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How We Test Power Supply Units

Protection Features Evaluation

All power supplies have to be equipped with various protections, which also includes the system that is fed with power. You can learn more about PSU protections if you read the corresponding section of our PSUs 101 article.

The most important protections are the following:

  • Over Current Protection (OCP): The single +12V rail PSUs usually don’t have OCP on this rail, but they should have OCP on the minor rails, including 5VSB.
  • Over Power Protection (OPP): Unfortunately most PSUs have the OPP triggering point set significantly higher than their nominal max power output. This is mostly done to deal with load spikes, which could trigger OPP and shut-down the system, however OPP is there for a reason and it has to be properly configured in order to serve its purpose.
  • Over Temperature Protection (OTP): Since the ATX spec recommends at least 50 °C operating temperature for continuous full power delivery, a PSU has to have an appropriate OTP triggering point, if it is equipped with OTP of course. In PSUs with only a 40 °C rating, OTP’s triggering point inevitably will be lower.
  • Over/Under Voltage Protection (OVP/UVP): These protections only kick in when voltages surpass or go below a specific level. Given that the corresponding ATX limits are practically dangerous for the system’s health, and the fact that there is no safe way to test these protections, we decided not to deal with them, for the moment at least.
  • Short Circuit Protection (SCP): This is a basic protection that all PSUs should have. If there is a short-circuit in any of the rails, the PSU must immediately shut-down.
  • Power Good Signal (PWR_OK): This signal has to drop when any of the +12V, 5V or 3.3V output voltages goes out of spec.
  • No-load Operation (NLO): The PSU must operate normally even when there is no load on its outputs.
  • Surge & Inrush Protection (SIP): The design must include a Metal Oxide Varistor (MOV) or a Transient Voltage Suppression (TVS) diode, both of which provide protection against voltage spikes coming from the mains network. In addition, the platform must be equipped with inrush current protection. The most common way of lowering the inrush current during the PSU's start-up phase is by using an NTC (Negative Temperature Coefficient) thermistor, along with a bypass relay, which allows the thermistor's fast cool down.

In our methodology we consider that the OCP or OPP aren’t configured properly if their triggering points are set above 130%. Please note that a PSU might be able to deliver much more than its nominal power under normal ambient temperatures, but at higher temperatures this won’t be the case, and if OPP is set too high then most likely the PSU will be destroyed, especially if OTP is absent. In addition, if we notice any load regulation or ripple suppression issues during our OPP tests, we consider this protection to be improperly configured.

An evaluation example of a PSU’s protection features follows.

Protection Features
OCP12V: - 5V: >130% 3.3V: >130% 5VSB: >130%
OTPYes (at 46 °C ambient)
SCP12V: Yes 5V: Yes 3.3V: Yes 5VSB: Yes
PWR_OKProper Operation
SIPSurge Protection: MOV Inrush Protection: NTC & Bypass Relay
Aris Mpitziopoulos
Aris Mpitziopoulos is a Contributing Editor at Tom's Hardware US, covering PSUs.