Power-Supply Characteristics

power-supply characteristics

There are several power-supply characteristics that affect their operation:

Drift: The variation in dc output voltage as a function of time at constant line voltage, load, and ambient temperature.

Dynamic response: A power supply may be employed in a system where there is a requirement to provide fast dynamic response to a change in load power. That can be the case for the load of high-speed microprocessors with power-management functions. In this case, the microprocessor may be in a standby state and upon command it must start up or turn off immediately, which imposes high dynamic currents with fast ramp rates on the power supply. To accommodate the microprocessor, the supply’s output voltage must ramp up or down within a specified time interval, but without excessive overshoot. 

Efficiency: Ratio of output-to-input power (in percent), measured at a given load current with nominal line conditions (Pout/Pin).

Holdup time: Time during which a power supply’s output voltage remains within specification following the loss of input power.

Inrush current: Peak instantaneous input current drawn by a power supply at turn-on.

International standards: Specify a power supply’s safety requirements and allowable EMI (electromagnetic interference) levels. 

Isolation: Electrical separation between the input and output of a power supply measured in volts. A non-isolated has a dc path between the input and output of supply, whereas an isolated power supply employs a transformer to eliminate the dc path between input and output.

Line regulation: Change in value of dc output voltage resulting from a change in ac input voltage, specified as the change in ± mV or ± %.

Load regulation: Change in value of dc output voltage resulting from a change in load from open-circuit to maximum-rated output current, specified as the change in ± mV or ± %.

Output noise: This can occur in the power supply in the form of short bursts of high frequency energy. The noise is caused by charging and discharging of parasitic capacitances within the power supply during its operating cycle. Its amplitude is variable and can depend on the load impedance, external filtering, and how it is measurement. 

Output voltage trim: Most power supplies have the ability to “trim” the output voltage, whose adjustment range does not need to be large, usually about ±10%. One common usage is to compensate for the dc distribution voltage drop within the system. Trimming can either be upward or downward from the nominal setting using an external resistor or potentiometer.

Periodic and random deviation (PARD): Unwanted periodic (ripple) or aperiodic (noise) deviation of the power-supply output voltage from its nominal value. PARD is expressed in mV peak-to-peak or rms, at a specified bandwidth.

Peak current: The maximum current that a power supply can provide for brief periods. 

Peak power: The absolute maximum output power that a power supply can produce without damage. It is typically well beyond the continuous reliable output power capability and should only be used infrequently.

Power-supply sequencing: Sequential turn-on and off of power supplies may be required in systems with multiple operating voltages. That is, voltages must be applied in a specific sequence, otherwise the system can be damaged. For example, after applying the first voltage and it reaches a specific value, a second voltage can be ramped up, and so on. Sequencing works in reverse when power is removed, although speed is not usually as much of a problem as turn-on.

Remote on/off : This is preferred over switches to turn power supplies on and off. Power-supply data-sheet specifications usually detail the dc parameters for remote on/off, listing the on and off logic levels required.

Remote sense: A typical power supply monitors its output voltage and feeds a portion of it back to the supply to provide voltage regulation. In this way, if the output tends to rise or fall, the feedback regulates the supply’s output voltage. However, to maintain a constant output at the load, the power supply should actually monitor the voltage at the load. But, connections from a power supply’s output to its load have resistance and current flowing through them that produces a voltage drop that creates a voltage difference between the supply’s output and the actual load. For the optimal regulation, the voltage fed back to the power supply should be the actual load voltage. The supply’s two (plus and minus) remote sense connections monitor the actual load voltage, a portion of which is then fed back to the supply with very little voltage drop because the current through the two remote sense connections is very low. As a consequence, the voltage applied to the load is regulated.

Ripple: Rectifying and filtering a switching power supply’s output results in an ac component (ripple) that rides on its dc output. Ripple frequency is some integral multiple of the converter’s switching frequency, which depends on the converter topology. Ripple is relatively unaffected by load current, but can be decreased by external capacitor filtering.

Tracking: When using multiple output power supplies whereby one or more outputs follow another with changes in line, load, and temperature, so that each maintains the same proportional output voltage, within specified tracking tolerance, with respect to a common value.


Post time: Apr-23-2018
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