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Tom's Explains: Why Power Supplies Fail

The component of your PC that's under the most stress is the power supply unit (PSU), because it's the power-conversion bridge between the system's components and the mains grid. What that means: It has to deal with every abnormality of the mains and make sure those abnormalities don't affect other components. That's a tough job, and it gets even harder if there's no power conditioner or uninterruptible power supply (UPS) installed.

In low-quality PSUs, the first parts to go are usually the electrolytic caps and the cooling fan. (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).

A Few Words About Multi-Layer Ceramic Capacitors

MLCCs are widely used in power-supply circuits, mostly for filtering purposes. They offer numerous advantages, including low cost, small size, low ESR, high reliability, and increased tolerance to high ripple currents. 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. 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. (Yes, coil whine is also generated by caps.)

Ceramic capacitors are piezoelectric, a word that derives from the Greek words πιέζω (squeeze) and ήλεκτρον (amber). So when the applied voltage on a ceramic cap changes, its physical size slightly changes as well, and this can result in an audible noise that users perceive as coil whine.

The Reasons Behind Problems In Quality PSUs

Multi-Layer Ceramic Capacitor Problems

According to our sources, the majority of failures in quality PSUs are because of cracked MLCCs. Even a single broken MLCC can result in issues, and they can crack due to any of the following:

  • Bad handling (i.e. improper PCB stacking during the manufacturing process)
  • PCB bending (which can happen during the solder-wave process, if extreme heat is applied)
  • Careless soldering repairs on the PCB
  • Bent pins of In-Circuit Test (ICT) fixtures, which are used in the manufacturing line to quickly evaluate the PCBs

Long PCB Mounting Screws

This may sound silly, but in some cases, mounting screws that are too long can actually cause shorts to the PCB.

IC And MOSFET Damages During Assembly

If the manufacturing line is set at higher than normal speeds, and the applied heat is high, there can be either fatal or minor damage to ICs and MOSFETS, both of which will in the long run (or under stressful conditions) eventually cause the PSU's failure.

High Voltage And Current Surges

Besides 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. 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). High voltage and current surges can be the cause of multiple component failures, including fuses, bridge rectifiers, diodes, and FETs. 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.

Cracked PCBs

If the shipping conditions are not ideal, and the PSU packages are handled too roughly, you might encounter cases of cracked PCBs that result in PSU failures. This is why protection of the unit itself inside the box is crucial. Thick layers of packing foam are the best way to protect PSUs (and other products, as well) from rough shipping.

A great piece of information that we got after contacting our sources is this: Shipping PSUs via air cargo increases the Dead On Arrival (DOA) rate significantly, because the products are usually shipped in "master boxes" in the belly of passenger aircraft. This transportation method is actually cheaper than shipping on pallets with cargo aircraft. All the loading, unloading, vibration, and possible falls of the master boxes can kill a notable number of PSUs, especially if they're not adequately protected in their boxes.

Bugs: Yes, Those Kinds of Bugs

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We are not referring to software bugs here, but actual insects. In the past, we've encountered some PSUs from Chinese brands that feature a piece of foam between the soldering side of the PCB and the chassis, and we wondered about its purpose. It turns out the foam is supposed to keep insects away, because in some environments, ants and roaches can cause fatal short circuits by entering the PSU's internals. But that foam is expensive, and it leaves the component side of the PCB unprotected. Thankfully, relatively few PSUs die because of bugs--the ratio is around 10% of total failures for a high quality and quite popular PSU line (the name of which we cannot reveal), so most companies don't use the foam.

Overview

To summarize, high quality PSUs can fail for the following reasons:

  • Broken MLCC components
  • Long mounting PCB screws
  • Damaged ICs and FETs because of soldering-wave issues
  • Careless soldering jobs/repairs
  • Cracked PCBs
  • High inrush currents
  • Creepy-crawlies
  • High surge voltages

You cannot do much about the first six, but you can keep bugs away from your system, and a power conditioner or UPS will protect the PSU from surge voltages, brownouts, and voltage sags, which also apply huge stress to the PSU's circuits. If you live in an area with an unstable mains grid, then the use of a quality UPS is essential.