Editorials

So-Called Mid-Power LEDs Becoming Interesting

The nomenclature of the
industry has oft-times ranged between nebulous and obscure, so exactly what one
means by a mid-power LED could certainly shift a bit from one person to the next.
We’ll hit on that a bit further below, but getting right to the point, the things
we’ll call mid-power LEDs are proving to be very useful when you need to create
a more diffuse light over a larger or longer emitting area of the lamp or luminaire.
A-lamps, for instance, are challenged to give you an omnidirectional light with
just one or a few “power” LEDs, each throwing out a somewhat bell-shaped
light distribution. The more reasonable approaches are to either mount a few lower
output LEDs around the circumference of a central internal stalk (Cree’s standard
A-lamp takes this approach), mount them on two or three internal side-shooting
plates (Philips remote phosphor A-lamps take this approach) or to distribute the
LEDs in a scattering of a general globe shape, behind the bulbs diffuser (Switch,
as an example). The LEDs in an 800-lumen 60-watt replacement will need to put
out around 100 lumens each if you used 8 LEDs, or 50 lumens each for 16 LEDs.
There are lots of combinations in-between, but the sense of it is these type of
applications are calling for sub-100 lumen devices, instead of 200, 500 or 1000
lumen LEDs. Back when a power-LED only put out 100 lumens, there weren’t any good
choices on how to do an A-lamp, which is why it took the technology advancements
of the last few years to get us to the point of making a decent, reasonably cost-effective
A-lamp replacement.

Other targets for mid-power solutions can be seen in
linear replacement tubes, as well as even good old troffers. The tubes are an
especially good example, since they have a long but narrow profile (4 feet by
1 inch) that needs to be smoothly illuminated. More LEDs makes the task of diffusing
and smoothing the light much simpler, and it also provides a better opportunity
to dissipate the heat coming off the devices themselves. If we picked something
like 2400 lumens as the representative output (for nice simple non-metric math),
we need to distribute that across 48 inches, which works out to 50 lumens per
inch. It’s pretty apparent how 50-100 lumen mid-power LEDs would make the work
out as a 1-2 inch spacing.

So getting back to a bit of the semantic discussion…
Traditionally, high-power (or better, simply “power”) LEDs were typically
those which were meant to be driven above 350mA, which multiplied by a typical
2.9 to 3.1V gives you about one watt. Watts are power, so one watt or higher became
known as a “power” LED. Since LEDs are current-driven (or “varying”)
devices, rather than voltage-varying devices, the power-supplying ecosystem built
itself up around this 350mA reference, including multiples thereof, including
700mA and 1200mA reference points. So current actually became more important than
power (current x volts) when it comes to what plays with what. Philips Lumileds
has designated this to be “mid-power March” with a series of product
announcements around new mid-power devices. Unfortunately, we haven’t heard any
associated promo for these being wild new approaches or liquidation sales, so
we can’t label it any kind of “March Madness” to line up with the nickname
for the USA’s collegiate basketball tournament. Guess we’ll have to design some
new greeting cards. For the most part, they are defining those as running off
nominal currents of 100 to 350mA. In the case of their 3030-2D product, announced
March 18, they chose the current-centric approach describing a 100 to 240mA two-die
implementation that you feed 6.1V, so it spans a .6 to 1.5 watt kind of range
and delivers around 125 warm-white lm/W (a power-mid-power device?) that is designed
to go drop-in head to the head with similar products from Seoul Semi and Nichia.

Lumileds’ announcement of a 3535-HV earlier in the month followed a power-centric
definition, talking in terms of 20mA drive currents in both 24V and 48V implementations
(.48W and .96W respectively, so contained around 1/2 to 1W range here). They achieve
the higher voltage by stringing together 8 3V elements on the die itself (8 “junctions”
per die), which gets rid of the bonded wires from die to die that some other high-voltage
LEDs use. To do the 48V version, they do bond 2 8-junction die together into one
LED package, so it becomes a more hybrid approach. The higher voltage comes with
certain advantages when it comes to the power supply, since the step-down from
110 or 240V to 24/48V is much more efficient than the larger step of 110/240 down
to 3/6V.

Rounding out the mid-power month products thus far was their 3014.
It’s a rectangular 3mm x 1.4mm package, which provides a lower profile that puts
out 20-23 lumens with just a 60mA drive current, pushing it closer towards a “low-power”
offering. It can range up to 100mA, so we won’t quibble, and most importantly,
it is what you’d consider a fully lighting-capable LED, really designed to cost-effectively
enable a lot of LEDs arranged in linear or rectangular fashion. In addition to
wall grazers and under-cabinet lighting, they shared a specific design example
of a 2×2 ceiling panel containing 200 LEDs in an optimized lumen/$$ approach.
We’ve discussed the tradeoff between driving more LEDs gently, vs. fewer LEDs
harder, and here was another good example that is actually driving more, smaller
LEDs harder, trading off some of their peak efficiency of 140 lm/w to make the
optics, layout and thermals as simple as possible. It should also be noted that
Lumileds went with a non-standard 1/9th micro color hot-binning approach on the
3014 and 3030, that looks a pair of concentric circles inside a standard ANSI
bin, with the inside circle being on the black-body and CCT intersection, and
the outer circle being chopped into 4 chunks. The space between the outer circle
and square-ish ANSI bin constitutes the outer 4 bins. All of the color binning
is done at 85-degrees C, which is good and toasty to better represent operating
conditions, so the color spec’d is the color delivered at temp.

Since
we always love the “US manufacturers” Lumileds vs. Cree technology battles,
it was an interesting coincidence that Cree introduced its own newest small footprint
LEDs this week in the form of their XB-H product line. The XB-H is around the
same size, at 2.45mm on a side, but goes the opposite direction with a squared
off dome optic on top, punching out 500 lumens at around 5W. Driven more gently,
it runs up to 175 cool white lm/W. That one would be about a lot of light in a
small space, such as a PAR or MR, as well as bigger spot-lighting applications.
Part of the positioning for it was as a footprint compatible replacement for their
own XP-D by sticking a larger die on the smaller pad. The result would be up to
3X the lumen output at the same efficacy. Obviously, you’ll be paying for those
lumens at a similar $$/lm, so while a cool migration path that opens up a whole
new performance level for an existing design, it won’t exactly be a dollar for
dollar replacement.

Since lighting applications can vary widely, it is really
a benefit to this industry that manufacturers have taken as diverse an approach
as they have. Whether it’s Mid-power March or March Much-lumen Madness, one size
does not fit all, and it’s nice having the choice.

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