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Options for powering LED luminaires

Posted: 10 Dec 2012 ?? ?Print Version ?Bookmark and Share

Keywords:electromagnetic compatibility? LED luminaire? power supply?

Compromises and workarounds undertaken in an effort to fulfil the requirements of electromagnetic compatibility (EMC) have had the potential in the past to markedly impair the reliability and efficiency, as well as 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 每 that is, the chassis plus its shield 每 and the reference ground of anundery 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 每 and this is now possible through the use of new power components that use soft switching to minimise ripple currents, or to spread the noise energy over a wide frequency band.

Figure 1: A 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.

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.

Figure 2: The power range in which each topology is most commonly used. (Click on image to enlarge.)

The Power Factor Controller (PFC) is the most common block in modern AC/DC LED drivers. A boost converter is inserted between the bridge rectifier and the main input capacitors. This regulator can operate in three modes. In Discontinuous-Conduction Mode (DCM), the energy stored in the inductor (L) during the conduction interval of the switch is equal only to the energy required by the load for one switching cycle (figure 3). The energy in the inductor drops to zero before the end of each switching cycle, resulting in a period of no energy flow, or discontinuous operation. In Transition Mode (TM) 每 also called Boundary Conduction Mode (BCM) or Critical conduction Mode (CRM), the converter operates at the boundary between DCM and Continuous Conduction Mode (CCM), reducing the idle time of DCM to close to zero.

Figure 3: Peak and average current in the inductor (IL) in a) discontinuous conduction mode b) transition conduction mode and c) continuous conduction mode.

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