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From: Victor Roberts
Organization: Roberts Research & Consulting, Inc.
X-Mailer: Mozilla 4.78 [en] (Win98; U)
Subject: Re: luxeon star leds
Date: Fri, 22 Nov 2002 02:45:37 GMT
NNTP-Posting-Date: Thu, 21 Nov 2002 18:45:37 PST
"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.
> How can I otherwise determine junction temp?
I agree with Don that you should use the thermal resistance
instead of the junction voltage, but believe there is
something else going on here. The data published by Lumileds
was most likely obtained with the Luxeon Star mounted to a
heat sink. When you operate the device without an external
heat sink, the whole package gets rather warm and you have
more radiative and convective heat losses from the parts of
the device other than the heat sink than Lumileds had when
they made their measurements. The thermal resistance to
"free space" of an unmounted device should therefore be
lower than the thermal resistance of a device mounted to a
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