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Mosfet question -- substrate connected to source

Discussion in 'Electronic Components' started by Tom McAndrews, Jan 20, 2004.

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  1. This question is geared toward the enhancement mode mosfet devices
    that have the substrate connected to the source.

    Since the device is not symmetrical, what is the downfall of driving
    current from the source to the drain and effectively switching their
    roles when the substrate is connected to the old source?

    Thanks in advance.
     
  2. Dave Platt

    Dave Platt Guest

    If you try to do this, the intrinsic substrate diode starts to
    conduct. As I understand it, it's not a terribly great diode in terms
    of either switching speed or forward voltage drop, but its presence
    pretty much eliminates the ability to use the MOSFET in its usual way.
    Remember, when you switch the connection roles around you do not
    magically change the internals of the MOSFET - the substrate is still
    connected to the "source" pin.

    I have seen one interesting application for a MOSFET wired up in this
    "backwards" fashion, in "Troubleshooting Analog Circuits" by Bob
    Pease (page 164). It makes a nifty polarity-reversal protector for
    power-supply-to-load hookups. For example, you install an NMOSFET on
    the ground side of the load, with the drain grounded and the source
    towards the load, and drive the gate via a resistor or voltage divider
    from the high side of the power supply.

    If the power supply is hooked up correctly, the positive voltage
    biases on the MOSFET, _and_ forward-biases the substrate diode, so the
    MOSFET switches on and allows power through to the load. If you use a
    MOSFET with a low Rds[on], you end up with less voltage drop than
    you'd have from a simple silicon-diode protector.

    If the power is accidentally hooked up backwards, the substrate diode
    is reverse biased (doesn't conduct) and the gate is pulled several
    volts negative with respect to the source and drain and switches the
    MOSFET off quite hard. The reverse-voltage standoff will be that of
    the MOSFET - probably a lot more than you could get with a
    low-forward-voltage-drop Schottky diode.

    To quote Pease, "Yes, this circuit looks funny. Yes, it is correct,
    and yes, it works well."
     
  3. Since the drain to source path is paralleled with a diode, using the
    fet as a conductive path only makes sense if it will always be turned
    on when the drain and source are reversed or the applied voltage will
    be less than about 300 millivolts. They are used this way as
    synchronous rectifiers.
     
  4. Ted Wilson

    Ted Wilson Guest

    Another notable example of an instance where we make good use of the
    integral diode is in totem-pole MOSFET drives for inductive loads.

    In these applications, there has to be a delay between switching OFF
    the conducting device and switching ON the device that was previously
    OFF, in order to prevent shoot-through. What then happens is that the
    inductor current commutates into the body diode of the device that was
    previously OFF, thereby providing a continued path for the current,
    and the diode is then shunted by the low Rd-s on of the MOSFET when it
    is later turned ON. (Yes, it does conduct in both directions).

    You kids have it easy these days though. Back in the eighties, you
    had to be really careful using power MOSFETs in totem-pole
    configuration. If one of the devices was OFF, but the current flowing
    through its body diode, you couldn't simply turn ON the other MOSFET
    to take over the current as this would 'pop' either the body diode of
    the OFF MOSFET, or the d-s channel of the MOSFET that you had just
    turned ON, due to the reverse recovery limitations of the in MOSFET's
    body diode.

    Regards

    Ted Wilson
     
  5. Veronique

    Veronique Guest

    Why is everybody talking about diodes? Power? Goodness.. there is only one
    thing that this will do. If the voltage on the drain of the FET is more
    negative then the source, then it's topologically the SAME as inverting the
    device (exchanging the drain and source) and tying the substrate to the
    drain. What that does is that it requires less voltage on the gate to turn
    the device on. In your question, you forgot to mention something.. it this
    pFET, or nFET?

    Unless I'm wrong, the device is symmetrical (It's a FET afterall). The only
    thing that changes is that you're effectively increasing the potential on
    the substrate, thereby lowering the Vt of the device.. the rest depends on
    whether you have a pFET or an nFET.

    Veronique
     
  6. Jim Thompson

    Jim Thompson Guest

    [snip]

    It's called a "body" diode for a reason. Unless the "body" is a
    separate connection, weird things can happen in reverse.

    ...Jim Thompson
     
  7. Tim Wescott

    Tim Wescott Guest

    I don't think that a MOSFET _has_ to be symmetric (you could build a
    symmetric bipolar transistor, after all, and even "regular" ones have some
    action when run in reverse). I just think that MOSFET designs tend to come
    out more symmetric than bipolars do.

    If you dig up a drawing of a MOSFET in a textbook you'll see that the
    interfaces between the substrate and the source and drain are PN junctions
    (a depletion-mode MOSFET will have one region that makes up the source,
    drain and channel). This junction will have some bias on it, and will
    conduct if it's a forward bias. So for a three-pin device you connect the
    substrate to the source, and all of a sudden your device has a built-in
    diode.

    When they first started using power MOSFETs in switching supplies this was a
    pain, because the diode is right where you put your snubber diode, only it
    has first crack at turning on. Since the device is optimized for
    MOSFETness, the intrinsic diode is usually a pretty bad snubber diode and
    can have an extremely long reverse-recovery time. Everyone has learned how
    to deal with this now, but it's still an issue to be ignored at risk.
     
  8. Can you talk some more about that? I noticed recently (while looking for
    replacements for a blown MOSFET) that some of them claim "ultrafast
    intrinsic diode" or the equivalent, so I assume that's one solution to the
    problem. You make it sound like there are other solutions - what are they?

    Thanks!
     
  9. Roy McCammon

    Roy McCammon Guest

    No problem when the device is used as a switch and is ON.
    But when it is off, you have to contend with the the diode.

    It is common to see two devices in series with their sources
    tied together. That way, when they are off, only one
    of them will have a forward biased substrate diode.
     
  10. I read in sci.electronics.design that Tim Wescott
    com>) about 'Mosfet question -- substrate connected to source', on Thu,
    22 Jan 2004:
    Philips did; the OC140 was a silicon alloyed device. Used in video
    processors, and designers were very annoyed when Philips suddenly
    discontinued it.
     
  11. Tim Wescott

    Tim Wescott Guest

    The only other way that can remember is to use a Shottkey in parallel with
    the MOSFET, pointing the same way as the intrinsic diode. Very fast
    transients will want to go through the MOSFET anyway, so you have to be
    careful that the diode sees a nice low-inductance path.
     
  12. Ian Buckner

    Ian Buckner Guest

    I recall there were some others, done like that for use as analogue
    switches.

    Regards
    Ian
     
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