Energy-efficient devices are important components for saving resources and protecting the environment. The more efficient the electronics, the longer the battery service life for mobile devices – and in large-scale industrial and server installations with thousands of loads, the energy requirement is markedly reduced. The basis for energy-efficient devices is essentially influenced by the power supply unit. While the linear regulators still most widely used in the past were voltage regulators, mainly switch mode power supplies are found in modern power electronics. The continuous reduction in processor voltages has played its part in this. A few years ago, switching frequencies up to 300 kHz were widespread, whereas modern switching controllers usually clock with frequencies of 800 kHz and more. The switching losses, on the one hand, but also the power inductor losses, on the other, are important aspects in the design of switching power supply units.