The power modules used in modern electronic equipment are all so-called switching power supplies. The figure shows the basic schematic diagram of the switching power supply. The input power is connected to a switch that controls the energy delivered to the load through intermittent switching, which is a bit like using a thyristor to control the power to the load in AC. Here, the rms value of the voltage on the load is controlled by the switch. The instant the switch is closed and opened causes a dramatic change in current in the power loop, as shown in loop 1 in the figure. This drastic current change will produce electromagnetic disturbances, on the one hand, the conduction emission on the power line, and on the other hand, the radiated emission caused by the current in the loop. As we will see later, the electromagnetic radiation emitted by the input line of the DC/DC module is much higher than the radiation of the module itself. In addition to the first loop, the second loop is also a sudden change of current, and the period of change is the same as that of the first loop. This also produces electromagnetic radiation. In fact, although the voltage on the capacitor is called a DC voltage, there are actually many small fluctuations on it. The period of this small fluctuation is the period of the switching action. When a long wire is connected to the output end of the power module, the wire constitutes an electromagnetic radiation antenna. The electromagnetic disturbance of the DC module is not only reflected in the module itself, but mainly conducted through the input and output lines, and produces strong electromagnetic radiation. In particular, some modules have metal shells, and the electromagnetic radiation of the module itself is not the main factor.