On the previous slide, it was shown that the diode becomes forward biased with the collapsing of the magnetic field of the inductor. This occurs without any reference to a clock and hence is known as an asynchronous regulator. The VF of the diode used represents a significant portion of losses within an asynchronous regulator, especially as the value of VF approaches the value of VOUT. By replacing the diode with another MOSFET, the losses in the system can be reduced. This requires a more complex controller to ensure the MOSFETs are not both turned on at the same time. Since the bottom MOSFET is now controlled, the topology is referred to as a synchronous regulator. In the schematic on the right, S1 is referred to as the top FET or the high-side FET while S2 is referred to as the bottom FET or the low-side FET.