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Question regarding thermal resistance of a UA7805 regulator

Andrew J

From the datasheet, the TO-220 package has the following thermal characteristics:

 

Rja = 19C/W

Rjc(top) = 17C/W

Rjp(top) = 3C/W  (jp in this case means Junction-to-exposed pad)

 

Am I right in thinking that for a TO-220, the Rjp value is a reference to the tab when used with a heatsink?  Typically, Rjc would be used, but if this was the case, then adding a heatsink of any thermal resistance > 2C/W would make it worse!  An older version of this datasheet just referred to:

Rja = 19C/W

Rjc = 3C/W

 

This would tie in with my interpretation.

 

Thanks,

 

Andrew

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  • 0

    It sounds like you are thinking the R between case and air is 2 C/W.

    I would have expected Rca can be greater than Rjc.

  • 0 in reply to dougw

    Not sure I understand Doug.  When I've been doing a thermal calculation it is along the lines of:

     

    Pdiss = V*I

    Tj = (Pdiss * Rja) + ambient  (no heatsink)

    Tj = (Pdiss * (Rjc+Rha)) + ambient (heatsink with thermal resistance of Rha)

     

    Typically, Rjc is a lot less for an IC than Rja because it's a value to use with a heatsink:  Junction-to-Case-to-heatsink-to-air (ignoring thermal paste or thermal pad).  In the datasheet, Rjc is only 2C/W less than Rja so adding a heatsink would make it worse.

    Example

    Ambient: 25C

    Pdiss: 2W

    Tj = 2w*19c/w + 25C = 63C

    Tj = (2W * (17c/w+10c/w)) + 25 = 79C (with heatsink Rha of 10c/w)

     

    If the Rjp value was used:

    Tj = (2W * (3c/w + 10c/w)) + 25 = 51C

     

    Hence my question.  In the slightly older version of the datasheet, TI only gave a Rja = 19c/w and Rjc = 3c/w.

  • 0 in reply to Andrew J

    Hi Andrew,

     

    I think Rjc(top) value can be ignored, it is most likely the thermal resistance for the junction to the top of the case (i.e. the plastic bit) which won't be in contact with the heatsink.

    This heatsink calculator website is helpful (useful for getting approximate values for popular semi packages, which should be in the correct ballpark). That website suggests a Rjc value which fits your Rjp value, so they are referring to Rjp. Also, although the TI doc says Rjp(top), that may be a typo, or may refer to interfacing on the top side of the metal bit, either way your heatsink will be contacting metal, so you can use the Rjp value.

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  • 0 in reply to Andrew J

    Hi Andrew,

     

    I think Rjc(top) value can be ignored, it is most likely the thermal resistance for the junction to the top of the case (i.e. the plastic bit) which won't be in contact with the heatsink.

    This heatsink calculator website is helpful (useful for getting approximate values for popular semi packages, which should be in the correct ballpark). That website suggests a Rjc value which fits your Rjp value, so they are referring to Rjp. Also, although the TI doc says Rjp(top), that may be a typo, or may refer to interfacing on the top side of the metal bit, either way your heatsink will be contacting metal, so you can use the Rjp value.

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