Input Specifications and Power Quality

AC Input Current The maximum flow of alternating current ($\text{AC}$) electricity drawn from the main supply under the highest specified $\text{DC}$ load conditions. This value must be listed on product labels for compliance with safety agency standards.

AC Input Frequency The rate at which the $\text{AC}$ voltage changes polarity, measured in Hertz ($\text{Hz}$) or cycles per second. Most $\text{AC}$ power is generated at 50 or 60 $\text{Hz}$. Switching Power Supplies typically accept a wide frequency range, such as $47–63\text{ Hz}$ (nominal).

AC Input Voltage The nominal $\text{AC}$ input voltage range the power supply will accept during normal operation. Safety standards often require a supply to operate reliably with an extended range, typically ±10% of the stated input range. For example, a supply labeled $100\text{ to }240\text{ Vac}$ nominal will typically operate over $90–264\text{ Vac}$.

AC Inrush Current The instantaneous, transient surge of current drawn when the power supply is turned on “cold” (initial power-up). It’s primarily due to the rapid charging of the bulk input capacitors. This value is critical when selecting an $\text{AC}$ switch or circuit breaker rating. Smaller power supplies often use a thermistor to limit this surge.

Active Power Factor Correction ($\text{Active PFC}$) A circuit technique that actively controls the power supply’s input current waveform to make it nearly sinusoidal and proportional to the $\text{AC}$ voltage waveform. This significantly reduces harmonic content, minimizing stress on utility lines, transformers, and preventing stability issues. It’s often required in Europe and for higher-power supplies globally.

Passive Power Factor Correction ($\text{Passive PFC}$) A simpler method using a network of capacitors and inductors on the input side to minimize the pulsed nature of the input current draw. While it improves the Power Factor compared to an uncorrected supply, it is generally less effective than Active PFC at minimizing harmonic content.

Harmonic Distortion The presence of unwanted frequencies (harmonics) that are integer multiples of the fundamental $\text{AC}$ frequency in the sinusoidal input current waveform. It is caused by the switching action of the power supply and is often stated as a percentage or expressed via the Power Factor (e.g., $0.95$).

Input Current The current drawn by a supply, typically listed as the maximum continuous input current measured under the lowest input voltage and maximum output load. This value is essential for determining the proper fuse rating.

Inverter A device that converts $\text{DC}$ (direct current) power from a source (like a battery or solar panel) into $\text{AC}$ (alternating current) power.

Off-Line Power A power supply design that operates directly off the $\text{AC}$ line voltage without an input power transformer prior to rectification and filtering.

Output and Regulation

Constant Current ($\text{CC}$) An operating mode where a power supply automatically regulates its output current to a specified limit, irrespective of changes in the output load resistance or voltage (within its specified range). Common in battery charging and $\text{LED}$ driver circuits.

Constant Current Limiting A protection circuit that holds the output current at a maximum safe value whenever an overload is experienced.

Current Limit A protective circuit that restricts the output current to a predetermined, safe value during an overload or short-circuit condition. Normal operation usually restores automatically when the fault is removed.

Current Limit Knee The point on the plot of output voltage vs. output current where the supply transitions from normal voltage regulation to current limiting (or foldback current limiting).

DC Output Voltage The nominal direct current ($\text{DC}$) voltage setting provided by a power supply or $\text{DC}$–$\text{DC}$ converter.

Drift The variation of the $\text{DC}$ output voltage over a specified time period (e.g., hours or days), following a warm-up period, assuming all other operating parameters (line, load, temperature) are held constant.

Efficiency ($\eta$) A measure of the power supply’s performance, calculated as $\text{Output Power}/\text{Input Power}$. It is a percentage and varies with load and input voltage. Higher efficiency means less wasted power (heat).

Floating Output A power supply or $\text{DC}$–$\text{DC}$ converter output that is ungrounded and fully isolated from the input, ground, and other outputs. The user can reference it as either positive or negative.

Line Regulation The maximum change in the $\text{DC}$ output voltage as the $\text{AC}$ or $\text{DC}$ input voltage is varied across its entire specified range, while the output load is held constant.

Load Regulation The maximum change in the $\text{DC}$ output voltage when the output current (load) is varied from its minimum to maximum range, while the input voltage is held constant at its nominal value.

Overshoot The temporary, momentary rise in the $\text{DC}$ output voltage above its regulated level that a power supply experiences when a sharp decrease in load current occurs.

Output Trim The facility, either internal (via a trim pot) or external (via a fixed resistor), to make a small adjustment to the nominal $\text{DC}$ output voltage.

PARD (Periodic and Random Deviation) A comprehensive term used for the sum of all ripple and noise components on the output voltage, measured over a specified bandwidth ($0-20\text{ MHz}$ is common) and stated in peak-to-peak (Vpp​) or $\text{RMS}$ (Vrms​) values.

Ripple and Noise Unwanted $\text{AC}$ voltage components superimposed on the $\text{DC}$ output voltage. Ripple is typically synchronized with the switching frequency of the supply, while Noise is the high-frequency content caused by the switching action. See PARD.

Transient Response (also Dynamic Response) The behavior of the $\text{DC}$ output voltage when the load current changes rapidly (dynamic load). It is usually defined by the magnitude of the temporary voltage deviation ($\text{overshoot/undershoot}$) and the Transient Recovery time.

Transient Recovery (also Dynamic Recovery) The time it takes for the $\text{DC}$ output voltage to return and settle within its specified regulation limits after an abrupt change in the output load current.

Protection and Reliability

Altitude The maximum height above sea level at which a power supply can be operated without performance degradation (derating). Derating is often necessary at high altitudes due to the thinner air, which reduces the efficiency of convection and forced-air cooling.

Breakdown Voltage The minimum voltage that causes an insulating material to fail and become electrically conductive. This is a crucial parameter for safety and isolation.

Brownout An abnormal operating condition where the $\text{AC}$ line input voltage temporarily drops below its specified nominal operating range.

Burn-In An essential manufacturing quality control process where newly produced power supplies are operated under defined load and environmental conditions (e.g., elevated temperature, input power cycling) for a specified period (typically $4$ to $48$ hours). Its purpose is to induce early failures (the infant mortality stage) before the product is shipped.

Crowbar A protective circuit against overvoltage that quickly places a very low-resistance shunt (a “short”) across the power supply’s output terminals if a predetermined voltage level is exceeded. This is a forceful shutdown method.

Current Limit Adjustment The range over which the protective current limit set-point can be manually adjusted, and the means for making that adjustment.

Derating The specified reduction of a power supply’s maximum continuous output current when operating under adverse conditions, most commonly elevated operating temperatures or reduced airflow.

Expected Lifetime The predicted average service life of a power supply, calculated based on the reliability data (e.g., operating temperature, $\text{ESR}$ rating) of its most critical components, particularly the electrolytic capacitors.

Foldback Current Limiting A protective circuit where, during an overload, the output current dramatically decreases as the overload increases, reaching a minimum current (often near zero) at a short circuit. This technique minimizes internal power dissipation during a fault.

Hold-Up Time The duration, specified in milliseconds ($\text{ms}$), during which the $\text{DC}$ output voltage remains within its regulation limits after the $\text{AC}$ or $\text{DC}$ input power is lost. This energy is typically stored in the bulk input capacitors.

Hiccup Mode (also Cycle to Cycle Mode) A robust operating mode triggered by a fault condition (like a short circuit) in which the supply cyclically turns on and off repeatedly. This keeps the average internal power dissipation at a very safe, low level until the fault is removed, minimizing thermal stress.

Insulation Resistance The electrical resistance offered by an insulating material to the flow of current. It is measured under a specified $\text{DC}$ voltage and must meet safety standards.

Isolation Test Voltage (also Hipot/Dielectric Withstand) A high-voltage test applied between isolated points (e.g., input-to-output, input-to-ground) to verify the integrity and strength of the insulation barrier. The test voltage is typically much higher than the Rated Isolation Voltage.

Leakage Current Any small, unintended current flowing between isolated points, such as from input-to-output or input-to-ground/chassis, at a specified operating voltage. It is critical for safety agency certifications, especially medical standards ($\text{IEC }60601$).

MTBF (Mean Time Between Failures) A statistical measure of a system’s reliability, representing the predicted average time a unit will operate without failure. It’s usually calculated exclusive of “infant mortality” failures.

MTTR (Mean Time to Repair) The predicted average length of time required to diagnose and repair a faulty unit.

OTP (Over Temperature Protection) A thermal protection system where the supply shuts down automatically if the temperature at a critical internal point exceeds a specified safe rating. It is a safeguard against cooling system failure and most supplies auto-recover when the temperature drops.

Over Voltage Protection ($\text{OVP}$) A protective circuit that shuts down the power supply output (or uses a crowbar circuit) when the $\text{DC}$ output voltage temporarily or permanently exceeds a specified maximum limit.

Overload Protection A general protective feature that limits output current or power to prevent damage to the power supply or the load. See also Current Limit.

Topology and Structure

Base Plate A dedicated metal surface to which heat-generating circuit components are mounted. It serves as a large heat transfer point, typically for conduction cooling.

Blind Mate A mechanical design feature that allows a power supply or module to slide into a system slot and automatically make electrical connection with its mating connector. Often used in Hot-Swap and redundant systems, it typically includes a short pin (last mate) to sequence power-up and prevent arcing.

Boost Converter (also Step-Up Converter) A $\text{DC}$–$\text{DC}$ power converter topology where the output $\text{DC}$ voltage is greater than the input $\text{DC}$ voltage.

Bridge Converter A high-power switching supply topology that employs either four switching elements (Full Bridge) or two switching elements (Half Bridge). They offer high output power and low ripple but are more complex than simpler topologies.

Cased/Enclosed A power supply housed within a metal or perforated metal cover assembly, typically covering all $6$ surfaces to ensure finger-safe operation.

Covered A supply that is constructed on a $\text{PCB}$ or an L-bracket/U-frame chassis and has an attached cover, often covering only $4$ sides, leaving the ends open for connections.

$\text{DC}$–$\text{DC}$ Converter A device that accepts a $\text{DC}$ input voltage and produces a regulated $\text{DC}$ output voltage, usually with galvanic isolation.

Design Topology The underlying power conversion principle employed by the supply, such as linear, switched mode flyback, half bridge, resonant LLC, etc.

DIN Rail Mount A standardized metal track used for mounting industrial control devices (like $\text{PLCs}$) within an enclosure. A DIN Rail Mount is the bracket/system that secures the supply to this rail.

Encapsulated A power supply or $\text{DC}$–$\text{DC}$ converter that is hermetically sealed and fully enclosed in a thermally conductive epoxy resin or similar plastic compound, often for environmental protection.

Flyback Converter A simple and cost-effective switching topology that uses a single transistor switch. Energy is stored in the transformer during the $\text{ON}$ time and then transferred to the load during the $\text{OFF}$ time.

Forward Converter A switching topology where energy is transferred directly to the load via the transformer while the transistor is $\text{ON}$ and stored in an output inductor.

Galvanic Isolation A design principle where two circuits have no ohmic (conductive) connection between them. Separation is achieved using components like transformers or optocouplers, often required between the input and output for safety.

Linear Regulator A voltage-stabilization technique where a control device (usually a transistor) is placed in series with the load to continuously vary its resistance. The voltage drop across the control device is dissipated as heat, making this topology less efficient than switched-mode designs.

MOSFET (Metal Oxide Semiconductor Field Effect Transistor) A type of transistor used as the main switching element in modern switched-mode power supplies. $\text{MOSFETs}$ offer much better switching characteristics (speed and efficiency) than older bipolar transistors.

Open Frame A power supply consisting only of the printed circuit board ($\text{PCB}$) and its components, without a chassis, enclosure, or cover.

Thermal Management

Ambient Temperature (TA​) The temperature of the still air immediately surrounding the power supply. This is the temperature used for convection cooling specifications.

Case Operating Range (TC​) The temperature range at which a power supply will meet its specifications, as measured at a specified point on the supply’s external metal casing (often the center of the top surface or baseplate).

Conduction Cooling The process of transferring and removing heat from the power supply’s hot components to an external surface (baseplate or heatsink) and then to the system chassis or enclosure via direct contact.

Convection Cooling The dissipation of heat into the surrounding air through the natural movement of air (unassisted by fans or blowers), driven by temperature and density differences.

Forced Air Cooling The use of an external fan, blower, or other air-moving equipment within the system to actively move air across the power supply’s components, significantly increasing its cooling capacity and maximum output power.

Free Air Convection See Convection Cooling.

Heat Sink A metal plate, extrusion, or case designed to increase the surface area available to dissipate heat away from sensitive internal components and circuits.

Parallel and Redundancy

Active Current Share (also Single Wire Parallel) A technique where an active control circuit is used to ensure that multiple power supplies connected in parallel accurately and equally share the total load current. This is required when paralleling supplies for increased total system power.

Current Share (also Load Sharing) The function where multiple power supplies with outputs connected in parallel (for redundancy or higher power) collectively and equally divide the total load current.

Droop Share A method for paralleling supplies that relies on a slight, engineered drop in the output voltage as the load increases. The difference in voltage set-points dictates current flow. It is typically only recommended for redundant operation ($\text{N}+1$), not for increasing total power, due to poor sharing accuracy.

Hot-Swap The ability to replace or insert a power supply unit into an operating system without shutting down the system. This requires a mechanical design with blind-mate connectors and a sequenced power-up process (e.g., via the short pin/last mate).

N+1 Redundancy A system design principle using $\text{N}$ number of power supplies to meet the required total load, plus one (+1) additional supply for immediate backup. This allows the system to continue operating without interruption even if one supply fails, greatly increasing reliability.

OR’ing Diodes (also Decoupling Diodes) Isolation devices (often $\text{MOSFETs}$ instead of diodes for efficiency) placed at the output of each supply in a parallel or redundant system. They ensure that current can only flow out of the supply, preventing a short circuit in one unit from collapsing the entire system bus.

Parallel Operation The connection of the outputs of two or more supplies of the same voltage to achieve a higher total output current, and thus greater total system power. Requires supplies specifically designed for Active Current Sharing.

Safety and Compliance

ATEX (Atmosphères Explosibles) European Directive 2014/34/EU (updated from $94/9/\text{EC}$). It defines the technical requirements for equipment intended for use in potentially explosive atmospheres ($\text{Ex}$ zones), such as areas with flammable gases or dust.

Basic Insulation A single layer of required insulation to provide protection against electric shock.

CB Report A standardized technical document generated under the $\text{IECEE CB Scheme}$. It certifies that a product has been tested to meet the requirements of a specific $\text{IEC}$ standard (e.g., $\text{IEC }62368-1$) and can be used to obtain national safety approvals in over $50$ member countries.

CE Marking (Conformité Européenne) A mandatory marking indicating that the power supply complies with all applicable $\text{European Union (EU)}$ directives, including safety ($\text{LVD}$), electromagnetic compatibility ($\text{EMC}$), and $\text{RoHS}$ (restriction of hazardous substances).

CEC (California Energy Commission) A leading US agency that established mandatory energy efficiency standards for external power supplies in California. These limits were widely adopted across the US through the EISA (Energy Independence & Security Act). See also $\text{Ecodesign}$ for the $\text{EU}$ equivalent.

CENELEC (Comité Européen de Normalisation Électrotechnique) The European Committee for Electrotechnical Standardization. It recommends harmonized technical standards for $\text{electrotechnical}$ products, which are often adopted into the $\text{EU}$‘s official directives.

Chassis Ground (also Common Ground) A connection to the main metal enclosure or frame ($\text{chassis}$) of a piece of equipment. It provides a common reference point, ideally connected to Earth Ground, against which all other circuit voltages are measured.

Clearance The shortest distance through air between two conductive parts or conductors.

Common A conductive path used as a return for two or more circuits. It is often used interchangeably with ground but is not technically a ground unless it is explicitly connected to Earth.

Creepage Distance The shortest distance between two conductive parts or conductors along the surface of an insulating material. This distance is critical for safety to prevent tracking and electrical shock.

CSA (Canadian Standards Association) The independent Canadian organization responsible for testing and certifying products to meet Canadian public safety standards. It performs a function equivalent to $\text{UL}$ in the USA.

Double Insulation An isolation system that comprises $\text{Basic Insulation}$ plus a second, independent layer known as Supplementary Insulation. This is a requirement for many medical and transportation applications to provide two layers of protection against electric shock. Also known as Reinforced Insulation.

Ecodesign (EU) 2019/1782 A mandatory $\text{European Union (EU)}$ directive that sets strict minimum no-load power consumption limits and minimum average operating efficiency levels for external power supplies sold within the $\text{EU}$ market.

EMC (Electromagnetic Compatibility) The ability of a power supply to function satisfactorily in its electromagnetic environment without introducing intolerable Electromagnetic Interference ($\text{EMI}$) to anything in that environment, and itself being immune to $\text{EMI}$ from other sources.

Emissions Unwanted electromagnetic radiation that leaves the power supply. $\text{Conducted Emissions}$ travel along the $\text{AC}$ line cord, while $\text{Radiated Emissions}$ are sent into the surrounding air. Both must meet regulatory standards ($\text{FCC}$, $\text{CISPR}$, etc.).

ESD (Electrostatic Discharge) A sudden flow of electricity between two electrically charged objects (e.g., a power supply component and a human finger) caused by contact or a dielectric breakdown. Supplies must be immune to specified $\text{ESD}$ levels.

FCC (Federal Communications Commission) The US government agency that sets and enforces standards for conducted and radiated $\text{EMI}$ emissions in the United States.

Ground An electrical connection that is made to the Earth (or to a conductor connected to Earth) to provide a zero-voltage reference and a path for fault currents.

Hipot See Isolation Test Voltage.

Hot-Swap The function of replacing a supply without shutting down the system. A supply is designed to be inserted or extracted and fit within a mechanically designed slot with blind-mate connectors. These types of supplies typically have a soft-start function and utilize a short pin to enable the supply which is last to mate avoid arcing and ensure the supply is firmly seated prior to powering up.

IEC (International Electrotechnical Commission) The global organization that prepares and publishes international standards for all electrical, electronic, and related technologies. Its standards (e.g., $\text{IEC }62368-1$) are adopted by most national safety agencies worldwide.

Immunity A measure of a power supply’s resistance to external electromagnetic disturbances, such as surges, $\text{ESD}$, and conducted/radiated $\text{RF}$ fields.

Isolation The parameter measured by applying a maximum rated isolation voltage between two points (typically input-to-output, input-to-ground or output-to-ground).

Rated Isolation Voltage The maximum continuous working voltage a device can safely withstand across its isolation barrier for a fixed time period, without degrading the insulation material.

UL (Underwriters Laboratories) An independent, global safety science company that tests products and components to ensure they meet US safety standards (e.g., $\text{UL }62368-1$).

UKCA Certification (UK Conformity Assessed) 🇬🇧 A mandatory marking for products placed on the market in $\text{Great Britain}$ (England, Scotland, and Wales) since January 1st,2021. It replaces the $\text{CE}$ mark in the $\text{UK}$ market for assessing conformity to $\text{UK}$ regulations, which are currently largely based on pre-existing $\text{EU}$ standards.

Other Technical Terms

DC-DC Converter A supply / converter that accepts a $\text{DC}$ input voltage and produces a $\text{DC}$ output voltage.

Dynamic Load A load condition that changes rapidly and often cyclically. Power supplies must maintain regulation during these sudden changes (see Transient Response).

ESR (Equivalent Series Resistance) The inherent resistance of a capacitor, which acts in series with the ideal capacitance. Lower $\text{ESR}$ is a desirable characteristic, especially for output capacitors in switching supplies, as it directly impacts $\text{ripple and noise}$ filtering performance.

External Synchronization The ability to synchronize the power supply’s internal switching frequency to an external clock signal. This is often used to prevent beat frequencies or interference with sensitive system components.

Faraday Shield An electrostatic shield (a conductive foil or wire screen) placed between the primary and secondary windings of a transformer. Its purpose is to reduce the coupling capacitance between the windings, which in turn reduces common mode noise.

Ground Loop An unwanted phenomenon caused by two or more circuits sharing a common return path (usually a ground conductor), resulting in noise or unwanted voltage differences between the circuit grounds.

Logic Shutdown (or Remote On/Off) A control feature that allows the power supply’s output to be remotely inhibited (turned off) or enabled (turned on) via an external logic signal (e.g., a logic high or low voltage level).

Minimum Load The smallest amount of output current that must be drawn from a power supply for it to operate within its specified regulation and ripple/noise limits. Preload resistors may be added to meet this requirement if the system load is too low.

Peak Load/Current The ability of a power supply to temporarily deliver an output current higher than its maximum continuous rating for a short, specified duration. This is essential for applications with high start-up currents, like motors.