Solid State Switching

Discussion in 'The Projects Forum' started by DC_Kid, Jun 4, 2009.

  1. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
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    1) i have a question about FET datasheets. are the graphs of Ids as f(Vds) with no load attached? as example, the IRFZ24N graph shows this function as 20us pulse. more Vds yieds more Ids (makes sense with no load attached). what does this graph mean?

    my understanding of nFET is that Ids is a function of Vgs. does Ids rely on Vds if there is sufficient Vgs to cause full saturation? in other words, in my attached schematic if Vgs=10v and Vds=0.2v does the nFET conduct the load fully?

    2) i need to switch a load between different current sense resistors (r1, r2, r3) or parallel them for different values. the fet gates would be switched using a dip switch. my current monitoring circuit (integrated into the LM1949) needs to read the voltage across the sense resistor. so if i monitor between fet common drain and ground the resistence will be fet(Rds-on) + sense, is this correct? the range of sense resistors (without fet) would be 0.1, 0.9, 0.8, 0.7, 0.6 (or parallel combinations). so there is a obvious issue of monitoring at the fet drain. could i use a multi nFET IC for each switch and parallel the FET's to reduce Rds?? any thoughts on a better way to switch the sense resistors in?

    with a single sense resistor the LM1949 will do peak&hold or just hold as follows (peak is typically about 1ms):
    R(ohms) peak/hold amps
    .10 is 3.85/0.94
    .09 is 4.27/1.11
    .08 is 4.81/1.25
    .07 is 5.50/1.43
    .06 is 6.42/1.67
     
    Last edited: Jun 4, 2009
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    If you are using switches (DIP or otherwise), why not use then to switch your sensing resistors to ground and avoid the additional FET Rds?
     
  3. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
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    can you recommend any dip switches that can handle 6 amps, and, have very low contact resistence? this would be ideal.
     
  4. beenthere

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    No DIP's but the NKK M series minature toggle switches are rated for 6 amps, as are the Tyco MT series.
     
  5. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
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    well, i thought of switches. cost wise much more expensive than a fet solution. but i'll check the tyco line to see if they are small enough to fit my small 3x2x1 case.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    You can buy MOSFETs with Rds(on)<.0019Ω. You probably won't find mechanical switches with such a low On resistance.

    EDIT: but not multiple units in one package with that low ON resistance.
     
    Last edited: Jun 5, 2009
  7. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
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    i will look into these fets. one that looks ok is IRF7904 http://www.irf.com/product-info/datasheets/data/irf7904pbf.pdf

    one last question. if Vds=0.021v and Vgs=10v does the mosfet conduct fully? the graph starts at Vds=0.1v. is it the case that if mosfet gate is driven hard Ids will ramp up irregardless of Vds? or at very low Vds, the current profile becomes f(Vds)=Ids?

    thanks
     
    Last edited: Jun 5, 2009
  8. beenthere

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    The device should be in full conduction at 10 volts Vgs, but applying another 2 - 5 volts is safe and insures full turn on.
     
  9. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
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    so what are your thoughts on using the fet itself as the sense resistor? i know they will vary slightly with temp change, but would they vary less than a TT Electronics OARS 1% sense resistor? http://www.irctt.com/file.aspx?product_id=7&file_type=datasheet

    using dual fet in so-8 i could put the dual in series with each other, or parallel them to obtain desired Rds.

    just wondering if a fet's Rds is more temp stable than a open air 1% sense resistor?
     
  10. Ron H

    AAC Fanatic!

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    They're not stable with temperature. IIRC, the tempco is about +0.7%/°C. They also vary from unit to unit, and as a function of Vgs.
     
  11. Ron H

    AAC Fanatic!

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    I was thinking that you could use MOSFETs to switch the sense resistors (as you are proposing), and add a multiplexer to select which resistor is read by the LM1949. More parts, but the Rds(on) is taken out of the measurement.
     
  12. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
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    yes, this was another thought i had, but using a shunt jumper instead.

    a multiplexer takes more footprint, but i dont think i even need a multiplexer, just a switch is needed, perhaps something like this http://www.fairchildsemi.com/ds/FS/FST3126.pdf

    thanks for the idea, will look into this.
     
    Last edited: Jun 5, 2009
  13. Ron H

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    A rose by any other name, etc., etc.
    When you configure a number of SPST switches so that one side is common to all, and only one comes on at a time, voila! A multiplexer!:D
     
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