Designers know that in addition to providing a stable DC (or AC ) voltage under conditions 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 damage to the load.
Protection involves many aspects, and many power supplies combine them:
Overload (overcurrent / short circuit) protection, including classic fuses (fuse connections), protects the power supply when the load path is short-circuited or when it begins to draw excessive current. Many power supplies are "self-limiting" because they can only supply a certain amount of current, so fuses are not necessary—but they are required in some regulatory situations. Standard fuses stop current flow by "melting" (opening the circuit), so they need to be manually replaced; this can be troublesome in some cases, but advantageous in others. There are also electronic fuses, which are self-resetting.
Current limiting and current foldback protection are extensions of overload protection. If the load draws current from the power supply exceeding the design limits, current foldback reduces the output current and associated voltage below normal operating limits. In extreme cases, if a short circuit occurs in the load, the current will be limited to a fraction of its maximum value, and the output voltage will obviously become zero.
Figure 1 : As the power supply transitions from a powered-off state to a fully powered-on state and then back to a powered-off state, it goes through different modes; during this period, if the input voltage of the power supply is lower than the minimum required for normal operation, UVLO ensures that the power supply does not attempt to turn on and provides output. (Image source: Texas Instruments)
Overvoltage Protection ( OVP ) – If an internal fault in the power supply causes its output voltage to rise above a specified maximum value, the OVP circuit will activate to protect the load from damage. When the voltage exceeds a preset level, the OVP will either shut down the power supply or clamp the output. The OVP circuit is often called a "crowbar" circuit, presumably because its function is similar to placing a metal crowbar across the power supply output. Ideally, the crowbar circuit should be designed to be independent of the power supply itself.
One type of crowbar circuit resets only when the power is turned off (once the circuit breaker trips). Another type resets automatically once the output voltage fault is cleared. The latter type is useful when the crowbar tripping is brief and not due to a serious power supply failure. While most power supplies now come with a built-in crowbar, many vendors offer a small, standalone crowbar circuit that can be added to an existing power supply if needed.
Thermal overload protection – Thermal overload can occur if the power supply's cooling system is poorly designed or malfunctions (e.g., fan stops, airflow is blocked). In this case, the power supply may exceed its rated temperature, severely shortening its lifespan or even causing immediate failure. The solution is simple: install a temperature sensing circuit inside or near the power supply. If the power supply exceeds a preset limit, it will enter a static or shutdown mode. If the temperature drops, some thermal fuses will automatically restore the power supply to operation, while others will not.
Reverse connection protection – If the load is reverse-connected (positive power output to negative load rail, or vice versa), reverse connection protection will stop current flow and bring the voltage to zero. This protection is particularly popular in applications where the battery is disconnected and then reconnected, such as in automotive applications or where the battery or its connector is not locked.
Practical protection devices include metal oxide varistors ( MOVs ), positive temperature coefficient ( PTC ) thermistors, transient voltage suppression ( TVS ) diodes, gas discharge tubes ( GDTs ), and polymer PTC resettable fuses.
What to protect, what to prevent, and how to do it—these questions are not as simple as they seem. What types of protection do we need to add to a power supply? As always, the answer is "it depends" on the power supply itself, the load, and the system. From ICs (including converters and regulators) to larger modules and even chassis / open-frame devices, while many power supplies and related functions include some of these, you may need to add others.
What is your preferred power protection? What types of protection have you added to your power supply, and is this based on your good engineering judgment or your trial-and-error experience?
