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Measured RDs(ON) is greater than that specified in datasheet

Sutej Reddy
Prodigy 30 points
Other Parts Discussed in Thread: CSD17312Q5, INA128

Hi,

I am using a 30V N-Channel Power MOSFET, CSD17312Q5, in a power management converter. Knowing the current through the MOSFET, I tried to measure the VDS across the MOSFET during ON state through the oscilloscope. Datasheet says RDs(ON) is 1.2mohm for a VGS of 8V. Current through the MOSFET is 10A . Hence VDS across MOSFET should be 12mV. But I am observing it to be around 80mV. I am clueless why the MOSFET is showing around 8mohm RDs(ON). Such a high RDs(ON) is impairing the converter efficiency severly. Any ideas/suggestions?

Regards,

Sutej Reddy.

  • 0
    TI__Genius 16870 points

    The datasheet specs Rds on at 3 combinations of VGS at Id = 35A, and temp = 25C being typical or the 1.2 mOhm your reference earlier. Your Id is 10A which is different than the spec Id and have you confirmed your Vgs is similar to the used for spec Vgs?

    Otherwise, this question would be better suited for the NEXFET team: http://e2e.ti.com/support/power_management/power_stage/f/208.aspx 

    If you are just running the FET at 100% duty cycle 10A, I am sure it is heating up. This would cause the Rds on to increase but still below the max rating for the max temp. 

    For PMUs questions in the future, please reference the device number that your are asking about in the title of your post to receive better support from the correct expert. 

  • 0 in reply to Michael Green
    Prodigy 30 points

    I did ensure the Vgs to be as mentioned in the datasheet. Even at a junction temp of 125C the specified RDs(ON) in the datasheet is 1.7mV ,still much deviated from what I am observing,i.e.8mohm. Also, the FET is not running at 100% duty. Anyways I have posted the same question in  NexFET Power MOSFETs forum.

    Thanks for your reply...

  • 0 in reply to Sutej Reddy
    TI__Mastermind 18200 points

    Sutej, 

    We need to determine whether this is a FET issue (which I hope it is not) or something else. Measuring voltages in the mV range with an oscilloscope is generally not recommended, and can produce somewhat unreliable data. 

    As a sanity check, can you test the Rdson of the FET on your board under DC conditions? Drive the FET at the same voltage, and measure using a multimeter. If you still are getting 80 mOhms, and are sure that there is not a soldering or connection issue, then we have a real problem. However, if you are seeing the datasheet values of Rdson, then we can narrow your problem down significantly to an issue with AC ripple and/or your method of measurement. 

    Rdson is a spec guaranteed by design. If a DC test still reveals Rdson of the part to be 80 mOhms, then we may ask you to send the part in so we can investigate. Thanks. 

  • 0 in reply to Sutej Reddy
    TI__Expert 7599 points

    RDSon test is one of the parameters that requires a very clean test setup.

    For RDSon you need a differential source FORCEHI/SENSEHI connected to the DRAIN and you need a FORCELO/SENSELO connected to the SOURCE. The current flowing between the FORCEHI/FORCELO should be 10A. The SENSEHI/SENSELO should be connected at the DUT.

    For 10A the Voltage between Drain and Source will be 15mV (1.5mOhm). You need a high impedance measurement instrument connected between SENSEHI/SENSELO. You can use something like INA128 with a gain of 100 so that the 15mV is gained up to 1.5V and you can use a DMM to verify your RDSon.