How to Choose the Right Power-Supply Protection?

Article By : Bill Schweber

Designers know that there is more to a power supply than its ability to provide a steady DC (or AC) voltage despite load and line changes, system transients, noise, and other aberrations...

Designers know that there is more to a power supply than its ability to provide a steady DC (or AC) voltage despite load and line changes, system transients, noise, and other aberrations. The supply must protect itself against temporary and permanent faults (internal or external), that could cause damage to its load.

There are many aspects to protection, and many supplies use a combination of them:

  • Overload (overcurrent/short-circuit) protection, including the classic fuse (fusible link), protects the supply if the load-path short circuits, or begins to draw too much current. Many supplies “self-limit” in the sense that they can only supply up to a certain amount of current, and so a fuse is not needed — some regulatory situations demand it. A standard fuse that “blows” (goes open-circuit) and stops the flow of current will need to be manually replaced; this is a problem in some situations but a virtue for others. There are also electronic fuses, which automatically self-reset.
  • Current limiting and current foldback are extensions of overload protection. If the current — which the load draws from the supply exceeds a design limit — current foldback reduces both the output current and associated voltage to values below the normal operating limits. At the extreme, if the load becomes a short circuit, the current is constrained to a small fraction of the maximum value while the output voltage obviously goes to zero.
  • Undervoltage lockout (UVLO) ensures that a DC/DC converter does not attempt to operate when the input voltage it sees at its input is too low, Figure 1. Why would this be a problem? First, the supply output may be indeterminant if its DC voltage is too low, which would cause system problems. Second, it prevents “vampire” draining of power from the source even though the voltage is low; this could deplete a battery that the system is trying to charge. UVLO also helps the power-on sequencing (if any) to function properly. Third, the DC/DC converter itself may be damaged if it attempts to turn while its own input is too low for proper functioning.
During the different modes of a power supply as it transitions from off to fully on and back to off, UVLO makes sure that the supply does not attempt to turn on and provide an output if its input voltage is below the minimum needed for proper operation. (Image source: Texas Instruments)
  • Overvoltage protection (OVP) engages if an internal failure in the supply causes its output voltage to rise beyond specified maximum with likely damage the load. OVP shuts down the supply or clamps the output when the voltage exceeds a preset level. The OVP circuit is often called a “crowbar,” presumably because it has the same effect as placing a metal crowbar across the supply output. A properly designed crowbar functions independent of the supply itself.

One type of crowbar will reset (once tripped) only if the power is turned off; in the other type, it will reset itself once the output-voltage fault is cleared. The latter one is useful when the condition that tripped the crowbar is transient rather than a hard failure in the supply. While most supplies now come with a built-in crowbar, many vendors offer a small, separate crowbar circuit, which can be added to an existing supply if needed.

  • Thermal overload will occur if the supply’s cooling approach is inadequately designed, or fails in use (such as fan stops, airflow is blocked). The supply then will likely exceed its temperature rating, which severely shortens its life and may even cause immediate malfunction. The solution is simple: a temperature-sensing circuit within or near the supply which puts the supply into a quiescent or shutdown mode if it exceeds a preset limit. Some thermal cutoffs automatically allow the supply to resume operation if the temperature drops, and other do not.
  • Reverse-connection protection blocks the current flow and zeroes the voltage if the load is connected backwards — positive supply output to negative load rail and vice versa. It’s especially popular in applications where the battery is disconnected and then reconnected, such as in a car or where the battry or its connector is not keyed.

Among the useful protection devices are the metal oxide varistor (MOV), positive temperature coefficient (PTC) thermistor, transient voltage suppression (TVS) diode, the gas discharge tube (GDT), and the polymer PTC resettable fuse.

The entire issue of what to protect, against what, and how to do it is not as simple as it appears. Which types of protection types do you need to add to your supply? As always, the answer is “it depends” — based on the supply itself, the load, and the system. While many supplies and related functions ranging from ICs (including converters and regulators) to larger modules and even chassis/open-frame units include some of these, but you may need to add others.

What’s your preferred power-supply protection? What types of protection have you added to a supply, whether because of good engineering judgement, or because you learned the hard way?

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