feedback in switching voltage regulator
Go look up Type 3 compensators, which you'll need for voltage mode control. TI has a great article on them http://www.ti.com/lit/an/slva662/slva662.pdf you're currently using type 1 which doesn't have enough of a phase boost for stability.

Edit:
Also, your error amplifier (what's circled) should have a constant voltage reference as opposed to a ramp. The op amp that it connects to after should have the ramp. Your current configuration changes the voltage set point constantly.