[Editor's note: Ed Rodriguez has accepted a challenge on this site to pull together everything we know about the various entities that go by the moniker "AC LED." This is the fourth of four installments.]
‣ Part 4
Some key design criteria for choosing AC LED versus a conventional power supply
I have noted that AC LED methodology can be attractive in one form or another for the following sorts of applications:
- 120 VAC residential product under 10 to 15 watts, with cost and profile most important.
- Application a light strip or linear array, as in a cove light, a low-power, slim-profile, under-cabinet light, or other fluorescent tube replacement. Here, the 40 to 50 tiny LEDs inherently provide the desired smooth "light bar" effect.
- A higher-powered luminaire where a very slim profile cannot be achieved with a regular power supply -- as in an ultra-slim fluorescent replacement troffer application.
The attractive thing about using regular constant-current power supplies is that they are available with UL approvals in an extremely wide range of voltages, currents, and shapes, with dozens of interchangeable vendors -- many with excellent documentation. One can approach a fixture design and really fine-tune the result by not only choosing the desired LED vendor, CCT, efficacies, package style, and wattage per LED, but one can also decide on a combination of series and parallel connections for maximum reliability continuance in the event of a short or open in one or more LEDs over the course of years.
With the lower voltage output of a conventional power supply, one can do all kinds of tricks to offset individual LED failure. That is just not possible when you are committed to an "electrically hot" string of 40 to 50 LEDs.
It goes without saying that an AC LED approach is generally incompatible with any luminaire design where a single COB is desired -- as in a down-light or track light where some type of reflector (either as an add-on or in the parabolic walls of the housing) is used for collimation or beam control.
We have noted how a light fixture described as "AC LED," having no regular switching-type power supply module, is not necessarily descriptive of any single concept. It can be:
- A basic 2- or 3-watt component, with a diode bridge inside or outside the SMD package
- A 5- to 50-watt light engine, sold as a finished product (with metal or FR4 substrate) having on it one or more series high-voltage strings of LEDs, with auxiliary components to control the power to them -- with or without a power-factor-fixing sequential driver IC
- A sequential-switching IC to which the designer mates one of a series of high-voltage LED strings, each of perhaps 40 to 50 LED chips (one or more LEDs per SMD package) onto a PCB or MCB in one of an infinite number of possible shapes per its purpose. The IC need not actually be on the same PCB. A number of firms now offer such items on the basis of custom/contract manufacturing.
We've noted that things can become problematic in commercial applications above 30 to 40 watts where universal input range (90 to 277 VAC) is expected, whether absolutely needed or not. We've also suggested that an AC LED approach brings little to the party above 100 watts unless having a very slim profile is needed but unachievable with any modular supply, with price being secondary.
Just to be clear, there is technically no upper wattage limit at this time for a luminaire design having AC LED-type functionality. It's just that one would have to use: a) multiple sets, inside the luminaire housing, of whatever driver/LED array circuitry one would use for a basic 15-, 30-, or 50-watt luminaire; or b) one's own proprietary slave circuit -- and doing so would likely be more expensive and tedious than using a conventional power supply.
It becomes apparent that one of these versions -- barebones 2004 vintage or the more sophisticated 2014 version -- of the AC LED concept can be an excellent tool for the right job if a designer applies it intelligently, with a full awareness that much of the documentation out there may be ambiguous or not tell the whole story.