# LED Forward Voltage VS Junction temp relationship



## pavithra_uk (Mar 12, 2012)

Can someone explain how to determine Junction temperature / increase in junction temperature by LED Forward Voltage ??


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## AnAppleSnail (Mar 13, 2012)

pavithra_uk said:


> Can someone explain how to determine Junction temperature / increase in junction temperature by LED Forward Voltage ??


Read the data sheet, and use current as the determining factor in your LED instead of voltage. LED forward voltage varies from part to part. If you drive LEDs by voltage then one LED may be underdriven while the other turns angry blue and dies. Cree datasheets have nice "Typical Vf vs Current" and "Vf change vs. Temperature" graphs.


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## Kinnza (Mar 13, 2012)

You need to know accurately how the LED varies Vf as temperature raises. 

Usually, that info is obtained by running the LED at very low current (or short current pulses, usually 25ms) on a hot plate. Once that factor is known accurately, it is easy to know increase of Tj just measuring drop in forward voltage from the cold pulse to the desired operating condition.


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## uk_caver (Mar 13, 2012)

Isn't there a 'burn in' effect on new (ie 'unsused', rather than 'recent') LEDs, where the Vf permanently drops as a result of an initial period of high-power running compared to the Vf of the new device?

If so, that would suggest device-specific calibration might need to wait until after a device had been used for a while.


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## VegasF6 (Mar 13, 2012)

Are you trying to use the drop in forward voltage as a means of measuring die temperature? I am not sure it's that exact a specification to go by, and it would take some pretty accurate measuring equipment. But, for instance in the case of the Cree MX-6 the data sheet lists "Temperature coefficient of voltage mV/°C -3.3"

So I would guess if you were using a constant current source and measured voltage at that current when first applied then measure it again at another point in time, you could figure how much temperature rise. For every .0033 volts dropped would equal 1 degree Celsius temperature increase. I don't know what other contributing factors might affect this though.


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## pavithra_uk (Mar 14, 2012)

Here what I did, I got Forward voltage of 3.20V as soon as it turn on (700mA drive), after 10 minutes, heat sin temperature rise to 60-65C degrees and Vfd drop to 3.17V. Room temperature is 30C degrees. 

Roughly 30C rise in temp = 30mV drop Vfd.


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