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High-Side MOSFET gate driver voltage generation

Discussion in 'Electronic Design' started by wolti_At, Aug 23, 2008.

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  1. wolti_At

    wolti_At Guest

    Hello,

    I am looking for some advice for controlling an N-channel MOSFET
    (IPD64CN10N). Right now for testing I was using an IR2112 driver which
    I already had available and a small isolation transformer. I do not
    like this because this is a) quite expensive and b) a not so elegant
    solution. My application is special in the case that the source of the
    MOSFET does not stay on a constant potential - instead its source
    might change between -24V and 24V referenced to ground. This voltage
    is constant with respect to the switching times of the MOSFET(100kHz).
    The input voltage available is +24V. I have investigated the following
    solutions:

    a) Using a charge pump would allow me to generate a voltage above the
    positive supply rail to control the MOSFET. The main problem I see is
    that at the lower end of the operating point I would end up with a to
    high gate-source voltage and would destroy the MOSFET. Adding a zener
    in series to the gate would solve this problem but would add an extra
    and high load to the charge pump.
    b) Use a small DC/DC converter with proper isolation. In this case
    driving the mosfet is simple but the costs are high.
    c) I can not use the standard bootstrap methods provided for example
    by the IR2112 driver because I have no way to charge the bootstrap
    capacitor when the MOSFET is turned off.

    Thanks for all possible input,
    Christian
     
  2. legg

    legg Guest

    1) Don't float the source - use a p-channel mosfet in this location.

    2) Include the floating supply in the +/- 24V source generation.

    3) Re-examine your budget to put dollars and cents on your current
    generalizations and prejudices re cost. Include volume considerations.
    All other suggestions will have cost implications that you are
    with-holding from the analysis - wasting our time, and yours ($).

    4) Post a schematic. You may just be thinking yourself into a corner
    unnecessarily.



    RL
     
  3. wolti_At

    wolti_At Guest

    Hello,

    Sorry for my long reply - I have now decided to use a floating supply
    with a FOD3181 0,5A optoisolated gate driver. One reason for this was
    that the source of the n-channel MOSFET goes below zero volts and
    almost all high side drivers for MOSFETs don't tolerate a negative
    voltage with reference to their logic ground. Of course the source
    also goes above zero volts because otherwise I would have not needed a
    MOSFET at all. Posting a schematic is not possible for me right now
    because part of my work will be presented in a paper at the
    eurocon2009.

    Thanks,
    Christian
     
  4. Guest

    how about something like adum6132, supply,isolation and driver in one
    chip,
    though only +/-200mA

    -Lasse
     
  5. legg

    legg Guest

    You mean that the source goes below -24V, don't you? Most drivers are
    configured to operate referenced to the most negative power rail, not
    the 0V (system) reference.

    If you are attempting to use control logic referenced to system 0V,
    then this may be your trouble. Obviously, transformer-coupled or
    optically coupled drivers don't care where the signal input reference
    is, but lower-cost solutions will likely adopt the -rail driver
    reference convention, if only to reduce the cost of driving one set of
    switches; those switches tied to that rail.

    If one of the switched nodes is intentionally designed to exceed a
    supply rail, it may be of some advantage to configure the circuit so
    that the excess occurs with respect to the + rail, when using
    N-Channel or NPN switches.

    Level shifting from the system's zero voltage rail is most simply
    performed at signal logic impedance levels. These signals can then be
    buffered to the drive power stage without inherently involving the act
    of isolation. Combining the two functions may be a significant cost
    aggravator.

    RL
     
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