Dual-Spiral Common-Mode Filter for PCB EMI Suppression
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Solution Overview
Problem
Existing common-mode noise filters for PCBs are often costly, space-consuming, and ineffective across multiple frequency bands, particularly in smaller PCBs where vias and surface-mounted filters are impractical, and they can degrade differential signal integrity and increase the size and cost of electronic devices.
Innovation Solution
The integration of a dual-spiral common-mode filter into the PCB's reference plane, which is compact, via-less, and does not require a shield layer, providing effective RFI/EMI suppression and impedance matching by being patterned into the ground plane and situated under component pads to replace conventional PGS filters and cutouts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional common-mode noise filters (via-based or surface-mounted) are used, then EMI suppression is achieved, but PCB space is consumed and device size increases
Solution Approach 1:
The common-mode filter is merged with the reference plane structure itself, eliminating the need for separate via-based or surface-mounted filter components. The filter traces are integrated directly into the reference plane layer, allowing EMI suppression functionality to be combined with the existing ground reference structure, thereby saving PCB space while maintaining effectiveness
Solution Approach 2:
The reference plane serves dual functions: providing the standard ground reference for signal integrity and simultaneously acting as the common-mode filter through integrated filter traces. This multi-functionality eliminates the need for dedicated filter components, reducing overall device size and component count while achieving both grounding and EMI suppression goals
2Object-affected harmful factors
If conventional common-mode filters are used, then EMI suppression is achieved, but manufacturing cost increases
Solution Approach 1:
The filter structure is merged with the reference plane fabrication process, utilizing the same PCB manufacturing steps (copper deposition, etching, lamination) to create both the reference plane and the filter traces simultaneously. This integration eliminates the need for additional components and assembly steps, reducing manufacturing complexity and cost
Solution Approach 2:
The reference plane structure serves its own filtering function through the integrated traces, eliminating the need for separate filter components that would require additional procurement, placement, and soldering operations. The structure is self-sufficient, providing both grounding and EMI filtering functionality through its own geometry
3Object-affected harmful factors
If shield layers are added to suppress common-mode noise, then EMI suppression is improved, but device complexity and size increase
Solution Approach 1:
The EMI suppression functionality is merged into the existing reference plane layer through integrated filter traces, eliminating the need for additional shield layers. The filter traces are formed within the same layer structure that provides the ground reference, maintaining a simple two-layer or multi-layer configuration without requiring complex shielded constructions
4Object-affected harmful factors
If vias are used in common-mode filters, then EMI suppression is achieved, but the filter becomes unsuitable for smaller PCBs
Solution Approach 1:
The via-based filtering approach is extracted and replaced with a planar trace-based filtering approach that operates within the reference plane layer itself. This eliminates the dependency on vias and vertical through-holes, allowing the filter to be implemented as a two-dimensional pattern that can be scaled to very small PCB footprints while maintaining its EMI suppression function
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution offers improved RFI/EMI suppression across a wider frequency band, reduces PCB size, and maintains differential signal integrity without the need for vias or shield layers, making it suitable for smaller PCBs and reducing manufacturing costs.
Implementation Method 1
The dual-spiral common-mode filter comprises a first spiral portion connected to a second spiral portion
Implementation Method 2
the two signals in a differential pair create electromagnetic fields that are opposite in polarity. If the differential signal paths are identical and in close proximity to each other, the individual electromagnetic fields caused by the two signals will largely cancel each other
Implementation Method 3
providing effective RFI/EMI suppression and impedance matching by being patterned into the ground plane
Data Source
AI summary
Disclosed herein are dual-spiral common-mode filters, printed circuit boards (PCBs) comprising such dual-spiral common-mode filters, and devices comprising such dual-spiral common-mode filters and PCBs. A dual-spiral common-mode filter is patterned into the reference plane of a PCB. The dual-spiral common-mode filter comprises a first spiral portion connected to a second spiral portion. The spiral portions may be substantially identical, or mirror images of each other, or different from each other. One or more signal traces in a signal trace layer of the PCB pass over the dual-spiral common-mode filter. The disclosed dual-spiral common-mode filters can replace both conventional patterned ground structure (PGS) filters used for radio-frequency interference mitigation and the cutouts often used in the reference plane of a PCB to mitigate impedance mismatches due to DC blocking capacitors. Also disclosed herein are methods of making PCBs that include dual-spiral common-mode filters.