Trouble with LTspice model of PNP transistor

Discussion in 'General Electronics Chat' started by Tabur, Dec 19, 2013.

  1. Tabur

    Thread Starter New Member

    Mar 15, 2012
    Hello everyone!!!

    I'm having trouble with the PN2907A model in LTspice. According to the datasheet, it has a max current of at least 500mA(can get 800mA pulsed) and a hfe = 50 at that point.

    I'm trying to turn on 32 LEDs (wow!) using one of those devices, with a duty cycle of 1/8. I'm currently studying peak currents, so no duty is in place yet (worst case scenario).

    The thing is that, for a base resistance of 192Ω, I should be getting the 0.96A in the collector using 19.2mA in the base using a Vcc = 5V connected to the emitter (Please see the attached .asc). Instead, I get around 144mA no matter how much I vary the base current Ib, which makes no sense.

    Is the model wrong, or am I missing something?

  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    It is not a problem with LTSpice or the transistor model.

    Your problem is the Vf of the LEDs and the two resistors in series with them. You have too much voltage drop between the collector and ground, so the transistor is saturating.
    Here is what happens if you just sweep the transistor without letting it saturate.

    Note that I am plotting β for you. (red trace)

    ps, unless you have purchased specially matched LEDs, you cant operate them in parallel without having a current equalizing resistor in series with each one...
    Last edited: Dec 19, 2013
  3. Tabur

    Thread Starter New Member

    Mar 15, 2012
    Mike, thanks for the reply!!!

    It was very late at night and I made a trivial error! Thanks for pointing it out!!!!

    Those pair of resistances should be in every led... I'll change the design accordingly and let you know if things are still bad!

    Thanks a lot!