Most AC-DC power provides and DC-DC converters have internal current-limiting circuits to guard the power device, and to a point its load. Nearly all of over-current-protections embody an automated restoration feature. In follow, the present restrict feature sometimes begins working when the output present exceeds it most ranking by 10 to 20%.
In lots of instances, ought to an overload (e.g., quick circuit) be allowed to exist for a chronic interval, it will possibly scale back the product’s discipline life by temperature stressing the electrolytic capacitors, and in excessive instances, it will possibly harm the consumer’s printed circuit traces. Due to this fact, at all times verify the power provide’s “Instruction Guide” to make sure you perceive the precautions related to the power product’s over-current-protection feature. Additionally, if the power product has an Output Good sign, this can be utilized as a sign that the power provide is both defective or might be in an over-current mode.
There are a selection of the way to implement over-current-protection (OCP), and under are descriptions of the commonest strategies.
Fold-Again Current Limiting: When this methodology is employed if an overload situation exists, the output voltage and present scale back to secure ranges. As may be seen from the next curve, ought to an overload happen the provision will present present up its present restrict level (aka ‘knee’), after which the output present will fold-back to a decrease worth because the output voltage reduces in direction of zero.
This system is employed in linear power provides as a result of it reduces the pressure on the provision’s internal power units to minimal. One downside of fold-back present limiting is that if the provision turns on right into a heavy capacitive load, it may latch-up at a decreased present earlier than reaching its full output voltage. Relying upon the design, restoration from a fold-back present restrict situation may be automated, or after a built-in time delay when the overload situation is eliminated.
Fold-Ahead Current Limiting: On this methodology, when an overload is sensed the output voltage reduces in direction of zero, however the present will increase. When driving motors, pumps, or extremely capacitive masses, using a fold-forward present feature will help overcome inertia of those masses. Restoration from a fold-forward present limiting state of affairs is often automated when the overload is eliminated.
Fixed Current Limiting: On this methodology, ought to an overload happen, the output present stays at its restrict level and the output voltage reduces in direction of zero in a considerably linear style. This system is used in many switchmode power provide designs. Sometimes, the provision will robotically return to its regular output voltage when the overload situation is not current.
Current Restrict Shutdown: In some power provide designs, when an overload happens the power provide will start to enter a constant-current restrict mode, however when the output reaches a preset decreased voltage, the provision will shutdown. Restoration from this situation may be automated or require recycling of the enter power.
Hiccup Mode Current Limiting: Some low power provides have what's termed a hiccupcurrent-limit feature. Because the identify implies, if a present restrict is sensed, the provision will scale back its output voltage to zero after which, after a short while, it should try to offer its regular voltage. These On-Off makes an attempt at operation are known as a hiccup-mode. Ought to the overload situation be eliminated, the provision will once more function usually.
Peak-Current Power Supplies
It must be talked about that some power provides are designed particularly to offer giant peak-currents, which may vary from 200 to 300% of the utmost present ranking for a brief length, with out going right into a current-limit situation. These are particularly helpful when powering masses that embody electrical motors reminiscent of laptop laborious drives, followers, actuators, pumps, and so forth. When utilizing such a power provide you will need to restrict the “common power” that's delivered to load. Extra details about peak-current-rated provides shall be supplied in a separate article.