www.rutronik.com SWITCHED-MODE POWER SUPPLIES 25 www.power-mag.com Issue 3 2024 Power Electronics Europe Passive components in modern switched-mode power supplies Hidden specialists Switched-mode power supplies are often dismissed as low-budget applications, but they hide specialized components inside without which they are unable to operate efficiently. This technical article shows what these passive components are and how they can ensure operational reliability. Although the design of modern switched-mode power supplies is often characterized by newer semiconductor technologies, in particular wide band-gap power semiconductors, a wide range of passive components is required for them to work. It is important to match the characteristics of the components to the respective application. Rutronik presents the most important passives for switchedmode power supplies. Inductors The high-frequency (HF) transformer and other inductors form the core of a switched-mode power supply (SMPS). Located in the input section, the interference suppression components ensure suppression of interference voltages and currents on the power supply lines. Current-compensated chokes (common-mode chokes) suppress asymmetrical interference present on both lines in common mode. They are typically constructed with high-permeability ferrite cores or nanocrystalline core materials. Linear filter chokes or differential mode chokes attenuate symmetrical interference. Most models have an iron-powder toroidal core or a ferrite EE core with an air gap, but open core designs such as bar or thread chokes are also possible. In some cases, common-mode and differential mode chokes are combined into one component. This means fewer components and therefore less space requirement and lower cost. In this case, the leakage inductance of the commonmode choke assumes the function of the differential mode choke; a magnetic bypass can act as an amplifier. Sumida offers this combination, for example through its RK17S and RK23S series. Optionally, power factor correction (PFC) chokes can be added to the suppressor chokes to provide sinusoidal current consumption for power factor correction. Like the chokes, the active PFC stages contain either iron powder or ferrite cores with an air gap to smooth the output current. Gate driver transformers (trigger transformers) are used to drive the power transistor (MOSFET and IGBT). Typically based on smaller ferrite core geometries than the chokes, they are characterized by low winding and coupling capacitance and low leakage inductance. Normally, they are rated for isolation voltages ranging from 1.5 kV to 5 kV and are available in THT or SMD versions. Power transformers made of ferrite cores are the heart of a switched mode power supply. On the one hand, they ensure power transfer from the primary to the secondary side of the power supply, and on the other hand, they are responsible for the safe galvanic isolation of the primary and secondary sides. Since the output side of the power supply is often open, i.e. accessible, this isolation is required by safety standards and must be taken into account in the design of the transformer. Proven soft magnetic and low-loss materials with high saturation flux density are used for the power transformers. Their size is reduced as the switching frequency of the power supply increases. For switching frequencies between 500 kHz and 1 MHz, the pulse transformers therefore require fewer raw materials, which has a positive effect on the environmental balance and sustainability of the power supply – an aspect that is increasingly coming into focus. Customized inductors In addition to standard inductors, application-specific ones are also available. For power transformers, for example, these are models with several different output voltages. Sumida specializes in this area. These can be variants of existing components that are tailored to a customer’s specific application based on standard pre-materials and existing technologies. Standardized core shapes and magnetic materials (e.g. standard EE, UU, ETD, EVD, EFD, EP, RM, ER, PQ, toroid core shapes) and standard plastic components (coil formers, packages, and base plates) are also used. For this purpose, Sumida can partly rely on its own MnZn and NiZn ferrites as well as iron powder cores, which the supplier produces in Obernzell, Germany. The nanocrystalline and amorphous core materials are purchased from specialized raw material producers. Increasingly, however, there are requirements that can only be met with completely customer-specific components based on new magnetic core geometries and, in some cases, even new magnetic material compositions, proprietary plastic parts, and new manufacturing technologies. These completely application-specific geometries are only suitable for a very specific application – but they are perfect for that application in terms of both geometry and size as well as their electrical function. This applies, for example, to high-power transformers for half-bridge, full-bridge, or LLC topologies in a power range of up to 30 kW. They are used, for example, in photovoltaic inverters or DC/DC converters in e-vehicles or in high-power DC charging (HPC) applications. Capacitors Capacitors perform many functions in switched-mode power supplies. AC capacitors on the mains side (primary side) are mainly used to suppress or filter interference pulses. Ceramic or film capacitors can be used for this purpose. When they are connected between phase and neutral, it is important that they are X2 or X1 certified. For the connection between phase and protective conductor, a Y classification is mandatory. Since this provides greater electrical and mechanical safety than X capacitors, short circuits cannot occur due to a capacitor malfunction, for example. Since X capacitors are connected between phases or neutral conductors, they do not have the same high safety requirements as Y capacitors. X and Y capacitors are further subdivided into different test/pulse voltages according to the requirements of IEC 60384-14 and are referred to as X2 and X1 or Y2 and Y1 types. The most common combinations are X1Y2 and X1Y1. Standard subdivisions are shown in Table 1. In addition, test marks such as ENEC, VDE, UL, or CQC can be found on most X and Y capacitors, since the components
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