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From: email@example.com (Don Klipstein)
Subject: Re: luxeon star leds
Date: Fri, 22 Nov 2002 00:26:59 +0000 (UTC)
Organization: MGT Consulting
NNTP-Posting-Date: Fri, 22 Nov 2002 00:26:59 +0000 (UTC)
User-Agent: slrn/0.9.6.3 (SunOS)
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 (junction
>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 about
>30mS for the first reading) and subsequently monitor the forward voltage and
>the temp on the aluminium pcb I get a far greater temp rise on the pcb than
>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.
- Don Klipstein (firstname.lastname@example.org)
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