resonate to create a new unanticipated passband or stopband in the circuit’s transfer function.
Active EMI Filters
Active filters are the transistor-driven analog of passive filters. These filters use op-amps and passives to provide filtration in the desired bandwidth. These filters can also be constructed as higher order filters to provide steep roll-off with relatively flat passbands or stopbands. Any of the equivalent fundamental filters can be built with op-amps, and these filters are often integrated into SoCs for particular applications. They can also be configured to suppress common mode and differential mode noise in a single circuit.
This area of the field of electronics design is quite broad and is covered in many textbooks. One great textbook on this subject is Manual of Active Filter Design by John L. Hilburn.
Going Further to Microwave and mmWave Frequencies
EMI/EMC testing and compliance in this regime is more complicated, as are the devices being designed and tested. Consider for a moment two 5G-capable handsets. No one wants the two devices to interfere with each other through mutual reception of electromagnetic energy. Best practices for RF layout/routing, stackup design, and unique RF interconnect architecture will go a long way towards suppressing EMI emission and reception from these types of products.
When EMI from a product under test is a major problem, or when EMI within a product is difficult to solve, there are more advanced design choices to consider beyond the points mentioned above. Absorbing conformal coatings can help provide additional isolation, especially for EMI from the board edge at GHz frequencies. Unique isolation structures can also go a long way toward suppressing radiated EMI between different circuit blocks in a product, as well as provide additional isolation from external EMI.
No matter which types of EMI filters, interconnect geometry, or isolation techniques you use, you should simulate their effects on signal integrity using pre-layout and post-layout simulation tools. Pre-layout SPICE simulations are ideal for qualifying EMI filter designs and ensuring your filter provides the right level of attenuation within the desired bandwidth.
The schematic design, layout, and documentation tools in Altium Designer® are ideal for implementing design for testability steps in any new PCB. You’ll also have design data management and supply chain tools you can use to ensure your PCB is manufacturable and create deliverables for your fabricator. All these features are accessible in a single design environment, which helps you remain productive and get to market quickly.