555 PWM problem in PSpice

Discussion in 'General Electronics Chat' started by pan--, Aug 11, 2010.

  1. pan--

    Thread Starter New Member

    Aug 11, 2010
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    Hello I am trying to make a 555 timer PWM and connect it to a mosfet for a buck converter. My problem is that the PWM wont go down to zero.. also do you know how to make my duty cycle from 50% to 25%? This is my circuit:
     
  2. PRS

    Well-Known Member

    Aug 24, 2008
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    The usual way of using a 555 timer to create PWM is to use two. The first is an astable multivibrator which provides a square wave to drive the second 555 timer which is hooked up in the monostable mode. The mono varies the pulse length such that it can be used to vary the speed of a dc motor. Say the first stage creates a square wave with a PRR of 1 kHz, then the second stage is made such that it varies the on time from zero to 100 percent. The frequency of the first stage is under your control and you can calculate it, so too the pulse width of the second stage. Fixed capacitors are used in conjunction with variable resistors, but you can use fixed resistors with variable capacitors, too.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    Actually you can use lots of ways for a 555, two timers will work but it isn't necessary.

    Is the 25% to 50% a firm number? You mentioned going to 0% PWM.

    LEDs, 555s, Flashers, and Light Chasers

    Chapter 5 The 555 and PWM

    Is that MOSFET a logic level type? If it isn't then a 7404 won't cut it for driving the MOSFET. A CMOS 555 will do a much better job overall, and a standard 555 will work better than a 7404. The real problem is you need more than a 10V power supply if it is a standard MOSFET.
     
  4. Ghar

    Active Member

    Mar 8, 2010
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  5. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    You don't want to go to 0% PW very often because the inductors will ring. If you shoot too narrow a pulse of current into a coil it causes a higher proportion of noisy inefficient and often destructive resonance. Inductive motors have similar issues. It also depends on frequency and the coil (Lower frequencies and lower inductances tolerate lower %) but below about 2 to 10% PW you might use other methods like pulse blanking or other frequency reductions to reduce the output.

    If you were using 400 Hz and .1uH then you probably can go to .1%, but if you had 1H and 400kHz then you might have to stick to nearly 50%.
     
    Last edited: Aug 11, 2010
  6. pan--

    Thread Starter New Member

    Aug 11, 2010
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    Thanks everyone for your help. Do you know also why when I change the value of resistance R7 the output voltage will change as well? I mean.. an increase of R7 will cause a decrease of the output voltage. Do you know how can I explain that?
     
  7. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Increase of R7 causes a decrease in Output Voltage?

    R7 is a pull up resistor for the gate of the FET. If the resistor is larger then your obsolete 7401 NAND open collector output gate will sink all the current from R7 more easily but the gate charge will not get high enough quick enough. Your RC time for R7 with the FET gate being the C, will get too long.
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Also R6 at the output of the 555 is not needed, or at least the value can be much higher.

    Bertus
     
  9. pan--

    Thread Starter New Member

    Aug 11, 2010
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    ok thank you very much also.. please see the image below.. Do you know why the duty cycle makes that knee and its not rectangular like it should using the nand gate? I believe it has to do with the mosfet or something and the current but I am not quite sure.
    Thanks again for the previous answer

    http://www.mypicx.com/uploadimg/42865842_08282010_1.jpg
    The green line is after the nand gate
     
  10. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Looks like a capacitance charging curve. Is that with the larger value R7 because that could be the gate charging?

    I am surprised that it says that you will get that close to the rail when the gate drive is from the rail.

    The IRF150 is rated for 2 to 4 Volts threshold I think.
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    A side comment, a 7400 series quad NAND gate is not meant to drive a MOSFET, ever. What prompted you to select this device?
     
  12. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    I assume that he is in the same class as Vultac who also wanted to use the same type of NAND for his switcher project.

    Almost identical projects and they started with similar parts.
     
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