I agree that replacing T8 flourescent bulbs with LEDs is a formidable challenge, especially considering the lumens/W rating of a typical 32W flourescent bulb. The cost to implement an LED solution that has a comparable lumens/W rating is much higher than that of a standard flourescent bulb. That is why, as of today, there is little support material created for this application as the LEDs have yet to reach the level of efficiency to make a solution that makes financial sense. But, LED technology is improving very fast as is the driver technology, and eventually we'll be able to create something that makes sense. Check back from time to time on our website for additional information.
Take a look at this brief article from 2010. http://ledsmagazine.com/features/7/6/6
I am wondering why there is a need to have the secondary-side electrolytic (or any capacitor) at all? If the flyback circuit is operating in a constant-power mode at tens or hundreds of KHz, is it not OK to allow the LEDs to "flash" at that rate? As long as the average LED current is constant, doesn't our eye integrate the light and ignore high-frequency flicker?
Hi Hubertus Notohamiprodjo,
Please can you consider writing article on direct replacement LED fluorescent without ballast transformer T8 versions. I am sure there many design challenges; especially, thermal design and safe operations which many of us would like to know more. I know there are many building owners and facility managers whom eager switch but there are concerns for security lighting uses and reliability. I liked you last article on LED lamps very much and thought you extend your effort to direct replacement LED fluorescent without ballast transformer T8 versions.
Merv Perry MBA MSc CISSP
Thanks for your nice comment on the article and for posting the link to your detailed article about electrolytic capacitors and their use in solid state lighting applications. The intention of this article is to bring to light the fragility of the electrolytic capacitors, the potential impact on the operating life of an LED-based luminaire due to this fragility and ways where active solutions can be implemented to maximize that operating life. Your article nicely covers the end-of-life characteristics of the electrolytic capacitors and it is important for engineers to understand the impact of the reduced capacitance rating as it reaches its end of life. This part of electrolytic capacitor characteristics was not covered in my article as it would have made for a much longer and more extensive article than was originally intended. Luckily, the iW3626 LED driver mentioned in this article has a patented power factor correction circuit that allows the designer to customize the design for high power factor, low output ripple current or a balance of the two, unique in single-stage, low cost off-line LED drivers. LED bulbs using this product can meet power factor and output ripple current requirements over the full operating life of the LED luminaire while protecting all components in the system thermally.
With regards to the exploded-view of the A19 type bulb in figure 1, take into consideration that this is a graphical rendering of an A19 bulb and not an actual photo, hence the reason why it may not look exactly like any A19 bulb that you are familiar with.
Nice article pointing out the key, and often overlooked, issue with electrolytics in LED lighting.
The lightbulb shown is one I am very familiar with. It is not an A19 but rather is an R20 - the smallest of the "CAN" lighting bulbs used for down-lighting. Note that the enclosure is a heat sink with a fair amount of surface area due to the fins. This was a substntial improvement over earlier LED light bulbs that had no fins thus causing the case temperature to be significantly higher.
The LEDs themselves, the largest source of heat, are mounted on the single metal piece that is the heat sink. This arrangement dissipates the heat pretty effectivey to the outside world.
The circuit in this particular unit I am familiar with - or at least the version that was in prouction a couple years ago. It used no electolytics. Also only components that were rated to 125C or higher were used.
I have a few of these in my house in ceiling cans - the worst thermal environment - and none of them have failed in over 3 years, including one that is on all the time - that is about 30,000 hours so far.
You have hit on an important point about electrolytics - the 2x lifetime for 10C.
however there are some more subtlties to the 10C vs. 2x lifetime relationship that are covered in my recent artilce in EETimes sister publication, EDN; http://www.edn.com/design/analog/4411475/Ensure-long-lifetimes-from-electrolytic-capacitors--A-case-study-in-LED-light-bulbs