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Transistor Duty Cycle

Discussion in 'Electronic Basics' started by [email protected], Jul 11, 2005.

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  1. Guest

    Can you overdrive transistors in the same way you can overdrive LEDs,
    if you keep the duty cycle low?

    I need to drive 16 LEDs through one PNP transistor, with each LED
    connected to a constant-current sink of up to 55mA per LED (using Maxim
    Max6978's to sink), for a total of 880mA through each PNP transistor.
    But there's a 1/16 duty cycle (this is a 16x16 matrix), so the average
    power disipation won't be too high.

    I'd like to use 2N2907's (TO-18 case) or similar, but the datasheets
    say the max continuous current allowed is 600mA. The word "continuous"
    makes me think I could have higher peak currents, but I don't see
    anything in the datasheets as to what that might be (like I do with LED
    datasheets, where it's assumed you'll be overdriving them in
    multiplexed apps). Think it's ok to use the 2N2907 here, or do I need
    to move up to a higher-power transistor?

    thanks in advance,

  2. mike

    mike Guest

    When designing circuits, it's customary to DERATE devices rather than
    overstress them. If you're only gonna build one and you don't care if
    it fails, go right ahead. If you're building more than one, you're
    asking for a nightmare.

    Do some research on secondary breakdown. You don't say what voltages
    are involved, so it may or may not apply.
    Saw some research 30 years ago showing the effects of thermal cycling.
    Transistor chips have a thermal time constant that depends on the
    package, heat sink, etc. If you drive
    'em within the spec at a rate that maximizes the thermal excursion,
    few Hz rate as I recall, you can kill 'em in short a few
    hundred hours.

    I'd vote for the bigger transistor.

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  3. The devices may well stand the abuse, but the main reason for a 600 mA
    limit is that, above that, the current gain falls off dramatically, so
    that you need lots more base drive for a little more collector
    current, and the on saturation voltage goes to hell.

    You will be much happier with a ~3A device.

    I suggest something like one of these:

    Look at the gain and saturation voltage at .98A, compared to the 2N2907: Micro/Web Data/2N2905A, 2N2907A.pdf
  4. Guest

    Thanks John and Mike. I'll go with a larger transistor. The ZTX789A
    definitely looks interesting. One of my reasons for not wanting to just
    go with a typical 3A transistor was that I didn't want to give up the
    space that 16 TO-220's would take up.

  5. John  Larkin

    John Larkin Guest

    It'll be OK as long as you provide lots of base current, and some
    software stall doesn't leave it on full-blast.

    The Hfe may not be spec'd at 800 mA, so enough base drive will have to
    be a guess. But a fatter transistor would simplify life.

    Power FETS are great; some are spec'd to handle hundreds of times
    their DC power dissipation for short pulses.

  6. Guest

    The Max6978 has a watchdog that kicks in after 1 sec if my program
    hangs and stops sending data to the chip, so I'm not too worried about
    the continuous current.

    Is there a common Power FET you'd recommend for this type of
    application? Anything in a DIP package? Really all I need is a switch.
    My LEDs drop 4V at 55mA.

  7. John Larkin

    John Larkin Guest

    Just peek at a Digikey or Mouser catalog. You could use a P-channel,
    low-threshold ("logic") mosfet, like Zetex or IR or whatever.

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