Thermal power dissipation

Vignesh M26

Dear TI Team,

In datasheet I have found Rjc is higher than Rja. But what I understood is Rja=Rjc+Rca. In this sense Rja should be higher than Rjc right?? Please help me to solve my doubt..

How to calculate Tc(case temperature),

Below is correct ??

1) Tc = Rjc x Pd

2) Tc = Tj - (Rjc * Pd) , If this 2nd case is correct means, I get negative temperature for example ( ie,

At 0Deg.C ambient,

Tc= 0 + (Rja * Pd) - (Rjc * Pd)

Tc= (Rja -Rc) * Pd

Tc = (42.5- 56.1) *Pd

Tc= -13.6 Pd

Please advice.. How to calculate.

Thanks.

Vignesh.M

over 5 years ago

0 Andy Chen over 5 years ago

TI__Mastermind 33765 points

Hi,

Please note the Rja and Rjc are measured at different configuration. you must know the actual power loss distributed from Junction to Case top when calculate temp with Rjc

For example, assume LMR14030SDDA power loss Ptotal is 2W,

about Pd1=85%*Ptotal power loss will spread from junction to thermal PAD,
about Pd2=15%*Ptotal power loss will spread from junction to case top.

At 0 Deg.C ambient, Tc= Tj - (Rjc * Pd2) = 0 + (Rja * Ptotal) - (Rjc * Pd2)=42.5*2W--56.1*0.3W=85--16.83=68.17 degreeC.

B R

Andy

0 Vignesh M26 over 5 years ago in reply to Andy Chen

Intellectual 590 points

Thanks Andy for your clear explanation.

But, Can you please let me know where can I get this percentage kind of power loss distribution??

Thanks,

Vignesh.M.

0 Andy Chen over 5 years ago in reply to Vignesh M26

TI__Mastermind 33765 points

Hi Vignesh,

You can check difference between R-theta-JC (top) and (bottom) on data sheet, the percentage can be roughly estimated by R-theta-JC (bot) over R-theta-JC (top).

We suggest to use Psi_JT to calculate case temp as it based on total power loss.

B R

Andy

0 Vignesh M26 over 5 years ago in reply to Andy Chen

Intellectual 590 points

Hi Andy,

Thanks for your responce.

But I'm not sure how to calculate Tc with Psi_JT.

If it is like below,

Tc = Tj - (Psi_JT. * Pd) ?

Thanks,

Vignesh.M

0 Andy Chen over 5 years ago in reply to Vignesh M26

TI__Mastermind 33765 points

Hi Vignesh,

Yes, use Tc = Tj - (Psi_JT. * Pd) to calculate Tc, the Pd is total power loss of converter IC.

B R

Andy

0 Vignesh M26 over 5 years ago in reply to Andy Chen

Intellectual 590 points

Thanks Andy,

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Thermal power dissipation