EMI-RFI Filters in DC Applications

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Corcom has designed and developed many filters for use in specific DC applications.

Many of these filters have been special designs for DC only operation
Many have been modifications of AC filters
Many have utilized AC filters in DC applications as described below

For technical help with applications of EMI-RFI filters in DC applications, please feel free to contact one of our engineers at CorcomTechhelp@tycoelectronics.com or give us a call at 847-573-6579.

INTRODUCTION

To consider usage of standard components in an unconventional operational mode for that type of component, it sometimes helps to understand the historical basis of the "how and why" in the circuit/equipment design cycle. To address the noise level specifications of new designs, final consideration and remedial action almost always occurred near the end of the design cycle - prior mathematical or analytical predictions have not been reliable, and accurate screen-room noise measurements depend on all components of the equipment to be in final form. It was both relatively easy and prudent to allocate some physical space near the power entrance point - easy, because circuit card designers have little PCB real estate available (and some space usually could be found amidst the back or front panel wiring), and prudent, because this location provided "last chance noise emission filtering. Filtering closer to any internal radiation noise source could be defeated by noise induction on the filter input wiring. Thus AC filtering was usually the choice.

DC APPLICATIONS

One historical exception to AC filtering has been telephone circuits; 48 VDC power sources (backed-up by batteries) for Central Office operations, have required DC filtering. Some recent trends have caused designers to re-evaluate filter location. With the advent of inexpensive, small, and efficient modular DC-DC Converters, the placement of this converter on each PCB eliminates the design of a multiple output centralized power supply. Reasons are:

Improved regulation - voltage drops due to input wiring resistance's can be minimized. This can be likened to Utility high voltage power transmission lines; i.e., higher input voltage with corresponding lower current. This results in less power loss and superior regulation.
Standardization - this results in shorter design cycles.
Filtering can be closer to noise source to reduce overall radiation levels - radiation induced into leads near the power entrance mentioned earlier, can be treated by a simple AC "clean-up" filter, especially the type with an integral input receptacle.
Reduce overall design time; AC-DC Converter design is reduced to a simpler single output version. This approach is generally valid for larger systems only, because in the smaller systems, standardization is more prevalent (previously designed supplies can be utilized) and with loads physically closer to input power, a central multiple output supply presents the best tradeoffs.

FILTER SPECIFICATIONS

AC rated filters can be operated up to the same numerical DC current level. Reliability will be slightly improved: inductor core hysteresis loss will be lower, and there will be an absence of capacitor reactive current. Internal filter component temperatures will be correspondingly lower. Also, line-to-ground capacitor current, (leakage current) will be near zero, and will therefore, not contribute to overall system leakage current. Filter selection can parallel that for AC with leakage current ignored (refer to Selector Chart in the Corcom catalog).

SAFETY AGENCY CONSIDERATIONS

Although AC rated filters are labeled accordingly, DC operation will not violate the spirit and intent of UL and CSA Safety agency requirements, provided that the numerical values of both current and voltage are not exceeded. There may be individual cases when system safety approval is sought, that manufacturers may be required to list the DC operation mode. It is expected, that in only very rare cases, Safety Agencies will insist on additional filter testing.

For technical help with DC Filter Application, please feel free to contact one of our engineers at CorcomTechhelp@tycoelectronics.com or give us a call at 847-573-6579