What's Protecting Your Power Supply?
July 15, 2019
Be sure your system is protected against power-supply faults as well as the complementary scenario.
Inexperienced electronic-design engineers often assume that a good power-supply rail just “happens,” while more-experienced ones know that a rock-solid, noise-free rail doesn’t come easy but is essential for stable, consistent, glitch-free system performance. But there’s more to a power supply than just its ability to provide a steady DC voltage despite load and line changes, system transients, noise and other aberrations.
How so? A good power supply does more than just deliver, it is also both protected against temporary and permanent faults that may occur internally or externally and protects against causing irreparable damage to the system, which is its load.
Before we look at the various types of protection, it’s worth a quick look at the four classes of DC-output supplies, also called regulators or DC-DC converters; note that the current-output ratings cited are just approximate regions and there are no hard or official boundaries:
1) for larger loads, on the order of 20 A and above, there are many off-the-shelf open-frame or fully metal-enclosed supplies, for both AC-DC and DC-DC applications
2) for moderate loads around 10 to 20 A, there are modular supplies; these are often potted in epoxy for physical protection
3) under 10 A, there are many available ICs that require a few external passive and active components to function as complete supplies
4) finally, you can build a basic supply from individual components, such as diodes and capacitors, often in conjunction with a small LDO or switching-controller needed
So, what are the various types of protection?
a) Overload (overcurrent/short-circuit) Protection (OP), including the classic fuse, 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. 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 on others. There are also electronic fuses that automatically self-reset.
b) Current Limiting and Current Foldback are extensions of overload protection. If the current from which the load draws 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.
c) 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.
d) Overvoltage Protection (OVP) engages if an internal failure in the supply causes its output voltage to rise beyond the 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 independently from 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 which 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 that can be added to an existing supply if needed.
e) Thermal overload will occur if the supply’s cooling approach is inadequately designed, or fails in use (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 that 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 others do not.
f) 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 battery is not keyed.
So, which types of protection do you need to add to your supply? It is partially determined by the application, of course, but it also depends on the supply construction (Items 1 through 4 above). For metal-enclosed or modular supplies (supply types 1 and 2), most of these protection modes are usually standard and included (except the fuse). For type 3, supply ICs may offer some or all of the protection features, but they can also be disabled (necessary in some special cases but also risky). Reverse connection is a special case and only added where it makes sense. It can be implemented via a simple diode but that adds voltage-drop losses, so an ideal diode circuit is needed.
Treat your supply with the respect it deserves: be sure that it is protected and also protects your circuit. Your design and system will thank you.
Texas Instruments, Application Report SLVA769A, “Understanding Undervoltage Lockout in Power Devices”