Oh No ! I bought a Mppt Solar Controller

That round thing with wires is called an "inductor" (a torroid high efficient inductor to be specific) it is a device that stores energy and then releases it - current flowing in the wires creates a magnetic flux - when the current stops flowing the magnetic flux decays and produces a current flow in the wires. Yes there are other ways to get the buck conversion accomplished say a traditional less efficient transformer, buck conversion requires some form of inductor. Take a look at the Rouge controller guts that big nice Induuctor is the heart of the buck conversion stage. That thing costs money and when higher current is needed the price rises for that part - a good reason for multi phase controllers using many smaller inductors in parallel