The Usual Suspects
2. Inductance and Capacitance
The main culprits behind MOSFET ringing are inductance (L) and capacitance (C). These two properties are inherent in every electronic circuit — they're like the yin and yang of electronics. When you have both inductance and capacitance in a circuit, you create the potential for resonance, which is essentially the electronic equivalent of pushing a child on a swing at just the right frequency to make them go higher and higher. In the case of ringing, you don't want the signal to go higher and higher; you want it to settle down quickly and quietly.
Inductance comes from things like the leads of components, PCB traces, and even the MOSFET itself. Capacitance comes from things like the gate-source capacitance of the MOSFET, the drain-source capacitance, and any stray capacitance between traces on the PCB. Think of inductance as the resistance to changes in current, and capacitance as the resistance to changes in voltage. When these two properties interact, they can create a resonant circuit that oscillates at a particular frequency. This frequency is determined by the values of L and C, and it's this oscillation that we call ringing.
The interaction between inductance and capacitance creates a kind of energy storage and release cycle. Inductance stores energy in a magnetic field, while capacitance stores energy in an electric field. When the MOSFET switches, energy is transferred back and forth between these two fields, creating the oscillation. Imagine a spring-mass system: you pull the spring, release it, and the mass bounces up and down until friction eventually dampens the oscillation. The same thing happens in an LC circuit, except the "friction" is in the form of resistance.
So, the recipe for ringing is simple: add inductance and capacitance, stir well, and then switch the MOSFET. Okay, maybe it's not that simple, but you get the idea. The key to preventing ringing is to minimize the inductance and capacitance in the circuit, or to add enough damping (resistance) to dissipate the energy before it can cause significant oscillations.