Power Supply

The LPSP is defined as the fraction of simulated years Neval in which power failure occurs within the year, meaning external energy is required to satisfy demand.

From: Computer Aided Chemical Engineering , 2016

Power Supplies

Morgan Jones , in Valve Amplifiers (Fourth Edition), 2012

Publisher Summary

A power supply is a device that converts one voltage to another more convenient voltage while delivering power. Power supplies are designed from the output back to the input. Since they are designed after the amplification stages, it is tempting to think of them as an afterthought; indeed, some commercial products reflect this attitude. It is most important to realize that an amplifier is merely a modulator and controls the flow of energy from the power supply to the load. If the power supply is poor and has insufficient energy to meet the amplifier's peak demands, then the most beautifully designed amplifier will be junk. Valve amplifiers need a DC High Tension (HT) supply and one or more heater, or Low Tension (LT) supplies, which may be AC or DC. Often, the supplies for the pre-amplifier and power amplifier will be derived from the same power supply, which is frequently integral to the power amplifier, but this need not be so. The advent of Power Supply Unit Designer Version 2 freeware in 2003 has transformed the design of linear supplies, but an understanding of the underlying principles enables much faster convergence to an optimum design.

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Power Supplies

Peter Wilson , in The Circuit Designer's Companion (Fourth Edition), 2017

7.4.3 Safety Approvals

Major safety risks for power supplies are the threat of electric shock due to contact with "live parts," and the threat of overheating and fire due to a fault. Safety is discussed in greater depth in Section 9.1 . One of the important but forgotten functions of a power supply is to ensure a safe segregation of the low-voltage circuitry, which may be accessible to the user, from the high-voltage input, which must be inaccessible. Segregation is normally assured in a power supply by maintaining a minimum distance around all parts that are connected to the mains, including spacing between the primary and secondary of the transformer. This, of course, adds extra space to the design requirements. Insulation of at least a minimum thickness may be substituted for empty space.

There are many national and international authorities concerned with setting safety requirements. Foremost among these are UL in the United States, CSA in Canada, and the CENELEC safety standards, implementing the Low Voltage Directive in Europe. As designer, you can either choose to apply a particular set of requirements for your company's market, or if you plan to export worldwide, you can discover the most stringent requirements and apply these across the board. A common specification is EN 60950-1 (IEC 60950-1), which is the safety standard for information technology equipment and which is quoted by default by most off-the-shelf power supplies. If no safety specification is quoted, beware.

Most of the time it is legally necessary to have your product approved to safety regulations, often it is also commercially desirable. Using a bought-in supply that already has the right safety approval goes a long way to helping your own equipment achieve it. Note that there is a difference, on data sheets, between the words "designed to meet…" and "certified to…" The former means that, when you go for your own safety approval, the approvals agency will still want to satisfy themselves, at your expense, that the power supply does indeed meet their requirements. The latter means that this part of the approvals procedure can be bypassed. It therefore puts the unit cost of the power supply up, but saves you some part of your own approval expenses.

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Power supplies

D.I. Crecraft , S. Gergely , in Analog Electronics: Circuits, Systems and Signal Processing, 2002

13.3.1 Introduction

The purpose of a mains power supply is to convert the power delivered to its input by the sinusoidally alternating mains electricity supply into power available at its output in the form of a smooth and constant direct voltage. This is usually achieved in a number of stages as illustrated in Figure 13.4. In conventional power supplies, the 50 or 60 Hz mains is connected directly to the transformer inputs. In the more modern switch mode ones, the a.c. input shown in Figure 13.4 will be at a frequency of 20 kHz or more as described later in Section 13.4.

Fig. 13.4. The block diagram and typical waveforms of a power supply.

The purpose of the transformer is to convert the 230 V (or 115 V) mains voltage to one which is suitable for further processing to a generally much lower voltage d.c. supply. Most electronic circuits require power supplies of 5 to 15 V. An ideal transformer converts the power supplied in the form of sinusoidal a.c. from one voltage and current level at its input to another at its output without any losses. Transformers can be thought of as 'electrical gearboxes'. The transformer also provides electrical isolation between its input and output and therefore between the mains supply and the electronic system connected to the power supply. This is an important contribution to the electrical safety of the user.

Passive low-pass filters (see also Sections 10.4.1 and 10.5) are often used to prevent the transmission of high-frequency interference (radio frequency interference, RFI) between the mains supply and the power supply circuit (including the electronic circuits supplied by it). These are connected between the mains supply and the input of the transformer. They are designed to have a cut-off frequency at several kilohertz in order to pass the 50 or 60 Hz supply current (of several amperes) but to attenuate the higher frequencies. These mains filters are often packaged together with semiconductor devices called transient suppressors. They are used for the suppression of high transient voltage 'spikes' that may appear on the mains supply. These transients can be caused in many ways such as the switching on and off of motors, fluorescent lights, etc.

The rectifier circuits use diodes (see Section 5.1) to convert the alternating current (which, as its name implies, flows in opposite directions alternately every half cycle, into unidirectional current which flows in one direction only. Note that the voltage and the current retain their 'sinusoidal' variation with time within each half cycle as shown in Figure 13.4. Rectifier circuits can be made to use only one-half of the cycle or both halves, called half-wave and full-wave respectively.

As mentioned above, the outputs of rectifiers retain their 'sinusoidal' variation with time within each half cycle as shown in Figure 13.4. The voltage and current are zero twice in every cycle, and they are at their peak value once or twice (half and full wave). Thus, there are times when the voltage is less than required for the d.c. output, or to put it another way, less energy is flowing into the power supply than is flowing out. This problem is solved by having a reservoir of energy within the power supply circuit which stores energy when the voltage is high and releases energy when the voltage is low. This is analogous to the water supply where water is stored in reservoirs during the rainy season and is then constantly available to feed the supply network. A capacitor, called the reservoir capacitor is used as the reservoir of energy. This may be used on its own or as part of a circuit which provides further energy storage. Such a circuit is, in fact, a low-pass filter (see also Section 10.5). The low-pass filter reduces the variation of voltage. It provides the function of smoothing. Note that only passive filters are used in power supplies (see Section 10.4.1).

All but the simplest of power supplies now include a voltage regulator stage. As its name implies this controls the output voltage such that it is maintained at the required value regardless of changes of both the load and the input of the regulator. Voltage regulators often include functions to protect the supply from damage by excessive loads and short circuits. The regulator may be set to a fixed voltage, or in the case of laboratory test supplies the voltage (and the maximum current) may be variable by the user.

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Power Supplies

Yuk M. Lai , in Power Electronics Handbook (Fourth Edition), 2018

20.1 Introduction

Power supplies are used in most electric equipment. Their applications cut across a wide spectrum of product types, ranging from consumer appliances to industrial utilities, from milliwatts to megawatts, and from handheld tools to satellite communications.

By definition, a power supply is a device that converts the output from an ac power line to a steady dc output or multiple outputs. The ac voltage is first rectified to provide a pulsating dc and then filtered to produce a smooth voltage. Finally, the voltage is regulated to produce a constant output level despite variations in the ac line voltage or circuit loading. Fig. 20.1 illustrates the process of rectification, filtering, and regulation in a dc power supply. The transformer, rectifier, and filtering circuits are discussed in other chapters. In this chapter, we will concentrate on the operation and characteristics of the regulator stage of a dc power supply.

Fig. 20.1. Block diagram of a dc power supply.

In general, the regulator stage of a dc power supply consists of a feedback circuit, a stable reference voltage, and a control circuit to drive a pass element (a solid-state device such as transistor and MOSFET). The regulation is done by sensing variations appearing at the output of the dc power supply. A control signal is produced to drive the pass element to cancel any variation. As a result, the output of the dc power supply is maintained essentially constant. In a transistor regulator, the pass element is a transistor, which can be operated in its active region or as a switch, to regulate the output voltage. When the transistor operates at any point in its active region, the regulator is referred to as a linear voltage regulator. When the transistor operates only at cutoff and at saturation, the circuit is referred to as a switching regulator.

Linear voltage regulators can be further classified as either series or shunt types. In a series regulator, the pass transistor is connected in series with the load as shown in Fig. 20.2. Regulation is achieved by sensing a portion of the output voltage through the voltage divider network R 1 and R 2 and comparing this voltage with the reference voltage V REF to produce a resulting error signal that is used to control the conduction of the pass transistor. This way, the voltage drop across the pass transistor is varied and the output voltage delivered to the load circuit is essentially maintained constant.

Fig. 20.2. A linear series voltage regulator.

In the shunt regulator shown in Fig. 20.3, the pass transistor is connected in parallel with the load, and a voltage-dropping resistor R 3 is connected in series with the load. Regulation is achieved by controlling the current conduction of the pass transistor such that the current through R 3 remains essentially constant. This way, the current through the pass transistor is varied, and the voltage across the load remains constant.

Fig. 20.3. A linear shunt voltage regulator.

As opposed to linear voltage regulators, switching regulators employ solid-state devices, which operate as switches: either completely on or completely off, to perform power conversion. Because the switching devices are not required to operate in their active regions, switching regulators enjoy a much lower power loss than those of linear voltage regulators. Fig. 20.4 shows a switching regulator in a simplified form. The high-frequency switch converts the unregulated dc voltage from one level to another dc level at an adjustable duty cycle. The output of the dc supply is regulated by means of a feedback control that employs a pulse-width modulator (PWM) controller, where the control voltage is used to adjust the duty cycle of the switch.

Fig. 20.4. A simplified form of a switching regulator.

Both linear and switching regulators are capable of performing the same function of converting an unregulated input into a regulated output. However, these two types of regulators have significant differences in properties and performances. In designing power supplies, the choice of using certain type of regulator in a particular design is significantly based on the cost and performance of the regulator itself. In order to use the more appropriate regulator type in the design, it is necessary to understand the requirements of the application and select the type of regulator that best satisfies those requirements. Advantages and disadvantages of linear regulators, as compared with switching regulators, are given below:

1.

Linear regulators exhibit efficiency of 20%–60%, whereas switching regulators have a much higher efficiency, typically 70%–95%.

2.

Linear regulators can only be used as a step-down regulator, whereas switching regulators can be used in both step-up and step-down operations.

3.

Linear regulators require a mains frequency transformer for off-the-line operation. Therefore, they are heavy and bulky. On the other hand, switching regulators use high-frequency transformers and can therefore be small in size.

4.

Linear regulators generate little or no electric noise at their outputs, whereas switching regulators may produce considerable noise if they are not properly designed.

5.

Linear regulators are more suitable for applications of less than 20   W, whereas switching regulators are more suitable for large-power applications.

In this chapter, we will examine the circuit operation, characteristics, and applications of linear and switching regulators. In Section 20.2 , we will look at the basic circuits and properties of linear series voltage regulators. Some current-limiting techniques will be explained. In Section 20.3 , linear shunt regulators will be covered. The important characteristics and uses of linear integrated circuit (IC) regulators will be discussed in Section 20.4 . Finally, the operation and characteristics of switching regulators will be discussed in Section 20.5 . Important design guidelines for switching regulators will also be given in this section.

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Power supplies

Gerard Honey , in Intruder Alarms (Second Edition), 2003

4.1 Primary supply

Although a power supply can mean a transformer, a battery or a rectifier filter with or without a charging circuit that converts alternating current (AC) to direct current (DC), alarm engineers usually apply the term to the components as a group. Most standby power supplies use rechargeable batteries as a secondary supply.

A power supply starts at its step-down transformer, which converts its 240 V AC supply to the voltage of 12-18 V AC used by most intruder alarm systems. The transformer is a device employing electromagnetic induction to transfer electrical energy from one circuit to another, that is, without direct connection between them. In its simplest form, a transformer consists of separate primary and secondary windings on a common core of a ferromagnetic material such as iron. When AC flows through the primary the resulting magnetic flux in the core induces an alternating voltage across the secondary; the induced voltage causing a current to flow in an external circuit. In the case of the step-down transformer the secondary side will have a lesser number of windings. From this transformer, power is provided through a two-conductor cable to a rectifier and filter circuit where AC is converted to DC. A charging circuit will be contained within the power supply so that the standby battery may be constantly charged so long as AC is present.

The power supply must always be voltage regulated and be able to hold a fixed voltage output over a range of loads and charging currents. Microprocessor components, especially integrated circuits, are designed to operate at specific voltages and are not particularly tolerant of fluctuations. Low voltages cause components to attempt to draw excess power, further lowering their tolerance, whilst higher voltages can destroy them. For these reasons the voltage should be measured at source and once again at the input terminals on the equipment point.

The critical factor in selecting a power supply is in determining the load it must support. The first step is to establish how much power will be required by all power-consuming devices connected to the supply. The length of time that the standby supply must be able to satisfy the system if the primary supply is lost is then calculated.

The primary supply is the electricity supply to the building, and which will support the system for most of the time. The secondary supply is the support system in the event that the primary supply fails, i.e. the batteries. The systems in which we are interested will tend to be powered by a transformer/rectified mains supply plus rechargeable secondary cells via a power supply unit or uninterruptible power supply (UPS). Other power supply systems may comprise a transformer/rectified mains supply plus non-rechargeable (primary) cells, or primary cells alone, but these two types are less widely used. It follows that the intruder alarm relies heavily on the mains supply, which must be a source that:

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will not be readily disconnected;

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is not isolated at any time;

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is from an unswitched fused spur;

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is free from voltage spikes or current surges;

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is supplied direct to the control panel and not via a switch or plug and socket or remote spur that can fail or be switched off.

The transformer must be sited in an enclosed position and be ventilated, and must not be placed on a flammable surface. Transformers are found within the control panel itself, or in the end station in the event that the system employs independent remote keypads. Within the same confines will be found the rectifier and charger unit. The system will have either a battery charger unit (BCU) or a UPS.

The UPS has a greater ability to negate interference and surges on the mains supply, and it tends to be widely used in computer power supplies that have back-up systems. The essential requirements of a battery charger are that:

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it can recharge all batteries to full charge within 24 hours whilst maintaining the system load;

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it is internally fused, both primary and secondary;

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it is free floating and includes audible and visible indications of failure.

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it includes a voltage trigger to activate remote signalling of failure;

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tamper protection of the cover is provided;

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it has short-circuit protection with a grounded negative on the secondary DC.

As previously stated, a UPS has greater protection to interference with increased recording and monitoring. It must also feature a safety isolating transformer and have the specified output plus recharge requirements under any combination of rated supply voltage and supply frequency at temperatures between –10 and 40°C.

The UPS will additionally have a low heat output fully rectified transformer, solid state voltage regulator, linear current regulator and high-temperature cut-out with continuous monitoring of the low-voltage alarm circuit. Mains suppression filters are used to remove transient high-voltage spikes. BS 4737 requires the following UPS units:

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that they be of sufficient capacity and recharge rate to cope with any prolonged mains isolation of the main supply related to work being done for fire safety, normal isolation or normal work on the electrical services;

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that they are located where maintenance can be easily performed;

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that sufficient ventilation is afforded to stop gas build-up on the vented battery occurring and causing damage or injury;

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that they not be exposed to corrosive conditions and that the cells be fully restrained to stop them falling or spilling;

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that the units must be marked with the date of installation.

Before considering the types of secondary supply in use within the intruder alarm area, the student may wish to pay some attention to the inspection of the mains supply and the tests that must be performed to prove it acceptable. These tests range from visual checks for cable damage to electrical proving requirements, and are covered in Chapter 8.

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Power

Marilyn Wolf , in Embedded System Interfacing, 2019

7.1 Introduction

The power supply is the unheralded hero of circuit design. The importance of a high-quality power supply becomes apparent only when we try to use a bad power supply. We sometimes build our own power supply. As with most types of circuits, a solid understanding of their design helps us to choose the proper power supply even when we use a prebuilt supply.

The next section considers the specifications for a power supply. Section 7.3 analyzes the design of AC-to-DC power supplies. Section 7.4 looks at the design of DC-DC power converters. Section 7.5 discusses batteries as power supplies. Section 7.6 designs a simple AC-to-DC power supply using discrete components. Section 7.7 considers thermal characteristics of electronics and heat dissipation. Fig. 7.8 discusses power management.

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Environmental considerations

Dr Frank Süli , in Electronic Enclosures, Housings and Packages, 2019

10.7.6.3 Power supplies

Power supplies are often designed as subassemblies of larger devices. Many power supplies are cooled by natural convection (Meng et al., 2018 ). The enclosure is usually fabricated from sheet metal or plastic. The enclosure could also have many openings. Power supplies can also be installed to form a separate dedicated power supply unit. This could be as large as a cabinet.

However, power supplies usually experience a relatively favorable corrosion environment according to Hahn et al. (2015). Power supplies are usually kept dry and warm. Unfortunately, some of the power supplies are directly exposed to external airflows as part of the heat management system. Such a situation can alter the power supply's environment drastically as the conditions become contaminating and thus much more corrosive.

High operational temperatures keep power supplies dry. However, this heat can damage the isolation and wiring materials. Emadi et al. (2017) demonstrate that the load of power varies, thus heat cycling becomes an issue. Large ΔT creates an environment that accelerates corrosion processes.

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Interfacing an Op Amp to an Analog to Digital Converter

In Op Amps for Everyone (Third Edition), 2009

15.3 Power Supply Information

Power supply rails can quickly rule out amplifier solutions ( Figure 15.1). This is similar to clothing shopping—the style may be desirable, but if the size doesn't fit, the style is useless. So a wise shopper finds the options in the size first, before becoming attached to a style. Similarly, an op amp with fantastic specifications at ±15 V may not operate at all from a +3.3 V power supply. Power supply information is collected first, because it simply and unequivocally narrows the choices:

Figure 15.1. Focusing on the power supply characteristics.
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What is the power budget for the overall system? Is power a concern or is performance the ultimate goal?

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What power supply voltages are available in the design?

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Is there a preferred power supply voltage for the amplifier circuitry?

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Can an additional supply voltage be added if performance could be improved? Often, the best amplifier performance can be obtained with split supplies.

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Is a precision reference available in the system? In single supply systems, it is important to supply a virtual ground to the op amp circuitry. If the system already contains a reference, it may be possible to utilize it.

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Are there any special characteristics of the power supply? For example, is the power supply a switching power supply? Although op amps usually have excellent power supply rejection, it could be a concern in high resolution system. Any widely varying loads could also affect the op amp supply voltage.

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Building Global Energy Interconnection

Zhenya Liu , in Global Energy Interconnection, 2015

4.2.3.2 Assuring Flexible Access and Operation of Distributed Power Sources

Distributed power supply is an important means of fully utilizing widely-scattered energy resources and also a key approach to the development and utilization of clean energy in the future. Development practices and policy environments in different countries all point to large-scale development of distributed power supply as an emerging trend. It is therefore of utmost importance that smart grids should be able to accommodate and promote access for and safe and economical operation of large-capacity distributed power supply.

Supporting large-scale, high-level access for distributed power supply. When the capacity of distributed power supply in a grid has reached a relatively high level (i.e., high penetration), a conventional grid will find it very difficult to ensure power balance and safe operation as well as supply reliability and power quality. Unlike their conventional counterparts, smart grids do not need to passively restrict access capacity of distributed power supply to ensure operational safety. Rather, they may allow effective access for, and support the plug-and-play capability of, distributed power supply in a way that can facilitate distributed power generation and bring down overall investment costs. By upgrading the protection and control system and standardizing the system interfaces of conventional grids, together with the support of an information and communication platform, smart grids can effect information exchange with distributed power supply and build an open, integrated platform for energy utilization to facilitate equal, convenient, and efficient utilization of distributed power supply.

Supporting safe and economical operation of distributed power supply. Through the data and information platform of a smart grid, data on distributed power supply and grid operation are collected on a real-time basis and highly integrated with offline management data to visualize and control distributed power supply and provide operators with advanced decision support capabilities covering grid operation monitoring, pre-warning and self-recovery. The functions of distributed power supply such as "plug-and-play," two-way measurement, output forecast, and optimized control, will effectively improve the operational characteristics and economics of distributed power supply, and reduce the costs of power system auxiliary service specifically required for distributed power supply.

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