Inverter oscillator duty cycle

Discussion in 'General Electronics Chat' started by danmec, Jan 25, 2011.

  1. danmec

    Thread Starter New Member

    May 4, 2010

    I am making some modifications to a discrete isolated DC/DC converter that is part of a high volume project. Currently I have a 555 timer, feeding into a flip-flop with those outputs going to a break-before-make mosfet driver circuit then to the transformer.
    I am trying to reduce the cost of this circuit and the two parts I plan on replacing are the 555 timer and the flip-flop.

    I saw this app note from TI that contained a low cost circuit

    The one issue that I see here is how do I ensure a 50% duty cycle from the three-inverter oscillator. I have seen this oscillator before and it is usually attributed with that property but I don't see that happening unless the logic threshold is half of Vcc. Most of the datasheets I see on inverters give a wide range for the threshold.

    So does anyone have any ideas for how to handle this issue?
    Possibly either recommending an inverter that is better specified for this circuit or some modification to the circuit that makes it less sensitive to the logic threshold.

  2. SgtWookie


    Jul 17, 2007
    Why don't you replace all of that stuff with something like an SG1525/SG3525, SG1526/SG2526?

    It's like you're trying to re-invent the wheel here. Regulating PWM IC's like these will cut your parts count and give you more functionality than you have now.
  3. Audioguru

    AAC Fanatic!

    Dec 20, 2007
    A CD4047 IC produces a perfect 50:50 duty-cycle because its RC oscillator is followed by a divide-by-2 digital divider. It also has 2 opposing outputs.
  4. Wendy


    Mar 24, 2008
    There are lots of ways to do this, what are you trying to do? Power a meter?