LC filter design for multiple pulse width modulation for dc/ac inverter

[maicael]

hello,
i am a student who has been experimenting on pwm since i am relatively new so i and my friend implemented a SPWM inverter using a micro controller using push pull topology and and we also implemented another inverter using multiple pulse width modulation where each half cycle is broken down into series of pulses of equal width.
Now for the Multiple PWM the switching frequency is 25khz at the output of the inverter and also same 25kHz for the SPWM inverter.
My main question is that for the Multiple PWM inverter using push-pull topology how do i design an appropriate LC filter to get the fundamental 50Hz sine wave.
is it thesame technique used in designing filters for SPWM inverters or does the fact the the pulses in this inverter do not vary in a sinusoidal manner(SPWM) but instead have equal widths meaning that each half cycle of sine wave is broken into a train of pulses with equal widths meaning that the filter will be altered a little bit or something.
for the Multiple PWM inverter we have tested various loads like 60watt bulb, 75 watt bulb, soldering iron and so far they work ok without filter at the transformer output.
so now i want to design an appropriate filter for the Multiple PWM inverter.
the specifications are
output voltage-220V
switching frequency-25KHz
power rating could be as high as-500W or higher.
thanks for the help.

 

[--BawA--]

To design a LC filter , the value of inductance must be chosen in such a way that the impedence offered by the inductor at fundamental frequencu is 2% to 3% of charecterstics impedence of the invereter , after getting the value of L , you can find C from the formula F=1/2*pi*root(L*C).

 

[maicael]

To design a LC filter , the value of inductance must be chosen in such a way that the impedence offered by the inductor at fundamental frequencu is 2% to 3% of charecterstics impedence of the invereter , after getting the value of L , you can find C from the formula F=1/2*pi*root(L*C).

thank you for replying but i have a couple of questions
when you mean impedance offered by the inductor at fundamental frequency do you mean output impedance?
also just for knowledge sake why do we need to ensure that it stays between 2% to 3%?
does it have anything to do with total harmonic distortion(THD)?

 

[BradtheRad]

This link has an informative article:
'DC/AC Pure Sine Wave Inverter'

It describes designing and constructing one.
(This ebook is several MB and may take a while to load. It can be found at more than one website, and it may pay to try a few, if one site bogs down.)

**broken link removed**

Page 39 tells how the team had to make an output filter (a task which appears the same as yours). They developed a filter as shown in Figure 29. It has two coils and two capacitors.

I have tried simulations of different filter networks. The one in their diagram appears successful.
It would be tempting to try to use 1 coil and 1 capacitor. However that does not work well.