Machine Design

Power-Supply Specifications

Input ratings: Typical power-supply modules are designed and rated to operate with their inputs connected to lines delivering 105 to 125 Vac. Most 115-Vac power systems maintain line voltage within that range. However, line voltage for heavily loaded systems can drop below 105 V, and line voltage for lightly loaded systems or for systems close to utility substations may rise appreciably above 125 V. When connected to a low-voltage line, a regulated power supply may not produce rated dc output voltage. And power supplies tend to overheat when subjected to high line voltage. Since power system loading and, thus, line voltage varies widely during operation, line voltage for a proposed power-supply application should be checked. If low or high voltage is expected, that fact should be included in specifications for a power supply.

Standard power-supply modules rated for 210 to 250-Vac input are also available. These are suitable for application on most U.S. standard 230-Vac systems. However, 210 to 250-V rated power supplies may not be suitable for European power systems. Those systems are typically rated 220 V, and the resulting 10-V margin for low voltage is not sufficient for most 220-V systems. Thus, if a power supply is applied to a power system rated 220 V, the power supply should be designed specifically for that voltage.

Several power supplies are designed and rated for operation on 50 Hz (a power frequency used in Europe and other parts of the world) and on 400 Hz (a power frequency used on aircraft and other vehicles) as well as on 60 Hz. However, when power supplies are used on those other frequencies, output current must often be derated and fuse sizes changed from those standard for 60-Hz operation. Thus, power-supply specifications should be checked if 50 or 400-Hz operation is contemplated.

Output ratings: Ratings for power-supply output voltages are nominal or approximate values, and voltage output may be adjustable. But the precise output voltage at any moment during operation depends on many factors. For an unregulated supply, output voltage varies in direct proportion to input voltage and is further modified by load changes. For regulated power supplies, output voltage more precisely matches the rated value but does vary slightly with changes in line voltages and load current. Furthermore, output voltage is influenced by power-supply temperature coefficient and stability. Instantaneous output voltage is affected by the ripple present.

If a power supply with adjustable output voltage is required, the minimum adjustment acceptable should be chosen. Power supplies with narrow adjustment ranges generate less heat and are less costly than those having wide-range adjustments. Power supplies having a narrow adjustment range cannot be grossly misadjusted, a common cause of equipment failures, especially during initial start-up.

Power-supply output-voltage circuits frequently are electrically isolated or "float" with respect to the power-supply chassis. Thus, low-voltage supplies can be connected in series to obtain higher voltages. However, power-supply components and insulation are often rated for 200 V maximum. Thus, power supplies should not be connected in series if the resulting output would exceed 200 V unless the power-supply maker approves that application.

Regulating line and load: Line and load-regulation ratings relate to voltage regulation. Line regulation is the maximum steady-state amount that output voltage changes as a result of a specified change in input line voltage. Line regulation is typically expressed as the percent change of output voltage caused by a 10% change of input voltage. Line regulation runs from 0.005% for the best series-regulated supplies to about 1% for ferroresonant supplies.

Load regulation is the maximum steady-state amount that output voltage changes as a result of a specified change in load. Load regulation typically is expressed as the percent change of output voltage caused by increasing load from half load to full load. Load regulation runs from 0.005% for the best series-regulated supplies to 10% for unregulated supplies.

Regulating circuitry normally controls voltage at power-supply output terminals, and is satisfactory if the load is near the power supply. But current in wires connecting a power supply to a load produces some finite voltage drop which may not be tolerable. However, remote sensing is available for most power supplies which regulates voltage at the load.

A regulated power supply can fail in a mode that results in excessively high output voltage. An overvoltage protection circuit, a crowbar, is available as an option for most power supplies. This protection should be obtained if load failure due to power-supply overvoltage would be unacceptable.

Ripple and noise: Ripple identifies undesired ac components in the dc output voltage, and includes the portions of the ac input voltage waveform that remains after filtering. It also includes electrical noise developed within a power supply. Ripple is generally stated in mVrms or in mVpp for series-regulated and switching-type power supplies. But ripple is given in percent of nominal-rated dc voltage for ferroresonant and unregulated supplies. Ripple in the output of series-regulated supplies is typically 0.25 to 5 mVrms and 1 to 15 mVpp. Switching supply ripple runs from 5 to 20 mVrms and 20 to 150 mVpp. Ripple for ferroresonant supplies typically is 0.5 to 5% and that for unregulated supplies is 5 to 10%.

Most power-supply applications require dc voltage containing very low ripple. For applications providing power for high-gain or low-power-level amplifiers, for example, power supplies should be chosen on the basis of Vpp specifications rather than Vrms.

Two characteristics that affect output voltage, but are important only in critical applications, are temperature coefficient and stability. Temperature coefficient is the percent change of output voltage as a result of a 1°C change in ambient temperature.

Power-supply stability is the percent change of output voltage as a function of time. A stability figure is often stated for warm-up, based on a short time period, typically 20 min, as well as for long term, typically a 24-hr period.

Current ratings: All power-supply modules have current ratings that are based on some specific ambient temperature range. These current ratings should never be exceeded. However, most manufacturers include a safety margin in their current ratings and overspecification with respect to current rating would be wasteful.

Power supplies are generally rated for either 0 to 55°C or -20 to 71°C ranges. But current ratings for most power supplies are based on a 55°C maximum ambient. Current ratings for higher temperatures are derated from those for 55°C. A power supply should not be operated in an ambient temperature exceeding its maximum rating unless it is approved by the manufacturer. For higher ambients, a manufacturer generally indicates a reduced maximum current rating.

The ambient temperature for a power supply is the temperature of the air immediately surrounding the power supply when it is operating with its associated equipment. That temperature generally is appreciably higher than the free-air temperature of the room in which the equipment is operating. A power supply is typically the hottest item in an electronic system, and its enclosure is often used as a heat sink. Thus, adequate air circulation must be provided to prevent ambient temperatures from exceeding ratings.

Fuses provide overcurrent protection for unregulated power supplies. In some cases, they are furnished as power-supply components. Fuses are sometimes used with regulated power supplies, but in such cases, they only provide backup protection. Primary overcurrent protection for regulated power supplies is provided by a current-limiting feature that limits output from 10 to 50% above rated current. When that point is reached, voltage drops off, preventing further current increase.

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