Q&A with Cree about 60W-replacement LED bulb

After looking inside the Cree 60W replacement LED bulb (here’s a link to last week’s tear down) I had several questions for Cree, so I talked with Mike Watson, Cree’s VP of corporate marketing.

My most pressing questions were about the electronic circuit design – how the heck was Cree able to build an LED driver that dimmed so well, with such an excellent power factor (.98), all the while driving an array of 80 LEDs so effectively that it is able to warranty the bulb for 10 years of heavy use?

The most common way for LED bulbs to handle triac dimmer switches is to decode the power from the triac through a high-frequency pulse-width modulation circuit.  Says Mike: “We don’t use that approach in this particular bulb. We have a very proprietary type of RMS process that involves our IP [intellectual property] and phase dimming.” Hmmm. Not very informative, but Cree’s reticence is understandable: Why make it easy for your competition to copy you?

As we saw in the tear down, the bulb’s only power management IC is the  STMicroelectronics 6561D transition mode power factor corrector. So back to the ST website. Here’s app note AN2711 (a pdf) 120Vac input Triac dimmable LED driver for the L6562A, a chip in the same family as the 6561, that explains the use of the 656X Chip to handle triac dimming as well as PFC in a flyback power converter topology. Granted, it’s for an isolated LED driver, but its detailed explanation of using the chip for dimming is still valid as an insight into how the Cree driver may work.

The 80 LEDs (4 LEDs per individual LED component; 20 individual LED components) are arranged in an array of two parallel strings each with 10 individual LED components, making 40 LEDs per string. I asked Mike about the long series strings of LEDs: One LED failing open would take out an entire LED string, causing half the bulb to go dark. He said the PPM (parts/million) failure for an LED in the field – especially one that would fail open — is almost vanishingly low, which gives Cree the confidence to warranty these bulbs for 10 years.

I asked Mike a question that reader Chris posed in the original tear down piece: Can you use this bulb in an enclosed fixture? As Sal Cangeloso suggested in the comments, Cree intends that you be able to use the bulb in any application you would an incandescent. Mike agreed and elaborated: As long as there is an opportunity for air to circulate, the bulb will be fine. So, in a can lighting fixture such as you’d find in a track light:

Partially-enclosed can light track fixtures

…where the fixture is open, it’s fine. If you have a totally enclosed fixture, say, one with a lens over the opening, you it would still be fine as long as the ambient temperature doesn’t rise too much. (Think: Phoenix, summertime, no air conditioning.) Watson didn’t back-pedal or hedge: My impression is that Cree wants to move people to LED bulbs as quickly as possible by showing complete confidence in LED bulb reliability.

I also got some emailed questions from readers about the manufacturability of the bulb, including:

– How is the metal-core pc board that holds the LEDs formed?

– How do the clips work? Is that a failure point compared to a soldered connection?

Here’s a photo that shows the LED matrix from a top view.

[LED matrix top annotated]Cree 60W LED bulb pc board

The metal-core pc board is scored between each LED pair, allowing the board to be bent into a decagon that then slips over the 10-sided column. Even though the column is metal, there is no heat conductive path, such as a heat-sink adhesive, between the metal-core pcb and the metal column. A thermal conductive compound would make for better heat transfer, but would also make for a more complex, messy and expensive assembly process.

Next up: the clips. Here’s a photo showing the plastic guide that runs up the center of the metallic column, carrying power from the driver on the pcb up through the column and then, through the clips around to the outer surface of the column.

Cree 60W LED bulb internal power distribution

Don’t make the mistake of thinking that clips are a dodgy way to make a power connection compared to a solder joint: Solder joints are, on average, the most likely failure point for LED bulbs, especially since these joints would be hand-soldered. A properly installed clip is more reliable than hand soldering and has the added advantage of allowing the components to snap into place cutting, thus reducing labor costs. (These bulbs are assembled in the US.)

As I said in the previous article, this bulb is my new favorite LED bulb, and at under $13 (Home Depot) it’s a great value. Is there room for another price drop? Most likely: Philips North America CEO Greg Sabansky, “…sees a “growth tipping point” with the debut this year of its 60-watt equivalent LED light bulb that will retail for about $10.” Other reports say the bulb will come out at the end of this year.

Comments

  1. William Swartzendruber says:

    Is there a maximum operating temperature? This would allow us to test if our enclosed fixtures get too hot by throwing a thermometer inside and running the bulb for a while.

  2. Margery,

    Perhaps you could inquire with Cree w.r.t. the reports of excessive flicker with their bulbs. Some are surmising this is an issue related to achieving high power factor through use of a smaller output capacitor as it will draw power more consistently throughout the sine wave. The flicker appears to be higher than CFL bulbs which is troubling as this was a concern to some consumers.

  3. I am glad to see CREE finally entering the consumer market. I have long been a fan (and stockholder) in CREE and think that they do a fantastic job. My question is this: Is anyone making a high quality, dimmable, LED light that is designed for recessed cans (so to replace a typical “reflector” type) with light output equivalent to 100W? Mostly they seem to top out around 75W equivalent. Thusfar I have found two that I am considering, but neither is affordable (if replacing a whole house), both are too dim, and I am concerned about the CRI (color rendering) on them:
    LED20DP38S827/40 $47 GE Bulb with 40-degree beam, dimmable, in/outdoor
    20P38/27LFL-UP $55 Toshiba 100W equiv, 2700K, CRI: 84, 35-degree beam
    Those two came from http://www.lampline.com and http://www.grainger.com … I would actually prefer Lowes or Home Depot, but they simply do not publish enough information about the LED products on their websites.

    Any help? I can wait another 6 months or so if something big is expected to be released…but by then it will be truly urgent to get something purchased. Maybe CREE will do a floodlight next?

  4. I have tried several different LED replacement bulbs and all have caused RFI interference in the house, some moreso than a cheap switching wal-wart power supply. In posting data for any LED bulb RFI has to be tested and stated IMO. Is there any RFI data for these new bulbs?

  5. Larry, here’s what Doug Leeper, my go-to LED test guru had to say about the Cree and RFI: “It radiates RF, so not recommended for Hams (without filtering) IMHO.  The Spectrum Analyzer is out for calibration, but [the bulb] has significant RF emissions in the Aviation radio band, and the hand held radio caused it to break squelch up to 300-400 MHz.  I don’t know if this is radiated or re-radiated through conduction into the 120V wiring.”

  6. I just installed five of the Cree 40w led bulbs in a bathroom light bar fixture and the buzz/hum is very noticeable. The connections are tight on the Lutron Maestro dimmer and some of the bulbs seem louder than others. Will they quiet down after awhile? What can I do???

  7. Brian, have you checked the connections from the sockets to the light bar? If they aren’t tight then buzzing is possible: The whole bar can turn into an audio resonator. Also, try the noisy bulbs in a lamp. If they are quiet in the lamp you know it’s something to do with the light bar. But if they are simply noisy bulbs, then take them back to Home Depot and get your money back. Bummer.

  8. Thanks for the quick response, Margery. If I unscrew all but 1 or 2 bulbs, the buzz is eliminated. Is it common for LED bulbs to make noise like this? I will test these in another light bar that uses 6 bulbs without a dimmer switch and let you know if anything changes.

  9. Brian, in my experience it’s not common for the Cree bulbs when solidly mounted. There are some LED bulbs which are chronic buzzers.

  10. I have a concern with RF interference: FCC regulations prohibit interference with the primary user of a frequency; historically, the FCC has forced removal of appliances, fluorescent signs, etc. that interfered with radio signals by virtue of spurious RF emissions.
    If these Cree bulbs interfere with any radio frequencies, you’re looking at a potential visit from the government, and at being legally mandated to remove all interfering devices.

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