JT> There are many similar arrangements, that only have a simple square
 JT> wave output in the second stage.  Generally, the above method is used
 JT> for the higher power inverters.
 JT> The second square wave type I mentioned is sometimes called a
 JT> "modified square wave inverter" although the ones I've
seen are quite
 JT> crude. 
 JT> The second stage switches like this:

 JT>  ----      ----
 JT>  |  |      |  |
 JT> -|  |-|  |-|  |-
 JT>       |  |
 JT>       ----

 JT> So much for "modified square wave".


That's exactly what that is.
To approximate a sine wave from a fixed voltage switching device there 
are two criteria to meet.
You can't drop the peak voltage otherwise you have problems with
say microwave ovens or other devices that rely on peak voltage.
At the same time you can't deviate from the RMS value since you
then burn out globes or heater bars.

So to satisfy both you have to meet 

Integral(0 to pi ) of (Vm sinwt)^2 dt = Integral(0-t) of Vm^2 dt


Fortunately I don't ofhand remember the integrals solutions anymore
leaving it to you to solve.

Regards,

Theo


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