Why Power Supplies and Semiconductors Fail- Causes and results of power disturbances


Why Power Supplies and Semiconductors Fail- Causes and results of power disturbances



 


Comments regarding power disturbances



• Power quality varies significantly from one area to another. Some countries



have very stable power grids while others are extremely short on capacity.



• Power disturbances are caused by the generation, distribution and use of



power; and lightning.



• A power disturbance can be defined as unwanted excess energy that is



presented to the load.


 


Causes of power disturbances



• Power disturbance originate both outside and inside customer facilities.



• Load switching causes surges because of collapsing fields (-e = l * di/dt).



• Over loaded power distribution systems can cause significant voltage variations



between peak and off-peak hours.



• Significant momentary load changes, such as heavy inrush currents can cause



severe voltage variations.



• Black-outs can cause severe voltage surges both on loss and return of power.



• Circuit-breaker tripping and fuse blowing can cause severe surge voltages.



• Large ups and variable-speed drives can cause various surge voltages inside



buildings.


 


Results of power disturbances



• Sags and under voltages can cause component overheating or destruction.



• Surges and over voltages can cause component overheating, destruction or can



trigger other electronic components such as SCR's.



• Component overheating reduces the life and deteriorates the real reliability as



opposed to the estimated reliability based on steady-state conditions of the



product.



• False triggering of other components can create nuisance alarm tripping or,



worse, can cause overheating or destruction of other electronic components.



 


Why semiconductors fail



• Most semiconductor devices are intolerant to surge voltages in excess of their



voltage ratings.



• Even a fast surge of a few microseconds can cause the semiconductor to fail



catastrophically or may degrade it so as to shorten its useful life.



• Damage occurs when a high reverse voltage is applied to a non-conducting



pn junction.



• The junction may avalanche at a small point due to the non-uniformity of the



electric field. In this case, thermal runaway can occur because of localized heat



build up and cause a melt-through which destroys the junction.


 


Problems with current solutions



• Common-mode voltage disturbances can be amplified in non-tn-s ac distribution



systems.



• Typical emi filters are not well damped. This has a dramatic effect on any voltage



disturbances, resulting in oscillations inside the emi filter under any transitional



conditions. Severe voltage surges may result from fly-back from saturated



inductors looking for a path to release energy.



• Boost converters can be destroyed by surges causing increased energy storage



in input filter, the output capacitor (c) is charged to an unsafe level depending on



capacitance value and load levels for the dc/dc converter connected to the output



of the boost.


 


Limitations of commercial UPS equipment



• The industry is driven by lowest cost.



• Users normally don't care about power quality.



• Users really care about saving data which means that they are satisfied with



orderly shut down to protect data files.



• Most UPS do not have quality battery chargers and chargers are nor designed



for long-term back-up.



• Most UPS equipment does not provide real power conditioning (there are



exceptions).



• Some UPS equipment is poorly protected against surge voltages.



• Extended back-up requires the addition of expensive rectifier/chargers and



battery packs.


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