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LM117: Thermal computation in vacuum

Art Mecina
Genius 16655 points
Part Number: LM117

Hello Guys,

Good day.

Our customer is using LM117H in their design. They are dissipating across the device 10V*20mA~0.2W. They are operating in a vacuum chamber (2*10^-6 mbar) at 25degC.

They mounted a thermocouple on the metal can (top flat surface) region and this thermocouple measured the temperature  (in above vacuum condition) to be 70degC. To calculate junction temperature, should they consider the above temperature with theta-ja (186degC/W) or theta j-c (21degC/W)?

In addition to this, is there any limit on the power dissipation of the PFM version? The datasheet says 2W, "but limited internally". What is this internal limit - at what power/ current/temperature?

Thanks in advance!

Art

  • 0
    TI__Mastermind 23361 points

    Hi Art, 

    They mounted a thermocouple on the metal can (top flat surface) region and this thermocouple measured the temperature  (in above vacuum condition) to be 70degC. To calculate junction temperature, should they consider the above temperature with theta-ja (186degC/W) or theta j-c (21degC/W)?

    --It seems to me that you are measuring the top case temperature, so you should use the jc number. 

    In addition to this, is there any limit on the power dissipation of the PFM version? The datasheet says 2W, "but limited internally". What is this internal limit - at what power/ current/temperature?

    --Please see note (1) on Page 7 in the datasheet. The limit is essentially on the maximum junction temperature and you should use this for the maximum power dissipation. 

    Regards, 
    Jason Song

  • 0 in reply to Jason Song
    TI__Genius 16655 points

    Thanks Jason!

    Is there any derating curve w.r.t. power/current/voltage/temperature? is there any curve for the Safe Operating Area?

    regards,

    Art

  • 0 in reply to Art Mecina
    TI__Guru 50025 points

    Hi Art,

    Figure 7 sums up how the output changes with temperature. To use this figure the customer needs to use the thermal metrics either from the datasheet or from their own measurements to estimate the junction temperature to see how much the reference voltage is expected to deviate. Also keep in mind that the reference voltage deviation is amplified by the feedback network. There isn't a curve showing a safe operating area, but we know it is a linear function for power vs. temperature so the customer could easily construct one using the thermal metrics they measure.

    Best regards,

    Nick 

  • 0 in reply to Nick Butts
    TI__Genius 16655 points

    Hello Guys,

    Thanks for looking into this.

    By the way customer is using the LM117QML-SP (SNVS356D) datasheet. But I think the answer above still holds true.

    Could you help check if the following computations are correct?

    1. His operating environment is in a vacuum chamber with an overall temperature of 55degC. When the chamber was 25degC, his LM117 case temperature was 70degC. So, assuming a linear extrapolation for chamber operation at 55degC, the LM117 case temperature would be 70+30 ~ 100degC. Taking your suggestion of using Theta-jc (21degC/W) for 0.2W dissipation, they expect Tj to be 100+(21*0.2) ~ 105degC ?

    2. He have not provided any heat-sink, the LM117 is mounted on a FR4 PCB. If he is monitoring the case lid temperature & dissipate 0.2W, he would reach Tj = 150degC at ~ (150-21*0.2) 145degC.

    Also, when operating in vacuum, would ambient temperature (and Theta-ja) be any more directly applicable for him?

    Thanks in advance!

    Art

  • 0 in reply to Art Mecina
    TI__Guru 50025 points

    Hi Art,

    Art Mecina said:
    1. His operating environment is in a vacuum chamber with an overall temperature of 55degC. When the chamber was 25degC, his LM117 case temperature was 70degC. So, assuming a linear extrapolation for chamber operation at 55degC, the LM117 case temperature would be 70+30 ~ 100degC. Taking your suggestion of using Theta-jc (21degC/W) for 0.2W dissipation, they expect Tj to be 100+(21*0.2) ~ 105degC ?

    Yes this calculation is correct. 

    Art Mecina said:
    2. He have not provided any heat-sink, the LM117 is mounted on a FR4 PCB. If he is monitoring the case lid temperature & dissipate 0.2W, he would reach Tj = 150degC at ~ (150-21*0.2) 145degC.

    Can you elaborate a bit more here? Where did the 150C come from? 

    Art Mecina said:
    Also, when operating in vacuum, would ambient temperature (and Theta-ja) be any more directly applicable for him?

    I think theta_ja would actually be less applicable in a vacuum because the thermal metrics are measured (or rather simulated) assuming atmospheric pressure, so when there is much less pressure there is much fewer molecular interactions that serve as the heat transfer mechanisms, so I think the theta_ja would be inaccurate here.

    Best regards,

    Nick

  • 0 in reply to Nick Butts
    TI__Genius 16655 points

    Thanks Nick!

    I'll let you know as soon as I got the answer to your inquiry on the 150C.

    Art

  • 0 in reply to Nick Butts
    TI__Genius 16655 points

    Hello Nick,

    Apologies for the delayed response. Customer is making a test setup in thermovac so it took some time for him to get back to us.

    As for question#2 "Where did the 150C come from?", the answer is that the customer assumed the 150degC of the junction & made the theoretical estimate of case temperature.

    Once his done with the thermovac setup, he will update us with the results.

    Regards,

    Art