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Under and Over Voltage Protections (UVP and OVP)

We are going to talk about under and over voltage protections together because they are built using the same circuit. These protections monitor the +12 V, +5 V and +3.3 V outputs and shut down the power supply in case any of these outputs are above (OVP) or under (UVP) a certain value, also called “trigger point.” These are the most basic protections available and almost all power supplies have them, including the ultra-low-end models. This happens because all monitoring integrated circuits (PWM controllers, on the case of low-end power supplies based on the half-bridge topology) implement these protections – and also because ATX12V specification requires OVP.

One interesting thing that most people don’t know is that the ATX12V specification requires all PC power supplies to have over voltage protection (OVP) but the under voltage protection (UVP) is optional.

The problem with these two protections is that they are usually configured at trigger points that are too far away from the output nominal voltage. To give you a better idea, consider the over voltage protection trigger points required by the ATX12V specification:

Output Minimum Typical Maximum
+12 V 13.4 V 15.0 V 15.6 V
+5 V 5.74 V 6.3 V 7.0 V
+3.3 V 3.76 V 4.2 V 4.3 V

One manufacturer could build a power supply with an OVP configured at 15.6 V at +12 V output or 7 V at +5 V and still be compliant with the ATX12V specification. So this power supply could be delivering say 15 V at its +12 V output and the over voltage protection would not kick in, and it would be probably damaging your components due to this very high voltage.

In other words, ATX12V specification says that voltages must be between 5% their nominal values, but when comes to over voltage protection, it allows manufacturers to configure this protection up to 30% on +12 V, 40% on +5 V and 30% on +3.3 V.

How manufacturers choose OVP and UVP trigger points? By choosing the monitoring integrated circuit (or PWM controller, on the case of low-end power supplies based on the half-bridge topology), because the values for these protections are hard-wired inside this circuit.

For a real example, consider the popular monitoring integrated circuit PS223, which is used by several power supplies available on the market. This circuit provides the following trigger points for over voltage protection (OVP):

Output Minimum Typical Maximum
+12 V 13.1 V 13.8 V 14.5 V
+5 V 5.7 V 6.1 V 6.5 V
+3.3 V 3.7 V 3.9 V 4.1 V

And the following values for under voltage protection (UVP):

Output Minimum Typical Maximum
+12 V 8.5 V 9.0 V 9.5 V
+5 V 3.3 V 3.5 V 3.7 V
+3.3 V 2.0 V 2.2 V 2.4 V

Other circuits will present different trigger points.

Once again we’d like to call your attention on how far from the nominal voltages these protections are usually configured. For them to enter in action the power supply must be facing a very serious condition. In fact in our experience low-end power supplies (which don’t have any other protection besides OVP/UVP) will burn before these protections have a chance to enter in action.

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Gabriel Torres is a Brazilian best-selling ICT expert, with 24 books published. He started his online career in 1996, when he launched Clube do Hardware, which is one of the oldest and largest websites about technology in Brazil. He created Hardware Secrets in 1999 to expand his knowledge outside his home country.