Re: Luxeon emitter forward voltage question


Ian Stirling wrote:

RHRRC <h.lewis@xxxxxxxxxxxxxxx> wrote:

Ian Stirling wrote:

RHRRC <h.lewis@xxxxxxxxxxxxxxx> wrote:
The datasheet(s) for the Luxeon 1 emitters gives the change in forward
voltage with temperature at -2mV/degC.
I have just measured 2 samples and found this to be -1.7mV/degC.
Both samples gave very similar results (a lot closer than experimetal
error) and were measured over the range of -17degC to +68degC with a
current of 80uA. The leds were soaked overnight at temperature, the

uA?
I would not be comfortable with measuring at a current of much under
80mA.
There are non-linearities in there.

Thanks for the response

I thought that I could remember Lumileds using 1ma but cannot turn up a
reference to this fact.
Are you sure about your numbers - throwing around 250uJ at a junction
with such a low thermal mass at a rate of 250 mJ.s seems excessive for
what is needed to be achieved.

Well, microseconds would actually be just plenty to measure it.

I was hoping that keeping the rate low (~200uJ.s) even though there was
a long (up to 2 secs) duration would keep self heating effects small.

Do you have further details on the non linearities of the thermal coeff
of Vf at low currents?

Look at the 'illuminance vs input current' graph somewhere on the
datasheet, and the note immediately after that says something like
'leakage currents may be very variable, to illuminate at low levels, use
PWM'.

At 80uA, you're not measuring the voltage drop of the diode at 80uA.
You're measuring the voltage drop of the diode, in parallel with the
parasitics that are very significant at this current.
There is no good reason to suppose these are linear, or follow any other
simple temperature dependancy.

Certainly Lumileds recommended to keep the current density up (and thus
dim by PWM if low average currents required) but I have not seen
reference to this being due to 'parallel parasitics'.
Adverse current distributions causing variable efficiencies,
particularly over time and temperature, was, I thought, the reasoning.
Its not just a question of quantum efficiency it is also getting the
photons to the outside world.

Is there something more going on?

Even so how would it affect my results?

.

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