Designers know that in addition to providing a stable DC (or AC) voltage in the face of load and line changes, system transients, and noise deviations, a power supply must also be able to protect itself from temporary and permanent faults (internal or external) to prevent load damage.
Protection involves many aspects, and many power sources use a combination of them:
Overload (over current/short circuit) protection, including classic fuses (fuse connections), provides protection to the power supply if the load path shorts or begins to absorb too much current. Many power sources are "self-limiting" because they can only supply a certain amount of current, so fuses are not required - but they are required in some regulatory situations. Standard fuses are "fuses" (open circuits) to stop current flow and therefore require manual replacement; This can be a problem in some cases, but an advantage in others. There are also electronic fuses, which can reset themselves automatically.
Current limiting and current return protection are extensions of overload protection. If the load absorbs current from the power supply in excess of the design limit, current retracting reduces the output current and associated voltage to values below the normal operating limit. In extreme cases, if the load shorted, the current would be limited to a small fraction of the maximum and the output voltage would obviously be zero.
Figure 1: The power goes through different modes as it transitions from off to full on and back again; During this time, if the input voltage of the power supply is below the minimum required for normal operation, the UVLO can ensure that the power supply provides output without attempting to turn it on. (Image credit: Texas Instruments)
Overvoltage Protection (OVP) - If an internal fault in the power supply causes its output voltage to rise above the specified maximum, the OVP circuit is in place to protect the load from damage. When the voltage exceeds a preset level, the OVP will turn off the power or clamp the output. OVP circuits are often referred to as "crowbar" circuits, presumably because they function in the same way as placing a metal crowbar at each end of a power output. The crowbar circuit is best designed independently of the power source itself.
One type of crowbar circuit is reset only when the power is turned off. The other type is that it automatically resets once the output voltage fault is cleared. The latter type is useful when the situation causing the crowbar trip is brief rather than a serious power failure. Although most power supplies now come with a crowbar built in, many suppliers offer a small stand-alone crowbar circuit that can be added to an existing power supply as needed.
Thermal overload protection - Thermal overload can occur if the cooling mode of the power supply is not designed properly or cannot be used (e.g. fan stops, airflow blocks). At this time, the power supply may exceed its rated temperature value, which can seriously shorten its life, and may even cause an immediate failure. The solution is simple: set up a temperature-sensing circuit inside or near the power supply. If the power supply exceeds the preset limit, the power supply is put into static or shutdown mode. Some hot-melt circuit breakers automatically restore power if the temperature drops, others do not.
Reverse connection protection - If the load is inversely connected (positive power output to negative load rail and vice versa), reverse connection protection prevents current flow and zeros the voltage. This protection is particularly popular in applications where the battery is disconnected and then reconnected, such as in cars or applications where the battery or its connector is not locked.
Practical protective devices include metal oxide rheostat (MOV), positive temperature coefficient (PTC) thermistor, transient voltage suppression (TVS) diode, gas discharge tube (GDT), and polymer PTC self-recovering fuses.
The question of what to protect, what to prevent and how to do it is not as simple as it seems. What types of protection do we need to add to the power supply? As usual, the answer is "depends" on the power supply itself, the load, and the system. From ics (including converters and regulators) to larger modules and even chassis/open devices, many power supplies and related features are included in some of these, but you may need to add others.
What is your preferred power protection? What types of protection did you add to the power supply, again out of good engineering judgment or out of trial and error?