Heat management for 100 x WS2812 leds - Page 1
Forum section: electronics / projects

author: ealex
author_rank: Frequent Contributor
timestamp: June 19, 2014, 12:22:47 pm
content: HelloI've started designing a board that will hold 100 WS2812 leds in a 10x10 matrix. I'm used the datasheet from adafruit:http://www.adafruit.com/datasheets/WS2812.pdfI'm thinking of using a 100x100 mm board with 10mm margin for screws, etc.I've got the following thing to clarify:- heat dissipation: from the data sheet there is no info. regarding this, only some current for each led - 60mA in total. At 5V this will be 0.3W per part, 30W in total. Will the board area be enough to dissipate all that heat ? I intend to design the top layer with a finger pattern to have as much copper area as possible.- communication: i can address each row of 10 leds or i can chain all of them together. the second version sound better- power: it will go up to 6-7A at 5V -> big connector on the board or a buck smps from 12 / 24V ? the first version will keep the board simple, allowing for more free copper and will not add extra heat + i will not have to bother for now with the smps.Are my decision correct for those problems ? if you had the chance to work on something similar what were the problems you encountered ?This will be for a demo board for now, but it might get a small scale production run.

author: tom66
author_rank: Super Contributor
timestamp: June 19, 2014, 12:38:31 pm
content: Here's some real world data if it helps you: The WS2812B uncalibrated consume about 45mA each at full white which for my LEDs gave a slight bluish tint. A neutral (by eye) white point is reached at about 75% blue, 100% red, 100% green, which is about 38mA per LED. At this white for a couple of minutes they do get quite warm, about 25C above ambient when mounted in free space. When displaying ordinary RGB the current is very low (~10mA per LED for an average rainbow hue pattern) and subsequently they do not heat up much. The quiescent current per LED is about 800uA, which means that the all-dark 100 x LED array will suck about 80mA.The LEDs operate down to about 4.3V without loss of colour. If you don't need blue or can compensate for a slight loss in blue intensity, you can run them down to about 3.7V. This will all help to reduce dissipation in the control IC for the LEDs.Remember each LED needs a 100nF capacitor (0603 or 0805) near it, because they are digital devices switching at 400Hz for the LEDs and 800kHz for the data bus.I would use a local DC-DC converter. You could consider breaking the display into two halves and providing power separately. You can get monolithic DC-DC converters that supply 2A each with no external FETs - finding one to supply 4~5A is harder.Chain all the LEDs together. I am addressing 210 of these LEDs in a project of mine. I can update the entire display in 6ms plus the LEDs are intelligent and wait for a reset command before latching data. This means they change simultaneously. You can control them from an SPI port.

author: ealex
author_rank: Frequent Contributor
timestamp: June 19, 2014, 01:55:53 pm
content: That data is useful.I've found this module:http://www.farnell.com/datasheets/1785100.pdf, quite expensive but cheaper that unsing 2 smaller modules.I will use 100nF 1206 caps for each led and maybe some smd electrolytics for bulk.I'll post an update here once i have the board and the parts

author: tom66
author_rank: Super Contributor
timestamp: June 19, 2014, 03:09:56 pm
content: Have you ever designed a DC-DC converter? Try using TI WeBENCH.Considerhttp://www.ti.com/product/TPS54329Eorhttp://www.ti.com/product/tps56628These devices are very easy to use and use generally big components - smallest will be 0805 which is pretty easy to do. If you follow WeBENCH recommended design, and datasheet layout recommendation, it's really easy.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 19, 2014, 11:16:25 pm
content: Unless you have a very local PSU, You probably want a DC/DC, but if you are concerned about heat, you may want to mount it off the LED board, so you don't have the DC/DC and LEDs heating each other up.

author: ealex
author_rank: Frequent Contributor
timestamp: June 20, 2014, 05:44:17 am
content: I thought about adding a local DC-DC converter, but i don't have the experience to debug one if it does not work as expected - i could not find a chip that can handle all that power by itself.There are a lot of controllers that use external transistors - i might look into one, but understanding and debugging the feedback loops / compensation network might take too long => the easy way is to use an external PSU, at least until i get a demo running.It's a good idea to spend the time until i have the PCB and LED's trying to get something like this working - i have some UC3843 / IR2153 that i want to use in a power supply.A practical question: i can get a SMT stencil for the board + i have some solder paste available - what are the chances of damaging the LED's with a hot air gun ? i don't know if i can gen a hot plate big enough ... or maybe i can use the hot air gun to heat the board from the bottom ?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 20, 2014, 08:06:42 pm
content: Quote from: ealex on June 20, 2014, 05:44:17 amJust buy a DC/DC converter module. Plenty to choose from with outputs up to 20A and more

author: tom66
author_rank: Super Contributor
timestamp: June 20, 2014, 09:00:55 pm
content: As for hot air... I destroyed one of these LEDs with about 20 seconds of soldering. They are pretty sensitive. Sample size of one of course but I would be careful. Follow reflow profile exactly if you do hot air solder them.