Identifying Dominant Conducted Noise: Differential vs. Common Mode in SEPIC (Boost Mode)
Updated: Aug 11
First, the measurement setup for evaluating common-mode and differential-mode currents is defined. Common-mode current is measured using a current probe clamped around both the positive and negative conductors simultaneously. Differential-mode current is evaluated by measuring each conductor individually (positive and negative).
Measurement Setup for Current Testing
This is the measurement setup used for current testing. CISPR 25 Class 5 limits were applied to assess the feasibility of optimizing the device and verifying its ability to comply with Class 5 requirements.
Current Measurements Overview
The light green trace represents differential-mode current, while the dark green trace represents common-mode current. Across the measured frequency range, the differential-mode current exhibits significantly higher emission peaks compared to the common-mode current.
1. Dominance of Differential-Mode Current
The higher peak levels in the differential-mode measurement indicate that the dominant noise current flows between the positive and negative supply lines. This behavior is consistent with documented SEPIC converter operation, where:
The input current is pulsating.
High di/dt currents circulate in the power loop.
2. Low Common-Mode Contribution
The common-mode current remains significantly lower across the spectrum. According to application notes:
Common-mode noise is mainly driven by parasitic capacitances between the switching node and ground.
Its level depends on coupling to chassis or reference ground.
The measured low levels indicate that this coupling is not dominant in the current design.
3. Correlation with Switching Operation
The differential-mode trace shows discrete peaks across frequency, which is consistent with switching converter behavior described in datasheets and EMI guidelines:
Noise is generated by the switching action.
It appears as spectral peaks related to switching frequency and its harmonics.
Conclusion
In summary, our analysis of common-mode and differential-mode currents reveals critical insights into the performance of the device under test. The dominance of differential-mode currents suggests that optimizing the design for these conditions could enhance compliance with CISPR 25 Class 5 standards. Furthermore, the low common-mode contribution indicates a well-designed coupling strategy that minimizes unwanted noise.
By understanding these factors, we can help companies develop reliable electronic products that meet stringent industry standards. Our goal at CircuitCopper is to become a trusted partner for companies developing complex electronic products, assisting them in efficiently bringing their ideas to market while ensuring compliance with relevant standards.
For further information on optimizing electronic designs, we encourage you to explore additional resources.
Struggling to Identify the Source of Conducted EMI?
If your design shows excessive conducted emissions and it is not clear whether the dominant problem is differential-mode noise, common-mode noise, switching harmonics, grounding, or parasitic coupling, CircuitCopper can help investigate the source.
We can support conducted-noise measurements, current-probe analysis, PCB and power-stage review, filtering evaluation, and identification of the dominant noise mechanism before the next EMC test cycle.
Send us your measurement results, schematic, PCB files, or a short description of the issue — we can review the available information and help determine the next engineering steps.
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