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MOSFET Switch Configuration

Discussion in 'Electronic Design' started by yy7d6, Sep 1, 2006.

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  1. yy7d6

    yy7d6 Guest

    Fairchind has several line of products for MOSFET Switches but i can't
    find one that can handle at least
    -1.5A @ -14V and
    +1.5A @ +26V, are there already such fet switch ( w/ simple on/off pin
    ) in the market? if there's none how do i configure/design an N-Channel
    along with P-Channel MOSFET to achieve such range.

  2. yy7d6 wrote...
    It's easy, but we have to understand what you want. Do you
    have +26V and -14V sources and want to switch them to a load
    that will draw up to 1.5A, controlled by a logic level, etc.?
    How rapidly? What about short-circuit protection? Etc.?
  3. John Larkin

    John Larkin Guest

    Maybe you could use a solid-state relay. NEC has gadgets up to 5 amps.
    Try CEL.COM.

  4. Winfield Hill wrote...
    I'm feeling the need to write a few ASCII diagrams. Here's
    a simple bipolar switch that will connect either a positive
    supply or a negative supply voltage to an output, controlled
    by a 5V CMOS logic level. The circuit uses MOSFET switches,
    with gate voltages created by current-source level shifters
    to make the FET gate voltages independent of supply voltage.

    Each gate has a diode and pulldown transistor. Combined with
    the MOSFET's high gate capacitance, this gives the switches a
    fast shut-off, slow turn-on property, which insures that both
    switches will never be on at the same time during switching,
    avoiding high supply-to-supply shoot-through current spikes.
    In the case of relay switches, that's called break before make.

    .. High-Current Bipolar Voltage Switch
    .. +Vin
    .. o-----------+-------+------- S D --------------,
    .. | | _|___|_ |
    .. +12 to 150V 4.7k | ____ |
    .. | C G Q1 |
    .. +---- B npn | P-type MOSFET |
    .. | E -------+ |
    .. | | |
    .. +----|<|---------' |
    .. inverter | |
    .. __| \ _______ | ______ |
    .. | | / | | | out
    .. | C 2.2k +---(O)
    .. o----+---------- B | |
    .. npn E E |
    .. HI = pos out Q3 | B ---- gnd |
    .. LO = neg out 2.2k C pnp |
    .. full 5V CMOS | | Q4 |
    .. gnd | |
    .. +-----|>|-------, |
    .. | | Q2 |
    .. | E ------+ N-type |
    .. +---- B | MOSFET |
    .. -10 to -150V | C G____ |
    .. 4.7k | pnp _______ |
    .. -Vin | | | | |
    .. o------------------+-------+------ S D -------'

    There is no short-circuit protection; it's assumed that the
    two power sources have that capability.

    If 400V MOSFETs and 200V BJTs are used, +/-150V may be switched.

    OK, I got that ASCII drawing out of my system.

    yy7d6 asked about Fairchild MOSFET switches with an on/off
    pin. There are a number of companies that make "intelligent"
    MOSFET switches. These operate from a logic level and include
    current limiting and thermal shutoff features. They're available
    in "high-side" form, performing the task of the Q1 p-type switch
    above, and in low-side form, which pulls down to ground. The
    latter could be used for the task of Q2, but a level shifter
    would be required to obtain a logic-level signal working off
    the -Vin rail. A scheme like Q4 above could do that job.
    A logic inverter would still be required as well.
  5. yy7d6

    yy7d6 Guest

    Ayon kay Winfield Hill:
    John Larkin,
    i cannot use such relay, because its costly for our board design than
    mosfets. We will use hundreds of it : (about 512-to-1024 per
    board)xNumber of board of the design.

    The requirement is that the control that will switch off is a 3.3V
    logic, while the voltage input (VIN) of the mosfet pair would be -14V
    to +26 (+/- 1.5A max), this is much like an AC Switch.About the
    switching speed, that would be 50ns(20Mhz) full speed.

  6. Fred Bloggs

    Fred Bloggs Guest

    Looks like he has a single Vin terminal varying between -14 to +26, to
    be chopped at up to 20MHz. Of course it has to be low component count,
    come in under a penny, and use no more than 10mm2 of board space...too
  7. legg

    legg Guest

    Oh psshaaaw.

  8. yy7d6 wrote...
    OK, that's a 40V signal swing, and you need a Ron of no more
    than say 0.5 ohms? Is more OK? What about Roff (and C_off)?
    Is there an OFF feedthrough spec (looking at the 20MHz below)?

    And you want low cost and simplicity. What're the source and
    load impedances like? That's important in order to understand
    what shortcuts can be taken. Perhaps you'd better just tell us
    what you're really trying to accomplish. Where does the 1.5A
    spec come from, what's its relevance? Is that a current spike?
    What about switch charge injection, do you have a limit there?

    For example, depending on the exact configuration, it might be
    possible to use a single MOSFET. It could be an enhancement
    mode with 50-volt maximum gate voltage, or depletion mode like
    Supertex dn3525, controlled by a simple diode-resistor scheme.

    OTOH, if your Ron spec isn't so low, e.g. 20 ohms, you could
    consider the Supertex 8 or 16-channel 200V CMOS switches.

    You mean the 40V input signal has up to 20MHz in it? Or do you
    mean you want to switch 1.5A in 50ns, and do so up to 20 million
    times a second (BTW, that's 25ns max per transition)? Either
    way that's a tough spec, considering the issue of high MOSFET
    capacitances. Not to mention inductance! This is a place to be
    careful and explicit about your requirements, and not overstate.
  9. John Larkin

    John Larkin Guest

    What in the world are you building?

  10. true, did you see the worlds fastest (diesel) JCB video?

  11. John Larkin

    John Larkin Guest

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