LED luminary EMC
A standard example of a power supply circuit for LED lighting equipment, using conventional power components is shown in figure 1. This kind of construction will lead to excessive radiated emissions. One of solutions is to shield entire housing. Shielded enclosure increases the parasitic capacity between the chassis and PCB and the reference ground of any EMC measurement equipment. Common-mode conducted emissions then becomes a large enough phenomenon to exceed conducted emission limit lines. In this case EMI filter is added to eliminate conducted emissions at the LED luminary AC power input. Similar situation is at the DC output. LED diodes are placed on heat sink, leading to common mode current that is responsible of radiated emissions from wires or PCB traces.
The main problem in LED lighting equipment is the power supply’s high-speed switching circuits, which create wide spectrum current and voltage ripples at the mains AC input and DC output. Proper shielding and filtering can mitigate the radiated and conducted emission problems, but do not eliminate it. A better solution would be to avoid generating high radiated emissions and conducted emissions at particular frequencies in the first place. This is possible through the use of new power components that use soft switching to minimize ripple currents, or to spread the noise energy over a wide frequency band in properly designed power supplies.
Figure 1: Typical AC/DC LED driver design. The H-field is the result of winding leakage, the primary loop area and the secondary loop area. The E-field is the result of high dV/dt on conductive surfaces and of high-frequency ripple in cables.
The Power Factor Controller (PFC) is the most common block in modern AC/DC LED drivers. PFC is responsible for power frequency harmonic current emissions. This is one of the mandatory EMC emission tests, defined by EN 61000−3−2. According to EN 61000−3−2 PFC is mandatory if LED luminary power exceeds 25W. In figure 2 harmonic emission results are presented for LED lighting equipment with and without PFC. In case of power factor correction current drawn by the LED luminary is close to sine wave, while without PFC current is drawn in peaks.
Figure 2 Harmonic emissions according to EN 61000−3−2
Conducted emissions are created by LED driver and conducted via all cables connected to driver. Conducted emissions for LED lighting devices are limited by international standard EN 55015 or CISPR 15. Emissions should be measured on all cables that are connected to auxiliary equipment– mains power, controllers, etc. Usually there is only one port for luminaries– that is AC mains connection. Excessive emissions are limited by EMI power input filter. Proper filter design will solve conducted emission issue. In figure 3 example of failing conducted emission results are presented. In figure 4 example conducted emission results passes the EN 55015 requirements. Conducted emission measurements are carried out by two EMI detectors– quasi-peak (QP) and average (AV). Therefore there are two limit lines on measurement plots. Quasi-peak detector measurement should be below QP limit line and average detector should be below AV limit line. Quasi peak limit line starts at 9kHz and quasi peak measurements should be measured starting at 9kHz, while average detector measurements should be started at 150Hz.
Figure 3 Conducted emissions according to EN 55015 (Fail)
Figure 4 Conducted emissions according to EN 55015 (Fail)
Usually first sweep is carried out by peak detector (blue) instead of quasi-peak detector, as peak detector measurements are faster. Afterwards, emission peaks are re-measured by quasi-peak detector– final measurements (blue squares). Average measurements are created only with average detector.
Radiated emissions are created by high frequency current components flowing in LED luminary conductors, long enough to be an efficient antenna (AC power input cable, LED array and connection wires, metallic enclosure etc.). Source of disturbances is LED driver allowing high frequency current flow outside the driver enclosure. Radiated emission measurement result examples are presented in figure 4 – 6. LED luminary radiated emissions are measured in frequency range 9kHz-30MHz using magnetic field antenna and 30MHz-300MHz using log-periodic or biconical antennas. Measurements are carried out using quasi-peak detector.
Figure 5 Radiated emissions 9kHz-30MHz according to EN 55015 (Pass)
Figure 6 Radiated emissions 30-300MHz according to EN 55015 (Pass)
Figure 7 Radiated emissions 30-300MHz EN 55015 (Fail)
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