Check out the inverter / grid tie inverter thread on this forum, buy a 150w inverter and dismantle it to learn how it works? There are some non PWM inverter schematics on the interweb, why do you specifically want PWM?
If you want to 'design' the circuit, figure out how you want it to work, ie. DC-DC step-up into PWM 50Hz modified sinewave. I've seen a design using two tl494 and a high frequency transformer for an inverter.