As the voltage on the capacitor increases, its capacitance decreases, and the current charging it goes into the smaller capacitor to get a higher voltage. In this case, a hot-plug event of a 12V power supply can result in a peak voltage peak of about 30V. There are several ways to reduce the amplitude of the voltage ringing signal during hot-plug: Most power supplies use large output capacitors The switch-mode power adapter of this type of circuit has a low output impedance and can quickly generate large currents in the event of a hot-plug event. Adding a common mode inductor and a small electrolytic capacitor with high ESR will increase the output impedance of the adapter and suppress the resonance process.