Radiocommunications Agency  EMC Awareness

Lighting

Description

Conventional tungsten filament lamps are entirely without radio frequency emissions, except in rare fault conditions. However, because they are inefficient, governments are promoting use of energy efficient alternatives in support of its climate change policy. These include energy efficient fluorescent lamps, which can produce high levels of RF interference.

For a fluorescent tube to operate at high frequency an electronic switching ballast is required. A typical operating frequency for an electronic ballast would be in the region of 20kHz to 40kHz. The high frequency ballast is essentially a switching power supply and has the potential for radiofrequency interference in the same way as any switched mode power supply. Dimming is achieved by reducing the power applied to the fluorescent tube. In order to maintain the discharge, the frequency of operation must increase as the light output level is decreased. Thus a typical electronic ballast which operates at around 30kHz for maximum light output will supply the lamp at increasing frequencies up to approximately 100kHz for minimum light output.

Measurements of sample lamps have shown that electronic ballasts can cause an increase in interference in long wave and medium wave bands to portable radio receivers. The minimum distance of a radio receiver to the lighting samples for good reception varied between samples. Of particular interest was the interference to AM services from the dimming ballast. Because the fundamental ballast operating frequency alters as the lamp is dimmed, so interference to individual radio stations depends upon the light output level of the unit.

VHF band 2 FM radio reception can be affected by electronic ballasted fluorescent lamps with batten type fittings. Modern lighting technologies and techniques do give rise to significant emissions in the VHF band. In practice these emissions are seen to be limited to an upper frequency of less than 300MHz at present, but since the EMC standard for lighting (CISPR 15) gives no tests above 30MHz, these emissions are effectively uncontrolled by any legislation.

Commentary

Switching power converters are a frequent source of RF emission problems, and those used in lighting products are no exception. In this case, the problem is compounded because fluorescent lamps are found in large numbers in many buildings and it is hard to find a location far enough away from them; and the price pressure on such products works against best practice in the design to minimise emissions. Because of their dimensions, batten-type fittings will radiate effectively at VHF, and efficient switching ballasts will quite easily generate energy at these frequencies.

References and links

Development of improved test methods for assessing the EMC emissions from luminaires and ancillary devices, York EMC Services, Radiocommunications Agency Project AY4125, March 2002: available from http://www.radio.gov.uk/topics/research/topics.htm#emc/

Links to Mitigation Techniques

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