# Measurment of Vf vs Tj



## space (Feb 12, 2009)

I did an experiment where I measured the Vf vs Tj for a Cree XR-E R2 (at least what it is stated as at DX) at 350mA.
The LED was mounted on an old CPU heat sink as seen in this picture:






In the heatsink I've embedded a type-K thermocouple. Power to the LED was provided by a CCS. Vf-measurement was taken at the LED-MCPCB. Current (via a shunt), Vf and temp was logged with a data logger (Agilent 34970A).

The LED was left on for roughly 5 min before start of logging. Then the heatsink was heated with a hair drier to 60 deg. C. Then I let the heatsink cool of by placing it outside my window (amb. temp -10 deg. C)

Tj was estimated by adding 10 deg C to the measured Tsink.

Plotting the data gives the results...





The experiment gave a Vf to Tj relationship of roughly -6mV / deg C.

space


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## marcopolo (Feb 13, 2009)

Superb experiment! That was exactly the information I was looking for

cheers.


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## ifor powell (Feb 13, 2009)

Very nicer work. What I would like the to know is how much variation is there in this between leds? If you had the inclination to test a few more leds that would be very interesting.

Ifor


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## JWP_EE (Feb 13, 2009)

I get -4mV/C for the slope of the line on your graph. How did you get -6mV/C?


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## space (Feb 13, 2009)

You are right. -6mV/C is only valid at Tj~0C. The slope is increasing as the temp increases. Taking the 1. deravative gives the exacts slopes for the different temps. This gives values in the measured areas from -5.5mV/C to -2.8 mV/C :







space


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## Justin Case (Feb 15, 2009)

Great stuff and consistent with the Cree XLamp7090 docs, which give the value for "Temperature coefficient of voltage - white, royal blue, blue, green" as -4 mV/°C.


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## JWP_EE (Feb 15, 2009)

When I first saw the graph I thought -4mV/C was too high. Some time ago I used -2.8mV/C as a measure to determine how well a heatsink I was using on a P4 star was performing. I got that number from a CREE data sheet. The sheet is for the XLamp 7090XR-E white only and is dated 2006. It list -2.8mV/C.

After reading the post above this one I went back and reviewed the data sheets I have. I found a newer one for the different color 7090XR-E LEDs and dated 2006-2007. It list -4mV/C. Thats a 43% increase.

I am not sure why it has changed. Did they find a better way (more accurate) way to make this measurement?


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## space (Feb 17, 2009)

I've done a new test at 700mA and it shows some interesting results in that the Vf is even more dependant on Tj. This really underline the fact that the harder one drives LEDS, the more important current and temperature controll becomes.






space


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## Daekar (Feb 18, 2009)

So, let me see if I've got this right... I think there are a couple of important points:
1) You obtained your data using a _constant current_ regulated power source. Regardless of what the voltage did, the current would stay the same.
2) The derivative curves indicate that, as Tj increases, the drop in voltage per degree C of Tj decreases... so at 350mA and 10C, there is a 5.4mV/C decrease in voltage, whereas at 350mA and 70C, there is a 2.4mV/C drop in voltage.
3) The slope of the 700mA derivative curve is less steep than that for the 350mA curve, and further down the scale, meaning that A) the higher the current is, the less temperature affects the decrease in Vf per C, and B) the higher the current, the greater the decrease in Vf per C.

So from an application standpoint, what do these graphs mean? Well... it looks like Vf goes up as temperature goes down... meaning power consumption decreases as the temperature increases. But... total flux also decreases as temperature increases. So might there be a happy medium?


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## space (Feb 18, 2009)

Thanks for your throughout analysis of the graph. 

Your point nr 3) made me realise I've made an error in the 1. derivative curve for the 350mA test. I've redone the graph below with what should be the correct curves. 

It now shows that both derivative curves are equally steep. The only difference is that the 700mA curve is levelshifted to lower values, meaning higher temperature dependence for Vf.







space


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## space (Mar 22, 2009)

A short update for adding a measurement done at low current (17.2mA) setting. It shows dVf/dTj of just under -2mv/C.







space


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## eprom (Mar 23, 2009)

Hi Space,

First thanks for your very informative effort,
I think I missed something about Tj estimation for 700ma test, How many C added to Tsink for estimated Tj

If we add 10C for 350ma test, we have to add ~30C for 700ma test. So if we also add 10C for 700ma test, 20C difference will carry the 700ma test curve horizantally.

.


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## space (Mar 23, 2009)

I've estimated Tj from Tsink + 10 C/W * Pinn. Tsink and Pinn are measured. The Thermal resistance of 10 C/W is guestimated.

This translates to:
(lowest to highest temp)
~ +0.5C for 17.2 mA
~ +11.3 to 10.3 for 350 mA
~ +23.4 to 21.5 C for 700 mA


space


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## zzonbi (Dec 15, 2009)

Looks like dV/dT depends mostly on V.

My idea was that between I, V, F(flux) and T only 2 are independent. If this is consistent for a given led model, Vf looks useful to estimate Tj once we know a (Vf,Tj) pair (say the starting point at some constant I). Then we have all 4.


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