I. Customer Case: Broadband Conducted Emissions Exceed the Limit for a 35 W Power Supply

The device under test employs a PI TOPSwitch switching‑mode power supply design. While the product functions as intended, broadband emissions exceeding limits were observed during conducted emission testing at the AC mains port:

II. Step 1: Spectrum Analysis—Precisely Identify the Characteristics of Excessive‑Limit Issues

The core premise of EMC remediation is to accurately identify noise characteristics, rather than indiscriminately adding filtering components. By dissecting spectral data, three key issues are clearly defined:

Frequency range: The broadband continuous emissions from 150 kHz to 10 MHz exceed the limits, indicating that this is not a problem confined to a single frequency or a narrowband signal.

Exceedance margin: It significantly exceeds the limits across the entire frequency band, with the highest level exceeding 23 dB ；

Rectification Objective: It effectively attenuates broadband noise in the 150 kHz–10 MHz range and provides a safety margin to account for device deviations, test errors, and operating‑condition variations.

III. Step Two: Interference Source Tracing and Identification of the Core Noise Pattern

Conducted noise at communication ports is categorized into differential-mode interference and common-mode interference. These two types of noise have entirely different propagation paths and suppression strategies, and the filtering components must be matched to the specific interference mode in order to be effective.

3.1 Core Differences Between the Two Types of Noise

3.2 Qualitative Verification of Interference in This Case

The spectral characteristics of this case are as follows: the entire broadband range from 150 kHz to 10 MHz exceeds the limit, with the degree of exceedance exceeding 23 dB This is a typical characteristic of common-mode noise in switch-mode power supplies (while differential-mode noise tends to be concentrated at low frequencies and within narrow bands). Therefore, it is preferable to select… Common-mode inductor Rectification;

IV. Step 3: Determine the Basic Parameters of the Common-Mode Inductor

1. Operating Voltage

In this case, the maximum continuous input voltage is 265 VAC. The rated operating voltage, inter-winding withstand voltage, insulation resistance, creepage distance, and clearance of the candidate components must meet the requirements corresponding to the actual overvoltage category, pollution degree, and the overall equipment’s safety standards; selection should not be based solely on the nominal inductance value.

2. Rated Current, DCR, and Temperature Rise

The rated current shall be based on the maximum input RMS current under the minimum input voltage of 90 VAC, full-load conditions, and the highest ambient temperature, while accounting for conversion efficiency, power factor, inrush currents, and thermal dissipation. The device’s current‑carrying capability must also satisfy requirements related to DCR losses, winding temperature rise, and long‑term reliability.

3. Common-mode impedance and parasitic parameters of the target frequency band

6.8 mH represents only the low‑frequency inductance under the specified test conditions. For this case, the key comparisons should focus on the common‑mode impedance ZCM(f) and the insertion loss curves within the 150 kHz–10 MHz range, while also verifying the leakage inductance, inter‑winding capacitance, self‑resonant frequency, magnetic core material, physical dimensions, and temperature stability.

The common-mode inductor recommended here is 6.8 mH , the specific parameters are as follows:

Attenuation Requirements and Margin: Prior to remediation, the maximum exceedance was approximately 23 dB , the device’s attenuation capability should exceed 23 dB; increase by at least 6 dB After accounting for basic allowances, the minimum requirement is approximately 30 dB. Considering that the impedance/insertion loss curves in the datasheet are based on specified standard test conditions, the effective attenuation under actual loaded conditions may be reduced; therefore, for the target frequency band, approximately… 40 dB The selection is more reliable.

4. Installation Location and System Compatibility

The common-mode inductor is connected in series within the AC input circuit; both the L and N conductors must pass through both windings simultaneously, and no high-frequency bypass paths shall be formed.

The common-mode inductor should be positioned as close as possible to the noise source, and the high-frequency loop should be kept short to prevent high-frequency coupling between the input and output traces of the filter.

For normal operating current: The currents in the L and N lines flow in opposite directions, causing the winding fluxes to largely cancel each other out; consequently, the common-mode inductor offers virtually no impedance to the power-frequency operating current.

For common-mode noise: The high-frequency currents in the L and N lines flow in the same direction, causing their magnetic fluxes to add constructively. As a result, the device exhibits a high common-mode impedance, providing strong suppression of common-mode noise.

V. Step Four: Conduct a Repeat A/B Test Under the Same Conditions

To eliminate interference from the test environment, this retest was conducted under strictly identical conditions—input voltage, full‑load current, LISN equipment, wiring configuration, detection mode, and test bandwidth—while only the common‑mode choke configuration was modified.

Lika Electric Control ensures every transmission is more reliable.

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