I am studying buck converters and have nearly grasped their ideal workings in the Continuous Conduction Mode however there is one last bit that I can't get a confident grasp with.
When the transistor switch is OFF, what makes the inductor go voltage go negative - Is it that at that instant the diode is not in forward bias mode, it is has therefore undergone a large negative di/dt (according to v=L(di/dt) and therefore a negative voltage is applied across it from say 5V to say -0.6V which is a typical diode forward voltage?? What restricts the maximum value of this voltage?
You are right in that there is certainly a negative voltage applied across the inductor. The diode is connected so as to be reverse biased on the normal flow of the circuit.
When the circuit has attained steady state with the switch closed, at that time the diode is still reverse biased and only passing some negligible current. At the time when the switch is opened, for a normal resistor, we know that the current will drop to zero almost immediately, but in the case of an inductor, it will try to restrict the decay of the current by applying a very high negative voltage which is dependent on both the inductor, time it takes for the switch to open and the core resistance of the inductor.
In fact, the reason why the inductor behaves that way is because it actually stores energy in its magnetic field. So when you stop the current from flowing, it returns all the energy it has stored. This will be seen as a voltage spike that may damage the switching component.
The purpose of the fly-back diode is that when the starts inducing a negative voltage, the diode will become forward biased and will have a forward voltage drop of 0.6v - 1v. Because of this, current will continue to be supplied to the inductor which will satisfy the inductor. Of course it is important that the diode should be able to handle the current flowing through the inductor.
Now because of power loss between the diode and the inductor core resistance, the inductor current will decay to zero after a while.