When designing a switching power supply, you may have heard of electromagnetic interference (EMI).
More and more applications must pass EMI standards in order for their manufacturers to receive approval for commercial resale. A switching power supply implies that there are electrical switchers inside the device, through which EMI radiates.
Sources of EMI in switching power supplies
First of all, you can’t beat physics. According to Maxwell’s equations, an alternating current produces an electromagnetic field. This occurs in every electrical conductor, which has by nature some electrical capacitance and inductance that forms an oscillating circuit. This oscillating circuit radiates electromagnetic energy into space with a specific frequency (f=1/(2*π*sqrt(LC))). The circuit acts as a transmitter of electromagnetic energy, but can also receive electromagnetic energy and act as a receiver. Antennas are designed in such a way as to maximize transmitted or received energy.
But not every application should act like an antenna, and negative side effects can occur. For example, switching-buck power supplies are designed to convert a higher electrical voltage into a lower voltage, but they also act as an (unwanted) transmitter for electromagnetic waves and can disturb other applications, such as interfering with the AM band. This effect is called EMI.