Power Supply Glossary

Ambient temperature - the temperature of the still air in the locality of a power supply. For fan cooled units, the ambient temperature would be measured at the air inlet.
Agency Approval: UL, CSA, and TUV are safety agencies that test specifications such as component spacing, HI-pot insulation, leakage currents, circuit board flammability, and temperature rating.
ANSI - American National Standards Institute: An industry body that publishes standards, such as standards developed by the IEEE.
Bandwidth: The range of frequencies over which a measurement is made. Typically used in regard to electromagnetic compatibility (EMC) testing and the measurement of output ripple and noise which is often measured over a bandwidth of 20MHz.
Basic Insulation: The insulation applied to live parts to provide basic protection against electric shock.
Battery Backup: a power supply system whereby a battery provides input energy to keep the dc outputs stable in the event of a mains failure
Burn In:The process of testing a power supply (typically at full load), in an elevated ambient temperature, immediately after manufacture. This process is often used to eliminate early life failures.
Bridge Converter: A converter topology employing four active switching components in a bridge configuration across a power transformer.
Brownout: A planned condition created by a utility company to counter excessive demand on their generation and distribution system, whereby the line voltage is reduced by approximately 10 - 15%.
CE: Certificate European. The CE-mark is a European Union regulatory symbol. which demonstrates the compliance of a product with all relevant Directives, typically for power supplies the Low Voltage and EMC Directives
CISPR 22: European Community standard which defines the allowable limits of radio frequency emissions for appliances and other electrical equipment. CPCI (Compact PCI)
CSA - Canadian Standards Organisation: Canadian government organisation that evaluates the safety of electrical equipment.
Common-mode noise: The component of noise voltage that appears equally and in phase on conductors with respect to a common reference for example earth.
Constant Current Power Supply: A power supply which is designed to regulate its output current within a specified range.
Constant Voltage Power Supply: A power supply which is designed to regulate its output voltage within a specified range.
Cross-regulation: in multiple output power supplies, the voltage change at one output due to a load change on another,expressed as a percentage of the nominal voltage.
Crowbar: a type of over voltage protection that shunts the power supply output to ground (usually with a SCR) when the output voltage exceeds a set value. Would normally require recycling of the input to reset.
Derating: reduction in one operating parameter to compensate for changes in another in order to maintain reliable operation. Most common in power supplies, the reduction in output power at elevated temperatures.
Differential-mode noise: the component of noise excluding common-mode noise, that is measured between two lines eg line to neutral or dc output and output return.
Double Insulation: Independent insulation system applied in addition to basic insulation in order to reduce the risk of electric shock should the basic insulation fail.
Dropout: The lower limit of input voltage where a power supply can no longer maintain output regulation. For linear regulators dropout voltage is load dependent whereas for most switch mode regulators it is largely design dependent.
ESR - Equivalent Series Resistance: For electrolytic and tantalum capacitors, the value of resistance in series with an ideal capacitor which represents the performance characteristics of a real capacitor. Usually measured at a specific frequency (100Hz or 100kHz).
Efficiency: the ratio of total output power to input power, expressed as a percentage. Efficiency must be specified at a specific combination of load and input voltage.
Electromagnetic interference (EMI): unwanted high-frequency energy (noise) that is either radiated into space or conducted via cabling. Can have an adverse effect on the operation of other electronic equipment that may not be directly connected to the interference source. Switching power supplies by their nature are a source of conducted and radiated emissions due to the presence of internal high frequency/fast rise time signals. Conducted noise into the mains cabling can be reduced to acceptable levels by the addition of filtering at the front end of the power supply. Radiated noise can be reduced by shielding and careful PCB design. As well as being a source of EMI, a power supply can also be affected. IEC standards define acceptable levels of radiated and conducted EMI both for emissions and susceptibility.
EMC(Electromagnetic Compatibility): The requirements for both electromagnetic emissions and susceptibility dictated by the physical environment and regulatory governing bodies in whose jurisdiction a piece of equipment is operated.
Fan Rating: Airflow in cubic feet per minute (CFM). A 100% increase in airflow will result in a reduction in system operating temperatures of 50% relative to ambient. For each 10°C reduction in operating temperature the life of a system is doubled. (Arrhenius equation)
Foldback current limiting: a type of power supply overload protection which minimizes internal power dissipation under overload. The output current and voltage decrease as the level of overload increases reaching a minimal value at short-circuit.
Flyback converter: Common converter topology for power converters up to 100watts. Popular due to relatively simple design, low cost and ability to do multiple outputs. Similar to forward converter topology, main difference being that energy transfer to transformer secondary during primary switch off period.
Forward converter: a power supply topology for power converters between 100 – 250watts. More complex than the flyback converter. Main difference being that energy is transferred to the transformer secondary when the primary switching transistor is conducting. A Forward converter circuit stores minimal energy in the transformer.
Ground Loop: Undesirable voltages caused by ground currents of several circuits flowing in a common ground impedance
Haversine: A waveform that is sinusoidal in nature, but consists of a portion of a sine wave superimposed on another waveform. The input current waveform to a typical off-line power supply has the form of a haversine.
HIPOT: Abbreviation for High Potential, refers to the high AC or DC voltages used to test dielectric withstand capability for agency safety requirements.
Holdup time: the time during which a power supply's output voltage remains within specified limits following the removal of input power. Usually specified in mS, holdup time is normally measured at full load and nominal line conditions.
IEC - International Electro-Technical Commission: International organisation that defines standards for safety of electrical and other equipment. Many IEC standards were adopted from the German VDE, which was the main standards-writing body in Europe. One goal of the IEC is to harmonise differing standards between European countries to facilitate free trade. The US Underwriters Laboratories (UL) and the Canadian CSA are members of the IEC.
IEC 601 (EN60601): The safety standard specified by the International Electrotechnical Commission, which covers medical and dental equipment that is intended for professional use.
Input Line Filter: An internally or externally mounted low-pass or band-reject filter at the power supply input which reduces the noise fed into the power supply and also from the power supply back to the mains.
IEC 950 (EN60950): The safety standard specified by the International Electrotechnical Commission, which covers IT equipment that is intended for professional use.
Isolation Transformer: A transformer in which one or more output windings is electrically isolated from the input winding and all other output windings by an insulation system equivalent to double or reinforced insulation.
Inrush current: the peak instantaneous input current drawn by a power supply when it is initially turned on. Dominated by charging current into empty input reservoir capacitor.
Inrush circuit limiting: a circuit that limits the inrush current when a power supply is turned on. Often a simple NTC thermistor that is initially high impedance at switch on and low impedance during normal operation.
Insulation resistance: the dc resistance between two defined points at a specific test voltage in a controlled environment (25°C temperature and less than 50% relative humidity).
Inverter: a power supply that converts dc power at its input into ac power at its output.
Isolation: the galvanic separation between the input and output of a power supply due primarily to the power transformer. Isolation is dependant on insulation materials and creepage/clearances within the supply.
Isolation voltage: the maximum ac or dc voltage that may be continuously applied between input/output, output/ground, primary/secondary etc of a power supply without failure of the insulation system
Leakage Current: Defined in UL 2601-1 as any current, including capacitively coupled currents, which may be conveyed from accessible parts of an appliance to ground or other accessible parts of the appliance and which is not intended to be applied to a patient.
Line regulation: the maximum percentage change in output voltage that occurs when the input voltage is varied over its specified limits.with load and temperature constant.
Load regulation: the percentage change in output voltage that occurs as the load is varied from minimum to maximum, at constant line and temperature. Load change may also be specified for step changes eg. 50% load to full load.
Magnetic Amplifier: Abbreviated "Mag Amp," a saturating inductor which is placed in series with a power supply output for regulation purposes.
Mean time between failure (MTBF): An indicator of the reliability of a power supply. MTBF is a statistical failure rate expressed in hours, either predicted as stipulated in Military Standard MIL-HDBK-217 or measured as stipulated in Military Standard MIL-STD-781C. Other methods can be used eg. for telecoms. equipment Bellcore TR-332, British Telecom HRD4. Demonstrated values are determined by carrying out accelerated life testing.
Minimum Load: The minimum current which must be drawn from a power supply output to maintain output regulation. Often requirement of multiple output supplies.
Off-line: A power supply which receives its input power directly from the AC mains, without use of a 50/60 Hz power transformer, hence "off line" power supply.
Optoisolator: An electro-optical device consisting of a light emitting diode and a photo-transistor which transmits a signal across a DC isolation boundary. In AC power supplies often used to transmit output voltage error signal across the isolation barrier to the control circuitry on the primary side..
Output Impedance: The ratio of change in output voltage to change in load current.
Output Noise: The AC component present on the DC output of a power supply. Output noise on a switching regulator has two components: a lower frequency component at the switching frequency and a high frequency component due to fast edges switching transitions. Standard method for measuring noise is to measure directly at the output terminals with a scope probe having an extremely short ground lead with the output decoupled with 10uF electrolytic and 0.1uF ceramic capacitors.
Output Good: A TTL compatible alarm signal common on may power supplies which indicates that the output voltage is within its specified regulation levels. If the output goes out of regulation, the state of this signal changes.
Output Noise: See ‘ripple and noise’. The differential-mode output ripple and noise as measured over a 20 MHz bandwidth.
Operating temperature range: the range of ambient or case temperatures over which a power supply may perform safely and within specified limits.
Output current limiting: a protective feature that maintains the output current of a power supply within predetermined limits during an overload to prevent damage to the supply or the load. The supply automatically reverts to normal operation on removal of the overload.
Overvoltage protection (OVP): a protective feature that shuts down the power supply output to prevent damage to the load should the output voltage exceeds a predetermined limit.
Parallel operation: connection of two or more power supplies of the same output voltage in order to obtain a higher output current than either supply can provide alone. Parallel operation requires power supplies that are specifically designed to load share to prevent one power supply sourcing very much more of the load current than the other.
Post Regulator: A linear regulator used at the output of a switching power supply to improve line and load regulation and reduce output ripple.
Ripple and noise: the unwanted periodic (ripple) or aperiodic (noise) deviation of the power supply output voltage from its nominal value. Ripple is a function of the input line and switching components. PARD is usually expressed in millivolts peak-to-peak or rms for switching power supplies and is usually measured over a bandwidth of 20MHz.
Peak Current: The maximum output current which a power supply will source for brief periods of time.
Power Fail: A TTL compatible alarm signal which indicates that the input AC or DC power has failed. This signal is very useful in microprocessor systems as an early warning of impending power loss. As the output voltage will still be stable for the holdup time of the PSU the user can either store important information or switch over to backup power before the system goes down.
Power Factor: The ratio of true ac input power to the apparent ac input power. If both voltage and current are sinusoidal, power factor is the cosine of the phase angle between them.
Power Factor Correction (PFC): Technique of increasing the power factor of a power supply. Switching power supplies without power factor correction have a low power factor (typically 0.6-0.7) due to the non-linear ‘peaky’ nature of the input current. Power factor correction uses either passive or active techniques to make the input current waveform more closely follow the voltage waveform therefore making the power supply appear to the line as a resistive load (PF =1)
Pulse-width modulation (PWM): the basis of many switch mode power converters. A method of regulating the output voltage by varying the conduction time of the primary power switch
Reinforced Insulation: A single insulation system with such mechanical and electrical properties that it provides the same degree of protection against electrical shock as double insulation.
Rated output current: the maximum load current that a power supply is designed to source at a specified ambient temperature.
Remote sensing: a technique for compensating for the volt drop along the load cabling by sensing the output voltage at the load rather than the terminals of the PSU.
Safety extra low voltage (SELV) : A power supply output which is so designed and protected that, under normal and single fault conditions, its voltages do not exceed what is considered a safe value.
Safety Ground: Connection to earth that is designed to protect persons from electrical shock by shunting away any dangerous currents that might occur due to malfunction or accident.
Sequencing: Method of activating the outputs of a multiple output power supply in a desired order.
Short-circuit protection: a protective feature that limits the output current of a power supply to prevent damage in the event of an external short circuit
Soft start: a feature that ensures the smooth rise of the output voltage at switch-on. This feature protects the main switching transistors from transients
Switching frequency: the rate in Hertz at which the dc voltage is switched in a dc-dc converter or switching power supply.
Switching regulator: a high-efficiency power supply circuit employing operates in closed loop control to regulate the voltage across a load, generally by means of pulse-width modulation.
Temperature coefficient: the average change in output voltage per degree change temperature, expressed as a percentage of nominal output voltage, over a specified temperature range.
Thermal protection: a protective feature that shuts down a power supply if its internal temperature exceeds a predetermined level.
Transient recovery time/transient response: the time taken for the output voltage of a power supply to settle within specified accuracy limits following a step change in load current or input voltage.
Transfer Time: Time period taken (in milliseconds) for a standby or off-line type UPS to sense a power interruption and switch from utility to inverter power.
True Power: In an AC circuit, true power is the actual power consumed in watts . It is distinguished from apparent power (Volt Amps) by eliminating the reactive power component (VAr) that may be present.
TÜV: Safety-testing laboratory with headquarters in Germany. TÜV tests products for compliance with IEC or VDE requirements. Products that have the TÜV logo have been tested by TÜV for compliance with applicable standards for sale in the European market.
UL2601-1: The safety requirements specified by Underwriters Laboratories Inc. that cover electrical medical and dental equipment that is intended for professional use by personnel in hospitals, nursing homes, medical care centers, medical and dental offices and similar health care facilities. These requirements cover portable equipment rated at 300 V or less and permanently connected equipment rated at 600 V or less. An artificially induced failure of a component, usually as a result of "abnormal" testing for regulatory agency safety compliance.
UL - Underwriters' Laboratories: US organisation originally founded as a result of the need for insurance companies to help consumers choose safe electrical and safety equipment. UL evaluates equipment against standards which UL has written for the particular equipment category. Equipment, which is evaluated and found to meet the safety requirements, is either UL Listed or UL recognized.
UL Approved: This is a widely used term that is technically incorrect. The correct terms are UL LISTED or UL RECOGNISED.
UL Listed: UL approval for equipment that will be user installed or operated and meets relevant UL safety standards. If a product is UL Listed, then it must be marked with the UL symbol.
UL Recognised: UL approval of component equipment that is not intended to be used stand alone but installed into some other system by a manufacturer or other end-user.
UL2601-1: Safety requirements specified by Underwriters Laboratories Inc. that cover electro medical and dental equipment that is intended for professional use by personnel in hospitals, nursing homes, medical and dental facilities etc. These requirements cover portable equipment rated at 300 V or less and permanently connected equipment rated at 600 V or less.
UPS - Uninterruptible Power Supply: A power supply which continues to supply power following a loss of input power. During power loss power is supplies from a battery source
Warm-up time: the time period after a power supply is initially turned on before it operates according to specified performance limits.
Warm-up Drift: The change in output voltage of a power supply after switch-on until thermal equilibrium is reached.

Power Supply Glossary

YEG Powerline