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WEBENCH-POWER-DESIGNER: LM5012 thetaJA in WEBENCH POWER DESIGNER

Part Number: WEBENCH-POWER-DESIGNER
Other Parts Discussed in Thread: LM5012

Hello experts,

I'm trying to build a 48Vin-15Vout-2AIout buck converter using LM5012. As the ambient temperature is 90 degree, we should be careful not to exceed Tjmax.

Thus, I took advantage of WEBENCH to calculate the loss, efficiency and Tjmax.

webench.ti.com/.../SDP.cgi

However, some questions come to my mind. Then, Could you answer the following questions?

1. Why is the ICThetaJA 18 degree/W in WEBENCH? In the datasheet of LM5012, both ThetaJA in EVM(29degree/W) and ThetaJA in JEDEC PCB(34.8degree/W)) are above 18 degree/W. Where does this value come from?

2. Can we change the ICThetaJA in WEBENCH to calculate IC Tj appropriately?

3. As you know, a loss from converter increases with increasing temperature, and I found IC Pd in WEBENCH can consider this effect.

Which temperature is considered in the estimation of IC Pd? Ambient temperature or IC Tj in the operating condition?

  • Hello Bob,

    Thank you for reaching out,

    Re-assigning the thread to the right product line.

    Next time please specify the device number at the part number category so that it will automatically assigned to the respective product line.

    Thank you & regards,

    Moheddin.

  • Hi Kento,

    There isn't a single number for RthethajA. Is dependent on several parameters (ie: board size, power disipation, ambient temperature and more).

    In this appnote, contained is a thermal capture of the IC on a board design which was designed for better thermals than the EVM. The EVM was designed to showcase the small solution size.

    In figure 2-6, the case temperature of the device is recorded with a 48V input, 12V output, and 1.75A load. This is almost nearly identical to your application.

    I'll try and make WB and datasheet conistent or clarify the operating conditions for which the particular parameter was collected.

    Thank you for pointing that out.

    Pdiss is calculated with this entry

  • Hello Beck,

    Thank you for your reply. I clarified Q.1.

    Could you help me with Q.2 and Q.3?

    2. Can we change the ICThetaJA in WEBENCH to calculate IC Tj appropriately?

    3. As you know, a loss from converter increases with increasing temperature, and I found IC Pd in WEBENCH can consider this effect.

    Which temperature is considered in the estimation of IC Pd? Ambient temperature or IC Tj in the operating condition?

  • Hi Kento,

    You are welcome.

    With Q2/Q3

    18degC is probably obtainable. I don't see it necessary to change.

    If an 18degC/W rthetha is not applicable for your board design and you would like to approximate the junction temperature below is a figure for various rthetha with differing copper board area. Choose that which is applicable for your design. Max load current can be found by the following equation.

    The temperature for which Pdiss is calculated is from the highlighted entry below

    If you change it slightly the value of Pdiss changes. 

  • Hello Beck,

    Thank your for your reply. I also clarified Q2 and Q3!! 

    Thanks,

    Kento

  • You are welcome, Kento.

    Have a good day.