Harmonics in a switching power supply are non-fundamental components present in its output current or voltage. A switching power supply uses high-frequency switching devices, such as MOSFETs, to convert the input voltage into a high-frequency pulse signal, which is filtered and regulated to obtain the required output voltage or current. The switching action of these devices introduces higher-frequency harmonic components into the output current or voltage.

Harmonic components are generally expressed as multiples of the fundamental frequency. Common examples are the second harmonic (2 times the fundamental frequency), third harmonic (3 times the fundamental frequency), and fourth harmonic (4 times the fundamental frequency). These harmonic components distort the current or voltage waveform and may interfere with other electrical equipment and power systems.

Harmonic currents in switching power supplies result from the nonlinear characteristics of switching devices and the frequency response of circuit components. To meet electrical safety and electromagnetic compatibility requirements, the amplitudes of these harmonic currents must be limited. Common ways to reduce harmonic current generation include designing filters, applying modulation techniques, and optimizing circuit design.