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TPS53515: Thermal Analysis of the PCB with TPS53515

Vijetha HN
Intellectual 675 points
Part Number: TPS53515

Hi,

We are using 4 nos of TPS53515RVET for generating 4 nos of 3.3V in our board design and we have finished Layout activities. Now we are checking the Thermal analysis of the design by doing Thermal analysis.

We observe hot spot on these 4 ICs. Without considering Fan, junction temperature comes up to be 250 deg C and with fan of speed 1000ft/min Tj comes down to nearly 127 deg C.

Before put down my question, I will mention the design considerations below. These values are considered as electrical inputs in the simulation tool:

Output Voltage : 3.3V

Output current : 6A

Efficiency : 90%

Input Voltage : 12V

Input Current : nearly 2A

Power dissipation : 2.2W

Theta JC : 34.

Maximum Ti allowed : 150 deg C

In layout we have provided the proper thermal pad and thermal vias and Ground shapes (there are 4 GND layers in the stackup). And also parameters are properly entered in the tool too.

In spite of all these cares taken, why the junction temperature is going to 250 degC (in the absence fan) ? Whether the design considerations are correct or is there any flaw in the design considerations as mentioned above?

I am checking is there any layout issue or Simulation Tool related issue. Through this forum I just wanted to confirm whether the parameters considered as above make logical sense. 

I am also attaching the TI Webench design report as per which the schematics is designed.TPS53515RVER_3.3VFPGA.pdf

Regards,

Vijetha

  • 0
    TI__Guru**** 191445 points

    Vijetha,

    The Webench report states Pd = 1.384 W and Tj = 68.925 C.  If you are getting 250 C. in your simulation, I think there must be a gross error in the simulation.  I would suggest you check it closely.

  • 0 in reply to JohnTucker
    Intellectual 675 points

    We need to see the total Pd right?

    Theoretical Pd value comes to be 2.2W for us based on the Formula no 14 in the Application note http://www.ti.com/lit/an/slva390/slva390.pdf.

    If we consider the formula for THETAJC (= (TJ-TC)/Pd), then TJ value comes out to be 120+ deg C (considering case temperature of 50 degC , Pd=2.2W, THETAJC=34).

    We have considered Fan of 500RPM during the simulation and now we are seeing 120+deg C on the junction of the IC. We have used many more TI regulators in this design and we do not see any hotspot on those IC locations. We find red area only around TPS53515.

    Regards,

    Vijetha

  • 0 in reply to Vijetha HN
    TI__Guru**** 191445 points

    Vijetha,

    If the case temperature is 50 C. and Pd = 2.2 W, then Tj = (PsiJT * PD) + Tc = (1.8 * 2.2) + 50 = 53.96 C.  ThetaJC top is not really a useful parameter for devices with a thermal pad.

  • 0 in reply to JohnTucker
    Intellectual 675 points

    Hi,

    Ok, our thermal simulation tool asks for Junction to case thermal resistance parameter. And we had entered the ThetaJC(TOP) value (=34). 

    If the ThetaJC(TOP) is not a useful parameter then we must consider THETAJC(Bottom) value in the simulation tool. Using 2.2 in place of 34 will definitely yield a great thermal result. So I want to confirm are we entering wrong value here?

    Regards,

    Vijetha 

  • 0 in reply to Vijetha HN
    TI__Guru**** 191445 points

    Vijetha,

    You will probably want to use RθJB Junction-to-board thermal resistance to account for the solder connection.

  • 0 in reply to JohnTucker
    Intellectual 675 points

    The tool has no option for entering the THETAJB value. The tool takes only THETAJC, Pd, Vout and maximum allowed Tj for the chip.

    When I checked with support team of the tool, what I herd is that the tool will internally calculate the THETAJB based on the entered values and the dimension details of the IC (L,W,H,Pin count and dimensions, Thermal Pad dimension and Thermal vias, Pd etc). 

    Hence I considered THETAJC as 34 and entered the same in the tool. Now I am having doubt on the entered value of THETAJC, whether it has to be 34 or 2.2?

    Regards,

    Vijetha

  • 0 in reply to Vijetha HN
    TI__Guru**** 191445 points

    Vijetha,

    As I am not familiar with your modeling software, I cannot give you a definitive answer.  The data sheet shows a thermal imaging of the device with Vin = 12 V , Vout = 5 V, Vout = 12 A and Fsw = 500 kHz.  The IC top case temp is 75.6 C., the power dissipation is 2.19 W and the junction temp is 75.6 + 2.19* 1.8 =79.54 C at Ta = 25 C..  I would try to match this conditions and adjust your thermal parameter until the junction temperatures match.

  • 0 in reply to JohnTucker
    Intellectual 675 points

    In your calculation of finding Tj, you have considered '1.8'. What is this 1.8?

    I guess you used the formula of RThetaJC=(Tj-Tc)/Pd to calculate Tj.

    But you considered RthetaJC=1.8. Why? It has to be 34 or 2.2 right? Or is it variable?

    Regards,

    Vijetha

  • 0 in reply to Vijetha HN
    TI__Guru**** 191445 points

    Vijetha,

    1.8 is the PsiJT characterization parameter.  1.8 * Pdiss + top case temperature = Tj.  I estimated the junction temperature for the above conditions based on the thermal image in the datasheet figure 32.  I would not use 34.1.  The Theta JC does not give realistic junction temperatures for devices with a thermal pad.  You can use 2.2 if your software can estimate the thermal resistance of the solder attachment to the PCB.  See this document for details about the thermal metrics;

    www.ti.com/.../spra953

  • 0 in reply to JohnTucker
    Intellectual 675 points

    Hi,

    Sorry for no reply for last few days. We concluded the thermal analysis considering the RThetaJC=2.2 .

    But we would like to know one thing. In the datasheet (page 13) there is a Thermal report captured on Thermal Image camera for Pout of 60W (5V,12A). The observed Case temperature is 75.6deg C. We would like to know what was the environmental condition of the board when it was captured (whether it was kept in open environment, with or without fan? If there was a fan then what was the speed of fan) ? Please inform.

    Regards

    Vijetha

  • 0 in reply to Vijetha HN
    TI__Guru**** 191445 points

    Vijetha,

    It is impossible to say with 100% certainty as that image was taken over 6 years ago, but we would typically take those images at room temperature ambient and no forced air.