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Understanding transistor Ptot in data sheet

  1. Jun 11, 2014 #1

    Reffering to the attached image, I wonder how total power dissipation is defined in each case; "Tamb < 25C" and "Tc < 25C" respectively. I suspect that Tc is referring to usage with a heat-sink , but it makes no sense as the TO92 case is not well suited for heat sinks.

    Thanks in advance!

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  3. Jun 11, 2014 #2


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    Ta = ambient temperature, Tc = case temperature.
    Or make your own from a small metal plate held onto the transistor with heat-shrink tubing!

    It's unlikely a heat sink would keep the case temperature down to 25C, but the data sheet should give the derating factors for higher temperatures, as in http://www.onsemi.com/pub_link/Collateral/P2N2222A-D.PDF
  4. Jun 11, 2014 #3


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    Were those numbers for the TO-92 package version, or maybe for a different package option (like metal can)?
  5. Jun 11, 2014 #4

    jim hardy

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    There's a "Thermal resistance to case" and a different "Thermal resistance to ambient".
    Many transistors have their collector bonded right to the metal case for good heat transfer.
    As you observed - with a heatsink you'd add 'to case' thermal resistance to heatsink's thermal resistance to get thermal to ambient....

    I like the metal can (TO39?) for troubleshooting.


    The one on the left had its base wire melted off inside, emitter too if i recall. We attributed that one to human error not aging. Probably a 'scope ground probe got plugged into the wrong test point.
  6. Jun 12, 2014 #5
    Yeah, i think i might have mixed up the data sheets, that would be the TO-18 case. But are there any heat-sinks for TO-18?
  7. Jun 12, 2014 #6
    I totally missed the fact that there are heat-sinks for those casings. Thanks!
  8. Jun 12, 2014 #7
    Thanks for clarifying, think i understand now. But does the Tc based Ptot refer to _zero_ thermal resistance to 25C? That would be purely theoretical, right?

    Great idea, never thought about it =)
  9. Jun 12, 2014 #8
    What does "Derate above 25°C - 5.0 mW/°C" actually mean? Is it the SOA that falls with 5mW/C?
    Last edited: Jun 12, 2014
  10. Jun 12, 2014 #9

    jim hardy

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    Zero thermal resistance case to ambient, yes. If you could keep the case at 25 deg you could dissipate 3 watts.

    Take a look at the thermal resistances given here for TO-39 case

    compare to the plastic case....

    try the numbers and see if both conditions get you to max junction temperature at max power dissipation.
  11. Jun 12, 2014 #10


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    I would take the Ta values as meaning the device was on a circuit board in air, without any heat sink.

    If you have a heat sink, you should know the thermal resistance between the case temperature and the air. For example the RS link gives the thermal resistance as 60 degrees C per watt.

    So if you were dissipating 0.5W with that heatsink and Ta = 25C, Tc would be 25 + 60 x 0.5 = 55C.

    The derating parameter tells you how the power rating decreases as the case temperature rises. If the max power rating is 1.5W at Tc = 25C and the derating is 12mW/C, at Tc = 55C it would be 1.5 - 0.012*(55-25) = 1.14W.

    If you increase the actual power dissipated, Tc will rise and the max power rating will fall. When the actual power = the max power, that is the operating limit for the transistor with that type of heatsink.
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