Synchronous Buck Converter - Current sourced by the NCH mosfet
Well that's easy - the source is connected to the ground plane and the MOSFET is turned on, so the current comes through the MOSFET from the ground plane.
&quot;Wait&quot;, you say, &quot;it comes from the ground plane?&quot;
Yes. The left end of the inductor has a voltage below the ground plane. Remember the inductor really wants to keep the current the same, and it will make whatever voltage it needs to make in order to &quot;suck&quot; current.
This is how a buck converter works: The current goes around in a loop, through the inductor, the lower MOSFET, and the load/smoothing capacitor, until the inductor runs out of juice (until it's fully discharged). Every so often, the controller turns on the upper MOSFET instead, and the supply &quot;pushes&quot; current through the inductor, recharging it.
In a non-synchronous buck converter, the lower MOSFET is replaced with a diode. When the inductor sucks current through the diode, it creates a 0.6V voltage drop across the diode which wastes energy. In a synchronous buck converter, a MOSFET is used. The MOSFET's body diode may conduct for a moment, but then the controller turns on the MOSFET which allows it to conduct with much less loss.