4 EMI and EMI Filter Comparison

Power stage designs require extra attention to detail to avoid unsafe electromagnetic interference across noise sensitive circuitry especially in applications that have EMI requirements such as CISPR 32 and CISPR 25. While newer controller designs have EMI reduction features such as Dual Random Spread Spectrum (DRSS) and Active EMI Filtering (AEF) like found in LM5149-Q1, the layout of a power design is critical for EMI performance. High parasitic loops that are formed in between key components such as the controller, FET, input and output capacitors as shown in Figure 4-1, minimizing these loops with good layout improve EMI performance. See Improve High-Current DC/DC Regulator EMI Performance for Free with Optimized Power Stage Layout, application note.

Figure 4-1 Parasitic Loops Formed in a Controller Design

A converter design provides the most minimal parasitic loop between controller and FET. This factor alone contributes highly to the EMI performance between a controller and converter design and can be seen in the EMI filter required for each design. Figure 4-2 showcases the EMI filter needed to pass CISPR25 Class 5, a stringent EMI specification for automotive applications, using a converter design with DRSS. Figure 4-3 and Figure 4-4 showcases the much larger filter and shielding required for a controller design with no EMI reduction features to pass the same EMI specification.

Note: EMI filter is highlighted is approximately 230mm²

Figure 4-2 LM70880-Q1 EVM EMI Filter

Note: Part of the EMI filter is highlighted and is approximately 145mm²

Figure 4-4 LM5146-Q1 EVM Back-End EMI Filter

Note: Part of the EMI filter is highlighted and is approximately 390mm²

Figure 4-3 LM5146-Q1 EVM Front-End EMI Filter

EMI filters can be a large contributor to design size and total cost but determining this can be a challenge. Sometimes a designer needs to complete their initial design and layout before being able to test their circuit’s EMI performance and determine how much more EMI margin they require for their systems. This can be as simple as adding more passives to a filter such as input capacitors, ferrite beads or use larger filter inductors but it may also require additional design time to optimize circuit layout. A converter design will likely require the least amount of design work and EMI filter components required to pass EMI standards saving time, cost and design size.