creating dual output pwm with 555 timers

Discussion in 'General Electronics Chat' started by elec-eng-99, Jul 4, 2008.

  1. elec-eng-99

    Thread Starter New Member

    Jul 4, 2008
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    Hi Everyone,

    I am trying to design a pwm circuit that outputs two waveforms. Each is to have a 0.5 duty ratio. The two waveforms must be 180 degrees out of phase with each other. To this end I have been researching 555 timers. I now understand how I would use a 555 timer to produce a single square-wave output with the desired duty ratio.

    My question is this: If I were to use two 555 timers to produce two identicle outputs, is there a way to create the phase relationship i need between the two outputs?

    I'd be grateful for any help that anyone can give, from pointing me towards a good source for further research to suggesting a suitable circuit to use. Also, if anyone thinks that there is a better approach other than to use 555 timers, please let me know. :)
     
  2. jpanhalt

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  3. Wendy

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    Mar 24, 2008
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    You could also use a dual 555, a 556, and wire the other side as an inverter. 3 parts, a chip, a capacitor, and a resistor. So what are you going to use it for?

    [​IMG]
     
    Last edited: Jul 4, 2008
  4. elec-eng-99

    Thread Starter New Member

    Jul 4, 2008
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    hi guys,
    thanks for the tipps.
    In response to bill's question: I intend to use the pwm circuit to control a 2-cell interleaved boost converter. I want to be able to use the converter to charge some large capacitors. I then want to build on that to make a small coilgun.
    I just graduated as an electrical engineer, I have been fascinated with coilguns for ages and I thought building a low-power one would be a great way to continue to improve my practical skills. I also think that it provides a cool way to fill in some of the gaps in my education, for example; we were never taught anything about 555 timers.
     
  5. SgtWookie

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    Jul 17, 2007
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    You can also use a Schmitt-trigger input CMOS IC that has an inverting output to build a simple astable multivibrator, and another gate to invert that for the 180º out of phase signal.

    However, whether you use a 555 timer and a transistor to invert it, or a couple of CMOS inverter gates, you will not wind up with a perfect 180° out of phase due to propagation delays.

    If you attempted to use the output of such circuitry to drive an H-bridge, you would wind up with the dreaded "shoot-through" condition, where both the upper and lower halves of the bridge are on simultaneously, thus shorting out your supply, and releasing great quantities of smoke with a loud "Bang"! :eek:

    You need "dead time", where both upper and lower sides are off for a short period of time. The dead time ensures that shoot-through does not occur.

    To that end, I've been playing around a bit with using comparators with a sinewave input (it could be a triangle wave, or basically any waveform with a slow rise and fall time) Have a look at the attached. It's a bit crude; wasn't meant for general distribution - just exploration of a concept.

    "vbat" = 12v
    The input is a low-PP sinewave.
    C1 isolates the DC level
    R4 & R5 cause the average voltage to be 0.5v
    R1, R2, and R3 set the comparator trigger levels
    R2 adjusts the "dead time" by varying the difference between the comparator trigger levels.
    R6 & R7 are due to the open-collector outputs of the comparators.
    Q1 thru Q4 are voltage followers to drive the MOSFET gates.

    Note in the simulated O-scope display that there is a definite dead time between the MOSFETs on times. R2 makes this very adjustable.

    There are H-bridge driver IC's available that would be much more economical and reliable to use than this experiment I whipped up.
     
  6. Wendy

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    Mar 24, 2008
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    This will also take care of the shoot through issue, I think...

    [​IMG]

    It would require a bit of modification to be acceptable for the new application, but there would be a small delay between the off time on one output and the on time of the other. This was originally designed to be a dual output Class D Amp, remove the cap, feed in a DC signal, and it becomes a variable duty cycle output. The 47Ω resistors may need to be upped a bit for proper operation, I picked arbitrary values when drawing this.
     
    Last edited: Jul 4, 2008
  7. jpanhalt

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    This may sound like a broken record, but the LT1158 is a single chip solution. The gate feedback is claimed to ensure no shoot through. It needs just a single PWM input to create both the top and bottom (inverted) drive signals. It has a wide duty cycle range and at the extreme near 100% converts to 100%.

    There are probably similar chips out there, but I haven't checked. John
     
  8. SgtWookie

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    Jul 17, 2007
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    Yep, there are. ST Microelectronics, International Rectifier, and many others.

    I wasn't advocating anyone use my example; it was simply exploration of a concept.
     
  9. elec-eng-99

    Thread Starter New Member

    Jul 4, 2008
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    hello again,
    Thankfully shoot through is not an issue with my circuit.It shares some characteristics with a H-Bridge,but is not the same.
    I have done some research into using the 555 timer as an inverter as Bill Sugested and have come up with the attached idea for a circuit. I would like to hear peoples opinions on my idea. Do you think it would work?
    Also, thanks sgtwookie for raising the issue of propegation delay. The frequency i intend to work it should be no more than 10KHz so i am hoping any delays are negligable compared to the period of the waveforms. If they are not I will have to re-think.

    btw please excuse the crappy circuit diagram, i only have paint suitable for drawing diagrams.
     
  10. Audioguru

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    Dec 20, 2007
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    Your circuit with the 555 ICs does not produce a good square-wave. Its duty cycle is 77 to 70. It also does not have PWM nor any dead-time.

    A CD4047 IC has an oscillator and a divide-by-two. It has two outputs that are 180 degrees apart and have a perfect 50:50 square-wave. It also does not have PWM nor dead-time.
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    Feeding any clock signal into a simple flip flop will accomplish 50% duty cycle and two outputs exactly 180­­º from each other. The Schmitt Trigger RC oscillator is the only way I know of for a 555 to produce a true 50% duty cycle square wave.
     
  12. SgtWookie

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    Jul 17, 2007
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    If one wants real exactness w/o sacrifice, a TTL 54/74 265 IC will give up to four complementary outputs from an input signal that will be "right on", with no division of frequency.

    Of course, if you connect other items to the IC's output that have different propagation delays, then you're on your own.
     
  13. elec-eng-99

    Thread Starter New Member

    Jul 4, 2008
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    thanks for all the helpful suggestions guys!! I think the best way for me to proceed now is to actualy build some ideas and see what works for me in practice.
     
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