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In case I have a diode, and I want to know it could dissipate the power, I have two values, but I can't choose between them: - Junction to ambient thermal resitance \$R_{ja}\$ - Junction to lead thermal resitance \$R_{jl}\$

Which one should I use to know if my diode is good for my application?

Thanks

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  • \$\begingroup\$ Depends if you have forced cooling or just the component standing in free air. \$\endgroup\$ – winny Jan 15 at 15:10
  • \$\begingroup\$ What if we assume the is no forced cooling (no fan, just the diode on the board)? \$\endgroup\$ – Tack Jan 15 at 15:16
  • \$\begingroup\$ If you don't have significant heat sinking attached to the leads or temperature-sensitive components connected to them, probably R[ja] is what you're interested in. \$\endgroup\$ – JimmyB Jan 15 at 15:20
  • \$\begingroup\$ Ok, thanks. How do I do if I don't have the junction to ambient? What do you mean by heat sinking? \$\endgroup\$ – Tack Jan 15 at 15:28
  • \$\begingroup\$ +1 on what JImmy said. Heatsinking could be copper area on the PCB from your leads, forced air via fan, any external heatsinks attached to the component and so on. \$\endgroup\$ – winny Jan 15 at 15:33
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Some of this is partially answered in the comments to the question, but here's the complete (IMHO) answer:

\$R_{ja}\$ is the thermal resistance from the diode junction to ambient air assuming (1) no air movement (or perhaps natural conduction), and (2) there's no significant thermal coupling between the diode's leads and the ambient environment. It says, in effect, "if you take no special effort to couple (or insulate) this diode to (or from) the environment, this is what your thermal resistance is".

\$R_{jl}\$ is the thermal resistance from the diode junction to the leads -- or, if the diode is fully specified, to some point on the leads (i.e., 10mm from the case, or 5mm -- there's no standard, read the data sheet carefully). Using this you can arrange your application (somewhat) to suit the diode, by arranging to couple the leads to the ambient environment. Usually this is done by making extra-big traces on the PC board, or extra-big pads, to conduct heat from the diode leads to the surrounding PC board.

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You can imagine thermal systems with resistors to help you relate. Ambient is like ground (although it moves around), and its value is not zero, its usually 25C (or whatever the datasheet indicates. The heat wants to flow through thermal resistances to a lower heat level (just like current flows with resistors)

The heat can flow out of the top of the package through the air (junction to ambient) or through the leads of the device and to the PCB (junction to lead)

If these thermal resistances are too high (or your device uses too much power) it can heat up beyond the absolute maximum ratings on the datasheet and cause the part to fail. You use both of the resistances to calculate what the final temperature of the device. You'll also need to know how much power the part is dissipating.

schematic

simulate this circuit – Schematic created using CircuitLab

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