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Old 13th September 2009, 01:47 AM   #1
Bigun is online now Bigun  Canada
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Default LTSpice: how to model temperature ?

I'm not talking about how to set the global variable TEMP to model the circuit behaviour over temperature - done that.

The issue is that some components get hot (e.g. power devices) and will be mounted on heatsinks. Other devices are not mounted on the heatsink and will be at a different ambient.

Now, consider an amplifier design where a power BJT is on a heatsink at one temperature and another BJT setting the biass current for the power device and it is on a separate heatsink at a different temperature. [Don't worry about whether this is good design practice, I have a very good reason for these devices being at different temperatures.]

In order to model this I have to set different temperatures for two devices, which I can't figure out how to do
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Old 13th September 2009, 03:46 AM   #2
andy_c is offline andy_c  United States
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Here's an example of passing "temp" to an individual model. In this specific case, the results should be exactly the same as if temp were not passed in. That's because temp is set to the nominal value of Tnom for a BJT, which is 27 deg C. See the documentation of Tnom in the BJT model parameters help.
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Old 14th September 2009, 10:27 PM   #3
Bigun is online now Bigun  Canada
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Thanks Andy - brilliant, it works like a dream. I even used a parameter to allow me to step through some values for only those parts mounted on the heatsink.

p.s. It allowed me to verify that the temperature compensation in my circuit works (theoretically at least), which I couldn't do with a global temperature setting.
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