The buck regulator is an adaptation of my synchronous buck project posted here: 125vbuck.  That design provided 25 watts at 5 VDC.  It's capable of handling more power (I've tested to 100W), transient response is good, and it can function as an LDO (Low Dropout Regulator) - necessary to support the boost stage of the inverter.

Modifications:

• Removed the output Solid State Relay (SSR) - not necessary and reduces conduction losses.
• Removed PCB fuse F1.  Replaced with a thermal user push-to-reset 7A fuse mounted in the enclosure.
• Added 4000uF of bulk capacitance to the output.  This aids transient response and supplies the current necessary to respond to the inrush currents of user AC equipment.
• Adjusted the voltage divider for the control loop.
• Modified the control loop RC network.  Changed from type 3 to type 2 filter given the addition of higher ESR filter caps.
• Modified the MCU control program to support the higher output voltage and current.

The low losses in the power & output section, along with the control program, allow it to function as an LDO.  It's actually configured for a 13V regulated output and can hold this with an input of about 13.5V, depending on load.  Below this the output will track the input with a maximum 1V drop.j

The MCU will enter under-volt lockout (UVLO) around 11V by shutting down the regulator.  This turns the bridge & sine sections, and hence the AC output off.  The MCU will hold the unit in UVLO until 12V which allows for battery recharge in the PV system.

The regulator can maintain its regulated output with input voltages up to 20V.  Above this the MCU shuts down the regulator and enters over-volt lockout (OVLO).  This is necessary to protect the gate driver & 5V logic regulator.  Beyond the MCU shutdown there's no circuitry to disconnect the input so higher voltages will damage it.