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Obtaining a sinusoidal signal from a generated square wave

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Newbie level 1
Feb 20, 2013
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Hi y'all

I need some urgent help. For our senior design, we are creating a digital UPS system, meaning that all our control signals and switching signals will be generated by an FPGA. So far we have finished designing the charging circuit and the DC to DC converter. We have also been able to do the comparison of the square wave with the triangle wave, to produce the necesary square wave. All that remains to be done is to pass the signal through a filter, which should then produce a nice sinusoidal wave form, except for the fact that we cant figure out how to determine the values of the inductance and capacitance. The output frequency should be 50HZ and with a power rating of approximately 150W. I would appreciate it if someone could help me understand how exactly I should decide on the value of C and L. Thank you!

... . with the triangle wave, to produce the necesary square wave....

I am assuming you mean PWM wave here.. instead of square wave ?
Surely you are using a PWM output ? Otherwise it might turn out to be a very bulky filter

In theory the schematic below will convert a square wave to a sine.
This will deliver 150W at 50 Hz and 230VAC. Your load calculates to 352 ohms.
The coil and capacitor values must be tailored to the load.

This is a second order filter. It must never be operated without a load. If you remove the load, resonating action will get out of hand, and cause extreme volt levels to develop on the components.

It only works in simulation. The coil and capacitor should be just right, so that the resonant frequency coincides with the switching device shutting off.

If things are not just right, then they may try to cause continued current flow after the switching device shuts off. I do not know how good this method will be for the switching devices.

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