DC Power Supplies Information
How to Select DC Power Supplies
Image Credit: Advantech Corporation | Carlo Gavvazi | Extech Instruments Corp.
DC power supplies are power supplies which produce an output DC voltage. Power supplies are devices that deliver electric power to one or several loads. They generate the output power by converting an input signal into an output signal (in this case, a DC output).
Composition 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 DC Power Supplies
DC 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. Linear power supplies provide excellent regulation, very small ripple, and very little output noise.
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 (SMPSs). 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. A SMPS design results in high power density (smaller size for the same power output) and reduced power consumption (higher efficiency) in comparison to linear power supplies.
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.
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.
Input frequency is the frequency of the input signal.
Output voltage is the magnitude of the DC voltage at the output of the device.
Output current is the current associated with the output voltage.
Output power is the power (in watts) delivered to the load.
Regulation indicates the stability of the output voltage.
Line regulation is the maximum steady-state amount that the output voltage changes as a result of a specified change in input line voltage.
Load regulation is the maximum steady-state amount that the output voltage changes as a result of a specified change in load.
Mounting specifications are of less importance but should be considered as needed to properly fit the power supply to the application or system. Mounting options include:
DIN rail mount
Features for DC 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 DC 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
Over current protection
Over voltage protection
Under voltage protection
Over temperature protection
Several cooling methods are used to protect DC power supplies:
- Fan cooling
- Heat sink cooling
DC power supplies can also incorporate a number of other features:
- Battery backup
- Hot swappable
- Power factor correction
- Temperature compensation
For a more in-depth overview of power supply selection, visit the Power Supplies Selection Guide on GlobalSpec.