Why EMI Filter Selection Deserves Care

An EMI filter suppresses the conducted noise on the input of an equipment, so its current, its leakage and its attenuation decide whether the equipment meets the emissions limit and the safety limit. Choosing too small a current overheats the filter, and choosing too large a capacitor breaks the leakage limit. This guide walks through a repeatable method for selecting a COSEL EMI filter.

Step 1: Fix the Current and the Voltage

Choose the current rating just above the maximum load current, because the filter carries the full input current and its loss raises its temperature. Choose the voltage rating from the line, whether AC or DC, and confirm the rating suits the worst case. The COSEL NAC and EAC series cover single-phase 250 V AC and DC at a range of currents, so a designer can match the load.

The Phase

Confirm the number of phases: a single-phase filter suits most equipment, and a three-phase filter is used for a larger machine. Choosing the wrong phase leaves a line unfiltered, so confirm it against the input.

Step 2: Choose the Capacitor Code

The line-to-ground capacitor code sets the leakage current and the attenuation. A larger capacitor gives more attenuation at high frequency but a higher leakage current, and a smaller capacitor the reverse. A medical or a sensitive application needs a low leakage, so the code is chosen to meet the safety limit while still giving the attenuation the EMC standard requires. Confirm the leakage against the limit for the equipment.

Leakage and Safety

The leakage current flows to ground through the filter capacitors, so it is a safety limit for an equipment that touches a person, and it also matters for a residual-current device. Confirm the value at the rated voltage and the frequency, and choose the code with margin below the limit.

Step 3: Choose the Stage and the Attenuation

A one-stage filter attenuates the common-mode and the differential-mode noise across the band, and a two-stage filter adds attenuation for a noisier source or a stricter limit. The COSEL NAC series is a general-purpose compact filter, and the EAC series has a defined attenuation from about 150 kHz to 1 MHz with a set of safety certifications, so the choice follows the source and the standard.

Matching the Source

Match the filter to the noise the source produces: a fast switching supply needs more attenuation at high frequency, and a differential-mode-heavy source needs a larger differential-mode element. Where the filter alone is not enough, a two-stage filter or a better layout is the answer.

Step 4: Plan the Installation

The filter is only as good as its installation, so mount it close to the input connector, keep the input and the output wiring separated and bond the case to the chassis. A long wire between the connector and the filter lets the noise couple around the filter, and a poor ground raises the impedance that the filter works against, so the placement and the grounding are part of the design. Choose the chassis or the DIN-rail mount from the enclosure.

Grounding and Wiring

Bond the case to the chassis with a short, low-impedance connection, keep the input and output wires apart, and do not run the filtered and the unfiltered lines together. A neat installation is what turns a good filter into a compliant design.

Verification

Validate the design on the bench by measuring the conducted emissions with the real load, by measuring the leakage current at the rated voltage and by checking the filter temperature at the worst-case current. Our FAE team can review the measurements and the installation, so the equipment meets the limit on the first attempt.

Getting Help

If you send your load current, the line voltage and phase, the leakage limit and the attenuation target to our FAE team, we will propose a filter, help choose the code and the stage and review the installation. BeiLuo holds mainstream COSEL filters in regional stock and ships them with an import declaration, a certificate of origin and a RoHS compliance file, and our engineers will review the choice with you before you commit to production.