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From: "R. Lewis"
Subject: Re: luxeon star leds
Date: Fri, 22 Nov 2002 18:57:04 -0000
Organization: Nextra UK
NNTP-Posting-Date: Fri, 22 Nov 2002 18:55:53 +0000 (UTC)
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
"Don Klipstein" wrote in message
> In article , R. Lewis wrote:
> >I seem to have a problem with the luxeon star - or its data.
> >Luxeon quote a tempco of forward voltage of -2mV/degC and a theta
> >to board) of 17 degC/watt.
> >However, with a constant current source, if I power up a device (in free
> >air- no additional heatsinking) and measure the forward voltage (takes
> >30mS for the first reading) and subsequently monitor the forward voltage
> >the temp on the aluminium pcb I get a far greater temp rise on the pcb
> >the temp rise of the junction.
> >How is this so -- or what am I doing wrong.
> I would not expect the datasheet's tempco of any electrical or optical
> parameters to be accurate for the purpose of temperature measurement.
> It is merely for prediction of changes in device performance as a result
> of temperatures other than the usual 25 degrees C. And the datasheet
> tempcos are typical values and only useful as a rough guide.
> >How can I otherwise determine junction temp?
> I think thermal resistance is something they determine more accurately.
> If you know the board temperature and the amount of power going into the
> junction, then I think this is the best indicator of junction temperature.
The problem is that where the device is modulated the average power may be
For example if the led is 'flashed' for 200mS every 3 seconds how can I
tell what is happening to the die?
Luxeon appear to promote the fact that the die temp change may be monitored
by monitoring the forward voltage.
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