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need FET, P-ch, < 10nA off current @ 13Vds from 0 to 60C

Discussion in 'Electronic Design' started by [email protected], Apr 12, 2007.

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  1. Guest

    does this exist? I need the fet to act as a very open switch when off
    so a very sensitive circuit breaker does not trip. The circuit breaker
    remains tripped unless it sees less than 30na to the load.
     
  2. Winfield

    Winfield Guest

    acannell wrote:

    Subject: need FET, P-ch, < 10nA off current @ 13Vds from 0 to 60C
    You may be OK with most MOSFETs. I've measured leakage currents at
    room temperature of under 10 to 30pA for many MOSFETs, IIRC. With
    a current-doubling every 10C, that would translate into under say
    0.5nA at 60C. But you won't find this performance spelled out on an
    official datasheet. Have you taken any measurements? Oh, depending
    on the time-scales, etc., you may have to worry about capacitance.
     
  3. Guest

    I guess I just got unlucky? The first two mosfets I picked were the
    IRF4905 and SU65P04, the first has 25uA zero Vgs drain to source
    current, the second has 1uA. Are you perhaps referring to gate
    current?
     
  4. Guest

    Oh wait I thought you meant the datasheet would only show leakage
    current at one temperature and not the doubling per 10C, but you meant
    the datasheet will show worst case leakages and not the actual
    leakages, which are orders of magnitudes smaller, am I correct?
     
  5. Joerg

    Joerg Guest

    Datasheets are really, really conservative in that respect. For example
    the BSS84 states 100nA max at 25C but I have found it to be much lower:
    http://www.nxp.com/acrobat_download/datasheets/BSS84_3.pdf

    I don't think there is much of a selection beyond that except maybe for
    some fancy boutique devices. You may have to hand-pick or contract that
    out but calling a few mfgs to find the best contenders can't hurt.
     
  6. Winfield

    Winfield Guest

    Yes, I meant, go measure actual devices. Forget the datasheet;
    do your own characterization. For a production design, create
    a test fixture and add the tests to the manufacturing rules.
    However in that case, wise up and understand your part well.
    Understand the production spreads, and a half-year of supply
    in advance, so you can weather a crash in the fab processes
    while you select and get in stock an alternate part.
     
  7. You could use a second switch to help any leakage prob.


    1. P-FET upstream of the current limiter.
    ____________ _____________
    | | | |
    13Vdc -----|Series P-FET|----+---|Current Limit|--->Load
    |____________| | |_____________|
    ______|_____
    | |
    |Shunt Switch|
    |____________|
    |
    0v----------------------------+--------------------->



    2. P-FET downstream of the current limiter.
    _____________ ____________
    | | | |
    13Vdc--+--|Current Limit|----+---|Series P-FET|--->Load
    | |_____________| | |____________|
    | _______________ |
    | | | |
    +--|Shorting Switch|--'
    |_______________|

    0v------------------------------------------------>
     
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