Posted on: July 1, 2014
What is Rated Output Current?
The rated output current is the maximum load current that a power supply can provide at a specified ambient temperature. A power supply can never provide more current that it’s rated output current unless there is a fault, such as short circuit at the load.
However, if there is an overload, the power supply such as an SMPS will quickly detect the fault and automatically shut down. Otherwise, if the power supply provides the excess current beyond its rated amount for a prolonged time, it may damage the components.
The maximum current is usually determined by the design, components and target load. However, the current drawn from a power supply depends on the rating of the load, and is always a good practice to use a load that has a lower rating than the power supply (i.e. selecting a higher power supply with a higher rating than the load).
Choosing the appropriate rating of a power supply
Operating below the maximum rating allows the power supply to provide enough energy, even when there are momentarily power surges in the load. In addition, the components are not stressed and there is reduced heat dissipation when operating at the lower levels. When the power supply is used on loads of lower ratings, the rated output current is rarely reached during the normal operations.
A load requiring more current than rated output current of the power supply will not get enough power and may not operate as expected. For example, a computer that does not get enough power may not even boot, and if it does, it may keep on restating. This is especially so when accessing the hard disk, under heavy processing or when performing an activity that requires more current beyond the rated output current value of the power supply.
On the other hand, an exact load will draw the rated output current from the supply, leading to working the supply to its limits. The power dissipation and component degradation increases from this, as well as instabilities during the small power surges in the load.
In operating conditions outside the specifications for the rated output current, such as in high or very low temperatures, the rated output current must be lowered. The derating is usually necessary because the output current is influenced by other factors in the power supply and maintaining the value designed for ambient conditions may lead to stressing of the components or damage to the supply.
In switched mode power supplies, the feedback circuit monitors the operating conditions and automatically adjusts the current to suit the conditions. The derating depends on the output conditions such as the output voltage, the load power factor, frequency, etc.
Rated output current and output voltage – Image Credit
Relationship between the rated output current and the power supply rating
The output rated current is dependent on the power rating of the power supply, higher values translate to high power and more expensive units. As such, it is normal to see two power supplies that will work on equipment such as a computer; however, they may have different rated output currents. The one with high value may be more reliable and allow adding of extra components such as hard disks, power display cards and more while the lower rated may not handle some of the extra load.