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Basic Operating Principles of Digital Electronic Devices

Semiconductor Device Characteristics

The microprocessors at the heart of every computer are very large scale integrated circuits made by depositing trace amounts of various elements into the surface of treated silicon crystals. Software instructions and data (i.e., bits) are represented in these circuits by small electric charges. Instructions are processed by measuring patters of these small charges.

As tiny and delicate as these circuits are, they can theoretically run reliably for centuries in the right operating environment. There is nothing to wear out because there are no moving parts and no measurable consumption of materials—just electrons trading places in molecules of impure silicon.

At the microcircuit level, very small transient voltages can be mistaken for a digital signal. The transient can change the characteristic of a bit or group of bits, and if it has just the right characteristics and happens at just the right moment, critical instructions or data become altered. These circuits usually operate at 5 volts (V). A transient as small as 0.5 V can change a logical 0 into a logical 1. In some systems, altered instructions cause spurious hardware interrupts that can produce system errors or abends.

Severe transient voltages can completely destroy a microcircuit. At the chip level, as little as 10 V can damage a circuit that is designed for 5 V. Degraded circuits become hypersensitive and eventually fail altogether. The cumulative nature of component degradation is one reason that damaged equipment functions poorly in some sites yet operates fine in others.

Power Supply Requirements: Design Considerations

A computer’s digital logic circuits operate on +5 V direct current (DC). The purpose of the computer’s power supply is to convert the alternating current (120 V or 230 V AC) supplied in the building branch circuits to a steady, finely regulated 5 V to 18 V DC.

During the 1970s the computer industry used a power conversion device commonly referred to as a linear power supply. The linear supply’s first stage was a large 60 Hz transformer; its purpose was to step the incoming line voltage from 120 V or 230 V to some lower level between 7 V and 17 V. The low-voltage AC was then rectified, regulated, and filtered to DC. Regulating the DC output was a significant problem; in the end, supplemental AC line voltage regulation was often specified and as a holdover from these times many people today still assume power conditioning means voltage regulation.

During the early 1980s, a different power supply architecture came into broad use. The new design was smaller, lighter, inherently less expensive, and produced tightly regulated DC output over a wide range of AC input voltage. The advantage of these power supplies, called switching power supplies, made them a natural for use in microcomputers.

The new architecture makes use of a simple, inexpensive diode bridge as a first stage element, which converts the high voltage (120 V or 240 V) AC to high-voltage DC. A high-frequency inverter converts the direct current to high-frequency (10 kHz to 100 kHz) AC. High-frequency, high-voltage AC is then stepped down to lower voltage AC with a small high-frequency transformer, and rectified again to DC, to server the power requirements of the electronic and mechanical devices inside the computer.

Exhibit 1-6-6 shows a block diagram representation of a typical switching type computer supply. Variations to the basic switching architecture are being introduced to make the switching power supplies easily adaptable to the range of line voltages and frequencies in worldwide use.


Exhibit 1-6-6.  Block Diagram of a Switch Mode Power Supply

Switching Power Supply Performance Characteristics

Switching power supplies inherently are able to provide finely regulated low-voltage DC output over a wide range of high-voltage AC input. In fact, a standard switching supply operating in a nominal 120 V AC environment can usually sustain a well-regulated DC output when AC input falls well below 90 V for extended periods of time. Operation in such “brownout” conditions hampers the supply’s ability to compensate for momentary power failures. Operation at extremely low voltage may also stress the rectifier diodes. By and large, however, this power supply architecture does not need any supplemental voltage regulation. In fact, nearly all the available commercial voltage regulators have less tolerance for changes in voltage than a switching supply.

The diodes used in the first stage rectifier of a typical switching type power supply have an upper end rating of approximately 400 V. These diodes are attached directly to the incoming AC line. If there is no protection against transient AC line voltages this relatively delicate electronic front end will not survive long even in a normal operating environment. For this reason nearly all switching supplies incorporate at least some form of protection from transient over-voltages.

Electrical Noise Considerations

Any electronic device that generates signals at a rate in excess of 10,000 Hz and is intended for use in the United States is subject to regulation by the Federal Communications Commission (FCC). Similar requirements are in effect in Europe. The purpose is to keep radio communications free form interference. Most computer power supplies comply by having some form of radio frequency (RF) filter on the input of the power supply. The primary purpose of these input RF filters is not to protect the computer system from conducted AC line noise, but to prevent high-frequency noise that is created in the computer or power supply being conducted into the AC lines. These filters typically block noise from either direction, but their effect on inbound noise is predictable only when the noise is of similarly low energy content and frequency as the noise generated by the computer power supply and digital circuits.

Conducted AC voltage transients, or power line noise, can easily fall outside this range. The energy content of power line transients is typically significantly greater than the computer’s internal noise. Radio frequency rated filter circuits have no measurable effect on large incoming transients.

Noise can cause a loss of output regulation or failure of the power supply. High-frequency transients can also couple through the power supply circuit elements and find their way onto the low-voltage bus of the system mother board. Once on the mother board, these noise impulses can corrupt data or damage electronic components.


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