Even if the PSU is equipped with an MOV (surge protection) and an NTC thermistor (inrush current protection), it can still malfunction, especially if the voltage or current surge is too high. High voltage and current surges can be the cause of multiple component failures, including fuses, bridge rectifiers, diodes, and FETs. Those high currents are also called "inrush currents," and in power supplies, the main reason for them is the charge of the bulk cap(s). (Yes, coil whine is also generated by caps.)īesides high voltage spikes, which can be caused by weather conditions (i.e., lightning) or other problems in the mains grids, high currents can be sunk from the mains due to sudden (transient) energy demands caused by or during the system's startup. Something that many people don't know is that MLCC caps (and all other ceramic caps, for that matter) can be the source of coil whine. An interesting feature of high-dielectric-series-type MLCC caps is that their capacitance changes according to the applied DC voltage the higher the voltage, the less the capacitance. They offer numerous advantages, including low cost, small size, low ESR, high reliability, and increased tolerance to high ripple currents. MLCCs are widely used in power-supply circuits, mostly for filtering purposes. A Few Words About Multi-Layer Ceramic Capacitors (You can read more about electrolytic cap life calculation in our "PSUs 101" article, where we also discuss the various fan bearing types.) So those are the parts that tend to fail first in low-quality PSUs, but what causes failures in PSUs that use higher-quality components? We'll get to that, but first let's take a look at an especially important component in today's PSUs: the Multilayer Ceramic Capacitors (MLCC). In low-quality PSUs, the first parts to go are usually the electrolytic caps and the cooling fan.
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