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Original question:

[FAQ] How to Calculate TPS23755 and TPS23758 IC Power Dissipation

TPS23758EVM-080: TPS23758EVM-080 thermal image and power dissipation

Ching-Fei Ho
Prodigy 150 points
Part Number: TPS23758EVM-080
Other Parts Discussed in Thread: TPS23758,

Tool/software:

Hi team,

I have a project that use TPS23758(36VIN~57VIN --> 5VOUT/2.5A), and we would like to calculate the power dissipation for thermal estimation. The power loss should includes TPS23758, transformer, conduction loss of secondary mosfet.
I think the power dissipated by the IC would be as below, so the total power dissipation of TPS23758 should be around 0.66W, right? please correct me if any wrong about my calculation.

1. Power at the pass FET (IRTN*IRTN*RDSon) : 0.8A * 0.8A * 0.68ohm = 0.4352W (Base on datasheet)

2. Power to drive the gate of the switching FET (VCC*ICC) = 8.6V * 2.7mA = 0.02322W (Base on datasheet)

3. Conduction loss of the switching FET (RDSonFET*Irms*Irms) = 1.28ohm * 0.4A*0.4A= 0.2048W (Irms is the input current which calculated by 5VOUT/2.5A)

Conduction loss of secondary mosfet : (RDSonFET*Irms*Irms) = 14.5mohm * 2.5A * 2.5A=0.09W (The RDSon of the mosfet that I use is 14.5mohm)

What about the dissipation calculation for the transformer? Is there any tool that could use?

And could you provide me the detail thermal information(maybe thermal image) of TPS23758EVM-080 Evaluation Module for reference?

Thanks.

  • 0
    TI__Mastermind 37031 points

    Hi Ching-Fei,

    Thanks for your reaching out!

    I will get back to you early next week. Thanks for your patience.

    Best regards,

    Diang 

  • 0 in reply to Diang Xing
    TI__Mastermind 37031 points

    Hi Ching-Fei,

    Thanks for your patience.  

    TPS23758's primary FET and secondary side FET also has switching loss, which is related to gate resistance, load condition, and switching frequency, shoot-though... It is hard to calculate. You may consider it equaling to the 1x - 3x conduction loss (very rough estimation, inaccurate).

    For the transformer power loss, it is better to reach out the the transformer vendor. Normally it includes core loss, copper loss, and eddy current loss.  

    For the thermal of TPS23758EVM, you can refer the below results based on thermal image: Vin = 48V, Output = 5V/2.5A, ambient temperature: 23C, running time: 10 mins, natural convection. Rectifier is the hottest point.  

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    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Prodigy 150 points

    Hi Diang,

    Thanks for your feedback, I will try to reach transformer vendor to get the power loss of transformer. It seems hard to calculate the power loss of POE, I will take the thermal image as reference. Thanks.