This is an idea for a true sine inverter to get 120 or 240VAC from lower voltage DC. This could be handy for running a mini-fridge from your car all the way up to powering your house with photovoltaic cells and lead acid batteries. Most inverters found in stores use an approximated sine wave or square wave which is alright; but devices are much happier when they have a sine wave, especially inductive ones. After looking at a few Vector units, which are all square wave, and some filter circuits, I came up with a preliminary design idea. So far it's just a block diagram. Maybe someday it may get a schematic or even built; but there are lots of projects going on around here.
You can see the circuit is divided into 4 sections: voltage booster, 60Hz generator, voltage swinging, and half cycle polarity changing.
In the voltage booster section, the low voltage is taken up to 180 or 350VDC. This is to allow for peak AC output voltage since it will be 1.414 times the RMS, and then another 10V on top of that as a safety margin.
The 60Hz generator section has the 60Hz sine wave oscillator which provides the basis for our output. Some components also take this and rectify it into pulsing DC.
Now the "voltage swinging" section is my idea. Basically, the PWM takes the pulsing DC and amplifies it through the filter. It works similarly to the way the voltage booster section does except it has an input reference which is always changing. This causes the pulse width to change. The filter smooths out the high frequency (say around 40KHz) pulses. What we get is a swinging voltage following the pulsating input, except a lot higher power. It's sort of like a constantly varying buck converter.
The "half cycle polarity changing" section is another idea. This is necessary to turn the 120Hz swinging voltage back into a sine wave. It just flips the H-Bridge over when the 60Hz reference signal crosses zero. The zero crossing detector outputs a pulse when the voltage crosses zero. The pulse is picked up by a flip flop which changes state, flipping the H-Bridge. Dead time in the H-bridge shouldn't be necessary because the switching is occuring at or very close to 0V. Even if all 4 transistors in the bridge were to be on, briefly making a dead short, it still shouldn't have much effect.