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.

 

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.

 

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?