Everything You Need to Know About Power Supply Protections

Over Temperature Protection (OTP)

Over temperature protection, as the name implies, will shut down the power supply if the temperature inside the power supply reaches a certain level. Although several monitoring integrated circuits have this capability, not all power supplies implement this protection. This is an optional protection.

Opening a power supply you will easily spot a thermistor attached to the secondary heatsink (some power supplies use a tiny sensor soldered on the solder side of the printed circuit board, though). This thermistor is connected to the fan controller circuit, making the power supply to adjust the fan speed according to the power supply internal temperature. This thermistor is not used for the over temperature protection: power supplies with OTP usually have two thermistors, one for the fan circuit and a separated one for the OTP.

Figure 6: Power supply with two thermistors and thus featuring OTP.

The trigger temperature for the over temperature protection is configured by the power supply manufacturer by choosing the value of a resistor that is connected to the monitoring integrated circuit (RT in Figure 5; on this same figure NTC is the thermal sensor – NTC stands for Negative Temperature Coefficient, meaning that the resistance of this component decreases with the temperature).