# What is “foldback short circuit protection” in a power supply?

I am looking at buying a 24V power supply. Two of the options are these:

The first offers "foldback short circuit protection" and "overvoltage protection," but it is $85 dollars. The second is much cheaper but does not mention these. What are these types of protection, and how important are they? Are they worth an extra ~$50? Thanks!

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I need you to give me one very important spec. What are you buying it for? If you are tinkering with cheap electronics or if you are planning to use other commercial products to connect to it makes a big difference. – Kortuk Jun 9 '10 at 20:41
I will be using the supply to power a PLC and a touch-screen HMI. Together they draw a max of 2A at 24V. Ideally only the PLC and HMI will be connected to the supply. Input power will (theoretically) be coming from a dedicated 120VAC line. – Steven Jun 9 '10 at 23:10
I prefer constant current limiting to foldback in my supplies -- sometimes I want to short it out. – tyblu Mar 4 '12 at 17:28

overvoltage protection: protects the input from exceeding the specifications, this would 'attempt' to protect the device from a voltage surge on the DIN rail. There may also be some protection on the output.

foldback short circuit protection: is a method used in power supplies to protect them from over current situations such as shorting out the output with a wire or attaching too much equipment to the power supply.

With normal (high side) current limiting there is a hard current cap that the supply is limited to to protect it. As the load resistance approaches 0 the current is limited to a fixed value and the voltage begins to drop. This can cause a large amount of power dissipation in the supply.

With foldback protection, as the voltage drops the current limit also drops fairly linearly. This provides safer protection from short circuits as a "really bad" short will result in very little current draw so the supply won't be sitting there baking at max current.

overvoltage is what it is, the response time and current carrying capabilities of basic overvoltage circuits aren't going to save you from lightning strikes but may protect you from small screwy surges from the power company.

A dedicated surge/power smoothing device at the source of the DIN power is a better option.

foldback style over current protection is simply better/safer than only having high side current limiting.

In both cases they are reliability/durability features, the question of if its worth it, depends on how critical downtime is for the equipment your connecting to it.

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So overload protection covers input spikes, and foldback protection covers short circuits/burnouts in the load? – Steven Jun 9 '10 at 23:12
Yes. I believe that is what he means in one sentence. Well explained mark. – Kortuk Jun 10 '10 at 0:51
yup, 1 additional note: Foldback current protection is much more important for a linear supply than it is for a switching supply such as those you listed. Linear supplies increase their power dissipation as the difference between Vin and Vout increases. Because of this, when the current limit is hit and voltage begins to drop, power dissipation increases even if the current is held constant, this can cause the supply to exceed its thermal limits. Switching supplies don't have this relationship so as long they have high side current protection they should be safe. – Mark Jun 10 '10 at 1:13
Thank you for this answer (which I'm accepting, because of its completeness), as well as for the explanation of linear vs. switching supplies! Is high-side current protection yet another type of protection? – Steven Jun 10 '10 at 13:48
sorry i've been using high side current protection and 'standard' current protection rather interchangeably. High side current sensing is detecting the current flow on the + side of the supply and implementing current limiting based on that level of current. This is often done but sensing the the voltage across a small resistor to determine the current then feeding that voltage into a comparator that will trip the over current protection when needed. Its also possible to do this on the low side, by having the sense resistor on the ground side of the circuit, but its not as common. – Mark Jun 10 '10 at 16:30

A graph will illustrate the difference between classical current limiting and foldback.

During normal operation of the PSU you move along the horizontal part of the graph: the voltage remains constant at variable current. Once the current exceeds the maximum allowed the voltage drops but the current remains (left graph), which may cause damage to the circuit, and also causes a high dissipation in the PSU itself.

With foldback current limiting, when the maximum current is exceeded the voltage still drops, but the current is decreased to a safe value. (right graph)

(note: $I_{SC}$ means short-circuit current)

edit (overvoltage protection)
Overvoltage protection means that the device suppresses spikes on the input voltage, so they can't disturb the output. This is often done by a VDR (Voltage Dependent Resistor, aka Varistor). The VDR is placed parallel to the input and will conduct if the input voltage exceeds normal values.

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In general, overvoltage protection refers to output protection. The main reason for overvoltage protection is to protect against someone doing something screwy with remote sensing lines (like shorting them) or the voltage feedback loop going open-loop, both of which will generally cause the output voltage to rise above its desired setting and potentially damage downstream devices.

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A worthwhile thing to note about multi-output supplies: some are designed so that a fault condition or failure on one output will not affect others, while others are designed so that a failure or fault condition on one output will not only shut down the faulted output, but others as well. The former style of design may allow for better overall system reliability (if the part that faults isn't needed for all operations), while the latter style may help protect downstream components from damage (e.g. if +12 leaks onto +5). – supercat Mar 15 '11 at 15:57

Devices may be designed with one or more of the following types of overload protection:

1. When overheated sufficiently, have something melt in such a way as to kill power before things catch on fire.
2. When overheated sufficiently, have something shut down before things are damaged.
3. When overloaded, cut back on power output sufficiently to prevent the supply from overheating.
4. When overloaded, cut back on power output sufficiently to prevent either the supply or the load from overheating.

Different supplies will do different things when trying to drive a load which behaves as a voltage clamp. Some supplies will limit the output current, independent of voltage. Some will limit the output power (so a load which clamps at 3 volts will receive twice the current of one that clamps at 6). Some will decide something is wrong and shut off completely (until input power is removed and reapplied) or partially (until even the reduced output current will suffice to allow the output voltage to reach its target).

It's probably worthwhile to get a supply which won't itself be damaged by an output overload condition. Depending upon what you're driving, it may or may not be good to have a supply that will attempt to protect the load as well. Indeed, in some circumstances, unexpected loss of power can be catastrophic. A power supply that shuts down from a transient overcurrent condition may in so doing turn what would otherwise have been a relative non-event into a disaster.

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