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Why Do We Need a Power Line Filter and Where to Place It?

EMI Power Line Filters are required due to the use of high frequency switching techniques used in today’s power conversion circuits. Harmonic noise created from fast changes in voltage and current from Switch Mode Power Supplies (SMPS), inverters, rectifiers, etc. require EMI filters to meet the conducted emission requirements as set forth in the various international and military EMI standards.

In addition to reducing the harmonic energy propagating out onto the power distribution system, an EMI power line filter serves the dual function of reducing noise already on the power distribution system from entering an equipment.

The design of an EMI power line filter involves knowledge of filter impedances and how they interact with circuit impedances across a broad frequency range. However, there are installation and mounting details that can equally determine the effectiveness of an EMI power line filter.

Three common issues are presented:

1. Filter case bonding to the host equipment: It is imperative that the metal case of the EMI filter is ‘metal to metal’ bonded to the equipment metal case/chassis. Painted or nonconductive surfaces must be eliminated as they will reduce the effectiveness of the line to ground capacitors in the filter.

2. Filter placement at the entry point of the incoming power lines: Placement of the EMI power line filter at the entry point of the incoming power lines is critical to filter performance and maintaining overall shield integrity of the equipment’s metal case/chassis. The further away a filter is mounted from the power entry point, the greater the possibility noise from the power supply or logic circuits can radiate and couple onto the power lines bypassing the filter.

3. System cable routing: In conjunction with item 2 above, it is important to physically isolate the incoming power lines to a filter from other system cables and wires. If system cables/wires are routed too close to a filter’s input wires, noise can couple onto the filter wires bypassing the filter.

At Premier Filters, our extensive line of standard and custom EMI power line filters are coupled with our unparalleled knowledge of EMI to deliver the optimum filter solution. Give us a call to see how Premier can deliver your project on time and on budget.

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