PK Design Lab
Results 1 to 14 of 14

Thread: What does "electronically isolated" mean when referring to LEDs and heatsinking?

  1. #1
    Flashaholic iamerror's Avatar
    Join Date
    Jan 2006
    Location
    Nevada, USA
    Posts
    253

    Default What does "electronically isolated" mean when referring to LEDs and heatsinking?

    Can someone give an explanation of this/what it is for and how it is achieved?

  2. #2
    Flashaholic
    Join Date
    Dec 2001
    Location
    Silicon Valley CA
    Posts
    241

    Default Re: What does "electronically isolated" mean when referring to LEDs and heatsinking?

    An LED is a special type of diode, which is the equivalent of a "one-way valve". It flows current in one direction, from the + side to the "-" side.

    The big LEDs have a heatsink to take the heat away, and the heatsink on Luxeons is connected to the + side of the diode.

    On Cree's, the bottom of the LED has a layer of insulating material so that the + and "-" side are not electrically connected to it but the heat still has a path to escape.

  3. #3
    Flashaholic iamerror's Avatar
    Join Date
    Jan 2006
    Location
    Nevada, USA
    Posts
    253

    Default Re: What does "electronically isolated" mean when referring to LEDs and heatsinking?

    Thanks for trying to explain, although I am still not getting it. What happens if the LEDs are not electronically isolated? How are the LEDs made isolated if they do not have the insulation (like the Cree). Which LEDs have or do not have the insulation?

  4. #4
    Flashaholic
    Join Date
    Dec 2001
    Location
    Silicon Valley CA
    Posts
    241

    Default Re: What does "electronically isolated" mean when referring to LEDs and heatsinking?

    You will get "mixed results" depending on the polarity of the heatsink (positive or negative). This is an issue with Lumiled Luxeons, and Not the Cree XR's.

  5. #5
    Flashaholic iamerror's Avatar
    Join Date
    Jan 2006
    Location
    Nevada, USA
    Posts
    253

    Default Re: What does "electronically isolated" mean when referring to LEDs and heatsinking?

    How does one fix this polarity issue when designing something using Lumileds Luxeons?

  6. #6
    Flashaholic
    Join Date
    Dec 2001
    Location
    Silicon Valley CA
    Posts
    241

    Default Re: What does "electronically isolated" mean when referring to LEDs and heatsinking?

    You have to keep the back of the Luxeons from direct contact with the heatsink (which is typically aluminum or copper) by using a thin layered thermal transfer sheet or by physically keeping the LED above the heaksink (just barely) with plastic film (tape with a hole in the center for the thermal goo) or something. I've done this and used the usual heatsink white goo in between. It is a tricky process.

    If you are using a converter board to drive the LED, you can directly place the LED on the heatsink as long as the + and " - " leads of the LED are isolated from the heatsink.
    Last edited by LightBright; 12-18-2006 at 11:16 AM.

  7. #7
    Flashaholic* FirstDsent's Avatar
    Join Date
    Jan 2006
    Location
    Columbia, South Carolina
    Posts
    560

    Default Re: What does "electronically isolated" mean when referring to LEDs and heatsinking?

    "Electrically isolated" means that the base of the LED is not electrically connected to the circuit. It is essentialy insulated from the circuit.

    In the case of Soul Semiconductor's new P4 LED, and their whole Z-Power line, the bottom is not electrially isolated. It is electrically connected to the positive side. That means that when you attach it to your metal heat sink, you have attached the positive side of the LED to your heat sink. That is a huge problem because in most flashlights, the heat sink is negative. It is connected to the body which in turn is connected to the negative side of the battery through the battery spring. That's an instant short circuit. To use this LED, you have to find a way to insulate the bottom of the LED from your heat sink, or reverse the polarity of the whole flashlight.

    I suppose an even layer of Arctic Alumina brand thermal epoxy would insluate the base, and conduct heat. Arctic Alumia is an insulator, however one would have to be very careful to keep any part of the base from contacting the heat sink. AA thermal epoxy also does not conduct heat as well as metal to metal contact, so thermal conductivity will be less efficient.

    If I had to mount a X-Power LED to my metal heat sink, I would put a thin, smooth, and even coat of AA thermal paste on the bottom of the LED base and let it cure. Then I would use more to mount it to the heat sink. Then I would be assured of insulating the LED from the heat sink.

    All white Luxeon LEDs are electrically isolated. It doesn't matter how you mount it. There is no electrical path to the base of the LED. The Cree XR-E and the whole X-Lamp line are also electrically isolated. There is more flexibility in how to mount them because you don't need to worry about the electrical path.

    On the other hand, If our favorite LED had a base that was electrically connected to the negative side, mounting it to a metal heat sink would automatically ground it. You would only have to run a positive wire to the LED. That would be great. No such luck though.

    Bernie
    Last edited by FirstDsent; 12-18-2006 at 11:18 AM.
    If it's worth driving, it's worth overdriving!

  8. #8
    Flashaholic
    Join Date
    Dec 2001
    Location
    Silicon Valley CA
    Posts
    241

    Default Re: What does "electronically isolated" mean when referring to LEDs and heatsinking?

    That's funny - in the pdf for the Lux III, it says right there - " the slug is NOT electrically neutral, DO NOT connect either the anode or the cathode of the LED to the slug...."

  9. #9
    Flashaholic*
    Join Date
    Apr 2002
    Location
    Boise, ID
    Posts
    2,639

    Default Re: What does "electronically isolated" mean when referring to LEDs and heatsinking?

    All white Luxeon LEDs are electrically isolated. It doesn't matter how you mount it. There is no electrical path to the base of the LED.
    NO - that is incorrect. Luxeon slugs are NOT isolated.

    See this thread: http://candlepowerforums.com/vb/showthread.php?t=91700

    (I'm beginning to sound like a broken record)

  10. #10
    Flashaholic* EngrPaul's Avatar
    Join Date
    Sep 2006
    Location
    PA
    Posts
    3,677

    Default Re: What does "electronically isolated" mean when referring to LEDs and heatsinking?

    It means you can combine the heat sink pad with either a (-) connection, a (+) connection, or heat-sink only without connecting to either.

    You have all these options, but of course you cannot connect to both (+) and (-) at the same time.

  11. #11
    Flashaholic* FirstDsent's Avatar
    Join Date
    Jan 2006
    Location
    Columbia, South Carolina
    Posts
    560

    Default Re: What does "electronically isolated" mean when referring to LEDs and heatsinking?

    Quote Originally Posted by evan9162
    NO - that is incorrect. Luxeon slugs are NOT isolated.

    See this thread: http://candlepowerforums.com/vb/showthread.php?t=91700

    (I'm beginning to sound like a broken record)
    Thank you for setting me straight Evan. Your reply is intriguing. My belief was based on the fact that nobody (including myself) has had any problem mounting bare white Luxeon emitters directly to the electrical path of the flashlight. Nor did I recall any discussion (other than your 1995 thread, which I never saw before) about the slug being electrically connected. I took it as given. Regardless, The answer that iamerror was looking for is, does he have to worry about shorting his LED. In the case of white Luxeons, the answer is no. So when I said "There is more flexibility in how to mount them because you don't need to worry about the electrical path", I stand by it, only now with an asterisk.

    I read your thread. Most of it was well over my head. Since all white Luxeons are binned at at least 350mA, what is the reason for this bizzarre connection vis a vis zener diode? Does it have any application in a flashlight? If an electrostatic charge is passed to an aluminum flashlight resting on a grounded surface, it won't pass through the middle of the light, it will track around the case right?

    Bernie
    If it's worth driving, it's worth overdriving!

  12. #12
    Flashaholic iamerror's Avatar
    Join Date
    Jan 2006
    Location
    Nevada, USA
    Posts
    253

    Default Re: What does "electronically isolated" mean when referring to LEDs and heatsinking?

    Thanks for the replies, this thread is interesting and helpful. When you said you could use a "thin layered thermal transfer sheet", were you specifically referring to using a thermal glue such as Arctic Alumina? You also said you could use tape, what kind of tape would be good for something like this (maybe a brand or product name)?

  13. #13
    Flashaholic*
    Join Date
    Apr 2002
    Location
    Boise, ID
    Posts
    2,639

    Default Re: What does "electronically isolated" mean when referring to LEDs and heatsinking?

    The answer that iamerror was looking for is, does he have to worry about shorting his LED. In the case of white Luxeons, the answer is no.
    But this is still not true. THere are many circumstances that even a single luxeon can fail if the slug is not isolated.

    If the slug gets connected to the power supply's positive node, then it will forward bias the ESD diode on the negative terminal, causing a high current short through the slug, through the ESD diode, and through the negative terminal, burning out the negative bond wire and destroying the LED.

    If the slug is connected to negative and the LED is floating with respect to negative because of the power supply topology, you could bypass the negative return path of the power supply in favor of the built-in ESD diode, causing the LED to be dim (at best), not light, or again, destroy one of its bond wires, rendering it useless.

    Finally, if the slug is connected to negative, and the power supply input is reversed (i.e. by inserting batteries backwards), it will forward bias one of the two ESD diodes, resulting in the first failure mode described above. While it takes ~2.5V to forward bias the luxeon (the first point it starts producing light), it only takes ~0.7V to forward bias the ESD diodes - so even a small voltage applied in this way can lead to damage.

    Since all white Luxeons are binned at at least 350mA, what is the reason for this bizzarre connection vis a vis zener diode?
    The ESD diodes have nothing to do with the current rating of the LED. They are there to shunt ESD charges away from the junction to somewhere safe. The ESD diodes will conduct before the LED will, dissipating the static discharge energy instead of the LED taking it. It is entirely possible that electrically connecting one of the power terminals to the slug actually reduces the effectiveness of this setup, causing the device to be more succeptible to an ESD discharge, increasing the likelihood of damage.

  14. #14
    Flashaholic
    Join Date
    Dec 2001
    Location
    Silicon Valley CA
    Posts
    241

    Default Re: What does "electronically isolated" mean when referring to LEDs and heatsinking?

    Quote Originally Posted by iamerror
    Thanks for the replies, this thread is interesting and helpful. When you said you could use a "thin layered thermal transfer sheet", were you specifically referring to using a thermal glue such as Arctic Alumina? You also said you could use tape, what kind of tape would be good for something like this (maybe a brand or product name)?
    The thermal transfer material is called Thermally Conductive Elastomeric Film, and probably other names too.

    Yes the Arctic Alumina works great, you just have to find a way to keep the LED slug from actually touching the heatsink. The thermal transfer goo works better the thinner it is (without touching).

    Tape - If you look at the back of a Luxeon, there's the metal slug, and the surrounding black plastic. I use tape and such, ONLY under the black plastic, to keep the slug from touching the heatsink. Then you can use Arctic Silver or such as a thermal transfer goo under the center of the LED.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •