Identifying Dominant Conducted Noise: Differential vs. Common Mode in SEPIC (Boost Mode)
- CircuitCopper
- 3 days ago
- 1 min read
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).
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.
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 CurrentThe 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 ContributionThe 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 OperationThe 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
appears as spectral peaks related to switching frequency and its harmonics
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