The thyristor or SCR can offer a very easy but effective method of providing a crowbar circuit to protect against this eventuality. One failure mode is for many analogue regulated supplies is that the series pass transistor can fail with a short circuit appearing between the collector and emitter.

If this happens the full unregulated voltage can appear at the output, and this would place an intolerably high voltage on the whole system causing many ICs and other components to fail.

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By looking at the voltages involved it is very easy to see why the inclusion of overvoltage protection is so important. A typical supply may provide 5 volts stabilised to logic circuitry. To provide sufficient input voltage to give adequate stabilisation, ripple rejection and the like, the input to the power supply regulator may be in the region of 10 to 15 volts.

Even 10 volts would be sufficient to destroy many chips used today, particularly the more expensive and complicated ones. Accordingly preventing this is of great importance. The thyristor crowbar circuit shown is very simple, only using a few components. It can be used within many power supplies, and could even be retro-fitted in situations where no over-voltage protection may be incorporated.

It uses just four components: a silicon controlled rectifier or SCR, a zener diode, a resistor and a capacitor.

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The SCR over voltage crowbar or protection circuit is connected between the output of the power supply and ground. The Zener diode voltage is chosen to be slightly above that of the output rail. Typically a 5 volt rail may run with a 6. When the Zener diode voltage is reached, current will flow through the Zener and trigger the silicon controlled rectifier or thyristor. This will then provide a short circuit to ground, thereby protecting the circuitry that is being supplied form any damage and also blowing the fuse that will then remove the voltage from the series regulator.

As a silicon controlled rectifier, SCR, or thyristor is able to carry a relatively high current - even quite average devices can conduct five amps and short current peaks of may be 50 and more amps, cheap devices can provide a very good level of protection for small cost.

Also voltage across the SCR will be low, typically only a volt when it has fired and as a result the heat sinking is not a problem.

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The small resistor, often around ohms from the gate of the thyristor or SCR to ground is required so that the Zener can supply a reasonable current when it turns on. It also clamps the gate voltage at ground potential until the Zener turns on. The capacitor C1 is present to ensure that short spikes to not trigger the circuit. Some optimisation may be required in choosing the correct value although 0.

If the power supply is to be used with radio transmitters, the filtering on the input to the gate may need to be a little more sophisticated, otherwise RF from the transmitter may get onto the gate and cause false triggering. The capacitor C1 will need to be present, but a small amount of inductance may also help. A ferrite bead may even be sufficient. Experimentation to ensure that the time delay for the thyristor to trigger is not too long against removing the RF.

Although this power supply overvoltage protection circuit is widely used, it does have some limitations. It is necessary to chose a Zener diode with the right voltage.

The firing voltage must be sufficiently far above the nominal power supply output voltage to ensure that any spikes that may appear on the line do not fire the circuit. The lower the voltage the greater the margins needed.To gain a broader understanding of power system reliability, it is necessary to understand the root causes of system faults and system failures.

A major reliability concern pertaining to underground cables is electrochemical treeing. Treeing occurs when moisture penetration in the presence of an electric field reduces the dielectric strength of cable insulation. When the dielectric strength is degraded sufficiently, transients caused by lightning or switching can result in dielectric breakdown.

Electrochemical treeing usually affects extruded dielectric cable such as cross-linked polyethylene XLPE and ethylene-propylene rubber EPRand is largely attributed to insulation impurities and bad manufacturing. To reduce failures related to electrochemical treeing, a utility can install surge protection on riser poles transitions from overhead to undergroundcan purchase tree-retardant cable, and can test cable reels before accepting them from the manufacturer.

Existing cable can be tested and replaced if problems are found. One way to do this is to apply a DC voltage withstand test approximately 3 times nominal RMS voltage. Since cables will either pass or not pass this test, information about the state of cable deterioration cannot be determined.

Another popular method for cable testing is to inject a small signal into one end and check for reflections that will occur at partial discharge points. Other methods are measuring the power factor over a range of frequencies dielectric spectroscopyanalyzing physical insulation samples in a lab for polymeric breakdown degree of polymerizationand using cable indentors to test the hardness of the insulation.

Cant see this video? Click here to watch it on Youtube. Not all underground cable system failures are due to cable insulation.

A substantial percentage occurs at splices, terminations, and joints. Major causes are due to water ingress and poor workmanship. Heat shrink covers can be used to waterproof these junctions and improve reliability. The last major reliability concern for underground cable is dig-ins. This is when excavation equipment cuts through one or more cables.

To prevent dig-ins, utilities should encourage the public to have cable routes identified before initiating site excavation.

IE#86: (PART 3/4) Realisation! Exploded power supply failure mode!

In extreme cases where high reliability is required, utilities can place cable in concrete-encased duct banks. Transformers are critical links in power systems, and can take a long time to replace if they fail.

Through faults cause extreme physical stress on transformer windings, and are the major cause of transformer failures. Overloads rarely result in transformer failures, but do cause thermal aging of winding insulation.

The five main reasons power supplies fail – and what can be done about it

When a transformer becomes hot, the insulation on the windings slowly breaks down and becomes brittle over time. Because of this exponential relationship, transformer overloads can result in rapid transformer aging. When thermal aging has caused insulation to become sufficiently brittle, the next fault current that passes through the transformer will mechanically shake the windings, a crack will form in the insulation, and an internal transformer fault will result.

This is because the hot spot can cause free bubbles that reduce the dielectric strength of the liquid. Even if free bubbles are not formed, high temperatures will increase internal tank pressure and may result in overflow or tank rupture. Many transformers are fitted with load tap changers LTCs for voltage regulation. These mechanically moving devices have historically been prone to failure and can substantially reduce the reliability of a transformer.

Manufacturers have addressed this problem and new LTC models using vacuum technology have succeeded in reducing failure rates. A lightning strike occurs when the voltage generated between a cloud and the ground exceeds the dielectric strength of the air.

This results in a massive current stroke that usually exceeds 30, amps. To make matters worse, most strokes consist of multiple discharges within a fraction of a second. Lightning is the major reliability concern for utilities located in high keraunic areas. Lightning can affect power systems through direct strikes the stroke contacts the power system or through indirect strikes the stroke contacts something in close proximity and induces a traveling voltage wave on the power system.

Direct strikes are virtually impossible to protect against on a distribution system. Trees continuously growcan fall over onto conductors, can drop branches onto conductors, can push conductors together, and can serve as gateway for animals.Yet aging UPSs can and do fail.

The life expectancy of VRLA batteries will fluctuate greatly depending on UPS placement, ambient temperature, cycling, maintenance, battery chemistry, and battery storage. Helping customers understand these conditions and how to be proactive will help maximize battery life and prepare them for any imminent power failures.

power supply failure modes

Ensure that customers keep the ambient temperature within the specified range, monitor the UPS for unusual or frequent cycling, and choose a UPS that can support the attached load.

Let customers know about the potential for failure so they can proactively identify and correct issues. Failure under normal circumstances is unlikely, but an incorrect or malfunctioning firmware setup and unusually high cycling can result in overuse and eventual failure.

Again, help your customer understand these conditions so they can adjust firmware settings before damage occurs. Your email address will not be published. Save my name, email, and website in this browser for the next time I comment.

Batteries The life expectancy of VRLA batteries will fluctuate greatly depending on UPS placement, ambient temperature, cycling, maintenance, battery chemistry, and battery storage. Fans Ensure that customers keep the ambient temperature within the specified range, monitor the UPS for unusual or frequent cycling, and choose a UPS that can support the attached load.

Common issues with power supply

Relays Failure under normal circumstances is unlikely, but an incorrect or malfunctioning firmware setup and unusually high cycling can result in overuse and eventual failure.

Leave a Reply Cancel reply Your email address will not be published.There are a number of problems which may occur with the incoming power supply to a site. These problems cause voltage and current instability which can have a significant impact on equipment operation and power usage. Voltage stability refers to the ability of a power system to maintain steady voltage levels after a disturbance. The main forms of voltage instability can be categorised as:. Undervoltage describes a sag in voltage for a short period or a sustained reduction in the system voltage level for a period of time sometimes described as a brownout.

Common causes of undervoltage conditions include the start-up of large loads or other long-term system faults. Conversely, overvoltage refers to a swell in voltage levels supplied which may occur over a short or sustained period and is the reverse of an undervoltage condition. Common causes of overvoltage include sudden or persistent load adjustments which are not compensated for, or phase fault events.

A transient is a sharp change in voltage or current away from normal operating levels for a short period of time usually no more than 1ms in duration. It normally occurs due to an event which causes a change in the circuit conditions, internally or externally to the circuit. Examples of events which may cause a transient to occur in an electrical system include lightning, inductive electrical equipment, electrostatic discharge, and circuit faults such as short circuits or circuit breaker operation.

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Waveform distortion involves the changing of the fundamental supply AC waveform due to influences such as:. The impact of the above mentioned power issues on an electrical system can be significant. They can cause disruption of power to equipment resulting in operational delays, equipment instability and failure. Poor power quality also results in higher than necessary energy costs due to the inefficient use of power in a power system.

Some of the main impacts of poor power supply are discussed below. When equipment within a system operates outside of manufacturer recommended levels it can shorten the life of equipment and result in unstable operation and failure. Motors — Motors operate most efficiently within the rated levels prescribed on the nameplate of the equipment.

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When operated outside of these levels the motor may not perform as efficiently or may malfunction. Undervoltage increases the operating current causing overheating which will shorten the life of the motor.

Common issues with power supply

Furthermore, large loads may fail to start if the starting torque is not sufficient due to low voltages. These stresses on the motor will eventually lead to mechanical failure. The impact of overvoltage is similar in that it pushes the motor into magnetic saturation, causing it to draw excessive amounts of current beyond its nameplate ratings to compensate.

power supply failure modes

Motors are also impacted by transient voltage conditions, harmonics and other waveform distortion events, causing unstable motor operation, overheating and noise which leads to equipment damage and malfunction. Sensitive electronic equipment — Electronic equipment including computers are prone to problems when exposed to noisy power supplies.

Power supply noise can cause components to operate outside of rated values, causing overheating and equipment operation issues such as data error or loss, equipment malfunction and component failure.If properly understood, MTBF can be a significant tool for understanding datasheets. Mean time between failures MTBF may be one of the more familiar terms seen in datasheets, yet there is still a widespread misunderstanding of the term and its application.

Consequently, some designers place too much emphasis on this parameter, others very little, and some have trudged through too many disparate datasheets to deem it any use at all. The truth is that the oft-maligned MTBF can indeed be helpful if one has a proper understanding of what it is and, maybe more important, what it is not.

MTTF mean time to failure may be substituted in some datasheets for units that will not be repaired. Reliability is further defined as the probability that given a certain failure rate, that a certain number of units will pass or fail within a specified period.

Failure rate is the central element that binds these terms. Its importance cannot be overstated. A common misconception is linked to a misunderstanding of failure rates as they apply to MTBF. Failure rate is expressed over time, and, more important, the reliability definition tells us that this time is designated within a specific, useful period. Not the entire life of the product. MTBF is often misquoted as life hours or service life and this is simply not the case.

A quick study of failure rates and their relationship to MTBF will further illustrate this point. Failure rates for all manufactured products can be characterized in the universal product reliability curve or bathtub curve seen in Fig.

The curve is an approximation of failure rates throughout the entire life of a product. Failures in the infant mortality or early period are generally comprised of poor workmanship or weak components and are usually screened out before hand.

While this information may be helpful in the analysis of early failures it is not relevant to reliability predictions. At the other end of the curve, the wearout or end of life period shows drastically increasing failures due to simultaneous component failures. This is beneficial in determining the true end of life for the product, but also not relevant to reliability information. The useful life period is the actual service life of a product.

Its length is dictated by the end of the infant mortality period and the onset of high component failures in the wear out period. Now that all terms have been defined properly, the relationship between failure rates, predicted reliability, and MTBF can be summed up with the exponential formula. Question: What is the predicted reliability for a unit that has a useful life of 5 years and an MTBF rating ofhours?

Total population tracking through field data or field data measurement method is the most accurate way to determine reliability, and should be used whenever possible. Historical field data are desirable for its ability to unearth real world failures that cannot be anticipated by calculations or other means. When field data are scarce or nonexistent, as in the case of new designs, then a predictability method should be used. There are several methods for predicting reliability.

The length of this article will limit us to exploring only a few of the more common. This handbook was first published in as a way to standardize reliability predictions.

power supply failure modes

It is still used by many manufacturers to this day, and therefore can serve as useful tool for comparing one product to another.The component of your PC that's under the most stress is the power supply unit PSUbecause 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 " 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? MLCCs are widely used in power-supply circuits, mostly for filtering purposes.

They offer numerous advantages, including low cost, small size, low ESRhigh reliability, and increased tolerance to high ripple currents.

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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. 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.

Even a single broken MLCC can result in issues, and they can crack due to any of the following:. This may sound silly, but in some cases, mounting screws that are too long can actually cause shorts to the PCB. 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.

Besides high voltage spikes, which can be caused by weather conditions i. 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 protectionit can still malfunction, especially if the voltage or current surge is too high.

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. 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.

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.In fact, UPS system failure ranks as the No. Prevention pays off, affording the opportunity to detect and repair potential problems before they become significant and costly. Whether you are operating aging infrastructure or looking to optimize the lifespan of a newer equipment, consider some of the most common UPS components that are susceptible to failure:.

Regardless of their age, batteries should be inspected semi-annually as part of a PM visit that includes testing for impedance or conductance, as well as assesses performance and evaluates any potential weaknesses.

A typical UPS contains a dozen or more capacitors, which are responsible for smoothing out and filtering voltage fluctuations.

However, because capacitors degrade over time, annual inspection helps to optimize their operation and extend their lifespan. Electrical or mechanical limitations and dried out ball bearings are common issues that can result in fan failures and subsequent UPS overheating.

Because replacing filters is an inexpensive component of an effective UPS maintenance plan, they should be inspected on a monthly basis and changed as needed.

Yet regular inspection can identify potential issues before they cause downtime. Contactors — Also susceptible to dust, UPS contactors should be inspected and cleaned regularly. At Unified Power, we are committed to delivering exceptional, timely maintenance performed by highly trained industry professionals. Unified Power offers critical power services, UPS maintenance services, DC power services, battery services, and more. Request Service. Get a Quote.

EducationUPS. Whether you are operating aging infrastructure or looking to optimize the lifespan of a newer equipment, consider some of the most common UPS components that are susceptible to failure: 1.

power supply failure modes

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