Electrical Power Supply
Electrical Power Supply
An electrical power supply is a device that provides the energy needed by electrical or electronic equipment. Often, electricity is directly available only from a source with inappropriate electrical characteristics— alternating current (AC) instead of direct current (DC), for example—and a power supply is needed to alter the power to meet the equipment’s requirements. Because digital devices, which are so numerous, run on fairly low DC voltage while power is most commonly available as fairly high-voltage AC, power supplies commonly change AC into DC raise and lower the voltage as required. They are also needed to condition power and current from batteries to sensitive devices. A flashlight, for example, does not contain a power supply, but a digital camera does. Power supplies often provide protection against power source failures that might damage the equipment. They may also provide isolation from the potentially damaging electrical noise that is usually found on commercial power lines.
An electrical power supply can be a simple battery or may be more sophisticated than the equipment it supports. An appropriate power supply is an essential part of every working collection of electrical or electronic circuits.
Batteries could be used to supply the power for almost all electronic equipment if it were not for the high cost of the energy they provide compared to commercial power lines. Power supplies were once called battery eliminators, an apt name because they made it possible to use less expensive energy from a commercial power line where it is available. Batteries are still an appropriate and economical choice for portable equipment having modest energy requirements.
Two basic types of chemical cells are used in batteries that supply power to electronic equipment. Primary cells are normally not rechargeable. They are intended to be discarded after their energy reserve is depleted. Secondary cells, on the other hand, are rechargeable. The lead-acid secondary cell used in an automobile’s battery can be recharged many times before it fails. Nickel-cadmium batteries are based on secondary cells.
The electrical energy supply for homes and businesses provided through the commercial power lines is delivered by an alternating current (AC). Electronic equipment, however, almost always requires direct-current power (DC). Power supplies usually change AC to DC by a process called rectification. Semiconductor diodes that pass current in only one direction are used to block the power line’s current when its polarity reverses. Capacitors store energy for use when the diodes are not conducting, providing relatively constant voltage direct current as needed.
Poor power line voltage regulation causes lights in a home to dim each time the refrigerator starts. Similarly, if a change in the current from a power supply causes the voltage to vary, the power supply has poor voltage regulation. Most electronic equipment will perform best when it is supplied from a nearly constant voltage source. An uncertain supply voltage can result in poor circuit performance.
Analysis of a typical power supply’s performance is simplified by modeling it as a constant-voltage source in series with an internal resistance. The internal resistance is used to explain changes in the terminal voltage when the current in a circuit varies. The lower the internal resistance of a given power supply, the more current it can supply while maintaining a nearly-constant terminal voltage. An ideal supply for circuits requiring an unvarying voltage with changing load current would have an internal resistance near zero. A power supply with a very-low internal resistance is sometimes called a “stiff” power supply.
An inadequate power source almost always compromises the performance of electronic equipment. Audio amplifiers, for example, may produce distorted sound if the supply voltage drops with each loud pulse of sound. There was a time when the pictures on television sets would shrink if the AC-line voltage fell below a minimum value. These problems are less significant now that voltage regulation has been included in most power supplies.
There are two approaches that may be used to improve the voltage regulation of a power supply. A simple power supply that is much larger than required by the average equipment demand will help. A larger power supply should have a lower effective internal resistance, although this is not an absolute rule. With a lower internal resistance, changes in the current supplied are less significant and the voltage regulation is improved compared to a power supply operated near its maximum capacity.
Some power supply applications require a higher internal resistance. High-power radar transmitters require a power source with a high internal resistance so that the output can be shorted each time the radar transmits a signal pulse without damaging the circuitry. Television receivers artificially increase the resistance of the very high voltage power supply for the picture tube by adding resistance deliberately. This limits the current that will be delivered should a technician inadvertently contact the high voltage which might otherwise deliver a fatal electrical shock.
Voltage-regulated power supplies feature circuitry that monitors their output voltage. If this voltage changes because of external current changes or because of shifts in the power line voltage, the regulator circuitry makes an almost instantaneous compensating adjustment.
Two common approaches are used in the design of voltage-regulated power supplies. In the less-common scheme, a shunt regulator connects in parallel with the power supply’s output terminals and maintains a constant voltage by wasting current the external circuit, called the load does not require. The current delivered by the unregulated part of the power supply is always constant. The shunt regulator diverts almost no current when the external load demands a heavy current. If the external load is reduced, the shunt regulator current increases. The disadvantage of shunt regulation is that it dissipates the full power the supply is designed to deliver, whether or not the external circuit requires energy.
The more-common series voltage regulator design depends upon the variable resistance created by a transistor in series with the external circuit current. The transistor’s voltage drop adjusts automatically to maintain a constant output voltage. The power supply’s output voltage is sampled continuously, compared with an accurate reference, and the transistor’s characteristics are adjusted automatically to maintain a constant output.
A power supply with adequate voltage regulation will often improve the performance of the electronic device it powers, so much so that voltage regulation is a very common feature of all but the simplest designs. Packaged integrated circuits are commonly used, simple three-terminal devices that contain the series transistor and most of the regulator’s supporting circuitry. These “off the shelf” chips have made it very easy to include voltage regulation capability in a power supply.
When a single power supply serves several independent external circuits, changes in current demand imposed by one circuit may cause voltage changes that affect the operation of the other circuits. These interactions constitute unwanted signal coupling through the common power source, producing instability. Voltage-regulators can prevent this problem by reducing the internal resistance of the common power source.
When an alternating current is converted to direct current, small voltage variations at the supply frequency are difficult to smooth out, or filter, completely. In the case of power supplies operated from the 60-Hz power line, the result is a low-frequency variation in the power supply’s output called ripple voltage. Ripple voltage on the power supply output will add with the signals processed by electronic circuitry, particularly in circuits where the signal voltage is low. Ripple can be minimized by using more elaborate filter circuitry but it can be reduced more effectively with active voltage regulation. A voltage regulator can respond fast enough to cancel unwanted changes in the voltage.
Power-line voltages normally fluctuate randomly for a variety of reasons. A special voltage-regulating transformer can improve the voltage stability of the primary power. This transformer’s action is based on a coil winding that includes a capacitor which tunes the transformer’s inductance into resonance at the power line frequency. When the line voltage is too high, the circulating current in the transformer’s resonant winding tends to saturate the magnetic core of the transformer, reducing its efficiency and causing the voltage to fall. When the line voltage is too low, as on a hot summer day when air conditioners are taxing the capabilities of the generators and power lines, the circulating current is reduced, raising the efficiency of the transformer. The voltage regulation achieved by these transformers can be helpful even though it is not perfect. An early TV brand included resonant transformers to prevent picture-size variations that accompanied normal line-voltage shifts.
Resonant power transformers waste energy, a serious drawback, and they do not work well unless heavily loaded. A regulating transformer will dissipate nearly its full rated power even without a load. They also tend to distort the alternating-current waveform, adding harmonics to their output, which may present a problem when powering sensitive equipment.
Voltage-regulated power supplies are necessary equipment in scientific and technical laboratories. They provide an adjustable, regulated source of electrical power to test circuits under development.
Laboratory power supplies usually feature two programmable modes, a constant-voltage output over a selected range of load current and a constant-current output over a wide range of voltage. The crossover point where the action switches from constant voltage to constant current action is selected by the user. As an example, it may be desirable to limit the current to a test circuit to avoid damage if a hidden circuit fault occurs. If the circuit demands less than a selected value of current, the regulating circuitry will hold the output voltage at the selected value. If, however, the circuit demands more than the selected maximum current, the regulator circuit will decrease the terminal voltage to whatever value will maintain the selected maximum current through the load. The powered circuit will never be allowed to carry more than the selected constant-current limit.
Alternating current is required for most power lines because AC makes it possible to change the voltage to current ratio with transformers. Transformers are used in power supplies when it is necessary to increase or decrease voltage. The AC output of these transformers usually must be rectified into direct current. The resulting pulsating direct current is filtered to create nearly-pure direct current.
A relatively new development in power-supply technology, the switching power supply, is becoming popular. Switching power supplies are lightweight and very efficient. Almost all personal computers are powered by switching power supplies.
The switching power supply gets its name from the use of transistor switches, which rapidly toggle in and out of conduction. Current travels first in one direction then in the other as it passes through the transformer. Pulsations from the rectified switching signal are much higher frequencies than the power line frequency, therefore the ripple content can be minimized easily with small filter capacitors. Voltage regulation can be accomplished by varying the switching frequency. Changes in the switching frequency alter the efficiency of the power supply transformer enough to stabilize the output voltage.
Alternating current— Electric current that flows first in one direction, then in the other; abbreviated AC.
Direct current (DC)— Electrical current that always flows in the same direction.
Filter— Electrical circuitry designed to smooth voltage variations.
Harmonic— Whole-number multiple of a fundamental frequency.
Hz— SI abbreviation for Hertz, the unit of frequency (1 Hz = one cycle per second).
Internal resistance— Fictitious resistance proposed to explain voltage variation.
Modeling— Analysis of a complicated device with a simpler analogy.
Ohms— Unit of electrical resistance, equal to 1 Volt per Ampere.
Parallel— Side-by-side electrical connection.
Rectification— Changing alternating current (AC) to direct current (DC) by blocking reverse flow of charge.
Ripple— Repetitive voltage variation from inadequate filtering.
Switching power supplies are usually not damaged by sudden short circuits. The switching action stops almost immediately, protecting the supply and the circuit load. A switching power supply is said to have stalled when excessive current interrupts its action.
Switching power supplies are light in weight because the components are more efficient at higher frequencies. Transformers need much less iron in their cores at higher frequencies.
Switching power supplies have negligible ripple content at audible frequencies. Variations in the output of the switching power supply are inaudible compared to the hum that is common with power supplies that operate at the 60-Hz AC power-line frequency.
Electrical power supplies are not the most glamorous part of contemporary technology, but without them the electronic products with which we are surrounded could not function.
Lenk, Ron. Practical Design of Power Supplies. New York: Wiley/IEEE, 2005.
Mark, Raymond A. Demystifying Switching Power Supplies. Oxford, UK: Newnes, 2005.