Gryloc said:
Hello Newbie. I was watching this thread for a while and I finally felt like posting. Again, nice work with the testing. Is it appropriate to be so harsh and judgmental over an early production LED? I was just wondering. Do early production parts ever change much from the more mature products that may be available like 6 months later? I was thinking about trying one when more available for some projects, but I hope I am not going to be disappointed.
Normally, when designers design things in, and run them through their own tests, thru qualification testing, HALT/HASS, and in many industries, certification. As such, you expect the parts to have a certain amount of consistency and not to be changing, as a simple change to something like a primer that is used for silicone adhesion, can cause issues when going back thru testing. These tests take months or even years to do, so you don't want things to be changing, or in a constant state of flux. This is where some designers get in a bit of hot water, where they will design and qualify things with prototype samples, then they find out (even though the manufacturer of the device said...) that something has changed, or often discover in themselves that the parts actually changed in their product production or the field, or that some attribute changed, and is causing failures now.
This is the whole reason for production parts, as the manufacturer has now dialed things in to a point that the part will be stable for a number of years, and will not change. You design using the stated specifications, and run qualification tests to assure the parts will work and hold up in the field. So, you expect/need a part to be the same for a period of time.
Unfortunately, my past experiences so far with Seoul Semiconductor is that they are always fiddling with their production parts, much like you were getting evaluation samples as things are dialed in, which makes them more difficult to use than LEDs from Nichia, LumiLEDs, OSRAM, Toyoda-Gosei, or CREE.
One of the indicators of how well a manufacturer has a part dialed in is the consistency from part to part. I am not seeing that much consistency, imho.
Critical? These are production parts and are what they are. I've not formed an opinion on these parts yet, but it is forming. I'm just checking them out.
Gryloc said:
That color shifting in your videos are pretty extreme, like the first one (on you very latest set of videos). I am usually pretty cool over various tints, as long as they are not an ugly color, but the super blue that they turn at higher currents are pretty nasty for me (if the white balance on your camera is correct).
Well, the parts are much more consistent at 350mA as far as the tint from part to part. Above that, some parts start to shift drastically at 500mA and some go clear up to 1000mA before they shift much. There is also a lot of color shift over the current range.
If you go back and look at the Vf curve, you will notice there is some tilt to it, as compared to the CREE XR-E, where they both use the same CREE EZ1000 die. As an LED die heats up, it's Vf drops, typically something like 2.0 mV per degree C. This may be what is going on, is the die is getting hot, and the thermal resistance is not what is stated on the datasheet. Another item one can look at is the change in wavelength with temperaturature of the blue die, which I hope to get to.
On the fourth production part which I soldered down (instead of thermal epoxy like the other three), just like the other three, it shifted right when it got up to the current (some cases only 500mA), not something that happened after some use.
Gryloc said:
What are they doing wrong with the phosphors? You mentioned using a different type, but something seems wrong about the extreme shifts. It scares me because it reminds me of accidentally overpowering the old Luxeon I and it turns that angry blue like it will fail at any moment.
Wrong? They may just be using a different mix for the YAG than others. Phosphors is a big area of patent activity, as well as how it is mixed in it's binders, and how it is applied. All this affects how well it works.
I was hoping that using agressive thermal techniques would help this part, it doesn't.
It could also be that the thermal resistance to the die is not what is stated in the datasheet. YAG phosphors will typically start dropping in output around 60C die temperature, and continue to drop as the temperature rises. This is the material that makes the broad band yellow (as well as the rest of the spectrum), and with the blue from the LED, what we interpret as white. Some YAG mixes are better than others. For an example of how a typical YAG phosphor performs, look here:
http://ledsmagazine.com/press/14132
I don't design LEDs for a living, I just use them in my designs, and one needs to look at things carefully, or risk jumping in the cauldron. As far as what is actually going on with the Seoul P4 LEDs, one would have to talk with their designers.
Gryloc said:
The pictures of the P4 emitter is discouraging, too. Thought the actual quality doesnt seem too bad, the way it was designed is odd. People are against the "gummy" domes. If it is like the K2 domes, I would fully accept them. How close are the "gummy" domes of the P4 to the K2?
There is quite a bit of difference, and not so much, depending on your point of view. I have not had the production parts long enough to do any sort of actual abuse or life testing.
Gryloc said:
What could they have done better with the internals of the P4 above the die? I see how the soft gel diffuses and disperses the light in a lousy way. Why isn't it clear like Luxeon and Cree?
No idea, this is another area where the Seoul parts have had a good amount of variance. Depending on how you treat the light source, this could help or hinder, depending on your goals.
Gryloc said:
Its way too milky, as though they didn't allow the silicone with the yellow phosphor to cure before they gooped on the filler gel. Also, why does the bond wires have to bee so long that they arch up and get in the way?
As to the cause, there are a number of things that could make this happen. It could be the supplier, the ingredients the supplier used, the production process that Seoul is utilizing, interaction with materials/steps in the production process, and a number of other things. No real idea as to the root cause.
Gryloc said:
I bet with that long, narrow bond wire, there is some good resistance. Maybe not. Finally, is the conformal coating of the die (where the phosphor is perfectly square and even like the shape of the die) patented by Lumileds? Why did they use that puddle of goop-like phosphor and not like how even Cree did it?
Keep in mind, there are some optical effects that magnify things, however, they are long enough and tall enough to cast shadows or be in the image of the die in a reflector. Yes, LumiLEDs has a patent on the way they do the phosphor application, and at least two other ways of accomplishing the same thing.
Gryloc said:
Oh, one more thing. Newbie, when are you going to take measurements of the flux of these P4s? I would like to see how they compare to the XR-E, even if its a similar die. That would be cool. I suppose that even if the LED is horribly blue, and it is super bright, I might have to try some of them. Alright, good luck and thanks...
-Tony
If I get access, I'll do some lumen testing, but I am not promising anything.
Meanwhile, there is some variance from part to part, on the production devices I have. Some batches may be better, some may be worse. And since Seoul has a history of constantly fiddling with their parts/process, hopefully we will see the parts evolve or improve. A risk one takes is that unseen or unknown items may cause issues later in the field. Another risk is that you may get in another batch that are not suitable for your product. Luckily, for modders and small time flashlight companies is that it isn't that big of a deal, you can just use something else, or choose and pick from what is available.
It will be very interesting to get parts in from other supply points, and see if they show the same characteristics, or if the Seoul parts just typically have the same types of variance that they have had in the past. It will also be interesting to see how they do over time with their constant evolution on their parts.
