Balancing power transistor output

Discussion in 'General Electronics Chat' started by jrdoner, Apr 25, 2015.

  1. jrdoner

    Thread Starter New Member

    Feb 21, 2012
    I have a power supply arrangement using two D1088 (6A. PNP Darlingtons) the bases of which are controlled by an LM317. Thus the two transistors are carrying the current, not the LM317. It works very well, giving tight regulation over widely varying load. But here's the rub: one of these two transistors runs about twice as hot as the other, which I suppose is due to normal manufacturing differences from one to the next. If I were trying to balance the two to carry about the same current, would I put a resistor in series with the base of the hotter transistor? That should restrict base current and cut the heating of that transistor. However, I don't know if there is any downside to such an arrangement.

    Any thoughts would be appreciated.
  2. dl324

    Distinguished Member

    Mar 30, 2015
    Do you have a schematic? Technically the LM317 doesn't drive the base of your pass transistor. The current going through the regulator goes through a resistor that turns on the PNP.

    I suspect you have the two darlingtons wired in parallel. If this is the case, that isn't recommended because Vbe variations will cause current hogging in one of them.
    Last edited: Apr 25, 2015
  3. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
    Without a schematic the best advice I can offer is that you consider use of a low-value ballast (cip 'degeneration') resistor in series with the emitter of each pass transistor...

    Best regards
  4. Ramussons

    Active Member

    May 3, 2013
    ...... which is the standard way of balancing parallel configuration.