Mosfet Rdson

Discussion in 'General Electronics Chat' started by joemmech, Jan 22, 2010.

  1. joemmech

    Thread Starter Member

    Jan 22, 2010
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    Hi to all. I have a question about my Rdson testing. I have been reading in the internet but until now I haven't found the answer. We have a device tested 4 times for Rdson. Below are the test configurations:

    1st test: Vgs = 10 and Id = 30
    2nd test: Vgs = 10 and Id = 15
    3rd test: Vgs = 4.5 and Id = 30
    4th test: Vgs = 4.5 and Id = 15

    The difference of the Rdson reading for the 1st and 2nd test is less than 1mohm but for the 3rd and 4th test they have greater than 1mohm difference. I also noticed that the measured Vds for the 1st and 2nd test has a ratio of less than 1, but for the 3rd and 4th test it has a ratio of greater than 2. Can somebody explain to me why the device is having this kind of response? What I am expecting is that the 3rd and 4th Rdson test will also have a less than 1mohm and a Vds ratio of less than 1, just the same with the 1st and 2nd test. Please advise.

    Thanks and Best Regards
     
  2. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    What MOSFET part are you using? The MOSFET may not be completely ON with a Vgs of only 4.5V.
     
  3. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
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    Look at Rdson against Id graphs at different Vgs for a MOSFET. Any MOSFET will do. You will see the answer to your question.
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Unless you are using Logic Level MOSFETs, 4.5V isn't enough to run the MOSFET as a switch. Vgs needs to be rather high, near Vgs (10V+).

    For this, there are several solutions for "High Side Drivers" to get Vgs to the required level from the output of a microcontroller. A search for High Side Driver on this forum will show many results, some discrete component, and some using HSD IC's.
     
  5. joemmech

    Thread Starter Member

    Jan 22, 2010
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    The mosfet part that I was using was the FDS6680A.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    See the annotated graph of Rds(on), attached.
     
  7. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
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    The FDS6680A is a logic level MOSFET, usable at gate voltage below 5V.

    However, it is still better to drive it with 10V gate voltage if one wants to get the lowest Rds ON.
     
  8. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    Here is the Rds(on) V. Id graph from the data sheet. Rds(on) varies with Id and the variation is larger with Vgs = 4.5V than at 10V.
     
  9. joemmech

    Thread Starter Member

    Jan 22, 2010
    32
    0
    Thanks for the information. It really help a lot.
     
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