Power Supplies

View A Video on Power Supplies - A Quick Introduction

Power supplies can be integrated into a device or externally attached, portable modules, depending on their operating temperature and risk of overheating. Power supplies are necessary to the operation of just about every electrical device, including desktop and laptop computers, cell phones, lasers, telecommunications like radio, phone lines and the internet, medical equipment, lamps, appliances and industrial machinery. They provide either AC power supply, which is an alternating current, or DC power supplies, which offer a direct current. Today, most electronic devices in the home and office are powered by 12v power supply, while industrial applications employ high voltage power supply. Different devices and electronics require certain types of current, frequency and voltage. In these cases, AC to DC converters are used to switch the type of current, which are also called power inverters. They use rectifier, which contain diodes that alter and regulate the electrical current. Frequency converters and switching power supplies are integrated into the power supply unit in order to give off the appropriate output. Some variable power Supplies are able to adjust the output voltage to specific requirements for product testing and design. Most electronic products today require regulated power supply, a type that produces stable and constant output at a certain, unwavering voltage regardless of power outages, brown outs or surges. Most power supplies are protected by a backup battery. These are called uninterruptible power supplies, and are reliable even when there is no power available.

There are two main types of electrical current that are regulated, controlled and altered by power supplies-alternating current and direct current. Both are used to power different kinds of electrical products, but the input into a power supply from a battery or other power source is almost always AC. Alternating current exhibits electrical charge that consistently and periodically reverses direction. It moves forward then backwards over and over. This form of current is used in commercial businesses and residential buildings. The alteration of the current's direction is measured in Hertz. For example, 60 Hertz refers to the number of alternative directions the current takes in a second. Direct current, on the other hand, refers to electric charge flow that runs in a single, linear direction. It flows in metal conductors like wires, semiconductors, insulators or even a vacuum. Cell phones and laptop computers use DC, as well as medical equipment, video technology and process control systems. Direct current units are usually external from the electronic device and held within a protective casing.

Power supply manufacturers offer many different designs and configurations of power supply units, which range depending on their application, type of current, frequency and voltage level. Some of the unit designs and styles are external, meaning they are separate components to the electronic device. These include board, cabinet, desktop, module, open frame, enclosed, rack mount and wall mount. If their operating temperature is low enough, many power supply units are integral parts of the device, located inside them. Some of the display choices available, which provide information about the voltage and current that are the result of measuring and monitoring, are digital numerical displays, analog visual indicators and graphic or video displays. Some of the various features include adjustable voltage, which can be increased or decreased by a dial or knob, adjustable frequency, computer interface, fan cooling systems, as well as integral heatsink and overcurrent protection. Many also work in conjunction with a back up battery that is employed in case of a power outage. Other features include overvoltage protection, power factor correction, pure sine output, remote on and off switch, short circuit protection and water cooling. When looking at power supply units, consider some of these specifications: the number of outputs, DC output voltage, DC output power, AC output voltage, AC output frequency, operating temperature and apparent power.

One of the most common applications for power supply manufacturers is electrical power for computers. This vital component is a smaller, black metal box typically located on the back of the computer in a corner of the case. The power supply unit also contains the power-cord receptacle and the cooling fan and is usually visible from the back of a system. Power supply units use switcher technology to convert AC input to lower DC voltages, so they are commonly referred to as switching power supplies. The voltages commonly supplied are 3.3 and 5 volts, which are used by digital circuits, and 12 volts, which run motors in disk drives and fans. In order for the power button to work when the PC is off, the power supplies have a circuit that supplies 5 volts, called "standby voltage" or VSB.
There are three types of uninterruptible power supplies (UPS). An offline UPS is basically a standby system that provides battery power to equipment when the main power supplies fall below a set limit. These power supplies do not cost much and are recommended for home office use. A line interactive UPS is similar to an offline UPS in that it switches to battery mode during a blackout. However, this UPS actually boosts the main power supply when it falls, using a regulator. These power supplies are ideal for business applications. The highest level of protection for an electrical device is an online UPS unit that converts AC to DC and then back to AC to supply critical power loads. These UPS units, often referred to as double conversions, contain an automatic bypass to ensure continuous power supplies during a short-term overload or UPS failure. On-line UPS systems are perfect for critical loads and sensitive equipment, such as medical technology.

---

Power Supply Types

• AC to DC converter converts power from an AC input, such as a wall outlet, into DC current.
• AC to DC power supplies are units that provide power to an electronic device by converting AC current, such as that which comes from a wall outlet, to DC current at the proper voltage.
  
• Constant current power supplies control the output current for alterations in load, line and ambient temperature and time within particular limits.
  
• Constant voltage power supplies control the output voltage in load, line, ambient temperature and drift resulting from changes over time
  
• DC/DC converters are used to increase or decrease the voltage level of DC electrical power, because, unlike AC, DC cannot be changed using a transformer.
  
• DC power supplies, such as linear power supplies, switching power supplies, DC/DC converters and high voltage power supplies, receive an input power and output the required form of DC power.
  
• Ferroresonant power supplies are utilized at higher levels in static applications, due to their weight. Ferroresonant power supplies are effective only when the line frequency is extremely stable, as they are sensitive to changes of input AC frequencies.
  
• Frequency converters, a special type of transducer, are simply electrical currents that convert periodic signals into their digital or analog equivalents. The most common frequency converters are frequency-to-digital and frequency-to-DC converters.
  
• Half bridge converters are power switching circuits consisting of two transistors and two capacitors. Half bridge converters function in similar fashion to full bridge converters.
  
• High voltage power supplies are capable of providing hundreds or thousands of volts from one or more DC outputs, using linear technology. Some high voltage power supplies have adjustable local or computer interface outputs and are used in specialized applications, including telecommunications, video technology and medical equipment.
  
• Inverters change DC current to AC current and may be mechanical (e.g. motor), ferroresonant and solid state.
  
• Linear power supplies have a bulky steel or iron laminate transformer that acts as a safety barrier for the low voltage output from the AC input and reduces that input to a much lower voltage. The AC output is then rectified by two or four diodes, and electrical converters change the output into low voltage DC, which is regulated into the required output voltage by dropping the difference in voltage across the shunt regulator.
  
• Modular power supplies are comprised of a number of separate subsections, such as power, input and filter modules.
  
• Off-line power supplies operate directly off the AC line. Off-line power supplies do not use a power transformer before the process of rectification and filtering.
  
• Operational powers supplies have a high open loop gain regulator, for which passive components can be used to program. The regulator acts like an operational amplifier.
  
• Power inverter converts DC current to AC current.
• Rectifiers are electrical components containing sets of diodes that change AC into DC.
  
• Switch mode power supplies rectify and smooth AC voltage using diodes and capacitors, resulting in a high voltage DC, which in turn is converted by a small ferrite transformer and FETs or transistors into a safe, low voltage, high frequency voltage. Another set of diodes, capacitors and inductors convert that DC voltage into the required voltage, the corrections of which are done by adjusting the pulse width of the high frequency waveform.
  
• Uninterrupted power supplies (UPS) are power supply units that continue to provide power during the loss of AC input power, which is achieved through a back up battery and a DC/AC inverter or DC/DC converter. A stand alone UPS unit is external to the equipment being powered, while a battery back up UPS is implanted in the equipment.

---

Power Supply Terms

Autoranging Input - A sensing circuit for the input voltage located within the power supply that automatically switches to the necessary voltage range.
 
Breakdown Voltage - The highest AC or DC voltage that may be applied from the input, output and/or chassis of a power supply.
 
Burn-in - Operating a newly manufactured power supply, usually at rated load, for a period of time in order to force early failures or other latent defects of the component before the unit is delivered to a customer.
 
Common-mode Noise - Noise that is typical of DC output and return lines with respect to input ground.
 
Compliance Voltage - The voltage output from a constant current power supply.
 
Cooling - The removal of heat in a power supply by convection, forced air, radiation or liquid. Heat comes from regulation, transformation, filtering and rectification.
 
Cross Regulation - The percent of voltage change at one output of a multiple output power supply resulting from the load change on another output.
 
Current Limiting Circuit - An overload protection circuit, which controls the highest output current of a power supply to safeguard the power supply or the load.
 
Design Life - The projected lifetime of a power supply during which it will run at its stated specifications.
 
Differential Mode Noise - Also referred to as "ripples," it is the noise measured between the DC output and the output return.
 
Drift - With operating parameters including line, load and ambient temperature held constant, it is the change in output voltage, following a warm-up period, over a certain period of time.
 
Efficiency - The ratio of power in terms of the input power against the output power. Efficiency is measured at full load and nominal line conditions.
 
Electromagnetic Interference (EMI) - Also known as "radio-frequency interference (RFI)," it is unwanted high frequency energy conducted through the input or output lines of switching power supplies or radiated through space. EMI is caused by the switching transistors, output rectifiers and zener diodes.
 
Foldback Current Limiting Circuit - A current limiting circuit that, when under overload conditions, will gradually decrease the output current to a specified minimum current level under a direct short circuit.
 
Ground - An electrical connection to earth that has a zero voltage or another conductor connected to earth.
 
Inhibit - The capability of remotely switching off the output power of a power supply.
 
Input Line Filter - A low-pass or band-reject filter used to decrease the noise fed to the supply. Input line filters are located at the input of a power supply and may be external.
 
Isolation Voltage - The highest AC or DC voltage that can be continuously run from a power supply chassis or from input to output.
 
Margining - Altering a power supply output voltage, either higher or lower from its minimal setting, in order to confirm the system performance margin in respect to the supply voltage. Margining is typically done electrically via a system generated control signal.
 
Minimum Load - The least amount of load current or power that needs to be drawn from the power supply in order for the supply to meet its performance specifications.
 
Non-Shutdown Over Voltage Protection - A feature of a converter such that it continues to provide voltage to a load at a set upper limit without turning off and without necessitating a reset when the overvoltage event ceases.
 
Overvoltage Protection - A circuit that either shuts down the power supply or shorts the power supply to ground if an overvoltage condition occurs.
 
Parallel Operation - The connection of the outputs of multiple power supplies with the same output voltage that are designed to share a load. The parallel operation generates a higher output current than would be available from a single supply.
 
Peak Power - The absolute highest output power that a power supply can create without immediate damage. Typically, peak power is much higher than the continuous reliable output capacity and ought to be utilized rarely.
 
Power Fail - A signal from the power supply interface that relays a warning that the input voltage is not sustaining full power regulated output.
 
Reverse Voltage Protection - A protection circuit that prevents damage to the power supply if a reverse voltage is applied at either the output or input terminals.
 
Safety Ground - A conductive path to earth intended to safeguard people from electrical shock by shunting away any dangerous currents that could happen from accident or malfunction.
 
Turn-on Time - Also known as "warm up time," it is the time a converter needs to start running within specification after proper power has been applied.

---