When choosing and buying a power supply it is necessary to be able to understand the specifications given in the data sheet. In this way a good understanding of its performance will be gained and it will be possible to determine whether the power supply will be able to meet the requirements of the application in mind.
There are several specifications used to detail the performance of the power supplies. Each one details a different aspect of the performance of a power supply, and dependent upon the application, some will be more important than others.
Voltage and current specifications
The primary power supply specifications are the voltage and current output parameters. In terms of the voltage, the power supply may be fixed or it may have a variable output. It is necessary to check whether the power supply has a fixed or variable output.
If the power supply has a fixed output there will be a small adjustment that can be made and it may be necessary to check that it can be adjusted to the required value if the voltage required is not exactly that quoted on the specification sheet. If the power supply ahs a variable range then it is necessary to ensure that it covers the required range.
In terms of the current it is necessary to ensure that the power supply will be able to provide the required level of current and have a degree of margin beyond that minimum requirement. When calculating the requirement for the power supply specification for current it is necessary to take account of what is termed the inrush current. This in-rush current occurs when an item is turned on and a large surge of current is drawn to charge up capacitors, etc. This inrush current can be several times the ordinary operating current.
Power supply specifications detail figures for a parameter entitled "line regulation." It is found that when the line or input voltage changes then a small variation may be seen on the output. The line regulation figure details this change. The specification is normally quoted in millivolts for a given input variation. It may also be expressed or as a percentage of the output voltage and it should typically be a few millivolts (e.g. 5 mV) or around 0.01% of the maximum output voltage for most supplies for a change of line voltage anywhere within the operating range.
Another important power supply specification is called the "load regulation." It is found that when a load is added to the output of a power supply the voltage at the terminals can fall slightly. This is obviously not desirable because the output voltage should remain exactly constant in an ideal world.
The power supply load variation is normally quoted as a in millivolts variation or as percentage of the maximum output voltage It might typically be a few millivolts (e.g. 5 mV) or 0.01% for a step load change from 0 to 100% load. It is normally quoted for a constant line voltage and at steady temperature.
Ripple and noise
The ripple and noise parameters are another important power supply specification. It is possible that noise and other impulses on the power line can be transferred to the output of the circuit that is being powered. In order to minimise this, especially for sensitive circuits it is necessary to ensure that the power lines are as clean as possible.
The ripple and noise on the output is combined as a single specification. For linear supplies, ripple frequency is normally be at twice the line frequency. For switching supplies ripple and spikes will arise from the switching action of the supply. The ripple components are often given as rms figures, but for switching supplies a peak-to-peak measurement is more useful because it shows the extent of the spikes arising from the switching. Most good supplies should offer noise and ripple figures of better than 10 mV rms and for switching supplies figures of 50mV or less should be achievable in many cases, although very high current supplies may have slightly higher values.
Stability with time
It is found that the output voltage of a power supply may change a small amount over a period of time. Although the amounts of change are normally small, they may be important in some applications. As a result figures for the voltage output stability with time are sometimes quoted within the overall power supply specifications.
For the stability specification, the output voltage of the power supply will be measured over a period of time under constant load and input voltage and the voltage drift measured. Typically, this will be a few millivolts (e.g. five to ten) over a period of ten hours.
The output voltage from any power supply will also change slightly as the ambient operating temperature changes. Again this can be important for some circuits that might be voltage dependent. As a result, figures are often given in the power supply specifications. The parameter is measured as a percentage or absolute voltage change per degree C. Typically, this might be in the region of 0.02% / degree C or 2 mV / degree C.
Power supply specifications
Although the power supply specifications mentioned above are generally the most widely used, others may also appear, and these may be important for some more specialised applications. In general it is possible to interpret them, at least in general terms, and gain a good idea of the required operation of the power supply.
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