Understanding transistor Ptot in data sheet

  1. Hi,

    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. AlephZero

    AlephZero 7,298
    Science Advisor
    Homework Helper

    Ta = ambient temperature, Tc = case temperature.
    Really?
    http://uk.rs-online.com/web/p/heatsinks/7124320/
    http://www.rapidonline.com/electronic-components/to-92-heatsink-77178
    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. berkeman

    Staff: Mentor

    Were those numbers for the TO-92 package version, or maybe for a different package option (like metal can)?
     
  5. jim hardy

    jim hardy 4,371
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    Gold Member
    2014 Award

    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.

    [​IMG]

    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. 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. I totally missed the fact that there are heat-sinks for those casings. Thanks!
     
  8. 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. 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. jim hardy

    jim hardy 4,371
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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
    http://www.st.com/web/en/resource/technical/document/datasheet/CD00003223.pdf

    compare to the plastic case....


    try the numbers and see if both conditions get you to max junction temperature at max power dissipation.
     
  11. AlephZero

    AlephZero 7,298
    Science Advisor
    Homework Helper

    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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