Re: OSRAM LUW HWQP data sheet;
http://www.osram-os.com/Graphics/XPic4/00151034_0.pdf/LUW HWQP - OSLON Black Flat.pdf
With I
fwd = 1A and V
fwd = 3.35V the LED will have 3.35 W power dissipation.
Knowing from the table on page 4 that the thermal resistance is 5.5°C / W.
We can expect a temperature rise of 5.5°C * 3.35W = 18.5°C
To limit junction temperature to 125°C requires the environment remain below about 105°C.
Relative Forward Voltage versus temperature is shown at the top left of page 10.
It shows that relative to 25°C, at the maximum operating temperature of 125°C, the voltage will have fallen by only 0.18V
This is less than the manufacturing tolerance grades, 8F, 8G & 8H listed at the bottom of page 5.
The Characteristics table on page 4, shows a 0.75 volt manufacturing spread.
It will therefore not be possible to determine temperature from V
fwd without a calibration process.
You will probably need to calibrate every LED with short "on" pulses to measure V
fwd at 25°C.Relative spectral emission, at the top of page 8, shows a peak at 440nm in the blue.
Knowing that " Energy = Plank's constant * frequency " we can compute V
fwd = 1240 / 440nm = 2.818V.
Ignoring the Gaussian thermal energy spreading expected, that should be the intercept of the I
fwd curve with V
fwd axis.
Looking at the graph of I
fwd versus V
fwd, (top left of page 9), we see it passes through the point 3.4V at 1.4A
The series resistance of the LED will be Rs = (3.4V - 2.818V) / 1.4A = 0.416 ohms
We can predict V
fwd at I
fwd = 1A as being 2.818V + ( 1.0A * 0.416R) = 3.234
The graph shows that it is a close enough model.
So the LED can be modeled as a 1.818V bandgap in series with a series resistor of about 0.416 ohms.
