Protection Features Evaluation

Every PSU should be equipped with a protection set to not only protects the PSU, but the whole system. You can learn more about PSU protections by reading though the corresponding section of our "A Detailed Look Into PSUs" article.

In a snap, the most important protections for PSUs are the following:

• Over Current Protection (OCP)
• Over Power Protection (OPP)
• Over Temperature Protection (OTP)
• Over/Under Voltage Protection (OVP/UVP)
• Short Circuit Protection (SCP)
• No-load Operation (NLO)
• Surge & Inrush Protection (SIP)

We should note that OCP isn't usually offered in single +12V rail PSUs, especially high capacity ones. In these units, OPP takes over a part of OCP's role. OTP is also very important, and we won't mess with OVP/UVP until we find a way to safely evaluate them.


OCP is high on all the rails but 5VSB, and the OPP triggering points are set at a little over 130%, which we consider safe for PSU's of high build quality. With that said, passive PSUs use higher tolerance parts in order to cope with the increase in internal heat.

The 5VSB rail went pretty high, but there were no performance problems, like increased ripple or large voltage drops, so all is good with this rail. What troubles us, however, is that we couldn't make the PSU switch itself off even though we pushed it really hard. The secondary side started to smell bad because of these increased temperature, yet the PSU was still going. We contacted Mistel about this and they informed us that the component responsible for providing temperature information to the OTP circuit is relatively far away from the +12V FETs (it is right next to the supervisor IC). However, our experience has shown that this thermistor must be as close as possible to the +12V FETs since those FETs are stressed the most. It is imperative for a passively cooling PSU to have its OTP working normally. We should add here that according to Mistel, the PSU shuts down due to OTP at full load and very high operating temperatures (60–65 °C), which we never hit because we never had the PSU exceed 50–55 °C in any of our tests; we barely exceeded 40°C because of its passive cooling.

DC Power Sequencing

According to Intel's most recent Power Supply Design Guide (revision 1.4), the +12V and 5V rails must have voltages equal to or greater than the 3.3V rail's output at all times, during power-up and regular operation. For our first measurement, we turn the unit off and switch it back on without any load on any of the rails.



As you can see, there is a problem with the 5V and 3.3V rails. Voltage on the 3.3V rail is higher than it should be for a significant amount of time, which is a problem according to the ATX specification.