battery pack is a set of any number of (preferably) identical batteries or individual battery cells.

An Ampere or an Amp is a unit of measurement for an electrical current. One amp is the amount of current produced by an electromotive force of one volt acting through the resistance of one ohm. Named for the French physicist Andre Marie Ampere. The abbreviation for Amp is A but its mathematical symbol is "I". Small currents are measured in milli-Amps or thousandths of an Amp.

A unit of measurement of a battery's electrical storage capacity. Current multiplied by time in hours equals ampere-hours. One amp hour is equal to a current of one ampere flowing for one hour. Also, 1 amp hour is equal to 1,000 mAh.

The number of ampere-hours which can be delivered by a battery on a single discharge.

During discharge, the negative electrode of the cell is the anode. During charge, that reverses and the positive electrode of the cell is the anode. The anode gives up electrons to the load circuit and dissolves into the electrolyte.

Batteries with water-based electrolytes. The electrolyte may not appear to be liquid since it can be absorbed by the battery’s separator.

The total battery capacity, usually expressed in ampere-hours or milliampere-hours, available to perform work. 

An electrochemical device used to store energy. The term is usually applied to a group of two or more electric cells connected together electrically. 

The electric output of a cell or battery on a service test delivered before the cell reaches a specified final electrical condition and may be expressed in ampere-hours, watt- hours, or similar units. The capacity in watt-hours is equal to the capacity in ampere-hours multiplied by the battery voltage.

A device capable of supplying electrical energy to a battery.

The current expressed in amperes (A) or milli amps (mA) at which a battery is charged.

The prescribed lower-limit voltage at which battery discharge is considered complete.

Used to signify a charge or discharge rate equal to the capacity of a battery divided by 1 hour. Thus C for a 1600 mAh battery would be 1.6 A, C/5 for the same battery would be 320 mA and C/10 would be 160 mA. 

Is an electrode that, in effect, oxidizes the anode or absorbs the electrons. During discharge, the positive electrode of a voltaic cell is the cathode.

An electrochemical device, composed of positive and negative plates and electrolyte, which is capable of storing electrical energy. It is the basic “building block” of a battery.

The conversion of electric energy, provided in the form of a current, into chemical energy within the cell or battery.

 The process of supplying electrical energy for conversion to stored chemical energy.

A charging process in which the current applied to the battery is maintained at a constant value.

A charging process in which the voltage applied to a battery is held at a constant value.

One sequence of charge and discharge.

A cycle in which the discharge is continued until the battery reaches it’s cut-off voltage, usually 80% of discharge.

Charge and discharge cycles which do not allow the battery to approach it’s cutoff voltage. Shallow cycling of NiCd cells lead to “memory effect”. 

For rechargeable batteries, the total number of charge/discharge cycles the cell can sustain before its capacity is significantly reduced.

The type of electrical current that a battery can supply. One terminal is always positive and another is always negative.

The conversion of the chemical energy of the battery into electric energy.

Capacity in Ampere Hours (Ah) that is discharged from a fully charged battery, divided by battery nominal capacity . DOD is normally presented in percent (%).

Withdrawal of all electrical energy to the end-point voltage before the cell or battery is recharged.

Withdrawal of large currents for short intervals of time, usually at a rate that would completely discharge a cell or battery in less than one hour.

Withdrawal of small currents for long periods of time, usually longer than one hour.

Withdrawal of current from a cell.

A primary cell in which the electrolyte is absorbed in a porous medium, or is otherwise restrained from flowing.

The system of active materials within a cell that provides electrical energy storage through an electrochemical reaction.

An electrical conductor through which an electric current enters or leaves a conducting medium, whether it be an electrolytic solution, solid, molten mass, gas, or vacuum.

A chemical compound which, when fused or dissolved in certain solvents, usually water, will conduct an electric current. All electrolytes in the fused state or in solution give rise to ions which conduct the electric current.

The degree to which an element in a galvanic cell will function as the positive element of the cell. An element with a large electropositivity will oxidize faster than an element with a smaller electropositivity.

The voltage of the battery at termination of a discharge.

Output Capability - expressed as capacity times voltage, or watt-hours.

Ratio of cell energy to weight or volume (watt-hours per pound, or watt-hours per cubic inch).

Method of recharging in which a secondary cell is continuously connected to a constant-voltage supply that maintains the cell in fully charged condition. Typically applied to lead acid batteries.

A combination of electrodes, separated by electrolyte, that is capable of producing electrical energy by electrochemical action.

The evolution of gas from one or both of the electrodes in a cell. Gassing commonly results from self-discharge or from the electrolysis of water in the electrolyte during charging.

The resistance to the flow of an electric current within the cell or battery.

The terminal of a battery from which electrons flow in the external circuit when the cell discharges.

Cells that do not contain water, such as those with molten salts or organic electrolytes. 

Condition of a battery which is neither on charge nor on discharge (i.e., disconnected from a circuit).

The difference in potential between the terminals of a cell when the circuit is open (i.e., a no-load condition).

A chemical reaction that results in the release of electrons by an electrode’s active material.

The arrangement of cells in a battery made by connecting all positive terminals together and all negative terminals together. The voltage of the group remains the same as the voltage of the individual cell. The capacity is increased in proportion to the number of cells.

Refers to the charges residing at the terminals of a battery.

The terminal of a battery toward which electrons flow through the external circuit when the cell discharges. 

A cell designed to produce electric current through an electrochemical reaction that is not efficiently reversible. The cell, when discharged, cannot be efficiently recharged by an electric current.

The number of ampere-hours a cell can deliver under specific conditions (rate of discharge, end voltage, temperature); usually the manufacturer’s rating.

Capable of being recharged; refers to secondary cells or batteries.

State in which the gases normally formed within the battery cell during its operation, are recombined to form water.

A chemical process that results in the acceptance of electrons by an electrode’s active material.

The structural part of a galvanic cell that restricts the escape of solvent or electrolyte from the cell and limits the ingress of air into the cell.

A battery made up of secondary cells. See Storage Battery; Storage Cell.

Discharge that takes place while the battery is in an open-circuit condition.

The permeable membrane that allows the passage of ions, but prevents electrical contact between the anode and the cathode.

The arrangement of cells in a battery configured by connecting the positive terminal of each successive cell to the negative terminal of the next adjacent cell so that their voltages are cumulative.

 For a dry cell, the period of time (measured from date of manufacture), at a storage temperature of 21 degrees C (69 degrees F), after which the cell retains a specified percentage (usually 90%) of its original energy content.

A condition that occurs when a short electrical path is unintentionally created. Batteries can supply hundreds of amps if short-circuited, potentially melting the terminals and creating sparks.

That current delivered when a cell is short-circuited (i.e., the positive and negative terminals are directly connected with a low-resistance conductor).

An assembly of identical cells in which the electrochemical action is reversible so that the battery may be recharged by passing a current through the cells in the opposite direction to that of discharge.

 An electrolytic cell for the generation of electric energy in which the cell after being discharged may be restored to a charged condition by an electric current flowing in a direction opposite the flow of current when the cell discharges. 

A charge regime delivering moderately high-rate charging current when the battery is at a low state of charge and tapering the current to lower rates as the battery becomes more fully charged.

The parts of a battery to which the external electric circuit is connected.

A condition whereby a cell on charge or discharge will destroy itself through internal heat generation caused by high overcharge or high rate of discharge or other abusive conditions.

A method of recharging in which a secondary cell is either continuously or intermittently connected to a constant-current supply that maintains the cell in fully charged condition.

A normally sealed mechanism that allows for the controlled escape of gases from within a cell.

The unit of measurement of electromotive force, or difference of potential, which will cause a current of one ampere to flow through a resistance of one ohm. 

Voltage at the end of useful discharge.

 Cell voltage below which the connected equipment will not operate or below which operation is not recommended.

Voltage of a fully charged cell when delivering rated current.

A measurement of total power. It is amperes multiplied by volts. 120 volt @ 1 amp = 12 volts @ 10 amps.

Remaining capacity in the cell/ battery as a percentage of initial capacity.

Remaining life time of the cell/ battery as a percentage of initial life time.

Used in terms of the battery's internal resistance a test during which a battery is subjected to alternate periods of discharge and rest according to a specified discharge regime.

The opposition exhibited by a circuit element (cell or battery) to the flow of an alternating current (a/c.) of a particular frequency as a result of resistance, induction and capacitance.

A voltage drop associated with the electrical resistance (R) of a battery or current flow (I). The voltage drop is the product of the current (in amperes) and the resistance (in ohms).

Absolute state-of-charge; ability to take specified charge when the battery is new.

Absolute state-of-health; ability to store specified energy when the battery is new.

Battery Management System used inside or outside a battery to manage charge, discharge and provide SoC; forms an essential part to assure battery longevity and safety.

Charge followed by a discharge and recharge. No standard exists as to level of charge and discharge to constitute a cycle.

Reflects battery readiness that verifies capacity, current delivery, voltage, SoC, self-discharge and more; measured in %. (Capacity, current delivery and SoC are most basic.)

Advanced chemistry/technology for primary and secondary batteries. Offers increased performance and twice the energy density of nickel-based batteries. There are several major varieties of lithium ion battery technology, each of which has unique properties. Lithium ion secondary batteries can charge to full capacity in as little as 3 hours.

A variety of lithium ion chemistry/technology that offers high discharge rate capability, long cycle life, and long calendar life.

A variation of lithium ion battery which differs only construction—chemistry is the same. Lithium polymer allows for very flexible packaging, lower cost, and safer operation.

They have the highest specific energy (energy by weight) and energy density (energy by volume) of all primary battery types. Have open circuit voltages (OCVs) between 2.7 and 3.6V. Their relatively high internal impedance limits them mostly to low drain applications.

The discharge current provided by a battery, or drawn by a battery powered device.

A special sensor which ends discharge at a specified voltage level.

Voltage-sensing device to automatically disconnect a battery or cell from a load at predetermined voltage. Low-voltage disconnects prevent cell reversal during discharge.

Method for maintaining the charge of a battery or cell by continuously charging it at a rate sufficient to balance its self-discharge.

The flow of electricity pulled under the highest DC load (value must be listed on product labels for compliance to safety agency standards).

Most AC power is generated at either 50 or 60 Hz (cycles per second). Switching Power Supplies typically accept any frequency between 47-63Hz (nominal).

The nominal AC Input Voltage that a power supply will accept during normal operation. Safety agencies require a power supply to operate an extended 10% of the stated input range on the product label. For example, a switching power supply labeled with a nominal 100 to 240VAC input range will actually operate over 90–264VAC input range.

The instantaneous current drawn when the power supply is turned on. Larger power supplies generally incorporate a thermistor to limit this amount. It is most important in considering an AC switch rating.

Traditional switched mode power supplies draw current from the AC line in short pulses, and as a result, the input current of such basic switched mode power supplies has high harmonic content. This creates extra load on utility lines and increased heat of utility transformers and may cause stability problems to the entire AC Line. Active Power Factor Correction controls the input current of a power supply so that the current wave form is proportional to the AC waveform (a sine wave).

The maximum altitude at which a power supply can be operated without derating. Supplies must often be de-rated due to the thinner air which is required to cool the power supply.

The temperature of still air surrounding a power supply.

(A) The base unit of current.

A metal surface to which circuit components are mounted in such a way as to draw heat away from components.

A single layer of required insulation to prevent electrical shock.

An electronic equipment subsystem that provides temporary power in the event of input power loss. Battery backed systems range from short term options for AC/ DC power supplies to high Uninterruptible Power Systems. See Uninterruptible Power Supplies (DC-UPS).

A resistor that provides a path for current drain. Often used in filter circuits provide a discharge path for capacitors.

The mechanical design of a power supply to slide into position and make connection with its mating connector when properly seated. This is particularly common in Hot-Swap/Warm-Swap power supplies or modules in parallel/redundant applications. It is common with this type of connection for there to be a short pin called last-mate which enables the supply to be on, in order to prevent arcing or powering of the supply until it is firmly seated into its mating connector.

A boost converter (step-up converter) is a power converter with an output DC voltage greater than its input DC voltage.

The breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to fail and become conductive.

A switching supply topology that employs four switching elements (full bridge) or two switching elements (half bridge). Bridge a supply provide high output power and low ripple, but are significantly more complex than other types of supply topologies.

When AC Line Voltage drops below nominal levels.

A basic switching converter topology that uses a series switch to chop the input voltage. The resulting pulses are applied to an averaging LC filter. Buck regulators will only produce an output voltage lower than the input voltage level.

Operating newly manufactured supplies under defined load conditions for a specified period to eliminate faulty power supplies from shipping. Most power supplies will typically fail in the first few hours of operation (referred to as infant mortality). The time period and conditions (input power cycling, load switching, temperature, etc.) will vary from product to product.

Range this is the temperature range at which a supply will meet its specifications as measured at the center of the top surface of the power supply.

Cubic Feet per Minute is a common measure of the volume of air flowing in a system. The conversion of cubic feet per minute to linear feet per minute is dependent upon the cross-sectional area through which the air flows.

A chassis ground is a connection to the main chassis of a piece of electronic or electrical equipment. It is sometimes called common ground. It provides a reference that can be considered to have zero voltage. All other circuit voltages (positive or negative) are measured or defined with respect to it. Ideally, all chassis grounds should lead to earth grounds. Chassis Mounting The power supply offers one ore more surfaces for mounting a power supply directly to a system chassis or other metal .

The shortest unimpeded distance between two conductors or circuit components.

Timing pulses (see Figure 5) used in electronic systems to synchronize circuit operation. In a power supply, clock pulses synchronize operation of the pulse width modulator (PWM).

A conductive path used as a return for two or more circuits.

The term Common is often used interchangeably with ground, which is not technically correct unless it is connected to earth.

The noise at an electrically fixed point (usually chassis ground) common to both DC output and return lines.

An input line filter that includes a differential wound transformer often used within power supplies. They show a high impedance to common mode signals and a low impedance to differential mode signals.

The process of cooling or removing heat via a baseplate or heatsink.

A power supply that regulates its output current to within a specified current range regardless of changes of output load. These type of supplies are commonly used in battery charging application and LED Driver Circuits.

Current limiting circuit that holds output current at some maximum value whenever an overload of any magnitude is experienced.


Circuit that controls certain operating parameters of the power supply. Used to maintain output regulation.


A device that accepts an ac line input voltage and produces a dc output(s). Often referred to as a “switcher” (although linear converters are available), switching regulated power supplies are used in the majority of applications. AC/DC supplies are available in a variety of form factors, power levels and feature/performance envelopes. Converter (DC/DC) A device that accepts a regulated or unregulated dc input voltage and produces a dc output that is typically at another voltage level. At times DC-DC Converters are used to provide noise isolation power bus regulation, etc., at which converters will have the same input and output level.

The process of removing heat generated by normal operation of a power supply. Typical methods are convection, forced air and conduction.

The shortest distance between two conductors (typically one conductor primary and one conductor secondary).

For an ac waveform, the ratio of peak value to RMS value. If the waveform is pure sinusoidal, this value is 2. Crest factor was used to approximate the current stress in an AC mains circuit.

On many multiple output power supplies, the secondary outputs may be affected by the loading conditions of the primary output(s). Multiple output power supplies often require minimum loads in order for the supply to achieve stated regulation specifications. Preload resistors may be employed at the system level to overcome these issues.

An overvoltage protection circuit which places a low resistance shunt across the power supply output terminals, if a predetermined voltage is exceeded.

The range over which output current can be adjusted and the means of making that adjustment.

The point at which current is limited (foldback) on the plot of output voltage vs output current. 

Multiple power supplies or DC/DC supplies are often connected redundantly (to increase system reliability) or in parallel (to increase system power). Outputs are connected together and each supply “shares” the load current.

The nominal output voltage setting of a power supply.

A supply / converter that accepts a DC input voltage and produces a DC output voltage.

In power supplies and DC/DC converters, derating is the specified reduction of the output current when operating under defined conditions, typically elevated operating temperatures.

The conversion principle employed (eg. linear, switched mode flyback, half bridge etc).

A supply may not bear any safety agency approvals, but when installed and used properly, should meet the official safety requirements of an electronic system.

The maximum voltage an insulating material can withstand before suffering punch through or arcing across.

A current that flows in one direction.

For power supplies, the ratio of “on” time to “off” time of the semiconductor switch (in PWM systems) or clock signal.

That component of noise measured with respect to output or input to its returns; it does not include common mode noise. 

The variation of output voltage of a supply over a specified period of time, following a warm up period, with all other operating parameters such as line, load and ambient temperature held constant.

The function of paralleling two power supplies to share a load without any active circuit to control how the load is shared. The current-sharing accuracy is directly related to the output-voltage set-point accuracy of the paralleled supplies. Considering that output voltage will vary based upon varying line, load, temperature and other conditions, it is strongly advised that droop sharing only be utilized for redundant operation, not for increased power.

A load condition that changes rapidly. During this load change, the output voltage may fall out of regulation (overshoot and / or undershoot) temporarily.

A measurement of Output power divided by Input Power. The values will vary depending on the load and AC input voltage. Typical Values shown are usually measurements of a supply at nominal input and output conditions.

Electronic interferences that impair the performance of electronic device is referred to as Electromagnetic Interference.

Conducted EMI is unwanted high frequency energy caused by the switching transistors, rectifiers, and transformers in power supplies and DC/DC. The noise that is generated on the input and output lines of a power supply is known as Conducted EMI. Most Conducted EMI measurements are done between 150.kHz and 30.MHz.

Switch mode power supplies and DC/DC converters may be filtered by using an EMI filter on their input or primary side to meet applicable EMC standards. While supplies may be designed to meet an EMC standard, the final equipment may have a dramatic effect upon a power supply’s EMC performance. It is the final equipment that must conform to EMC regulations. So, specifying a supply which meets the EMI classes is not a guarantee that final equipment will be compliant and does not remove the need for testing and possible additional filtration required for final equipment.

Radiated EMI is unwanted high frequency energy caused by the switching transistor, output rectifiers, and zener diodes in switching power supplies and DC/DC converters and emitted into the area surrounding the supply.

Electromagnetic radiation emitted into the atmosphere from the power supply. Conducted is that energy sent down the AC line cord; radiated is sent into the air. Final equipment must meet both standards while many supplies may be rated only for conducted emissions.

Hermetically sealed and contained in a thermally conductive epoxy resin or similar plastic.

A current produced by the static charge of two objects when they are close enough to produce a discharge or arc.

The expected average lifetime of a power supply, which may be calculated upon the reliability data of a supply’s individual components or demonstrated through Highly Accelerated Life Test (HALT) and Highly Accelerated Stress Screening (HASS).

Synchronizing the supply’s switching frequency to an external oscillator either in an external circuit or within a master supply.

An input or output circuit designed to attenuate ripple and noise generated by a supply.

A power supply or DC/DC supply output that is ungrounded and not referenced to another output. Typically, floating outputs are fully isolated and may be referenced as either positive or negative by the user.

Sometimes called a “Buck Boost” supply, this topology uses a single transistor switch and eliminates the need for an output inductor. Energy is stored in the transformer primary during the first half of the switching period when the transistor switches “ON”. During the second half of “Flyback period when the transistor is OFF”, this energy is transferred to the transformer secondary and load. This technique is cost effective because a minimum number of components is required.

The use of a fan (or other air moving equipment) within a system to reduce the ambient temperature and cool the electronic equipment.


Also called a “Buck Derived” supply, this topology, like the flyback supply, typically used a single transistor switch. Unlike the flyback supply, energy is trans ferred to the transformer secondary while the transistor is “ON”, and stored in an output inductor.

An operating environment in which the natural movement of air (unassist ed by fans or blowers) is sufficient to maintain a power supply’s temperature within its operating limits.

A topology that typically operates as a forward supply but uses a bridge circuit, consisting of four switching transistors, to drive the transformer primary, used to handle high power levels.

The maximum continuous output current a power supply is rated for under nominal operating conditions.

A circuit (bridge or centre tapped) that rectifies both halves of an AC waveform.

A circuit protection device, consisting primarily of a low melting point wire. If current passing through the wire exceeds a set level (as in the case of a fault condition), the wire melts and the circuit opens.

Two circuits which have no ohmic connection are considered to be “galvanically isolated” from each other. Galvanic isolation (separation) is achieved by using a transformer, opto coupler, etc.

An electrical connection that is made to earth (or to some conductor that is connected to earth). A power supply or DC/DC supply “common” is not actually ground unless it is connected to earth.

When two or more system components share a common ground line, a feedback (ground) loop is induced. This can cause unwanted voltage levels within the system.

A power switching circuit similar to the full bridge supply except that only two transistors are used, with the other two replaced by capacitors. 

Single diode rectifier circuit that rectifies one half the AC input wave.

The distortion characterized by the presence of multiple harmonics of the fundamental frequencies in sinusoidal AC current waveforms and caused by the switching action of the power supply typically stated as a percentage of the sinusoidal wave form, eg: 0.95 Power Factor.

A metal plate, extrusion, or case that is provided to increase surface area to dissipate heat away from sensitive components and circuits.

The test voltage between the input and output, and output to ground.

When there is a loss of input power to a supply, this is the time during which the output voltage remains within regulation. To protect against momentary power outages in switching power supplies, energy is typically stored in bulk capacitors referred to as hold-up capacitors.

The function of replacing a power 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.

Within a specified temperature range, the maximum moisture content permissible in the surrounding air of a supply. Two values are typically provided, operating humidity and storage humidity.

Indicates that the supply has been tested and is “immune” from electromagnetic or electro static discharge (ESD).

The apparent impedance presented by the supply to its output terminals.

An electrical property that opposes the flow of current in a circuit when a voltage is applied (or a change in an established current). Measured in henries Input Line Filter A low-pass or band-reject filter on the power supply input (internal or external) that attenuates noise from the system power bus.

The current drawn by a supply, which can be measured under a range of input voltage range and output load conditions. Typically listed as the maximum continuous input current under lowest input voltage and maximum output load so that proper fusing may be determined.

A performance curve illustrating how the input current varies with line input voltage. See Efficiency versus Input.

A modulating signal injected into a supply operating at nominal line and full load. The signal is attenuated by the supply’s feedback loop (loop gain) and propagates to the output. The ratio (in absolute terms) of the input to the output signal is expressed in dB and listed as the input ripple rejection. This is specified for a DC to 120 Hz input so that the effects of a full wave rectifier circuit can be evaluated. For example, if the Ripple Rejection of a supply is 60 dB (1000:1) and a 1volt, 120 Hz signal is superimposed on the supply’s input then the output will have a 1mV, 120Hz signal superimposed on it. This specification is sometimes referred to as “Audio Susceptibility.”

The isolation capacitance from the input pins to the output pins. This measurement is done with a 1 kHz, 1 VRMS capacitance bridge.

A spike or rapid voltage change of the input line to a supply. Input transient protection circuits are used to shield sensitive components (such as semi conductors) from possible damage.

Nominal Input value(s) of either AC or DC Input voltage for which the supply is connected.


The highest and lowest input voltages from which a supply may operate. Inrush Current When supplies are first turned on, a high surge input current is experienced caused by the charging of the bulk input capacitors. Also called Input Surge Current most commonly referenced in AC/DC Power Supplies.


A circuit which limits the inrush current during initial turn on of a supply.

The resistance offered by an insulating material to current flow.

The power dissipated (as heat) within a supply during normal operation. Primarily a function of the efficiency a supply.

A device that generates AC power from a DC power source.

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

The voltage test to determine the breakdown voltage of a transformer or supply. It is performed by applying a high voltage between two isolated test points. The isolation of a supply is typically tested to not cause stress to the insulation material.

Rated Isolation voltage is defined as the maximum voltage across the isolation barrier a device can withstand for a fixed time period. The actual breakdown voltage is typically in excess of 1000V higher than the rated isolation voltage. The reason for rating a conservative isolation voltage is to ensure that the isolation testing of supply does not degrade the isolation barrier in any way .

The current flowing from input to output or input to ground/chassis or output to ground/chassis of an isolated power supply or DC/DC supply at a specified voltage level.

The combined effect of varying the DC load and AC input voltage.

A power supply that uses linear regulation. Linears provide excellent regulation, low output noise and fast transient response. However, they are typically much heavier, larger and less efficient then “switching power supplies.

A common voltage-stabilization technique in which the control device (usually a transistor) is placed in series or parallel with the power source to regulate the voltage across the load. The term ‘linear’ is used because the voltage drop across the control device is varied continuously to dissipate unused power.

The components or circuitry drawing current from the output of a supply. The characteristic (resistance, reactance, etc.) of the load determines the amount of power drawn from the supply typically referred to as output current.

A supply may offer remote on/off functions to inhibit or enable a supply’s output. This is function is typically achieved by pulling the associate logic pin hi or low (depending upon the remote on/off circuit in use). Specifically, most power supplies have a natural state of on when input is applied and may be inhibited by the remote on/off circuit.

The change, over time, in the output voltage of a power supply with all other factors (line, load, temp, etc.) remaining constant, expressed as a percentage. This output change is primarily due to component aging.

The connection of two or more power supplies (see Figure 19) in which one (master) controls the operation of the others (slaves). Master/slave configurations provide higher output power, wider input voltage ranges, synchronized operation, etc.

Limit of specifications that, if exceeded, could cause the shutdown or damage to a supply.

The minimum amount of output current required for a supply to operate within its specified regulation.

Term used to describe the physical construction of power systems that consists of separate subassemblies. Modular construction tends to lower the design turnaround for custom products, but increases cost and lowers MTBF.

Metal Oxide Semiconductor Field Effect Transistor. The device of choice for the main switch in switched mode power supplies having much better switching characteristics than Bipolar Transistors.

Mean Time Between Failures is the predicted length of time before failure of a supply, exclusive of infant mortality failures.

The predicted average length of time to repair a faulty unit with the specified spares kit.

The use of multiple supplies to achieve higher reliability levels through system redundancy. The system consists of a number (N) of power supplies to satisfy the load plus one (+1) to provide redundancy and allow continued operation through the fault of one of the supplies. These supplies are typically isolated via an isolation device such as an OR’ing diode to ensure that a short within one supply will not cause the entire system to fail.

The output noise is specified at nominal line and full load. This specification is very difficult to measure correctly due to the measurement bandwidth (0-20 MHz). Short leads and proper grounding techniques must be used. The output noise is specified in mV peak-to -peak. The majority of the noise reading is due to the switching action of the supply and is at very high frequencies, whereas the peak-to-peak amplitude at the fundamental switching frequency is usually much less.

Power supplies which are constructed only of a PCB. Operating Temperature The operating temperature range of a supply measured as either Ambient (surrounding Air) or base plate.

These diodes ensure current can flow in only one direction – out of a power supply. Without the diodes, one problem supply could cause all supplies to go into over current protection in a current sharing scenario and generate a catastrophic failure within a system.

Over Temperature Protection. A protection system for supplies where the supply shuts down if temperature exceeds specified ratings. OTP is intended to save the supply in the event of a failure of the cooling system. OTP usually measures the hottest spot in the supply. Most supplies will automatically recover when temperatures return to normal.


The variation of output voltage to change in load current.

The adjustment of the nominal output voltage via an external fixed resistor or an internal trimpot on the supply.

The allowable tolerance of the output voltage of a supply when it is set at the factory.

A circuit that will typically shutdown the power supply when the output voltage exceeds a specified range.

A protective feature that limits output power or current demands to prevent damage to the supply.

This is the momentary rise in output voltage a supply experiences during a decreased load condition.

The connection of the outputs of two or more power supplies or DC/DC supplies of the same output voltage to obtain a higher output current. Only supplies specifically designed to share the load should be utilized to gain higher power (see active current sharing).
PARD Periodic and Random Deviation. A term used for the sum of all ripple and noise components measured over a specified band width and stated in either peak-to-peak or RMS values.

Traditional switched mode power supplies draw current from the AC line in short pulses, and, as a result, the input current of such basic switched mode power supplies has high harmonic content. This creates extra load on utility lines and increased heat of utility transformers and may cause stability problems to the entire AC Line (Especially in Europe). Passive Power Factor Correction is a network of capacitors and inductors to minimize the pulse so that the current waveform is more proportional to the AC waveform (a sine wave).

A supply that is designed for direct mounting on to a printed circuit board. The ability of a supply to provide higher output currents for short periods of time. A power supply with high peak loads is desirable in many applications, such as motors that draw high currents at start-up and then draw substantially reduced loads during normal operation.

A linear regulator used on the output of a supply to improve line and load regulation and reduce output ripple voltage.

The ratio of a power supply or DC/DC converter output power to its volume -typically displayed as Watts per square inch or Amps per square inch.

The ability of a power supply to operate at outside normal parameters, (such as eleveated temperature, reduced cooling or low input voltage) reduced or derated output currents.

Switching power supplies draw input current in pulses around the peaks of the AC line voltage frequency. Power factor is a measure of the input current draw and how closely it matches the sinusoidal phase of the AC input frequency .

PFC circuits improve how a supply draws input current to more closely match the sinusoidal line voltage. This reduces harmonics disturbance on the AC line. Reduced harmonic disturbance is a common requirement throughout Europe, and Power Factor Correction is the method to achieve reduced disturbances.

A logic (hi/low) compatible signal warning that input power has been lost and that DC outputs will soon fall out of regulation.

A logic (hi/low) compatible signal that indicates that DC Outputs are present and within specified regulation.

A system that converts AC current into the DC current or currents required by electronic circuits.

The regulation at the primary side of a power supply, generally by a type of switching regulator; followed by output regulation, usually by a linear type regulator.

The adjustment control of supply’s output voltage and/or current via an external parameter such as a control voltage or resistor value.

The internal fuse method for a supply, such as single or dual fused, typically rated for a maximum voltage, current and fuse type trip time (fast or slow acting). This fuse is usually rated to allow the use of an external fuse which can be mounted on an accessible panel, because replacing the fuse inside supply may prove difficult.

A circuit used in switching power supplies or DC/DC supplies where the switching frequency is held constant and the width of the power pulses is varied, controlling both line and load changes with minimal dissipation.

A supply topology that typically is configured as a forward supply but uses two transistor switches and a centre-tapped transformer. The transistor switches turn on and off alternately.


The transfer of heat between two materials at different temperature levels. Radiant heat does not play a significant role in the cooling of distributed power systems.


This is the maximum rated output current/load capability of a supply from minimum through maximum values under normal operating temperatures and cooling conditions. Operation below minimum load should not harm a supply in any way, but load regulation may suffer. Operation above the maximum rated load is not recommended and may degrade specifications, trip an overload protection circuit, and/ or reduce a supply’s life.

The ability to connect power supplies or DC/DC converters in parallel so that if one fails the other(s) will provide continuous power to the load. This mode is used in systems where a single failure cannot be tolerated. See also N+1 Redundancy.

The ability to vary output voltage and/or current over a specified range by an external signal, typically a control voltage. Often referred to as margining.

A logic (hi or low) signal to turn on/turn off a supply.

A circuit within the supply to compensate for the reduction of output voltage through connections and wires (voltage drop) which can vary under temperature, connection strength, and wire stresses. Typically, a twisted pair of wires is attached to the load to “sense” the voltage at the load, enabling the supply to automatically compensate for varying voltage drop.

A class of power topologies which reduces switching losses by forcing either zero voltage across, or zero current through the switching device when it is turned on or off .


The name for the return current of output voltage(s) and/or logic signals.


A built in circuit (or element) that protects the supply from a reverse polarity, applied across the input or output terminals of a supply.


This is the AC component superimposed over the DC output voltage and is the traditional “hum” at 60 or 120 hertz. In swit ching power supplies, it is a complex waveform and can increase at maximum loading and minimum input voltage.

The electrical separation between the primary and secondary circuits and the safety standards to which the supply conforms in this respect.

Standards laid down by various national and international regulatory agencies.

A twisted pair of wires connected to the load in order to route output voltage back to the remote sense control circuit of the power supply. See Remote Sensing.

The ability of two or more supply outputs to be connected and provide a higher output voltages (two 48V power supplies in series to generate 96V). The load should not draw more current than the maximum rated output current of any single supply .

The allowed variance of the output voltage as set at the factory during the manufacture of a supply.

A short circuit is an unlimited load potential far exceeding a supply’s output current capability. Under a short circuit condition, most supplies are designed to shut-down and typically recover to normal operation when the short is removed.

With the supply fully warmed up at room temperature with constant line, load and temperature, the output will not vary by more than this amount.

Current drawn by the supply from the supply when its outputs are disabled, Often referred to as no load input current or standby current.

A circuit in which multiple power supplies share current when paralleled. The supplies communicate through a single wire connection daisy chained to all the supplies. This circuit allows a specified amount of like supplies to be connected in parallel within a defined accuracy range.

Metal shielding fully encompassing a supply to minimize any noise radiation from the supply components. Shielding can be solid or perforated.

A feature which limits the inrush current of a supply and causes the output voltage to rise gradually to its specified value.

The input current drawn by a supply when it has been inhibited off or is under no load conditions.

An operating environment in which the air surrounding the power supply or DC/DC converter is restricted in small enclosures (often sealed) where it cannot move freely.

The safe storage temperature for the device. Long term exposure within these temperature ranges should not degrade the supply’s performance.

A technique whereby components are soldered onto the surface of a PCB instead of pins or leads which must protrude through a PCB.

SMD Components including some power supplies designed to be assembled using surface mount technology.

The rate at which the input voltage is switched or “chopped” in a power supply. Sometimes referred to as frequency of operation.

A power supply that uses switching regulation. Switchers are typically smaller, more efficient and lighter than linear supply.

A non isolated DC/DC converter consisting of inductors and capacitors to store energy and switching elements (typically transistors or SCR’s), which open and close as necessary to regulate voltage across the load. The switching duty cycle is generally controlled by a feedback loop to stabilize the output voltage, generally by means of a Pulse Width Modulation.

The peak to peak amplitude which occurs at the switching frequency on the output of switched mode supplies.


A circuit arrangement where the output rectifier diodes of a supply are replaced with active switches such as MOSFET’s. The switches are turned on and off under control and act as rectifiers. This results in considerably lower losses in the output stage of a supply and increases efficiency.

The average percent change in output voltage per degrees centigrade change in ambient temperature over a specified temperature range, with load and input voltage held constant.

The temperature rise of the case for each watt dissipated in the supply. The power dissipated is the difference between the input and output power.

A circuit within a supply that shuts down the supply if the internal temperature exceeds a predetermined limit (see over temperature protection – OTP).

The temperature specified at which the supply will shut down operation until the temperature decreases – typically measured at the hottest spot within a supply .

The combined voltage deviation a supply could experience due to any change within the specified tolerances of input voltage, output current and temperature change.

A characteristic of multiple output power supplies where one or more outputs ollow another and where there are changes in line, load and temperature, so that each maintains the same proportional output voltage, within specified tracking tolerance, with respect to a common return.

A spike or step change in a power supply parameter. Commonly used in describing input line and output load characteristics.

The time required for the output voltage of a supply to recover within a specified regulation following a transient load.

A percentage of the maximum output voltage deviation during a transient load.

Measurement of both transient deviation and transient recovery time after a transient load step.

The time it takes for the output voltage to reach the specified accuracy when the outputs are fully loaded into resistive loads.

A supply constructed within a bracket with a “U” shaped profile. Typically, all three planes of a U-Frame/U-Channel supply offer threaded holes for affixing the supply to a chassis/plate.


The momentary dip of output voltage a supply experiences during an increased load condition.

This indicates that a supply is able to operate with AC Power available in most countries without any changes in settings to the supply itself. This input range is typically 90–264 VAC.

Uninterruptible Power Supply. A system designed to supply power during the loss of AC line power. This is accomplished by means of a back up battery and a DC/AC inverter or DC/DC power supply.

A protection circuit that shuts a power supply off if the output voltage falls below a preset level. Sometimes used as an input protection circuit in wide input range power supplies (ac and dc) to prevent overheating if the input voltage sags below a predetermined level. Sometimes called under voltage shutdown or under voltage lockout.

The maximum voltage level that can be applied between circuits or components without causing a breakdown. See Breakdown Voltage and Isolation.

The range over which the output voltage can be adjusted.

Voltage balance is often specified on dual supplies as being the difference in absolute terms between the positive and negative output and expressed as a percentage. For example, if the positive output is at 12.00 Volts and the negative output is at 12.12 volts then the balance would be 1.0%.

The range(s) of input AC or DC voltage(s) over which the supply(s) operates within specification. plug directly into the AC socket and with a dangling output cable to mate with the system it is powering.


This usually refers to an N+1 Redundant powered system. This defines the ability to replace one of multiple supplies tied in parallel. The supply being replaced must be powered with it’s AC Input Removed off while the others may remain on. This type of supply is typically mechanically designed to fit into a slot with a blind mate connector, most commonly with an AC Inlet on the front panel.

The time required, after initial turn on, for a power supply or DC/DC supply to operate within its specifications. Most supplies do not require a warm-up time when operating above 0°C. Some supplies will operate at less than 0°C t emperature or below freezing, with a stipulation of a warm-up period.

Rated working voltage or electrical strength, is the maximum continuous voltage that can be sustained continuously across the isolation barrier of a supply without causing stress to the isolation barrier. The rated working voltage is typically much lower than the rated isolation voltage.


Power supply is covered in a metal or perforated metal cover assembly. Typically, an enclosed power supply is covered on all 6 surfaces for finger safe operation.

An output protection circuit where the output current decreases with increasing overload, reaching a minimum at short circuit. This minimizes the internal power dissipation under overload conditions.