+ Post New Thread
Results 1 to 6 of 6
  1. #1
    Newbie level 1
    Points: 68, Level: 1

    Join Date
    Feb 2019
    Posts
    8
    Helped
    0 / 0
    Points
    68
    Level
    1

    Temperature measurement on power mosfets during the power consumption in motor contro

    Hi,

    I am measuring case temperature placing the thermocouples on mosfets in the bridge configuration of motor control board. During the motor rotation or power consumption the six power mosfets are getting warm in my experiment and to monitor if the case temperature exceeds rated junction temperature of power mosfet's i placed thermocouples to get the data in degree centigrades. The question is how long we have to let it in running condition? How long we have to wait to check if case temp exceeds jn temp of power mosfet?

    •   AltAdvertisment

        
       

  2. #2
    Advanced Member level 5
    Points: 22,685, Level: 36
    barry's Avatar
    Join Date
    Mar 2005
    Location
    California, USA
    Posts
    4,341
    Helped
    959 / 959
    Points
    22,685
    Level
    36

    Re: Temperature measurement on power mosfets during the power consumption in motor co

    There are a lot of factors at play here, specifically, the physical aspects of your thermocouple, how it's bonded to the MOSFET, etc. Look at this: https://www.omega.com/techref/Thermo...ponseTime.html

    But if it were me, I'd just let it run continuously until the temperature stabilizes. Aren't you expecting it to run continuously in normal operation?



    •   AltAdvertisment

        
       

  3. #3
    Super Moderator
    Points: 70,762, Level: 64
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    14,474
    Helped
    3293 / 3293
    Points
    70,762
    Level
    64

    Re: Temperature measurement on power mosfets during the power consumption in motor co

    Hi,

    Mosfets include a body diode. This is the same silicon as the mosfet's switch, obviously they have the same temperature.
    A silicon diode has a temperature coefficient of about 2.1mV/°C in forward voltage.
    Thus a temperature rise from 25°C to 150°C = 125°C results in about 260mV.

    If you can find a way to measure the diode's forward voltage - eg in times when the PWM switches the Mosfet OFF - you durectly measure the junction temperature. ...almost perfect without adding time delay or added thermal mass.

    This is not very easy, but If you want us to help you with this solution you should give us the half bridge schematic and the PWM timing.

    *******
    The theromcouple solution is more simple, but includes delays and can only read the case temperature instead of junction temperature.
    The thermal model from junction to case may be simplyfied as a simple RC delay.
    To get a clue about the junction temperature you need to do a multiplication (R part) and you need to take care about the rise rate of the temperature.
    This can simply made in an EXCEL chart.

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.


    1 members found this post helpful.

    •   AltAdvertisment

        
       

  4. #4
    Advanced Member level 5
    Points: 15,532, Level: 30

    Join Date
    Nov 2012
    Posts
    2,861
    Helped
    676 / 676
    Points
    15,532
    Level
    30

    Re: Temperature measurement on power mosfets during the power consumption in motor co

    If you can find a way to measure the diode's forward voltage ...
    A very elegant solution indeed.

    But perhaps you need to add /modify the circuit extensively; forward drops are best measured at constant current.

    I was about to suggest a silicon small signal diode pasted on the body instead of the thermocouple.

    Diode forward drops depend on band gap in some complex way and can give the absolute temp without the need of a reference junction.

    Measuring the temp using the built in body diode can become messy (but I am fully ignorant of the intricacies involved). But I feel that replacing the thermocouple with a simple diode may be simpler.



  5. #5
    Junior Member level 2
    Points: 557, Level: 5

    Join Date
    Oct 2016
    Location
    Volos, Greece
    Posts
    23
    Helped
    5 / 5
    Points
    557
    Level
    5

    Re: Temperature measurement on power mosfets during the power consumption in motor co

    Quote Originally Posted by vishjay View Post
    The question is how long we have to let it in running condition? How long we have to wait to check if case temp exceeds jn temp of power mosfet?
    Best test this is at environmental temperature 36C for 48 hours. It will pass the test if your Heat-sink it does qualify too.



    •   AltAdvertisment

        
       

  6. #6
    Advanced Member level 5
    Points: 38,361, Level: 47

    Join Date
    Mar 2008
    Location
    USA
    Posts
    6,208
    Helped
    1797 / 1797
    Points
    38,361
    Level
    47

    Re: Temperature measurement on power mosfets during the power consumption in motor co

    Datalog the thermocouple readings and look for an
    asymptote. Which may change with ambient temp,
    airflow and load, but you will find one of you keep
    conditions constant.

    RTDs are maybe a better signal in noisy environments.
    Or the diode approach.



--[[ ]]--