LEDs and Temperature

Just reading of LED needing to be kept cool and I was wondering why. Is it because standard components are used which will fail, or is there some electrical principle that will always cause failure at a particular temperature? Could some re-design make them happier at higher temperatures?
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DerbyBorn wrote:

All semiconductors need to be kept relatively cool. A max of about 200C comes to mind from years ago.
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Germanium was very bad indeed, you could not get that semiconductor very hot before thermal runaway happened. I don't know the figures for Galium arsanide and the other materials used in LEDs. Brian
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On 09/11/16 16:21, Bob Minchin wrote:

200C storage. Genereally 180C operating junction temps.
But lifetime is a lot less at that sort of temp.
Dopant actually migrates through the semiconducting barriers as they age, and heat speeds it up.
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Capacitors too have a shorter life with heat. And SWPS rely on fairly high stressed caps.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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I have some kitchen downlights (Halers) where the driver assembly is separate from the lamp unit. They have a 7 year warranty - I guess this is the way to go. Imagine the electronics in a "ceiling rose) and the lamp on a pendant. I guess that would work. However, makers have felt pressure to make everything fit a conventional lamp envelope.
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On 10/11/2016 10:07, DerbyBorn wrote:

Yes, we're in a transition phase at the moment. The problems are really with LED lamps which have to retrofit an existing, often unsuitable, luminaire. The purpose made LED lights should be much better in this respect, and with the greatly improved lifetime, replacing a whole unit rather than just a lamp shouldn't cost more in the long run.
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True. And the same applied to CFL. Making any energy saving type which needs a power supply of some sort with the PS built in is invariably a compromise. I'm rather surprised new standard lighting wiring for LEDs - perhaps with a central power supply or two - hasn't been devised.
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On 10/11/2016 10:43, Dave Plowman (News) wrote:

It has, they are 12V usually.
For some reason people fit 240V ones instead.
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I've not seen much of a range of 12v LEDs - apart from MR16 and those made for cars. Can you give a link to a site that sells them? But a 12v LED would also include a power supply of some sort.

They're likely to want something looking like they're accustomed to see.
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wrote:

http://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR12.TRC2.A0.H0.X12v+led.TRS0&_nkw v+led&_sacat=0
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wrote:

Already there, often with 12V for the LEDs themselves.
Same with low voltage halogens.
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But this does not solve the problem. Bare LEDs (the diodes themselves) are not happy at a fixed voltage, they need circuitry to control the current through them. So a 12V 'LED bulb" is little different from a 240v one in the complexity of circuit required.
And even if it was possible to run LEDs from a remote power supply, there would still be the problem of a keeping the actual diode, itself a semiconductor, adequately cooled.
The only good engineering solution is a light fitting containing the circuitry to work from a fixed voltags supply (which might just as well be 240v AC as anything else) and designed to allow appropriate cooling both for the LED and the associated electronics.
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On 11/11/2016 11:41, Roger Hayter wrote:

A DC supply would be better as you wouldn't need relatively large electrolytics in the internal LED switch mode PSU which have a limited life or a high cost. If you're using DC, 48V would be a good choice as it's high enough not to need too thick wires (given that LEDs use less current than incandescents) and low enough not to be a shock hazard.
No point replacing existing wiring, but maybe one day for new buildings.
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Is it reallly possible to design a DC to DC converter that is both stable and efficient without using large value electrolytics?
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On 11/11/16 12:44, Roger Hayter wrote:

Yes up to a point.
But you need to go quite high in frequency which makes some parts cost more, RF emissions get to be an issue, and efficiency can drop as well.
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On 11/11/2016 12:44, Roger Hayter wrote:

It probably wouldn't be done that way. You can design highly efficient voltage to constant current converters which provide the drive that LEDs actually require. They are current based devices with a terminal voltage that varies somewhat with applied current and temperature.
Crude resistor based current limiting is used in some torches but chips to do this job cheaply and efficiently are quite common now.
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They still reduce efficiency and produce heat unless there are multiple LEDs which need a voltage near to the supply. Maybe that this the way to do it, like the LED strips.
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Roger Hayter

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wrote:

Yes, but you don’t need to when you have a central power supply or two and have that isolated from the heat that the LEDs produce.
That’s how the LED strips work, the power supply is physically separate from the LEDs.
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On 11/11/2016 11:41, Roger Hayter wrote:

For a constant current drive then yes, but one advantage of a lower starting voltage is that some multiple of LED voltage drop plus a simple passive resistor can be made to work without any failure prone electrolytic capacitors. Plenty of torches use this method although an increasing number now use a cheap small flyback step up current source and just a single cell battery.

The problem invariably boils down to thermal management of the waste heat generated in the LED chip itself otherwise the phosphor gets degraded and they are less efficient when run hot. It isn't for nothing that manufacturers maximum output claims are based on 25C and a nearly infinite heatsink compared to what a consumer lamp will have.

It is invariably the capacitors that go first when used in existing luminaires designed for hot incandescent bulbs.
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