www.rutronik.com SWITCHED-MODE POWER SUPPLIES 27 www.power-mag.com Issue 3 2024 Power Electronics Europe capacitors filter possible output-side RF interference. Resistors Resistors perform a variety of tasks in switched-mode power supplies: Among other things, they are used as bleeder or leakage resistors and as pre-charge resistors, for protection against overvoltages and overcurrents, and for current measurement. The bleeder resistor is used to discharge the capacitor, as this could otherwise cause an electric shock even when the power supply is switched off. It is not absolutely necessary in regulated low-voltage power supplies, and it is not needed in linear voltage regulators or switched-mode power supplies with fast duty cycle control to maintain a constant DC voltage. Highohmic/high-voltage series are used for this application. Axial, leaded safety resistors are typically used as pre-charge resistors for the buffer capacitors. This is because they bring low resistance value and high pulse strength. In addition, resistors are used to detect the phase position of the AC voltage to achieve a more accurate divider ratio. Thin film MELF resistors with outstanding pulse load capability and flat chip precision resistors in thin film technology are suitable for this purpose. Varistors “clamp” overvoltages to protect the non-inverting input of the comparator. Overvoltage metal oxide varistors perform this task. Thanks to their halogen-free, high-temperature-resistant silicone coating, they operate at an operating temperature of up to 125 °C and have a maximum current-carrying capacity of 13 kA. Resistors for overcurrent protection and current measurement When powerful loads are switched on, very high currents occur for a short time, which can cause damage to the system. PTC and NTC thermistors are used as switch-on current limiters or overcurrent protection. They can also be used for temperature measurement since their electrical conductivity changes as a function of temperature. The easiest way to limit high switch-on currents is to use low impedance power resistors. In normal operation, however, a relatively high power loss occurs at these resistors. For this reason, the use of NTC or PTC thermistors is recommended. When combined they offer the greatest advantages. The most important selection criteria for the NTC thermistor are the maximum current and the nominal resistance (R25). The latter must be at least large enough, by circuitry in series with the load, to limit the current to a value that will not blow the fuse and cause damage to other components. The maximum current is determined by the power of the load. The derating of the NTC thermistor must also be taken into account. PTC thermistors are suitable for safe current limiting with high-capacitance capacitors in DC intermediate circuits. Due to the high current flow, they heat up and become highly resistive and thus intrinsically safe. As a result, they limit the current to safe values in the event of a short circuit in the DC intermediate circuit. They are designed for DC voltages of 260 to 560 V, offer resistances of 22 to 1100 Ω at 25 °C, and, depending on the type, have UL, IECQ, and VDE approvals as well as AEC-Q200 qualification. Another application of resistors in switched-mode power supplies is current measurement. Low impedance shunt resistors are used for this purpose. The current flow can be detected and evaluated via the voltage drop at the resistor. Full metal resistor elements made of manganese-copper and nickelchromium-aluminum alloys are ideal in this respect. Their material properties mean that they have very low temperature coefficients and are also low in inductance. If the resistive element is a metal strip, resistance values of as low as 15 µΩ can be achieved. Summary The importance of passive components in switched-mode power supplies should not be underestimated, as they perform a wide range of tasks. As their importance continues to grow, suppliers are working to improve their performance. As such, it remains exciting to see where the journey of technical developments will take us. All the components mentioned are available from Rutronik with a wide variety of choices in all designs and performance classes. Customer-specific solutions, especially for inductors, are of course also possible, depending on the required quantities. Figure 4: Section through an MLCC Figure 6: Resistor elements made of solid metal are suitable for current measurements.
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