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IMPROVING POWER QUALITY Surge Suppression Transient voltage surge suppressors (TVSSs) were originally developed to protect lights and motors from lightning-related voltage surges. They may limit immediate damage to a system from large lightning-like transients, but although it is widely believed that surge suppressors also block electrical noise, they do not. Exhibit 1-6-15 shows a portion of an alternating current sine wave. Fast transient events that occur at points B and C are not blocked by the surge suppressor.
Beyond their failure to block electrical noise, these devices introduce circuit problems of their own. The primary shortcoming of TVSSs is the tendency of these devices to convert normal mode events to common mode events. Most commercial surge suppression devices direct unwanted surge currents to ground, and the ground can conduct those currents into the computer system. For these reasons, TVSS technology is not recommended for standalone mission-critical electronic systems and especially not for networked systems with multiple ground connections. Filters Power Filters A new category of power treatment device offers a kind of technological middle ground between surge suppressors and transformer-based power conditioning systems. Varying performance and specifications that confuse even experts make filter selection a difficult task, however. At the low-cost end are various grades of surge suppressors with added electro-magnetic interference (EMI) or radiofrequency interference (RFI) filters. At the high end are series-connected power inductors combined with surge suppression and output filtering modules. Well-designed high-end filters offer peak voltage and edge-speed control that is significantly better than what is achievable through simpler surge suppression technology. A high-end power filters common mode performance can be better than a surge suppressor but will never be as good as isolating transformers that are part of a complete power conditioning system. Some systems or sites may require the level of common mode performance offered only by transformer-based systems. Voltage Regulators Earlier sections of this chapter explored why computer power supplies do not need supplemental voltage regulation. External voltage regulators are redundant at best, and at worst they introduce problems of their own. There are generally two types of regulators, tap changing transformers and constant voltage transformers. Both devices tend to be unstable in certain situations. This characteristic an extend the duration of an otherwise harmless power quality defect, which makes it harder for a computer power supply to recover after a momentary blink in AC power. Tap Changers Tap-changing, voltage regulating transformers operate by switching between multiple transformer output taps, boosting or dropping output voltage in increments. These devices run cooler than older ferroresonant regulators and operate relatively quietly. Still, they represent a complicated solution to a non-problem. The technology does not guarantee effective noise isolation (performance varies by design), and with poor designs voltage transients are created when the device switches between taps. Ferroresonant/Constant Voltage Transformers Ferroresonant transformers were invented in the 1930s to provide constant voltage to neon lights. They provide consistent output voltage over a range of input voltages and also offer good noise isolation. Noise isolation is desirable but voltage regulation is not very useful for microcomputers. Worse, a tank circuit effect actually causes competition with the computers power supply for energy to recover from a notch in the incoming power. This can extend the duration of the notch beyond the ride-through capabilities of the computer power supply, causing the computer to re-boot. Isolating Transformers and Power Conditioning Systems High-Impedance Types Ferroresonant transformers and other high isolation transformers provide very good protection from conducted line noisemuch more protection from transients than can be provided by surge suppressors or power filters. However, they typically have high output impedance which can reflect or amplify electrical noise generated by computers and related hardware. High transfer impedance makes them incompatible with the high current crest factors that are typical of switching power supplies. Problems of heat and audible noise make them unpopular in environments shared with people. Low-Impedance Types Power conditioning systems built using transformers with low transfer impedance were developed in the early 1980s. The design is much more compatible with the power consumption characteristics of switching power supplies. Low transfer impedance eliminates the efficiency and instability problems associated with earlier high-impedance isolating transformers. Low output impedance is made possible by an output filter circuit that eliminates all high-frequency noise. These systems typically provide complete protection from conducted transients without any bad side effects. They also provide a safe and absolutely clean reference ground in close proximity to digital electronic devices. UPSs An uninterruptible power supply (UPS) is a battery-based power backup device that typically incorporates one of the previously mentioned surge and noise suppression technologies. The type of power conditioning technology employed determines the UPSs power conditioning capability when the AC line is functioning properly. Other issues in choosing a UPS are transfer time, inverter wave shape, battery maintenance, and the UPS architecture. Transfer Time It should be faster than 16 to 20 milliseconds (ms) to transfer from line to battery power without affecting the computer systems power supply. Most commercial UPSs are fast enough: transfer times of 2 ms to 5 ms are typical.
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