Everything You Need to Know About Power Supplies

Protections

Protection is always desirable, but one thing that a lot of people don’t know is that according to ATX12V and EPS12V standards only over voltage protection (OVP), short-circuit protection (SCP) and over current protection (OCP) are required. All other protections are optional and it depends on the manufacturer to implement them. Of course, the more protections a power supply has, the better.

Let’s first list the most common protections available. Then we will explain some interesting facts about them.

• Short Circuit Protection (SCP): as the name says, it will turn off the power supply if any output is shorted. This is a required protection.
• Under Voltage Protection (UVP): shuts the power supply down if the voltage at any of the unit’s outputs falls below a trigger value. This is an optional protection.
• Over Voltage Protection (OVP): shuts the power supply down if the voltage at any of the unit’s outputs rises above a trigger value. This is a required protection.
• Over Current Protection (OCP): shuts down the rail it is monitoring if that rail is pulling more than the triggering current. This is a required protection. Read the previous page for a more detailed explanation about this protection.
• Over Power Protection (OPP) or Over Load Protection (OLP): shuts down the power supply if you pull more than a trigger power from the unit. This is an optional protection.
• Over Temperature Protection (OTP): shuts the power supply down if the temperature inside the power supply reaches a trigger value. This protection isn’t as common and is optional.

The idea of protections is to shut down the power supply if something wrong happens, preventing your power supply from burning and the risks of a fire in the event of an explosion. For example, if you pull far more power than a power supply is capable of handling it might burn if it doesn’t have over power protection (OPP) implemented. With this protection the unit will shut down instead of burning.

All protections are configurable at the discretion of the manufacturer. Take the over voltage protection (OVP). ATX12V and EPS12V standards suggests a voltage range that the manufacturer can use for triggering this circuit, but it is up to the manufacturer to choose which value they will use.

The problem is that some manufacturers will set their protections with values that are too loose, allowing something wrong to happen before the appropriate protection kicks in.

below are just a couple of real examples we’ve seen while we overloaded some power supplies.

One given power supply was operating with its voltages completely out of range, but the power supply was still on because although the voltages were wrong, they weren’t reaching the levels necessary to activate the UVP and OVP circuits.

Another example – unfortunately more common – is with power supplies where the OCP is configured at a value that is so high that the power supply is operating as if it didn’t have an OCP at all. The same holds true for the OPP circuit.