Power Supplies Information
How to Select Power Supplies
Image Credit: Analytic Systems | Digi-Key Corporation | Automationdirect.com
Power supplies are electrical devices that deliver electric power to one or several loads. They generate the output power by converting an input signal into an output signal with some different characteristics. The input and output power can both be a direct current (DC) or alternating current (AC) signal. Power supplies are used in every type of electrical system, such as computers, telecommunication devices, process control systems, industrial systems, wireless devices, and any other systems that require power sources to drive them.
Construction and Operation
To illustrate the general structure of a power supply, we will use a typical DC power supply. A basic DC power supply can be built with four circuits (or sections), as shown in the following diagram, where each block represents a particular circuit that performs a specific function.
Image Credit: Electrician Training - Integrated Publishing
Transformer - The input to the transformer is – normally – an AC signal that is generated by a line voltage such as the power from an electric outlet. The transformer's main function is to step-down (lower the amplitude) or step-up (increase the amplitude) the signal to produce the desired DC level required at the output of the power supply. The transformer also plays the role of an isolator. In many applications it is important to isolate the input AC signal from the signals generated internally by the device.
Rectifier - The signal at the output of the transformer is fed to the rectifier. This device provides a rectified pulsating DC signal. The rectifier can be a half-wave or full-wave rectifier. A pulsating DC signal is a signal (voltage or current) that does not change polarity, but its magnitude is a function of time. Typical rectifiers are built with diodes and resistors.
Filter - In order to convert the pulsating DC signal into a non-pulsating DC signal, a filter is needed. Normally a simple capacitor filter suffices. The output of the filter is DC voltage, which usually has some ripple or small AC variations.
Regulator - The regulator has two functions: (1) To smooth the signal from the filter producing a DC signal with no ripple, and (2) to produce a constant voltage at the output. The voltage at the output of the regulator remains constant even with variations in the input voltage or variations in the load (not shown in the diagram).
To illustrate the four steps or block needed to create a DC voltage from the line voltage, the following figure shows the conversion of a 115 V(rms) signal to a constant 110 V(DC) voltage.
Image Credit: Electrician Training - Integrated Publishing
Types of Power Supplies
Power supplies are categorized by the mechanism used to convert and transfer the input power to the output power. There are three main categories:
Linear power supplies accept AC inputs and provide one or more DC outputs for a wide variety of computer and industrial applications. They use an active element (normally a power transistor) operating in its linear region to generate a desired voltage. The output voltage is regulated by dropping excess input power in ohmic losses (heat) in a series dissipative component (resistor) or a transistor. Some important characteristics of linear power supplies are: excellent regulation, very small ripple, and very little output noise. Applications of linear power supplies include
- Low noise amplifiers
- Lab equipment
- Telecommunication devices such as sampler circuits, multiplexers, demultiplexers, etc.
- Data converters, such as A/D and D/A converters
Image Credit: Digi-Key Corporation
Switching power supplies use a switching element or regulator (normally a power transistor) to generate the desired voltage. They are also called switch-mode products or switching mode power supplies (SMPS). These power supplies incorporate electronic components that continuously switch ON and OFF at a very high frequency. This switching action connects and disconnects energy-storing devices (inductors or capacitors) to and from the input source voltage or the output load. SMPS design results in a smaller power supply since the size of the power transformers, inductors, and capacitors is inversely proportional to the switching frequency. Switch-mode operation also reduces the power consumption because when a switch is OFF, its current is almost zero. When a switch is ON, its voltage is very small. A switching power supply is more efficient than a linear power supply because the excess power is not wasted (in form of heat) but instead all the power is used to convert input power into output power. The switching elements normally are inductors, capacitors, or transistors operating in cut-off or saturation. There is no dissipating resistance in the system, so no waste of power occurs.
Image Credit: Digi-Key Corporation
SCR power supplies use silicon controlled rectifier (SCR) topology to provide well-regulated voltage and current output. Silicon controlled rectifiers are four-layer thyristors with an input control terminal, an output terminal, and a cathode or terminal that is common to both the input and output terminals. An SCR circuit is commonly used in applications involving high voltages and currents.
Image Credit: ABB Low Voltage Products & Systems
Types Based on Input/Output
Based on the type of input/output voltages, the power supplies can be classified into the following general categories:
DC power supplies (AC-DC devices) - Provide a DC output voltage when an AC voltage is applied to the input. These are normally known as simply "power supplies".
DC-DC converters - These are power sources that produce a DC output voltage from a DC input voltage. Strictly speaking, these devices are converters that convert a DC voltage into another DC voltage with different magnitude.
DC-AC converters (inverters) - These devices produce AC voltage from a DC input voltage. These are better known as "inverters".
AC power sources (AC-AC devices) - These devices convert an AC voltage into another AC voltage with (in general) different amplitude. They are also known as AC-AC power devices.
Types Based on Output Power
Power supplies can also be classified in terms of the magnitude and type of the output signal:
High voltage power supplies produce a constant high voltage in the range of 600V or more. These power supplies are typically linear types.
High current power supplies produce a constant high current. The output voltage is not relevant in these types.
Lighting inverters provide AC power for indoor, outdoor, and emergency lighting, as well as signage and associated controls.
Solar inverters or photovoltaic (PV) inverters are electrical devices that convert the DC current produced by a PV device (cell or array) into alternating current.
High frequency devices are designed for high frequency or radio frequency (RF) applications.
RF generator power supplies are high-frequency units that generate power signals in the kHz and MHz ranges. There are many applications for RF generators, including welding, induction heating, and many other industrial processes.
There are a number of unique types of power supplies that don't fit under other classifications.
- Current sources provide reliable current for electrical component testing and for powering specialized components. These power supplies provide a regulated, constant current at the output with, normally, variations in the output voltage.
- AC-DC adapters accept AC input voltage directly from a wall outlet, and output DC voltage. These are generally used to power small electronic devices such as computers and consumer electronics.
- Programmable power supplies are digitally controlled power sources providing accurate and adjustable levels of voltage, current, and frequency. They are often used in automated equipment testing, certification, and calibration applications.
- Voltage multipliers are special types of power supplies that convert a small-magnitude voltage into a higher-level voltage.
There are many parameters needed to fully characterize a power supply; however for most power supply types there are a set of parameters that are common. These include input and output voltage (specified in volts [V]), the output current (in amps [A]), the rated output power (in watts [W]), the input signal frequency (in Hertz [Hz], kilohertz [kHz], or megahertz [MHz]), and the regulation.
Input voltage is the magnitude and type of the voltage applied to the power supply. This can be an AC or DC voltage.
Input frequency is the frequency of the input signal. If the signal is a DC voltage, its frequency is zero.
Output voltage or supply voltage, is the voltage at the output of the device. This is the voltage that is being applied to the load at the output of the power supply.
Output current is the current associated with the output voltage. If the power supply produces more than one voltage (a multiple-channel power supply), for each output voltage there is a corresponding current that must be specified.
Output power is the power (in watts) delivered to the load. For a single output, the required power will be the supply voltage times the maximum current required. For multiple outputs, add up the voltage × current products for all of the supply voltages to determine the power rating.
Selection tip: Good engineering practice dictates selection of a power supply model with a rating that provides some design margin.
Regulation indicates the stability of the output voltage. There are two types of regulation that can be specified when selecting a power supply.
Line regulation is the maximum steady-state amount that the output voltage changes as a result of a specified change in input line voltage. Line regulation is expressed as a percent change of output voltage caused by changes in the magnitude of the line (input) voltage.
Load regulation is the maximum steady-state amount that the output voltage changes as a result of a specified change in load. Typically, load regulation is expressed as a percent change of output voltage caused by increasing the load from half load to full load.
There are other specifications of lesser importance that should be considered as needed. These include
Form factor - Form factor designates the packaging of the power supply design:
Open frame - components are fitting together on an open board with no outer packaging or enclosure.
Enclosed - components are contained inside an enclosure to protect users and equipment.
Encapsulated - components are tightly enclosed in a (typically plastic) capsule covering.
Mounting - Mounting designates how the power supply is fitted, based on both the physical and electrical connections of the system. Several mounting styles are available:
- Board (PCB) mount - Designed to mount on a printed circuit board (PCB) through through-hole technology (THT) or surface mount technology (SMT).
- Circuit mount - The power supply is a computer card itself.
- Wall mount - The power supply can be mounted on a wall.
- DIN rail - Designed to mount on a DIN-Rail.
- Rack mount - These power supplies are designed to be inserted in a standard rack.
- Desktop - These are stand-alone power supplies that rest on a bench of table.
- Display - Power supplies which provide a display output may have one of several types:
- Analog / visual indicator - devices have an analog meter or a simple visual indicator such as a needle.
- Digital numerical display - devices have a digital display for numerical values.
- Graphic / video display - devices have a cathode-ray tube (CRT), liquid crystal display (LCD), or other graphical or multi-line display.
Features for power supplies add functionality such as circuit protection and cooling which may be important for certain applications.
Several factors can affect the performance and/or the physical integrity of power supplies. Circuits to protect the power supplies are normally included in the design and construction of the device. Some of these are
Short circuit protection - Protect the device from an internal or external short circuit current that can destroy or imperil the device. Provides protection in the event of a short circuit on the load from an external source; it includes electronic current limiting and thermal resets with automatic recovery.
Overload protection - Protects the power supply from load faults such as short circuits and open circuits.
Over current protection - This is internal circuitry that limits or shuts down the current output in an overcurrent condition.
Over voltage protection - This is internal circuitry that limits or shuts down the voltage output in an overvoltage condition. When present, it is usually found on the primary output.
Under voltage protection - This is internal circuitry that limits or shuts down the voltage output in an under voltage condition.
Over temperature protection - Circuitry used to protect the device from an increase in heat above the devices rated temperature.
Several cooling methods are used to protect power supplies:
- Fan cooling - The power supply is cooled by forced air into the device, normally using a fan.
- Heat sink cooling - The power supply is cooled using an integral heat sink.
Water cooling - The power supply is water cooled for increased temperature stability.
Power supplies can also incorporate a number of other features:
- Battery backup - The power supply includes a battery backup for continuous output in the event of power failure.
- Hot swappable - The power supply can be replaced without shutting down the system, which is important for critical systems and those where downtime is unacceptable.
- Power factor correction - The power supply corrects the phase difference between voltage and current in order to optimize power output.
- Temperature compensation - The power supply contains circuitry that compensates for changes in temperature that can result in undesirable output.
- Weatherproof - The power supply can withstand prolonged exposure to the environment.
Standards and Compliance
For many applications, it is important that power supplies are compliant with certain organizational or national standards, such as those listed below:
Registration, Evaluation and Authorization of Chemicals (REACH) is a European Union standard for supply chain information exchange. The REACH standard provides a supplier portal to facilitate communication with suppliers. If a product is submitted via the trader portal, the supplier portal sends an e-mail to the relevant exporter requesting that they register and give details of the substances contained in the product. If more complex products are involved, exporters can also include their own sub-suppliers in the automatic substance query.
Restriction of Hazardous Substances (RoHS) is a European Union (EU) directive that requires all manufacturers of electronic and electrical equipment sold in Europe to demonstrate that their products contain only minimal levels of the following hazardous substances: lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyl, and polybrominated diphenyl ether.
Waste Electrical and Electronics Equipment Regulations (WEEE Regulations) is a European Union Directive that is designed to encourage the reuse, recycling, and recovery of electrical and electronic equipment, and to improve the environmental impact and performance of this equipment. WEEE also establishes the requirement and criteria for the collection, treatment, recycling and recovery of this equipment. It makes producers responsible for financing these activities, and retailers and distributors must provide a means for consumers to return used or obsolete equipment without charge.
Generally, power supply types are related to their applications. There are many applications where linear or switching power supplies can be used; however, there are some applications that require a specific category of power supplies. There are a number of common applications for power supplies:
Computer - Power supplies are used for powering computers or computer peripherals.
- Medical - Power supplies are designed for medical power applications. Medical applications typically require highly conditioned and isolated output.
- Telecommunications - Power supplies are used for powering telecommunications equipment.
- Instrumentation - Power supplies are used for powering instrumentation devices such as oscilloscopes, meters, etc.
- Military - Power supplies are designed for military power applications. Most products for military applications must meet minimum performance specifications designated by the military.