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Driving BJT's

Discussion in 'Electronic Design' started by James Rollins, May 16, 2009.

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  1. The terminal voltages of a BJT are drastically different. Is there an
    issue when driving BJT's as with MOSFETS gate source voltage? I've
    done some testing and it seems one doesn't have to worry too much
    about driving the base of a bjt because of the diode characteristics.

    One NPN I plan on using for a capacitor multiplier is rated at
    V_CB=1100V, V_CE=800V, V_BE=7V. Since this will be used in a sort of
    "high side" configuration I'm unsure of how to deal with keeping V_BE
    within spec. From what I understand it is not a big deal unless I'm
    running huge currents into the base?

    If this is the case then do I need to worry about extreme voltage
    spikes such as putting a zener across the base and emitter or is this
    even a waste?

    My question is mainly do I need to worry about the base-emitter
    voltage spec in a similar fashion as mofsets gate-source or for most
    applications is it not a big deal?

  2. legg

    legg Guest

    The Vbe rating of the transistor is it's reverse breakdown voltage,
    and should be avoided, just as exceeding the rated Vce should be

    Bipolar transistors require drive current, rather than drive voltage.

    Driving high voltage bipolar transistors as switches will require care
    to avoid saturation, if higher switching speed is intended. This can
    be achieved using Baker clamps to divert turn-on overdrive currents.

    Turn-off time requires reverse base current to remove stored charge
    from the quasi-saturated junctions. This must be voltage limited and
    is most effective if applied with a controlled di/dt.

    Because of the SOA limitations of bipolar switching transistors, the
    driving of capacitive loads is NOT recommended, without load-line
    tailoring through the use of inductive absorbers, or possible use of
    cascode emitter switching methods, which can extend the SOA.

    ST Micro (and possibly NXP and ON Semi) may have some good app notes
    that originate from products developed by Thompson-CSF, Philips and

    motorola AN873 AN875 AN951 AR119 AR120 AR131 AR317

    philips SC06

  3. Tim Williams

    Tim Williams Guest

    A baker clamp at Vce(sat) = 5V? I'd like to see that. (You'll need a
    feedback circuit to handle that, I think, or at least a zener and an
    ugly current shunt.)

    Curiously, I've entertained some possibilities, for the purposes of
    protecting beam tetrodes. Now, switching speed isn't a problem for
    tubes (they're like MOSFETs with a hell of a lot less C_miller!), but
    the screen pulls a hell of a lot of current if you let the plate
    voltage drop too low. So something baker-clampey would be good.

    In the end, I just went with a series screen resistor and a bypass
    capacitor. It's dumb and it works. %-) Vpk(sat) ~ 30V at 500mA isn't
    too bad for something that's essentially a 5kV, 1A MOSFET with 1/5th
    the gain.

    Switching speed may not be a problem; HOTs are usually spec'd for
    2-3us t_stg (and that's with Ib2 = -2*Ib1, IIRC). Which is pretty
    damn long, but typical for HOTs. Just means you have to turn off
    sooner, really; t_f is usually pretty sharp (~200ns), so switching
    loss doesn't have to be high.

  4. For Vbe watch out mainly for reverse voltage, keep the reverse voltage
    low, for example by connection a reverse biased diode across base and emitter.
    For the rest a transistor in common base configuration is a current amplifier,
    the current you drive into the BE junction in forward direction,
    is appearing beta times bigger in the collector.
    It would take a huge current to get more then about a diode drop for Vbe
    in forward direction... so that likely will not be your problem.
    There is some charge storage effect, you need to be able do sink
    (say short the base to the emitter) fast enough to get rid of the charge there,
    if you want to switch fast.
    This may require a special RC network in the base drive.
  5. Eeyore

    Eeyore Guest

    You rarely voltage drive a BJT and when you do, there's usually some
    source resistance present anyway. To get some Ic, you need base current.
    And yes it behaves like a diode so it sorts out its own Vbe vs Ib.

    See a data sheet.

  6. legg

    legg Guest

    The man is talking about driving bipolar transistors. Baker clamps can
    be designed for whatever saturation voltage is anticipated as
    acceptible. Vcesat of even KV bipolars is generally specified below
    All very interesting, but not of much immediate use to the OP.

  7. Jim Thompson

    Jim Thompson Guest

    Easy to do, if you _really_ want to ;-)
    ...Jim Thompson
    | James E.Thompson, P.E. | mens |
    | Analog Innovations, Inc. | et |
    | Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
    | Phoenix, Arizona 85048 Skype: Contacts Only | |
    | Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
    | E-mail Icon at | 1962 |

    Stormy on the East Coast today... due to Bush's failed policies.
  8. legg

    legg Guest

    From your other post, it seems that your 'capacitor multiplier' is
    actually a linear ripple filter applied to a HVDC souce.This has
    nothing to do with switching transistor drive requirements.

    Main problems will be behaviour under overload and fault conditions. A
    reverse EB diode and base current limiting resistor can protect the EB
    jn against overstress, but this does not address output short stress
    on the SOA of the regulating transistor.

  9. krw

    krw Guest

    Ever hear of a "voltage follower"? No, I suppose not.
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