Power FAQ

Most wall mounted external power supplies are designed with only 2 input conductors (line and neutral) so the power supply can be plugged in either direction in a wall or power strip socket. The dc output voltage of these supplies is floating relative to ground since ac safety ground is not present at the input.

A 2-conductor power cord contains conductors for line and neutral while a 3-conductor cord also contains a conductor for ac safety ground. The output of a power supply with a 2-conductor cord will be floating relative to ground since ground is not present in the power supply. By convention, the output of desktop power supplies with 3-conductor power cords connect the negative output voltage terminal to ground.

A longer dc output cable contributes to worse load regulation and worse power conversion efficiency. User applications often require a minimum load regulation value and international standards set minimum power conversion efficiency levels for external supplies. Sometimes a dc output cable with larger gauge conductors can be used to enable the use of a longer dc output power cable.

Most internal AC-DC power supplies are IEC Class I (see Class I vs Class II) and thus use 3-conductor ac input power (line, neutral, and ground). The ac input terminals may be labeled L and N. The input ground conductor may be connected to a third input terminal or it may be connected to an appropriately marked mounting screw. Some internal AC-DC power supplies are IEC Class II and thus will use only the two ac input conductors and will not have a provision for connecting an input ground conductor.

The fan voltage is used to provide power for a fan external to the power supply. This voltage normally not well characterized and may be switched on and off with the main output of the power supply. The AUX voltage is to provide power to housekeeping circuits which need to be on whenever the power supply is receiving external ac power.

LFM (Linear Feet per Minute) and CFM (Cubic Feet per Minute) are both measures of airflow in the context of power supplies. The value for CFM is calculated by multiplying the LFM value by the area through which the airflow is measured.

Open frame, u-frame, and enclosed power supplies are all versions of chassis mounted internal power supplies. An open frame power supply may have a base plate, but the sides and top are open. A u-frame power supply has the base plate wrapped around two of the opposite sides. An enclosed power supply has the metal chassis covering the top, bottom, and at least two sides of the power supply.

As the input voltage and load current are changed the output voltage will remain more constant with a regulated output power supply than with an unregulated output power supply.

DC-DC converters do not have standardized input voltages as are commonly encountered with AC-DC supplies (120 V ~ 240 V) and so there is a desire to succinctly express the input voltage capability of the converters. The ratios (1:1, 2:1, etc.) are the ratio of minimum to maximum input voltage over which the converter will meet the data sheet specifications.

Yes, some of the CUI non-isolated and all of the CUI isolated converters can be used to create a negative output voltage from a positive input voltage. For non-isolated converters the application notes in the data sheet will indicate if the converter is capable of creating a negative output voltage. All isolated converters can have the positive output connected to ground and the negative output will then produce a negative voltage.

In an isolated power supply the output circuit is galvanically isolated from the input circuit, normally the isolation is accomplished with a transformer. In a non-isolated power supply one terminal of the output is dc connected to one terminal of the input.

Isolation refers to how much voltage can be applied between two nodes with a minimal amount of current flowing. Insulation is a non-conductive material which prevents the flow of current when a voltage is applied.

In most ac input supplies the input voltage terminals (line and neutral) are galvanically isolated from the input ac safety ground. However, ground may or may not be galvanically isolated from the output terminals. A “floating” power supply is one where the output is galvanically isolated from ground. The output of a power supply with one of the output terminals dc connected to ground may be called “ground referenced.”

A balanced load should be applied to both outputs of a +V/-V converter for best output voltage regulation. The +V output is monitored and adjusted to be at the proper voltage as the load changes. The -V output is not monitored but is designed to match the characteristics of the +V output. A matched load should be placed on both outputs to cause the voltages to track properly.

Trimming the output voltage is accomplished by providing access to the power supply control loop internal feedback network. The trim range limit exists due to several different issues. One issue is the stability of the power supply control loop will change as the feedback network is changed. A second reason is the duty cycle of the internal modulator is limited and changing the output voltage changes the required modulator duty cycle.

IEC 60601-1 is a set of safety regulations governing medical electronic products. IEC 60335-1 is a set of safety regulations governing household electronic products. IEC 62368-1 is a set of safety regulations governing internet communication technology and audio-visual electronic products.

Level VI, EU 2019/1782, and CoC Tier 2 are standards which govern the minimum required power conversion efficiency for external AC-DC power supplies.

An LPS rated power supply is defined in IEC 62368 as one which delivers less than 8 A and 100 W. The use of LPS rated power supplies enable lower cost power cables to be used without risk of fire hazards.

An IEC Class I power supply has an ac safety ground connection and thus is allowed to have only a single layer of insulation between the user and hazardous voltages. An IEC Class II power supply does not have an ac ground connection and uses double or reinforced insulation between the user and hazardous voltages.

A Class II power supply is an IEC rating for an AC-DC power supply which does not have an ac ground connection and uses double or reinforced insulation between the user and hazardous voltages. A class 2 power supply is an NEC label for a supply which has a limited power delivery capability and thus the wiring between the supply and the load presents less of a fire hazard.

When measuring ripple and noise there are typically three conditions that need to be addressed. First, datasheets typically specify a pair of capacitors that must be placed across the output at the point the probe is placed. A 10µF low ESR Electrolytic and a 0.1µF ceramic capacitor are commonly used. Next, a 20MHz bandwidth must be applied to the oscilloscope used for the test. And most importantly, the loop area created by the probe connections must be as small as possible.

The output ripple and noise may be reduced by adding capacitance, reducing the effective series resistance of the output capacitors, and/or adding filtering to the output of a power supply. Output capacitance can affect start-up behavior and there may be limits on the amount that can be added.

Efficiency is calculated by dividing output power by the input power. For a given output power, a lower efficiency means that it takes more input power to achieve the same output. The difference between the power in and the power out is the amount of power that is dissipated, in the form of heat, inside the power supply. For ac input remember to use the active power, measured in watts, and not the apparent power.

Power factor is the ratio of active power (that which is performing real work) to the apparent power (the product of the RMS voltage and current) supplied to the system. The component of apparent power that does not perform work is called reactive power and circulates in reactive components (capacitors and inductors). Reactive power is made of two components, displacement factor and distortion factor. Displacement factor is a measure of the phase offset between the voltage and current. Distortion factor is a measure of the harmonic content of a signal.

A regulated output is one which is actively controlled to maintain a constant value, (voltage in the case of CUI’s AC-DC and DC-DC products) under varying conditions, with some tolerance specified. Load regulation is the measure of the change in output voltage that will be caused by a change in load. Line regulation is the measure of the change in output voltage that will be caused by a change in line (input) voltage. Sometimes a total regulation value is given, which combines all conditions to a single tolerance value.

MTBF is the “Mean Time Between Failure” and is a measure of a products reliability. It applies to repairable systems, whereas a similar MTTF (Mean Time to Failure) is used for non-repairable systems. The value can be useful in comparing different designs, but not as a direct estimate of a products expected lifespan. There are different standards for calculating the MTBF. Two commonly used standards are the MIL-HDBK-217F and Telcordia SR332.

Fuses are typically included in a circuit to protect the interconnect conductors but not to protect the components in the circuit. In the case of some CUI power supplies, there are fuses present on the input of the supply. The fuse on the input of the supply protects the conductors feeding the supply if there is an internal failure causing the supply to draw excessive current. Most CUI power supplies have internal circuits to limit the output current that can be delivered from the supply, protecting the supply from damage due to excessive output current.

Power supplies have a maximum input voltage limit to protect the internal component from being damaged by applying too high of a voltage. Minimum input voltage specifications are sometimes required to prevent damage to internal components due to excessive current. In order to deliver a specified output power, the input current will rise as the input voltage is reduced. Many switching supply control chips also operate with a duty cycle that is dependent upon the input voltage. The duty cycle limits of the control chips may also limit the input voltage operating range.

Yes, a power supply can be damaged by operating it with a load current greater than the maximum rated load current. We recommend checking the datasheet but most power supplies have internal circuits to protect the power supply if too much load current is drawn. The protection feature is often called overcurrent protection (OCP).

Yes, a power supply can be damaged by applying a short circuit to the output terminals. We recommend checking the datasheet but most power supplies have internal circuits to protect the power supply if the output terminals are shorted together. The protection feature is often called short circuit protection (SCP).

Yes, a power supply can be damaged by operating it at a greater temperature than the maximum rated operating temperature. We recommend checking the datasheet, some power supplies have internal circuits to protect the power supply if the temperature of an internal component gets too hot. This feature is normally found on more expensive or complex power supplies and will be called over temperature protection (OTP).

We recommend checking the datasheet, many power supplies have a feature called output over voltage protection (OVP). This protection is often designed to protect the load if the supply suffers an internal failure. This feature typically does not protect the supply if a voltage is back-driven to the supply.

Large value capacitors across the input to a power supply can be used to provide charge when the power supply places a fast transient load on the power source. Without the capacitance the input voltage may droop to an unacceptable low level due to the voltage drop caused by the input current flowing through the impedance of the power source. Small value capacitors across the input of a power supply can be used to limit the noise from the power supply from being conducted onto the power supply input conductors.

Large value capacitors across the output to a power supply can be used to provide charge when the load draws a fast transient current from the power supply. Without the capacitance the output voltage may droop to an unacceptable low level due to the time response of the power supply to a transient load. Small value capacitors across the output of a power supply can be used to limit the noise from the power supply from being conducted onto the conductors connected to the power supply output.

A capacitor place between the input and output terminals of a power supply is often used to minimize the EMC/EMI from the power supply. A larger capacitor may provide a better job of attenuating the EMC/EMI, but a larger capacitor will also increase the input to output leakage current. The capacitors used for this application are of a special construction for reasons of safety and labeled as Y capacitors.

Yes, on CUI internal AC-DC power supplies the output is isolated from the input, but the user can connect the ac input safety ground to the dc output ground. On CUI DC-DC power converters, if the output is isolated from the input, the user can connect the dc input ground to the dc output ground.

Output sense pins are present on some high current or precision power supply designs. The purpose of the sense pins is to maintain an accurate output voltage at the load. The sense pins will monitor the voltage at the load and feed that voltage back to the control loop internal to the power supply. Voltage drop caused by load current flowing through the output conductors will be compensated for by the sense pins. The sense pins should be connected to the supply output pins at the load for optimal performance but can be connected at any place along the output conductors if the voltage drop of the output voltage conductors is not an issue.

The control (CTRL) or enable pin on a power supply is used to turn the output of the supply on and off by a means other than applying or removing the input voltage.

Positive logic on the control (CTRL) or enable pin means that a high voltage will turn on the output of the supply and a low voltage will turn off the output of the supply. Negative logic on the control pin behaves in the opposite polarity; a low voltage will turn on the output of the supply and a high voltage will turn off the output of the power supply. The control voltage is typically referenced to the input voltage of the power supply.

Yes, either output voltage pin on an isolated supply may be tied to ground, thus enabling the ability to create a positive or a negative output voltage relative to ground.

Many power supply designs include output overcurrent protection (OCP) to protect the power supply if the load draws too much current. The OCP implementation often shuts off the output of the power supply when too much current is drawn and then automatically restarts the supply after a brief delay. If the load current continues to be excessive then the supply shuts down again and the process is repeated. This method of OCP is referred to as "hiccup mode." Starting a motor is an example of an event that may draw excessive current and cause OCP to be activated. The motor rotation may start during the period when the overcurrent is being detected and will slow down while the power supply output is shut down between “hiccups.” If the motor does not stop turning on each cycle, then the supply may turn on properly. If the motor stops turning each cycle, then the supply will remain in hiccup mode and not start up.

Many power supply designs include output over current protection (OCP) to protect the power supply if the load draws too much current. The OCP implementation often shuts off the output of the power supply when too much current is drawn and then automatically restarts the supply after a brief delay. If the load current continues to be excessive, then the supply shuts down again and the process is repeated. This method of OCP is referred to as "hiccup mode." Charging up a capacitor is an example of an event that may draw excessive current and cause OCP to be activated. The load capacitor will be charged during the period when the over current is being detected and will discharge while the power supply output is shut down between ”hiccups.”' If the load capacitor discharges less on each cycle than it charges, then the supply will turn on properly. If the load capacitor effectively discharges each cycle, then the supply will remain in hiccup mode and not start up.