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MosFet: Source-Bulk-Diode Problem...how to insure Bulk is at highest Voltage?

Discussion in 'Electronic Basics' started by Diego Stutzer, Dec 16, 2004.

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  1. Hi all,
    Lets say I intend to design a simple circuit that shoud connect a
    (large) capacitor with a pulsed voltage-source, if and only if the
    voltage of the pulsed source is at a higher level than the voltage of
    the capacitor (the cap is charged). To do this I want to use a
    (certain) MOS-Fet Transistor.

    Now I've got the problem, that there is no signal where the potential
    is allways at the highest level in the circuit. E.g. If the pulsed
    source is charging the capacitor with a large current, there will be
    quite a bit of voltage-drop across the MOS-Fet, and while the source
    is at level 0 the transistor will be turned off an there will be a
    voltage drop the other way.

    So how schould I connect the bulk of the mosfet in a way that the
    source-to-bulk-diode never becomes foreward biased (to high)?

    I thought maybe a kind of "bulk-switch-circuit" could help? Maybe also
    just a simple diode (with a foreward voltage lower than that of the
    transistors-diodes)?

    Any ideas, experience?

    So far, Thanks for reading,
    Regards,

    Diego
     
  2. Jim Thompson

    Jim Thompson Guest

    You have a "MOS-Fet" with a separate bulk connection?

    Post a schematic on a.b.s.e

    Your description is clear as mud.

    ...Jim Thompson
     
  3. Diego Stutzer wrote...
    Which one?
    Most MOSFETs we use these days are vertical or VMOS types, which
    have three terminals. Generally the drain is the substrate, with
    an intrinsic diode to the source. The pre-connected substrate
    is also present for the popular lateral MOSFETs used in audio
    amplifiers. Many of the old-style "ordinary" MOSFETs with four
    terminals, like the 2n4351, etc., had a fourth terminal connected
    to the case, not to the substrate. So in all these cases as a
    designer you get to (have to) simply ignore the substrate.

    If you're using a 4-terminal MOSFET with a substrate pin, in many
    cases you can still ignore the substrate, and let it seek its own
    voltage through occasional substrate-diode conductance and low
    leakage-discharge currents. In rare cases you'll take charge of
    this pin to set the voltage, to reduce capacitance, etc.
    Yes, this is what some IC designers did, especially with CMOS
    analog switches, e.g., as first shown in RCA's CD4066 datasheet.
    Here they wanted to improve Ron as a function of signal voltage
    and used a second analog switch to connect the n-channel signal
    FET's substrate to the signal when the switch was on and to Vss
    when the switch was off. This trick is still commonly used in
    linear CMOS switches.
    As explained, the necessary diode is already present.

    BTW, Diego, it's not good to crosspost to so many groups.
     
  4. Joerg

    Joerg Guest

    Hi Diego,
    Wouldn't a diode alone do that? Anyway, if you are concerned about the
    substrate diode shorting your cap back to the source the only way I see
    is to place a diode in series.

    Win is right about too many cross posts. Also, I wouldn't go across
    language barriers, at least not a lot. Except if you worked at my
    favorite radio station when I was a student, where they happily jumped
    between four languages ;-)

    Regards, Joerg
     
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