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How can I convert a square wave signal from -15V to 15V to a positive DC signal?

Here are the details of my problem:

I'm trying to build a duty cycle for a buck converter in the following way: I want to compare (with an op amp) a DC signal with a sawtooth signal. When the DC signal is near the amplitude of the sawtooth signal => duty cycle will be high. When the DC signal is near 0V => duty cycle will be low.

So far, when I need a high duty cycle, ratter than a positive DC signal I have a square signal from -15V to 15V. I'll spare you the details but I have to deal with that input and can't expect anything else.

The easy solution would have been to have a signal between 0 and 15V and take the average with a low pass filter (then I would have a positive DC ouptut), but it isn't possible, I have to deal with a -15V 15V signal (thus I get a ~0V output, which is of no use since I also have a ~0V signal when I need a low duty cycle.) In fact, I have at my disposal only op amps powered by -15V 15V, thus I'm not seeing an easy way to get a positive DC signal from that.

Here are the components I have at my disposal: Resistors, capacitors, -15V/15V power op amps, diodes, electronic switches, buck converter.

For further detail, here is how I produce the 0V signal from 2 signals in phase (when I need a low duty cycle) so far: enter image description here enter image description here

And here is how I produce the square signal from 2 signals out of phase (when I need a high duty cycle): enter image description here enter image description here

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    \$\begingroup\$ Won't a diode rectifier help you convert the bipolar signal to unipolar? \$\endgroup\$
    – Andy aka
    Commented Feb 28, 2023 at 14:16
  • \$\begingroup\$ Options include offset the signal, rectify, and handle it in the time domain - I think microcontroller here. Please describe more about input and output required, and where "power" is supposed to come from. How do you also have a ~0V signal when I need a low duty cycle - I thought input was square wave and output a positive DC signal? \$\endgroup\$
    – greybeard
    Commented Feb 28, 2023 at 14:20
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    \$\begingroup\$ Just a diode like a 1N4148 should do the job. Bridges are generally not very good with PWM frequencies because they are slow devices. \$\endgroup\$
    – Andy aka
    Commented Feb 28, 2023 at 14:29
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    \$\begingroup\$ 2 square signals This question could significantly improve with a graph of input and output signals showing their relation. \$\endgroup\$
    – greybeard
    Commented Feb 28, 2023 at 14:34
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    \$\begingroup\$ @Andy aka found it, you're the boss :) \$\endgroup\$
    – c.leblanc
    Commented Feb 28, 2023 at 16:11

1 Answer 1

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For simple analog level shifting, such as used in video signals, an AC coupled negative clamp diode or transistor switch on edge with a pulse restores the negative level to 0V. Then the loading must produce negligible sag.

There exists both sawtooth and triangle OA circuits with phase control depending on what you need using SR latches or FF or gates.

Then Design by written parameters and plots and not just ideas or copy a great design and improvise.

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