LED lighting design--A checklist

Consumer demand and government initiatives to phase out inefficient lighting technologies are pushing expansion of LED lighting. Designers of these devices must satisfy not only end users, but also various agencies and published standards that dictate performance of the finished products.

Given that LED bulbs are intended to be form factor-compatible with current incandescent and CFL bulbs, they will have an AC/DC power converter, driver/controller IC for the LEDs, a heat sink for thermal management, and optics to optimize light quality. Similar considerations apply to the design of outdoor LED luminaires.

Pivotal design areas are the driver and its power converter. These sections of an LED lighting assembly have the greatest influence on achieving performance objectives. Similarly, protective devices for these circuits have a great deal of influence on the reliability and life of the assembly, particularly in outdoors applications. Moreover, safety and reliability demand adequate reliable overvoltage and overcurrent protection.

Overview of Industry Standards
Designers of luminaires and retrofit LED bulbs have several options to consider in the selection of a driver’s power converter topology, as well as protective devices for this circuitry and the LEDs. A good starting point is to review pertinent industry standards and test specifications.

Table 1 is an overview of global standards for those countries representing the greatest demand for LED lighting. As indicated in the table, two key areas to be considered are surge immunity and safety. The choice of isolated vs. non-isolated topologies with their corresponding power levels will dictate the level of safety required to protect against short circuit, overload and surge transients.

In the US, agencies are establishing uniform performance, safety and surge standards for LED lighting. The EPA’s Energy Star specifications apply to LED replacement bulbs for residential and certain commercial applications. The Dept of Energy’s (DOE’s) Municipal Solid State Street Lighting Consortium’s (MSSSLC) LED Roadway model spec, and the Commercial Building Energy Alliance Parking Lot and Structure Outdoor LED lighting performance spec are important documents released in 2011 and 2012 respectively that address surge immunity levels for outdoor LED luminaries.
Against this backdrop of standards and test specifications, designers must meet a number of performance objectives related directly to driver topology and the protective devices needed. These performance objectives include:

LED operating efficacy (lumen output per watt of input power)

Overall power efficiency of the lighting assembly

Thermal management of the LEDs and their driver circuit

AC power factor correction (PFC) for the driver circuitry

AC harmonics generation (distortion)

Meeting EMI restrictions

Whether or not dimming is required

LED driver reliability and service life (to match that of the LEDs)

Circuit protection devices needed

Electronics space efficiency (assembly size)

Cost/competitive position

Driver Power Supply Options/Tradeoffs
A typical LED lighting assembly uses a series wired string of LEDs powered by a switch mode constant current driver. This combination tends to be the best choice for the desired lumen output, color control, and adding a dimmer when needed. When supplied from AC mains, the constant current source is typically called an offline power converter, which is the case for most LED retrofit bulbs and outdoor luminaires.

Safety and UL recognition heavily influence power converter topology decisions, which in turn affect component selection, electrical insulation, and assembly size, how circuit protective devices are used, and of course – cost.

Topology also impacts power efficiency and other performance factors. While switch mode power converters are commonly used in LED drivers, certain application factors may suggest specific types of converter topologies. In general terms, the topology options for AC line powered assemblies could include:

• Isolated and non-isolated linear drivers with constant current regulation
• Buck and buck/boost converters (with or without PFC)
• Single stage flyback driver with PFC
• PFC + Isolated half-bridge constant V/constant I (eliminates linear or DC-DC conversion stage)
  

In many of these topologies, another consideration could be the use of synchronous rectification, which can improve efficiency at lower output voltages. Space doesn’t permit a detailed treatment of all these possible approaches to LED driver designs. However, a few examples will be presented to highlight some tradeoffs to be considered.