FN9246 | Pardon me, your Magnetic Field is leaking

May 03, 2023
David Armitage

EMI is so vigorously tested because the phenomenon is so unpredictable. A specific frequency might stray from a new design inexplicably, even when a powerful high performance filter is blocking the rest of the spectrum. In rare cases, an investigation into the mystery is needed—and a custom solution.

All but one Frequency

A leading audio-visual automation builder had a performance issue in a new design. The product already eliminated most of its noise with a suitable, high-performance IEC inlet filter (FN9246) from Schaffner. 

While the device was evaluated for conducted emissions to gain safety certifications domestically and for China Quality Certification (CQC), a mysterious leakage was discovered. A frequency below 1 MHz (which the EMI filter should be able to attenuate) caused the product to fail conducted emissions tests. 

The customer believed the issue was a toroidal inductor located very close to the filter. The magnetic field of the core appeared to be leaking just this narrow frequency, but how? 

Troubleshooting at Schaffner

The case was investigated by Schaffner’s own Dave Armitage, who attached it to testing equipment. He observed that gently pressing on the filter would decrease the amplitude of the noise peaking at 490 kHz. Maybe the component had a loose connection?

But he found that the filter was securely attached to the chassis and all its connections were strong. 

Pressing on the top of the filter, Dave discovered, improved the contact of the filter cover to that of the filter housing. The magnetic field of the core was actually seeping through the seams of the filter casing.

Caged in

The customer wondered if the shielding of the filter housing could be improved by sealing the seam of the filter housing to the filter cover. This would mitigate the magnetic field from the core from being able to couple into the filter. Schaffner offered to build a custom-sealed version to eliminate the leakage. 

Housing material was changed and instead of a couple of connection points to hold the filter cover to the filter housing, a continuous seam was placed around the housing and cover connection, making the filter housing essentially a Faraday cage.  

This sealed-up, more robust version of FN9246 enabled the customer to obtain the approvals for product release. 

With the mystery solved, the audio-video customer has placed its first production order, with the product quantity expected to grow year over year with a market peak in 3–5 years. 

Along with Schaffner EMI components comes the troubleshooting and customization expertise that can solve the most peculiar interference cases.