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Is there a simple way to change the width of a given square wave with duty cycle 50% without using microcontroller or ramp solutions? Actually, this square wave is switching frequency for my boost converter and I'd like to do PWM.

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    \$\begingroup\$ google for monostable multivibrator. \$\endgroup\$
    – jippie
    Commented Mar 6, 2016 at 19:49
  • \$\begingroup\$ As jippie says, look for a monostable, for example the good know IC 555 timer with astable output, there are a lof of of documentation about this IC in internet. google 555 timer. \$\endgroup\$
    – Alf
    Commented Mar 6, 2016 at 21:06
  • \$\begingroup\$ If you are hoping for voltage control of the new duty cycle, you should say so. It's pretty easy with a pot used as a variable resistance, not so much if you want computer control. \$\endgroup\$ Commented Mar 6, 2016 at 21:13
  • \$\begingroup\$ For a boost converter to work correctly, it will already need to do PWM (or pulse skipping) for the control signal. You may be going about this the wrong way..... \$\endgroup\$
    – jp314
    Commented Mar 6, 2016 at 21:57
  • \$\begingroup\$ Do you want to change frequency and have the duty cycle remain at 50%, have a fixed frequency and change duty cycle, or something else? \$\endgroup\$
    – EM Fields
    Commented Mar 6, 2016 at 22:20

2 Answers 2

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Buy one of these: -

enter image description here

I've used one exactly how you appear to be wanting to use one and it worked very well and saves a lot of hassle messing around getting component values right. Frequency is adjustable from low Hz to 1 MHz. Duty cycle variants exist that prevent 100% (flat line) or 0% flat lining.

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  • \$\begingroup\$ +1 for not recommending some kludgy analog circuit or a 666 timer. This is the answer that should float to the top. \$\endgroup\$ Commented Mar 7, 2016 at 12:01
  • \$\begingroup\$ Much cleaner than my solution but I don't see how the OP can synch it with his "given squarewave". The oscillator in this chip is free-running. \$\endgroup\$
    – Transistor
    Commented Mar 7, 2016 at 13:30
  • \$\begingroup\$ The OP can use this to generate the 'given square wave' and have duty cycle control; saves a square wave generator. \$\endgroup\$ Commented Mar 7, 2016 at 14:52
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. CMOS Schmitt trigger inverter based PWM control.

Using a CMOS Schmitt trigger such as CD41016 you can do this.

  • With input 'A' low, 'B' is high, 'C' is low and 'E' is low.
  • A rising edge on 'A' causes 'B' to be kicked low via C1 but it is pulled back high in time = \$R_1 \cdot C_1 = 100n \cdot 1k = 0.1~ms\$.
  • This short negative pulse is inverted by NOT2 and charges up C2 via D3. Output E switches high.
  • C2 discharges at a rate set by VR1 + R3. After the time constant \$C_2 (R_3 + VR_1) \$ NOT 3 will switch and 'E' will switch low again.
  • Pulse width at 'E' is adjustable by VR1.
  • D1 and D2 protect NOT2 against over-voltage.
  • R3 prevents a short circuit on NOT2's output.

Having looked at this you might decide that a monostable multivibrator is a much simpler solution.

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