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thermal resistance help

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Opticalworm:
Hi everyone,

I don't think I fully understand the information some manufactures provide on thermal resistance. Normally, I would take that resistance (lead to ambient) multiply it with the expected power and then add the maximum ambient temperature. Now if that temperature falls inside the normal working range then I'm happy to use as is.

However, take this Zener diode (1SMB5925BT3G  Datasheet), the datasheet says  that this device is a "3watt plastic surface mount Zener diode" which I can only assume it means that this diode can run find at 3Watts without heat sink. Now if you look at the table on the first page "maximum rating" it gives a thermal resistance from junction to lead of 25 °C/W with a maximum power dissipation of 3Watt where on the next row it says junction to ambient is 226 °C/W with maximum of 550mWatt.

Which thermal resistance should I be using if I want to pass 2.5W through it?

Psi:
The 3W device power rating is the max power the junction can handle without damage, all it really tells you is that it can handle 3W for an unknown length of time. It is likely that if you follow the datasheet for pcb pad size the component will handle near it's rated power without overheating but to find out for sure you'd need to calculate just how hot it will get at that power.

Most devices give both junction-to-lead and junction-to-ambient so you can work out the temperature rise for which ever option you use (heatsinked and/or in open air)

You will need to lookup the heat dissipation of the standard copper pcb footprint for that size component or the heat dissipation for a given area of pcb copper to work out the exact temperature rise at 2.5W

Have you watched Dave's video on this subject? it might answer some questions.
http://www.eevblog.com/2010/08/15/eevblog-105-electronics-thermal-heatsink-design-tutorial/

Opticalworm:
I see now why for junction to lead Thermal resistance is a smaller than Junction-ambient and why they have different power dissipation. If I want to achieve the full 3watt dissipation then I will need to look into a better way to cool this zener (For example like Psi suggested PCB footprint).

Psi Thank you.

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