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High side drive for MOSFETS

Discussion in 'Electronic Design' started by Eeyore, Jan 21, 2009.

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  1. Eeyore

    Eeyore Guest

    Can anyone put a date on when this became 'standard practice' ? Links to
    app notes would be nice for example.

    Graham
     
  2. Joerg

    Joerg Guest

    It was already done in tube amps, in the days when steam locomotives
    were state-of-the-art.
     
  3. Eeyore

    Eeyore Guest

    LOL ! :~)


    Graham
     
  4. Eeyore

    Eeyore Guest

    Seriously, with enhancement mode MOSFETs. Where Vdrive > Vsupply.

    Graham
     
  5. Joerg

    Joerg Guest

    Real men do not flinch when they drive a tube with a wee bit of grid
    current :)

    Oh, and the drive supply was bootstrapped.
     
  6. JosephKK

    JosephKK Guest

    Probably about the time the first MOS power transistors came out. Or
    slightly later when audio amplifiers sporting the devices came out,
    say about early 1970s. Yamaha IIRC.
     
  7. Joerg

    Joerg Guest

    Nah, before, just inverted polarity. Push-pull, one group of tubes with
    the plate at +900V, the other group with the cathodes at -900VDC. The
    grid of that 2nd group had to be below the negative rail. I built one of
    those myself, when I was a kid and blissfully unaware that doing two
    Cockroft-Waltons directly off 230VAC mains was, ahem, a tad on the
    brazen side. Woe to those who would not heed the dot on the power plug
    (they aren't polarized in that part of the world).

    This project was also the time I learned that electrolytics have a
    finite amount of ripple tolerance. The physics teacher at school never
    told us that, it was all ideal capacitors there so I assumed that would
    be the real world. One of the 470uF caps in the upper cascade decided it
    had enough of it and became a spacecraft. Fluorescent lights in the room
    dimmed. Now wait, there is no dimmer, and why does it all hum where
    there is no transformer ... phssooosh ... *BANG*. Crater in the ceiling
    plaster, burn hole in the carpet, missed my head (and right eye ...) by
    just a few inches.
     
  8. That reminds me of when I experimented a bit with using capacitor
    ballasts for high pressure sodium lamps around 1980 or in the veary early
    1980's.

    There are a few problems:

    1. Capacitor alone in series with a "discharge lamp" (arc and glow lamps
    in general) favors spiky current waveform, hard on electrodes of "hot
    cathode" ones (keep in mind "current crest factor"), and disfavoring
    efficiency of fluorescent lamps due to nonlinearities of those.

    A remedy that has been used is inductor in series with the capacitor,
    with series resonant frequency well above power line frequency but
    preferably below the 3rd harmonic.

    2. High pressure sodium lamps that are only rated for "inductive"
    ballasts have a millisecond or two of lag of current vs. voltage, making
    them "slightly inductive" (though there is no current-vs-voltage lag at
    zero crossings), and "more inductive at worst time with capacitive
    ballasts".
    Despite lamp voltage and lamp current zero crossings coinciding, I have
    had "HPS" (high pressure sodium) lamps with capacitive ballasts become
    "sufficiently inductive" to get both the lamp voltage and the ballast
    voltage exceeding the line voltage.

    3. More important still - AC usage of capacitors even of non-polarized
    types!

    Without an AC voltage rating, "You're On Your Own"!!! At Your Own Risk!
    70% or even 45% of the DC voltage rating can still have AC usage at 60 Hz
    making the capacitor get "awfully warm" to "egg-frying hot" and then fail
    spectacularly with a loud "bang"!

    With an AC rating, keep in mind frequency. If frequency is unstated in
    the AC voltage rating, do not assume safety at same voltage with
    frequencies above 60 Hz. However, I do still like to assume like to
    assume safety of rated AC voltage at 60 Hz, and at higher frequencies
    or with audio waveforms that RMS current same as achieved by max voltage
    60 Hz sinewave is safe as long as DC voltage rating has no peaks
    exceeding across the capacitor and RMS voltage across the capacitor does
    not exceed the AC voltage rating. (Keep in mind that most AC voltmeters
    read 1.11 times average voltage, and with most waveforms other than
    sinewave and squarewave RMS is more than 1.11 times average!)

    - Don Klipstein ()
     
  9. Joerg

    Joerg Guest

    This was a "real" ceiling. In Europe, concrete with a plaster layer
    underneath.

    Don't do that. If a young wild animal such as a fox kit finds it they
    can choke on stuff like that.
     
  10. Joop

    Joop Guest

    Besides that, you are likely to spill. So if the champagne is any good
    that is almost illegal...
     
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