SMD lighting led

Discussion in 'Lighting' started by, Aug 26, 2013.

  1. Guest

    My modest project consists in making myself a pendant lamp for my living room using linear lighting sources. I have questions and need your advice:

    It seems that low output 4000k white SMD led are more efficient that their high output big brothers. Could I get 1000 lumens out of a line of 10 or more SMD leds (philips), welded every 2 cm over a PCB, do I need a cooling device if I wish to have a correct lifetime (above 30000h with > 90% of initial light output)? Do I really need to buy a +100 euros power driver for those leds, or a cheap power driver would suffice ?

    I Also looked at 14w T5 tubular ramps with integrated electronic ballast supplied by a reputable brand name, and noticed that the power factor of theballast is only 60%. How is it so low while a longer tube, of the same product family boast a +90% PF ?

    Thanks in advance.
    , Aug 26, 2013
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  2. Also, some power LEDs are more efficient when underrun. Some of the
    Philips retrofit LED lamps use 3W emitters, running at 1W each, to
    improve the overal efficiency.
    In the EU, switched mode PSUs (of the type used in retrofit CFLs and
    LED lamps) don't need power factor correction if they are under 25W.
    Andrew Gabriel, Aug 26, 2013
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  3. Guest

    Thanks Andrew.

    But power leds are more expensive than SMD. If I underrun a power led, say 80% of the nominal forward current, do I still need heatsinks ? Because a factory made led bar with 20 SMD leds doesn't need heatsink yet delivers above 1000lm with 125lm/W at room temperature, is it realistic ?
    , Aug 26, 2013
  4. You'll have to read the datasheets to find what the power dissipation
    and operating temperature range are. LED efficiency is usually quoted
    at 25C junction temperature, which is pretty much unachieveable in
    practice. This is why LED lamps/luminares don't achieve the efficiency
    quoted for raw LEDs. When designing an LED light, the thermal design
    is often the most difficult part, and usually the limiting factor.
    80% of the forward current is probably going to land you with still
    something like 70% of the power dissipation (Vf drops a bit with
    reduced current, so the reduction in power isn't linear). It's also
    probably not enough drop to see a significant efficiency increase.
    Andrew Gabriel, Aug 26, 2013
  5. Guest

    On Monday, August 26, 2013 2:58:29 PM UTC+7, wrote:

    Reading datasheets, it seems that maximum efficiency is attained when leds are driven at 50% of the nominal current. Still the thermal resistance expressed in Celsius degree/W is hardly understandable to me: if a led with 6degree/W of thermal resistance runs at 1W, what does it imply ? How could I figure out the passive cooling system to be added to the led in order to keep the junction temp below the typical one, considering that the typical temperature of my room is 30 degrees ?

    By the way, I noticed that colour leds are far more efficient than white ones. Is it difficult to render white light using primary colours leds ?
    , Aug 28, 2013
  6. boxman Guest


    The thermal resistance tells you how much temperature rise you can
    expect across the LED junction from the amount of power flowing across
    it. In your case that means if you have 1 watt of power flowing through
    the LED then you junction temperature will be 6° higher than the contact
    point of the LED to it's mounting board. You want to keep the LED's
    junction temperature lower than it's maximum rated junction temperature
    to enable it to survive. If you want long lifetime performance it is
    better to design for a lower temperature like 80°C or so depending on
    the LED type etc.

    Once the heat leaves the LED, you will need to transfer it away from the
    LED to keep the temperature low by conduction. If you leave it in air
    for example, the air does not effectively transfer heat away by
    conduction and the LED will get very hot very quickly. To decide how
    big of a heat sink you need, you need to know how much power you need to
    dissipate and what temperature you are targeting. You can characterize
    that by the heatsink thermal resistance. For example this is a standard
    heat sink that you can buy that would give you close to 5°C/W in a small
    package. Also, you need to have a circuit board to power the LED which
    is another thermal interface that will have it's own thermal resistance.
    That's while you'll see metal core printed circuit boards used so
    often with LEDs rather than standard FR-4 boards because they FR-4 board
    can have rather high thermal resistances.

    Putting it altogether 1 led mounted to a metal core circuit board
    (assume thermal resistance of 2.5°C/W for example) which is then mounted
    to a heat sink and your ambient temperature was 30°C and you were
    running the LED at 1W your junction temperature would be 43.5°C
    (30+5+2.5+6+2.5). That's assuming you had good thermal contact between
    your LED, circuit board and the heatsink(usually requires high pressure,
    thermal grease or thermal adhesive).

    In the above example 43.5°C is pretty low, so the heatsink size is
    probably overkill. You could reduce it's size or choose another profile
    and get one with a larger thermal resistance that would still allow you
    to run your LED's cool enough.

    As for your other question, It can be difficult to render satisfactory
    white with color leds because their characteristics (color and
    efficiency) can vary significantly with both current and temperature so
    it can require complex circuitry to establish and maintain a particular
    shade of white.
    boxman, Aug 28, 2013
  7. Guest

    Thanks a lot for your reply. Now I can start my project.
    , Aug 29, 2013
    Anson Huang likes this.
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