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TPS7A92: Thermal metrics for TPS7A92/TPS7A94

Kelly Bai
Intellectual 1330 points
Part Number: TPS7A92
Other Parts Discussed in Thread: TPS7A94

Hi team,

I want to evaluate the thermal performance of TPS7A92. Here I have some questions about the thermal metric for TPS7A92

  • Why RθJB >> RθJC(bot)? to my understanding, the difference between this two is the resistance of solder, but why solder can influence such much?
  • If we want to evaluate TJ before board come back, which parameter should we use for evaluating?  
  • I also look into TPS7A94, and find out the Recommended Junction temp is up to 150C which is the same with absolute maximum Junction temp. Does this device designed for 150C use or this is a typo for 150C recommended junction temp?

Thanks.

  • 0
    TI__Genius 12511 points

    Hi Kelly,

    • It is a few things, the thermal interfaces between the solder and part, and between the solder and the board also add thermal resistance increasing the value alongside the pure thermal resistance of the solder itself. Additionally, the board measurement is taken a distance away from the device itself, not directly on the board contacting the device, further increasing the thermal resistance between the board measurement and the junction case bottom measurement. All of these add up to significant differences between the measurements.
    • Generally speaking most customers utilize the RθJA value in their own analysis of their board designs. This is done as it is the easiest value to measure, and most customers do not have the capability, or want to expend the resources to measure along the JEDEC standards on the board, or device, using thermocouples. So RθJA is the primary method we see customers using, as the ambient temperature of the application is known.
    • The part can most likely perform at 150C, however at this time we only have performance data up to 125C. As this is a preproduction part, this could change and more data could be collected to showcase it's performance up to 150C, but at this time, we only have supporting data up to 125C, so I would classify it is a typo and it will be standardized across the board before the part is released to market.

    Regards,
    John

  • 0 in reply to John Schneider
    TI__Intellectual 1330 points

    Hi John,

    we leverage thermal shutdown temp 160C at steady state together with power loss at this time, and place a thermocouple at the top case of TPS7A92 for case temp, but the thermal resistor calculated is about 16C/W which is much higher than the datasheet Psi_JT(0.8C/W), is this reasonable and why?

    thanks.

  • 0 in reply to Kelly Bai
    TI__Genius 12511 points

    Hi Kelly,

    No I would say that that is a bit too far off from how the part should be behaving.

    This document details the testing conditions to get a true Psi_JT value. Assuming 160C as the shutdown temperature is the first place that I would say error may be arising from, the next place would be that you state steady state operation, in thermal shutdown the part should be switching on and off, if it is not, then it must be below it's thermal shutdown temperature which would increase the perceived thermal resistance. If the part is switching than the fluctuations in power dissipated and temperature seen would also cause instability in the accuracy of the reading, which is why JEDEC testing requires the actual Junction temperature to be measured in order to assure you receive an accurate psi or theta measurement. Both of these cases would cause the thermal resistance to be computed as much higher than it is actually performing

    Regards,
    John