Cree’s consistency in using the same platform for all of its LED light bulbs allows us a unique peek inside of its engineering design process. We can see how its design has matured over time, and even make some guesses about what’s next.
I took apart the first Cree LED light bulb, an 800-lm, 60W-replacement bulb, a year ago in March 2013, and found an initial version of the hardware platform upon which Cree has based its entire family of LED bulbs. [SAFETY WARNING: Do NOT remove the bulb cover and operate the light. You will be exposed to the ac house power. I always have someone in the room with me when I check voltages and I work with extreme caution. For more on what a bad idea it is to remove the cover, see Tech blogger has trouble with LED bulb cover.]
A double row of LEDs was mounted on the bulb’s metal column, which Cree refers to as its “Filament Tower.” The column serves both as a heat path between the LEDs and the bulb’s metal heat-sink collar, as well as a way to elevate the LEDs for an omni-directional light pattern. The non-isolated LED driver‘s power control IC was an STMicroelectronics L6561D.
That was then. Here’s what the Filament Tower looks like now:
Last December, Cree tweaked with the design, most notably switching to LEDs that have more emitters (or LED chips) per LED package, thus dropping the number of LED packages on the 60W-equivalent bulb from 20 to 10. (This is the bulb that Chuck Swoboda, Cree CEO, referred to as “Gen 2” in his analyst call of October 2013.)
The old LED packages had four LED chips per package, seen as four tiny bumps in each yellow-colored LED package. The packaged LEDs in Gen 2 have 8 LED chips per package. So while there are now half as many LED packages, each package has twice as many LED chips and puts out about twice as much light. The LEDs look like they’re Cree’s XLamp XB-G high-voltage white. The string of LEDs in Gen 2 has the same total voltage drop as in Gen 1: 225Vdc.
Here’s the Gen 2 LED power driver pc board: And a closeup of the power management IC:
The power management IC is now the STMicro 6562A (PDF), which is in the same product family as the original 6561D. The L6562A is a newer device in the L656x PFC controller family; It uses newer process technology and has a tighter parameter spread. The L6562A also has improved power factor performance, according to STMicro.
There’s a significant change on the back of the pc board. Here’s the Gen 1 board with a two capacitors and an inductor:
…and here’s the Gen 2 board, which no longer has that inductor — a nice component to lose, especially in terms of cost:
Now let’s look at the 1100-lm 75W-equivalent version that Cree introduced in December. It’s quite similar in appearance to the 60W-equivalent Cree bulb – even up to the 10 LED packages on the column.
The LEDs are the same. The drive electronics looks the same. How does this bulb manage to squeeze 1100 lumens out of the same basic hardware that previously produced just 800 lm?
Let’s go back to the voltages across the LED string. We saw that for the 800 lm bulb, Vdc = 225V, or 22.5 for each LED of the 10 LEDs in series. This is the typical Vf for the package, and sustains about 44mA, according to the data sheet for the XB-G (downloadable PDF.)
For the 1100lm bulb, Vdc =230V, or 23V across each LED. Referring to the I-V curve below (Figure 1), at 23V the forward current through the LEDs (If) looks like it will be somewhere around 65mA, or about 20mA more than the 800lm bulb.
Figure 1. If vs Vf curve for XB-G
The additional current means that the 1100 lm bulb needs a significantly larger heat sink than the 800lm. And sure enough, the heat sink/LEDs combination for the 800lm bulb weighs 2.2oz, while the 1100lm bulb’s heat sink weighs weighs 3.4 oz. (See Table below.) The heat sink weight has increased by 55%, the light output increased by 38%, and the efficacy of the light decreased from 84.2 lm/W to 81.5lm/W.
Here’s the short answer to how the 75W-equivalent bulb produces those extra 300lm: Cree drives the LEDs harder in the 1100lm bulb, trading off efficacy to leverage the same basic electronics/LED platform at the expense of using a little more power and a larger heat sink.
There’s one more change, which is pretty major, and that’s the additional potting in the electronics in the base of the bulb. The 800lm bulb is not potted. (Although this is not totally new for Cree – its BR30 bulb is potted.)
There is also a foam shim between the top of the inductor – it’s back again! — and the case. It’s likely that the potting is there to damp out any vibrations caused by the higher currents in the electronics — especially the new inductor at the input. Here’s a side-by-side of the 1100lm (left) and 800lm (right) boards, showing how similar they are, as well as the disappearing/reappearing inductor.
Next up in the family: the 18W, 1600 lm 100W-equivalent Cree bulb.
It also has the potted base, but it doesn’t have the foam shim between the inductor and the case – because it’s lost that additional inductor again.
It uses twice as many LEDs to achieve twice as many lumens while using a little less than twice as much power.
The heat sink is proportionately lighter than the 800lm bulb’s, at only 2.9 oz. This bulb is more efficient in its use of power: Less of the power is wasted as heat, so it has less than a 2x increase in heat sink material, even though it produces twice as much light.
The power driver board looks surprisingly roomy: The bulb itself is larger, making for a slightly larger bulb base and pc board, but the number of components are also reduced.
The components and layout are generally similar, but the power controller IC is different.
While it’s still an 8-pin IC, it’s about half the size of the 6562A, and has a cryptic marking of E2 3zE .
This combination of letters and numbers doesn’t show up for any LED power management IC I’ve been able to find. My theory is that it’s a custom version of some form of the 656X family because of the overall similarities between the 1600-lm and 800-lm pc boards. And also because that seems to be the way Cree rolls – standardize and re-use wherever possible. I don’t think that Cree would abandon the basic design which has served it so well especially since it’s still the same basic board.
Interestingly, the 1600lm pc board has both an outline and the pads for an L3. What’s up with that?
The answer to that minor mystery may lie back with the 1100 lm bulb. When Cree introduced the 1100 lm, 75W-equivalent bulb you may have wondered: Why? It’s not like the world has been crying out for a 75W-equivalent LED bulb, since the 75W incandescent isn’t a staple for lamps as are the 60W and 100W bulbs. However, the reason for having a 75W-equivalent version may be related to another member of the Cree bulb family, the TrueWhite (TW) high-CRI bulb. (Really, this all has a connection the the 1600lm bulb’s missing inductor…)
Cree introduced the TW in September of 2013. This is a bulb that requires 13W to produce 800lm of lovely, high-CRI light. Yes, it’s less efficient in its use of power than the 800lm SW bulb, which requires just 9.5W to produce 800 lm, but the SW version is only 80 CRI, far less than the TW’s 93 CRI — you’re trading off lumens for a high-quality light. (The TW’s neodymium glass bulb cover filters out the yellow wavelengths, allowing reds to render more accurately, but effectively throwing away the power that goes into creating the yellow-ish wavelengths.)
Because of its high CRI, the TW bulb is the only bulb that’s eligible for rebates within California. If you go to a Home Depot in a county serviced by the largest California utilities – like SMUD and PG&E – you can buy this bulb for the subsidized price of $10. (Your only other choice for a $10 bulb in California is the Philips $10 SlimStyle, which sells unrebated for $10.) If you see this bulb in a Home Depot outside of California you can expect to pay $16 for it, since the TW version is not Energy Star certified which knocks it out of the rebates in most parts of the country.
So here’s Cree with a bulb that is rebated down to a great price in California – but what about the rest of the US? Cree’s choice, I think, is to take the same bulb and put on a non-neodymium, plain glass bulb cover that passes all the light produced by the LEDs. With the switch of a bulb cover, Cree now has an 1100lm 75W-equivalent bulb that it can sell anywhere in the country. And, because the 1100lm bulb recently became Energy Star certified, it’s eligible for many utility rebates.
Which takes us back to the missing inductor on the 1600 lm bulb. If Cree drives the LEDs a bit harder (which appears to require the addition of the input inductor) and adds a neodymium bulb cover, it would then have a 100W-equivalent 93 CRI bulb. True, it would require a larger heat sink, but a high-CRI 100W-equivalent LED bulb would be pretty attractive to many consumers, who miss the days of bright, high light-quality incandescent bulbs.
So there you have it: A look inside Cree’s LED light bulb family, some wild speculation about what we found, and how that may point towards future products.