Square wave AC control
bennettdan
Posts: 614
I would like some input as to control of a square wave AC voltage control like in a DC to AC converter I use a SX to generate a square wave and feed it into a pushpull IGBT set that makes a a nice square wave AC wave at 50% duty cycle but I want to be able to vary the amplitude of the AC signal and changing the duty cycle does not vary the AC signals amplitude. I am looking for some suggestions and like to hear from others with suggestions. Thanks.
Comments
In order to produce AC from a DC source, you either need an adjustable linear regulator (wasteful), an active reactive component (Resonate Coil Capacitor Combination - Usually for higher power applications) in your circuit, or a low-pass filter (Resistor Capacitor Combination - Usually for lower power applications) .... Or some combination of all of the above.
I have illustrated a few scenarios with different waveforms using a low-pass filter. As you can see the shape of the pulse can make a difference also. There are other factors that need to be considered such as how much power you are going to need at your AC voltage. (<-- How much load or demand you need)
Notice that there are two PWM wave patterns... PWM1 heterodynes two frequencies (800Hz and 860Hz) together by way of an XOR gate to create a 60Hz beat frequency. The result is a "sort of" sine that more closely resembles a triangle wave.
PWM2 uses a reference sine wave with a higher frequency triangle wave. Using a comparator, the output is a "better" PWM signal suited for generating a Sine Wave. With the PWM2, the sine wave, triangle wave, and comparator can all be done within software with a fast processor.
For Each group of signals (SQUARE,PWM1,PWM2) in the diagram, the top two waveforms represent voltage and current at the signal generation source (or the output of your push-pull IGBT's). The bottom waveform indicates the voltage across the capacitor.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Post Edited (Beau Schwabe (Parallax)) : 9/2/2008 6:12:27 AM GMT
I'd wondered how you do that. Thanks!
John Bond
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