control pulse width of a -15vdc pulse train @ 20khz

Discussion in 'General Electronics Chat' started by fringe, Jan 22, 2011.

  1. fringe

    Thread Starter New Member

    Mar 16, 2009
    1
    0
    Help needed to control the pulse width of a (negative) -15vdc pulse train at 20khz..

    Basically, the pulse is as described in the picture attached which is of the pulse itself all details are within picture.

    I will not need to vary the frequency only the pw..
    I would like to use something very simple... I have been searching everywhere.

    However, most designs are base on negative going pulses on a +dc supply and not on a wave form that actually goes positive from a negative -15vdc supply.....

    please idea's would be great appreciated.
    Ps: The pulse train controls a fet driven NEG Dc chopper circuit (high current)

    Thanks to all who take the time to at least read this post.
     
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  2. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,000
    1,512
    I'll take a shot at this.

    Use this circuit by Bill Marsden; http://forum.allaboutcircuits.com/showpost.php?p=117641&postcount=6 FIG. #4.3 to make your PWM wave form.

    Then using one of the following methods to switch your negative voltage;
    http://www.latticesemi.com/lit/docs/appnotes/pac/an6051.pdf

    http://www.zen22142.zen.co.uk/Circuits/Switching/negative_switch.htm

    http://www.edn.com/article/475807-Build_a_negative_voltage_power_side_switch.php

    You would build Bill's circuit using the correct resistors (R1 and R2) and capacitor (C1) to give the frequency you want. Hope this helps.
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    656
    Is this a pulse that already exists (you don't want or need to generate it)?
    It appears to be about 6 or 7 uS wide. Or is it 2.5uS?
    What sort of range do you need?
    Do you have a -15V supply?
     
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