Weird N-Channel MOSFET symbol

Discussion in 'General Electronics Chat' started by redbird_is, Jun 3, 2008.

  1. redbird_is

    Thread Starter Member

    Jan 13, 2008
    11
    0
    I've come across a symbol that I don't recognize for an N-channel MOSFET.

    As you can see in the picture, Q3 and Q4 use standard symbols, but Q2 has an extra wire (pin 4).

    Is pin 4 just another source connection?


    Thanks everyone
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    I am not sure but i guess the connection with the arrow on it (opposite the gate) is a fourth pin connect to the substrate of the MOSFET, so you can control the substrate voltage. With the ordinary MOS the substrate is connected internally to the source.
     
  3. redbird_is

    Thread Starter Member

    Jan 13, 2008
    11
    0

    Thanks for the reply. Yes, I agree that it must be some kind of substrate connection - perhaps it should be ignored when wiring (i.e. shouldn't actually wire the source to ground). Pin 4 may just be the one source connection.
     
  4. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    Those are sense-fets. That connection is used to sense the drain current.
    A small portion of the die area is bonded to a separate pad. If the gate to source voltage
    and gate to sense voltage match then the current out of the sense pin will be proportional
    to the drain current. Using a sense FET lets you eliminate a current sensing resistor.

    Search the IR and On-Semi websites for "sense fet" and you should be
    able to find some application notes. If you can't find an app-note send me an email
    and I will see if I have a local copy.

    (* jcl *)
     
  5. redbird_is

    Thread Starter Member

    Jan 13, 2008
    11
    0

    Thanks very much for the explanation, John. The circuit I'm building has a current sensing stage, so it makes sense that these sense FETs are used.
     
  6. DC_Kid

    Distinguished Member

    Feb 25, 2008
    638
    9
    is this type of FET used to clamp the current to a set point?
     
  7. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    Since the sense current is proportional to the drain current you could create a circuit
    that would reduce Vgs based on Isense.

    The important thing is to keep the sense voltage the same as the source voltage
    otherwise the Isense/Idrain ratio will change.

    (* jcl *)
     
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