Help with PWM Circuit

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vonsproken

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Hi,

I need a circuit for this please, or a hint of the type of circuit and components that I need.

I have an output from an oscillator that produces a square wave output, frequency range of 20 to 150kHz. The output needs to be split into an inverting and non-inverting output. Each of the outputs require an adjustable duty cycle. These outputs will be driving the transistor pairs of a H-bridge used to drive a transformer, the outputs obviously can have a maximum duty cycle of 50%.

I really only need help with a circuit that can adjust the duty cycle of a square wave signal. Adjustable in the range from 0 to 49%.

Help will be appreciated,

Thanks
 
Thanks for the reply.

The only problem is that the frequency is constantly changing in my circuit, so If I was to use this circuit the PWM would change as with the frequency, this cannot happen unfortunately.
 
What is the source of this varying frequency? It's very unusual to use PWM with a variable frequency.
 
The source is a voltage controlled oscillator from a 4046 running at 15V. The circuit will be running in a phase locked loop, outside changes will cause the phase to change and the frequency to increase or decreases. The PWM is so that I can control how much power the output circuit will draw.
 
You are likely not getting a response because no one can think of a simple way to maintain the duty-cycle of a PWM signal with a varying frequency. I expect you may have to use a microprocessor to detect the frequency and then generate a PWM signal at that frequency with the desired duty-cycle.
 
I had originally intended to use a microprocessor, but the only programming I know is picaxe Basic, I created a program but found it to be extremely slow. I'm guessing I'm going to have to learn C then.
 
PIC asm is REALLY easy to learn. It has like... 35 instructions. And you usually only use 3-10 of them frequently. The rest are used for when you need to get really trick with the code and are otherwise less frequent.

Myself and others can probably guide you through your project also.


Edit: What do you have as far as lab, equipment, parts, experience, budget? Also, whats your circuit and project. Give us a good overview of what your doing and we can know better where to go from there.
 
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Something based on this?
View attachment 68915
U1 inverts a duty-cycle control voltage.
U3,U2 compare a triangle wave respectively with the control voltage and its inverse. Their outputs provide drive voltages for amplifier stages (not shown) to drive the transformer. The duty cycle is independent of the triangle wave frequency.
 
Last edited:
What you might be able to do is amplify the phase locked loop error voltage and use it for comparison for a separate pwm.
 
Is there some reason that you can't just use a standard Full Bridge PWM controller IC?
 
That's OK as far as it goes, but here's the rub. How do you generate a constant amplitude triangle-wave controlled from an external variable frequency source?
 
How do you generate a constant amplitude triangle-wave controlled from an external variable frequency source?
Click to expand...
Yes, that could be a problem.
 
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