Sorry it is not 12 volt peak - to peak....it is 12 volts rmsA square wave at 12V peak-to-peak will only give you 6V RMS.
What kind of transformer do you have?
Why do you need 220V at low current?
step up transformer
i need high volt and low current in a next circuit needs high volt and low current
the problem is how to convert the variable dc (from 3 to 12 volt dc) to variable ac (from 3 to 12 ac)