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LM338: Calculation of heat sink thermal resistance

na na78
Guru 16770 points
Part Number: LM338

Hi

We are considering heat sink for LM338 (TO-220).

We uses following equations.

Vin = 12V
Vout=4.5V
Io=0.7V
Ta=60C
Tj=125C

Iq = 3mA as maximum ( According to Figure 9 )

Pd = (Vin - Vout) * Iout + (Vin * Iq) = 5.28W

Tj = Ta +(Rjc + Rcs + Rsa) * Pd ->  (Rcs+Rsa) = (Tj-Ta)/Pd - Rjc = 12.3 - 15.7 = negative value ?

Could you please tell me how to calculate heat sink thermal resistance correctly?

BestRegards

  • 0
    TI__Mastermind 23361 points

    Hi Na,

    In your calculation, the Rjc number used is from the thermal metric provided in the datasheet.  However, for the term (Tj - Ta)/Pd in the equation, this term is actually defined to be Rja and this Rja is 22.9. 

    The Rja and Rjc numbers are generated from the same thermal modelling through simulation. Their values only make sense when they were used with their defined models.

    Does it make sense?

    Regards,
    Jason Song

  • 0 in reply to Jason Song
    Guru 16770 points

    Hi

    Thank you for your reply.

    I noticed that Rjc nad Rjc(top) are different use.

    Actually, Rjc shown in old datasheet version offers 4C/W.

    I think if we want to calculate in a way I shown, old package model should be used for calculation and so we had to select old Rjc.

    Is my understanding correct?

    BestRegards

  • 0 in reply to na na78
    TI__Mastermind 23361 points

    Hi Na,

    You can use your way to do the calculation, but the Tj number needs to be calculated using Rja. 

    Tj = Ta + (12V - 4.5V) x 0.7A x 22.9C/W = 60 + 120  = 180C

    The maximum recommended temperature for this device is 125C. Since the thermal model was simulated, the conditions of the device need to be within the recommeneded maximum temprature for those thermal numbers to make sense.

    To answer your questions, can you share me the old datasheet you mentioned? I can do some investigation to see why the number has been changed.

    Regards,
    Jason

  • 0 in reply to Jason Song
    Guru 16770 points

    Hi Jason

    Thank you for your reply.

    As primary requirement, we want to calculate Rcs.

    I suppose the equation with following thermal metrics.

    Rja = Rjc + Rcs + Rsa

    Rjc : junction to case
    Rcs : case to heat sink
    Rsa : heat sink to ambient performance

    P.7 equation (6) 
    http://www.ti.com/lit/an/spra953c/spra953c.pdf

    We can refer old datasheet from National Semiconductor on the web.

    http://doc.chipfind.ru/html/nsc/lm338t.html

    BestRegards

  • 0 in reply to na na78
    TI__Mastermind 23361 points

    Hi Na,

    Thanks for providing the old datasheet for reference. Please allow some time for me to investigate the issue, I will get back to you in 1-2 business days.

    Regards,
    Jason Song

  • 0 in reply to Jason Song
    TI__Mastermind 23361 points

    Hi Na,

    As you mentioned, Rja = Rjc + Rcs + Rsa

    Rjc : junction to case
    Rcs : case to heat sink
    Rsa : heat sink to ambient performance

    Rja = 22.9, Rjc(top)  = 15.7; the thermal resistance Rcs include the two parts, one is the Rjc(bottom) which is around 0.7 (in latest datasheet) or 1 (in the old datasheet). Rjc(bottom) is contributing part of the Rcs; the other term is the thermal resistrance from your soldering or mounting compound. Rsa, however, is the resistance of the heat sink on your boards. In order to calculate the Rsa, you will need to actually measure the temperature of the PCB that holding the IC accurately and then you can use this equation to get the Rsa.

    Does it make sense?

    Regards,
    Jason

  • 0 in reply to Jason Song
    Guru 16770 points

    Hi Jason

    Thank you for your reply.

    >the thermal resistance Rcs include the two parts, one is the Rjc(bottom) which is around 0.7 (in latest datasheet) or 1 (in the old datasheet). >Rjc(bottom) is contributing part of the Rcs; the other term is the thermal resistrance from your soldering or mounting compound. 

    For example, 

    Tj = Ta +(Rjc + Rcs + Rsa) * Pd ->  Rsa = (Tj-Ta)/Pd - Rjc - Rcs

    Is it impossible to read value Rjc and Rcs from datasheet?

    BestRegards

    In this case, could not we refer what Rjc is from datasheet?

  • 0 in reply to na na78
    TI__Mastermind 23361 points

    Hi Na, 

    Sorry for the late response, to answer your question regarding Rcs. Rcs = Rjc(bottom) + Rsa. Rsa here is related to the thermal resistance from your soldering or mounting compound between the bottom of the IC to the PCB. Due to the different thickness and material used, it will be different. This information, however, is not available on datasheet as it changes in each application. 

    Regards, 
    Jason