The AC/DC power supply is a functional module that must be present in all AC-powered devices. Here, we analyze how it generates differential mode electromagnetic disturbance. If you have a designer specializing in power supply, you can refer to other more detailed information.
The AC/DC module mainly includes two parts of the circuit, one part is the rectifier circuit, and the other part is the DC chopping part.
The rectifier circuit converts the AC voltage into a DC voltage, and the DC chopper circuit chops the DC voltage into a pulse voltage, and controls the effective value of the output voltage by adjusting the pulse width of the pulse voltage.
There is a smoothing capacitor behind the rectifier circuit. The function of this capacitor is to smooth the output ripple voltage. However, this capacitor causes the input current to be pulse-shaped. Because only when the amplitude of the input voltage exceeds the voltage on the capacitor, there will be current into the rectifier bridge. This is a kind of differential mode disturbance current, which is dominated by low frequencies, and is actually dominated by higher harmonics of sine waves. This is called harmonic current emission, and it is called CE101 in the electromagnetic compatibility test GJB151.
A chopper circuit is a switching circuit that performs switching operations at a certain frequency. Therefore, this type of regulated power supply is called a switching power supply.
Obviously, the chopper circuit causes the pulsation of the input current. Due to the limitation of the series inductance, the current waveform is jagged. This is the high frequency differential mode disturbance, which is called CE102 in the electromagnetic compatibility test GJB151. The frequency of the disturbance depends on the frequency of the switching action.
In fact, the voltage on the switching device is pulsed, because when the switch is closed, the voltage is 0, and when the switch is opened, the voltage is the voltage on the DC bus. This pulse voltage is the common-mode disturbance voltage.