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We Need Cheap LED Drivers

On our sister site All LED Lighting, my colleague Keith Dawson (or K-Dawg as we affectionately call him) often talks about different versions of line-powered LED lighting. There is a lot of work being done on various types of LED light “bulbs” and fixtures — architectural, down-lighting, display case… I'm most interested in residential, incandescent bulb replacement lamps (the big market item). And I am specifically interested in whether anyone can make a dimmable LED lamp inexpensively.

There are several large companies manufacturing LED devices intended to replace the standard A19 incandescent bulb. Cree and Philips are probably the leaders. Prices are dropping, but for my money, they still have a ways to go. At the present prices, the ROI (cost of LED + cost of electricity vs. cost of incandescent) is still too steep.

Let's take a closer look at the problems being faced. To get a lamp that has the proper color temperature and properly renders surrounding objects' colors is tricky. Add to that the requirement that it must work properly on a triac or phase-cut dimmer… well, it ain't easy. If the cost of the drive electronics could be sufficiently reduced, the manufacturers might be able to hit the Planet Analog price sweet-spot. With sufficiently low-cost drive electronics, it would be easy to build lamps with a tri-color LED array. That way, you could get whatever color you wanted.

Let's look at the drive electronics. Based on the applications notes I've seen, some suggested circuits use a flyback topology switch-mode power supply. Some (though not all) of the suggested circuits use two or three ICs, a power FET, and the flyback transformer. Additionally, there are easily one to two dozen passive components.

If we could do this with a buck switcher instead, we'd only need an inductor instead of a flyback transformer. It would require two FETs if it's done as a synchronous switcher, although efficiency might be just fine if done as a non-synchronous switcher.

The real way to take the cost out is to push all this circuitry onto one IC. Can this be done? Note that besides being triac dimmable, the IC must tolerate line voltage. Assume that it's intended for “universal” input, so that means 240VAC, 340VDC nominal, or around 400V with a suitable margin (more if you want lots of margin). The output should be constant current, so there should be a way to monitor the output current. If that can be done without putting a resistor in series with the LED string and monitoring the IR drop, all the better.

This is a lot to ask for, but if someone can do it, the market is huge. Even at a slim margin, the multiple millions of units that would be sold will make for high sales dollars. Let us know your thoughts on this.

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31 comments on “We Need Cheap LED Drivers

  1. etnapowers
    September 11, 2013

    I have engineered many switching regulators to feed integrated leds, for their properties of exceptional brightness and clear colours. The problems to face in this development phase are very similar to the discrete LEDS case.

  2. Scott Elder
    September 11, 2013

    Brad, I think the problem is bigger than just the IC or even the cost of the LEDs.  Namely, the reliability of all the components needs to be equivalent to that of the LED.  The LED reliability is based solely upon low voltage stress.  Any IC will feel the ugliness of the power line.

  3. eafpres
    September 12, 2013

    Another factor to be considered is the efficiency of the driver.  Although the market is full of LED-based replacement bulbs, many of them are not very efficient.  The LEDs themselves are improving in efficacy all the time, The drivers need to have as little loss as possible to avoid wasting some of the advantages of LED lamps.

  4. etnapowers
    September 12, 2013

    @eafpres, I agree with you and I add that the reliability of the connectors driver-bulbs is another important factor to ensure the overall perfomance of the LED-based bulbs

  5. etnapowers
    September 12, 2013

    To integrate the circuit it is necessary to switch to a non insulated topology, so a proper circuitry is required to protect the system from spikes present on the AC supply.

  6. etnapowers
    September 12, 2013

    The buck topology requires a proper protection of the inductor because it is very sensitive to spikes and stress, it's the core of this system and its reliability is a key factor. 

  7. etnapowers
    September 12, 2013

    The flyback converter is just realized in an integrated DC/DC version, to complete the integration of the whole circuitry, an AC/DC converter should be added. This would solve the integration feasibility issue.

  8. kdawson
    September 12, 2013

    It's common wisdom in the LED lighting business now that the LED is rarely the limiter on the lifetime of a luminaire; usually it's the driver, and often it's a single component of the driver: an electrolytic capacitor. We have written about this quite a lot over at All LED Lighting — go there and search for “electrolytic” for the corpus.

    And by the way, to my knowledge only one person calls me K-Dawg. I tolerate it out of respect.

  9. samicksha
    September 13, 2013

    You are right eafpres, we cannot ignore the fact that its electrical properties change with temperature which can lead to poor performance or crash.

  10. etnapowers
    September 13, 2013

    The temperature effect has to be accurately monitored on each component of the system to avoid malfunctioning, the packaging is a really important step  of the engineering process. 

  11. etnapowers
    September 13, 2013

    The temperature could affect the correct working of the system not only for high temperature values but also for low operating temperatures: the presence of condensation is really an issue to be faced when the temperature of the ambient is very low.

  12. Davidled
    September 15, 2013

    Also, manufacturing variants and aging are carefully considered.  LED output could vary in the different production line so that a different light quality could be produced. In the course of time, quality is degraded.  To overcome these issues, intelligent controller might be used with LED drivers.

  13. RedDerek
    September 15, 2013

    How about this for an LED dimming concept… Use a detector to measure the brightness. Then adjust the drive to match the dimming requirement. At build, one would know the maximum intensity. Then below a certain dim voltage, turn the LED off (I have some dimming LEDs that do not go off on a digital dimmer because the circuit still sees voltage.). It would require a detector circuit, but it can be cheap. Thus the detail drive dimmable LED circuits are now can be reduced drastically.

    Integrate the detection and dimming into one circuit. Some biasing needed for the maximum brightness detection. But it could be cheaper as a fully integrated circuit.

    Hmm… patent idea here?

  14. etnapowers
    September 16, 2013

    I agree, also the production process has to be controlled by monitoring the most important parameters, this will improve the quality

  15. etnapowers
    September 16, 2013

    For sure this is a very interesting idea, however you have to integrate all this function and ensure the reliability of the combination dimmer+detector

  16. etnapowers
    September 18, 2013

    The led arrays are a new interesting solution utilized in automotive applications, having some led drivers that are at the same time reliable and cheap will be really promising for a led driver maker

  17. alziedood
    September 18, 2013

    Hey, the Cree is 10$ for a 40W equiv.

    I believe that the break even is only 1 yr, and

    they are triac dimmable.

    We have arrived.

     

    Re. xfmr operated, there is a goofy UL / CE requirement for isolation, even though

    incandescents dont have to worry about it, huh?!

    Either you put the cost into the xfmr or

    the extra insulation around the led array.

     

    Anyway, Cree has got it down.

    I have a few of them and

    the color is indistinguishable from incand.

     

  18. Brad_Albing
    September 18, 2013

    @Scott – you're right – and the power line is a harsh mistress.

  19. Brad_Albing
    September 18, 2013

    @kdawsonedn  -Yep, those pesky electrolytics. I know the IC guys are working on versions of their devices that will work OK with ceramic caps. They typically have lower ESR, so that means you need to redesign the loop for stability. Voltage ratings and capacitance vs. package size becomes an issue with the ceramics too. But if you can tolerate lots of 120Hz ripple at the LEDs there are work-arounds.

    I'll check my notes and see who the scoundrel is who insists on calling you by such an outrageous nom de plume.

  20. WKetel
    September 18, 2013

    More important than making the things cheap is making them reliable. A $20 light bulb that never fails does a lot more for the adoption of LED lights than a $7.50 device that fails after a few weeks. So that is the thing, which is that they must be reliable and not fail. 

    As for dimming, that is more complex a requirement, but still, even with the dimmable units, reliability should trum price, if the vendors don't get to very greedy.

  21. eafpres
    September 19, 2013

    @alziedood–the numbers are favorable for the $10 bulb, as you say.  It does, of course, depend on the use model.  If a lamp is on 8 hr/day, the LED wins even if only 3x the efficacy of the incandescent.  Yet the consumer says “I can buy 10 40W bulbs for the price of 1 of these (or maybe 20)”.  

    So as already stated, the real reliability is a big factor.  If you assume you buy it and never replace for 10 years, life is good.  If it fails in a year (or less), not so good.

    Regarding the color, if you like how it looks, that is fine, but the spectrum of incandescents is more pleasing, and shifts appropriately with dimming.  The LEDs at higher outputs, due to use of blue LEDs to pump yellow phosphor, etc., put out a peak in blue, which is bad for your sleep cycle.

    My view is that on a pure numbers basis the LEDs are there (if you don't buy junk–of which there is a lot), but we are still in wait and see on reliability, and many other factors such as true CRI and spectrum are still to be improved.

    I'm not convinced that cheaper is better right now.  In fact, there are some cheap LED bulbs that may be worse than incandescents!

    You might be interested in a survey article I did on a sister site:

    Internet Offerings of A19 LED Bulbs

  22. Netcrawl
    September 19, 2013

    Reliability always win here not cost, Pixi A19 LED is the ideal alternative to traditional incandescent and flourescent, Pixi is a world leader in LED so I go for the company's profile and track records when picking the right stuff. I'm not convinced of the idea that being much cheaper is a better right now, its not about cost therre's also something to do with reliability and efficiency.  

  23. alziedood
    September 19, 2013

    The problem with triac dimmers is that

    they need 50mA of holding current.

    That equates to 6W of pure resistance.

    Led lamps are on the edge of not being able to hold the triac.

    Eg. dimming can be strange.

     

    Whats also needed is a ckt that mimmics resistance

    when the triac is off in betweeen firings

    to maintain its phase shift mechanism.

    So, working with dimmers is difficult, but

    they have this stuff resolved for the most part.

     

    As above, the red spectral shift with dimming would be nice, but

    its a low priority over all.

    The non shift is some thing that we ll just have to get used to,

    just like when we gave up fire for illumination.

    Hey, did kero lamps red shift on dimming?  I dont think so.

     

    I d much rather have a reiable lamp

    thats cheaper in the Big picture than any thing else

     

  24. Brad_Albing
    September 19, 2013

    @WKetel – Good point regarding the reliability – we've seen exactlyb that issue with the cheap CFLs.

  25. Brad_Albing
    September 19, 2013

    @RedDerek – Maybe. I'm not sure you would need to monitor the light output to effect good dimming. Those darned triac (phase cut) dimmers are tough to deal with as you mentioned. But that just indicates a bad design. The designers should have set it up so that (e.g.) once the conduction angle is down around 5 degrees and the peak applied line voltage is down to around 15V, that would be the fully off point. Ramp up from there to full on at something just shy of 180 degrees.

  26. WKetel
    September 19, 2013

    Why in the world does anybody like that dismal brown-yellow color of fading lights?  What sort of marketing idiot decided that LED lights must emulate the very worst features of incendescents? Just let the light get dimmer, if you must. Or else, here is a radical concept, have an array of LED devices and switch some of them off for dimming, which it certaiinly would reduce the light output and not change the operating point. PLUS, switching off leds would make them last longer. And if you really wanted the light to change colors, have the remaining devices be red and yellow, and switch off the blue and white sources at the start of dimming. Of course it would add more devices to the BOM, which will probably doom the idea since it is not as cheap as the five-component light source that is the goal of many. BUT it might last for 20 years if it was done well, which would justify a somewhat higher price. Other than that, what about a much simpler system that winds up just verying the voltage over that narrow range where the LEDs put out light? Of course it would need a higher quality capacitor as the filter, so it would probably not be economical. But a simpler circuit might be the way to go.

  27. Brad_Albing
    September 26, 2013

    @WKetel – re dismal brown-yellow color of fading lights – I suppose we are just used to it, so we want what we've always had (except better, of course). Perhaps it draws from our memory of illumination by candles/oil lamps – somewhat romantic in its nature.

  28. WKetel
    September 27, 2013

    “Memory of illumination by oil lamps”? I have no such memory since that was well before my time, and in addition, my dad lived in an electrified neighborhood all of his life. My mother did live with the kerosene lamps when she was a child, but she never had anything good to say about them or about the dim light that they provided.

    It is primarily a marketing thing and the result of too many of those “Martha Stewart” types of opinion makers telling the gullible folks what they should want. Of course there are times when dim light is appropriate, but it does not need to be that yellow dim light. And if folks do want that color of light, well, red and yellow LEDs have been around for a long time. 

    So the simplest way would be to have brighter white lights and dimmer tellow-red lights and just use whichever one felt like. Why in the world attempt to produce a “one size fits all” product when the reality is that if one size really did fit all, it would not fit any very well. And you can certainly quote me on that.

    But reliable dimmers, or at least reliable drivers, are what is really needd for the LED lighting trend to really take off.

  29. Brad_Albing
    September 30, 2013

    @WKetel – When I referred to “memory” I meant that as genetic/racial memory. The idea of fire as illumination goes back quite a ways with humans, so I assume that explains pleasure in that form of lighting once the sun goes down. And of course many people perceive it as romantic. But that's not for me to presume.

  30. WKetel
    September 30, 2013

    I understand the theory, and it is sort of amusing. My ancestors were vikings and berserkers, two very colorful groups indeed. I avoid any activities such as they might have done. And I agree that dime firelight could be romantic, but at work romance is just not on the agenda. And at home there are a lot of activities in many areas that need a much less romantic kind of illumination. I really prefer to be able to see what I am doing, which normally requires full spectrum illumination, or at least reasonably white light, and enough of it.

  31. Steve K.
    October 16, 2013

    I haven't seen any mention of items such as Seoul Semiconductor's Acrich 2 LED module, or the Supertex Inc. CL8801 “sequential linear LED driver”.   Both accept AC line voltage and directly drive LEDs without need for a switching regulator. 

    For about a year now, I've been using a table lamp equipped with three of the Acrich 2 eight watt modules.  It replaces a CFL that was equivalent to a 100W incandescent, and I've been quite happy with it.  With no weak points such as electrolytic caps, and with good heatsinking (did I mention that I did the heatsinking and packaging myself?) it should last for a decade or two at least. 

    I'm not aware of any products utilizing the Acrich 2 modules or the Supertex CL8801, but they must be out there somewhere.

    Steve

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