Powering LED luminaires: What’s the best option?

Introduction

Compromises and workarounds undertaken in an effort to meet the requirements of electromagnetic compatibility (EMC) have had the potential in the past to markedly impair the reliability and efficiency, and increase the total production cost, of the power circuit in an LED luminaire.

A typical example of a power circuit for an LED luminaire using conventional power components is shown in figure 1. Excessive radiated emissions will normally lead the design team to shield the entire housing. In practice, however, this increases the parasitic capacity between the (now larger) conductive area – that is, the chassis plus its shield – and the reference ground of any EMC measurement equipment. Common-mode conducted interference then becomes a large enough phenomenon to require attention. While a low-cost EMI filter will eliminate this problem, this author has seen designs in which even these counter-measures are not sufficient, since higher-frequency emissions radiated by the mains cable persist, and must be blocked by a more expensive filter with higher attenuation.

The root cause of the problem in LED luminaires is the power supply’s high-speed switching circuits, which create wide spectrum current and/or voltage ripples. Shielding and filtering might mitigate the emission problem, but do not eradicate it. A better solution would be to avoid generating high emissions at particular frequencies in the first place – and this is now 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.


Figure 2 shows that there are broadly five architectures used today, each suited to different power outputs. Each of these LED driver topologies enables the designer to comply with the strict requirements of today’s EMC regulations. While figure 2 indicates the power range in which each topology is most commonly used, it should be noted that each can be adapted for use in a higher or lower power output range.