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 a significantly higher OPP triggering point set than their nominal max power output. This is mainly done to deal with load spikes, which could trigger OPP and shut down the system, but OPP is there for a reason, and it has to be configured correctly 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 equipped with OTP. In PSUs with only a 40 °C rating, OTP's triggering point will inevitably 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 essential protection that all PSUs should have. If there is a short circuit in any 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 go out of spec.
- No-load Operation (NLO): The PSU must operate normally even when no load is on its outputs.
- Surge and Inrush Protection (SIP): The design must include a Metal Oxide Varistor (MOV) or a Transient Voltage Suppression (TVS) diode, both of which protect 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 and a bypass relay, which allows the thermistor 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.
|OCP (Cold @ 27°C)||12V: 105.2A (126.29%), 11.973V|
5V: 35A (140%), 5.177V
3.3V: 30.5A (122%), 3.340V
5VSB: 4.6A (153.33%), 4.959V
|OCP (Hot @ 39°C)||12V: 105.2A (126.29%), 11.981V|
5V: 34.1A (136.4%), 5.163V
3.3V: 30.3A (121.2%), 3.340V
5VSB: 4.6A (153.33%), 4.958V
|OPP (Cold @ 28°C)||1239.04W (123.9%)|
|OPP (Hot @ 40°C)||1236.02W (123.6%)|
|OTP||✓ (151°C @ 12V Heat Sink)|
|SCP||12V to Earth: ✓|
5V to Earth: ✓
3.3V to Earth: ✓
5VSB to Earth: ✓
-12V to Earth: ✓
|PWR_OK||Accurate but lower than 16ms|
Inrush: NTC Thermistor
MORE: Best Power Supplies