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Why am I not getting the expected results in reducing conducted noise, even though I’m using a noise filter?
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It is possible that magnetic flux from the noise source is coupling into the wiring.
Increasing the distance between the noise source and the wiring, or applying shielding, is an effective countermeasure.
Cause
Interference Caused by the Switching Power Supply
Magnetic flux generated from the choke coil inside a switching power supply can couple into the input wiring of a nearby noise filter. When this occurs, the noise filter cannot provide its intended attenuation performance.
Resolution
Solution 1: Increase the Distance Between the Interfering Cable and the Power Supply
Separating the wiring on the input side of the noise filter from the switching power supply as much as possible helps enhance the filter’s performance.
point
Noise sources include switching power supplies, motor drivers, and many other devices.
Keeping wiring away from these noise sources is crucial.
Solution 2: Apply Shielding at the Point of Interference
In some installation environments, increasing the distance is not feasible.
In such cases, consider applying shielding (source shielding or cable shielding).
Note:
Some power supplies cannot accommodate a case cover.
When a case cover is used, output derating of the power supply may be required.
point
- Ensure that the shield is connected to a stable reference potential.
Evaluate potential side effects of shielding (e.g., heat buildup, cost) before deciding on implementation.
Explanation
The following explains the example above:
Explanation 1: Magnetic Flux from the Choke Coil Interferes with the Wiring
In this case, magnetic flux emitted from the choke coil coupled with the wiring, inducing unwanted voltage through magnetic linkage. This additional induced voltage caused the noise measurement results to worsen, preventing the noise filter from delivering its expected performance.
Explanation 2: Reducing Magnetic Interference by Increasing Distance
Increasing the distance between the switching power supply—the noise source—and the wiring helps reduce magnetic interference. As a result, the induced voltage decreases, which in turn reduces unwanted terminal noise.
Explanation 3: Reducing Magnetic Flux Leakage with a Case-Cover Shield
By installing a case cover to block magnetic flux from leaking outside the power supply, magnetic interference was significantly reduced. Consequently, the induced voltage decreased, lowering the terminal noise as well.
