•IMax - The first critical consideration in the selection of a thermistor is the maximum steady state current (AC or DC) of the power supply. thermistor are rated for maximum continuous current. The input power (Pin) is calculated as Pin = Pout/efficiency. In the case of a 75 Watt SPS with 0.70 efficiency, 100% load is calculated to be 107.14 Watts. The maximum input current is at the minimum input voltage. The effective input current (Ie) is equal to the maximum load divided by the minimum input voltage. In this case, a 75 Watt SPS, Ie = Pin/Vin(low) = 107.14 Watts/90 Volts = 1.2 Amps. Therefore, the thermistor must have an IMAX rating of at least 1.2 Amps.
•R@25°C. - The second step is to determine the minimum R value of the thermistor to be selected that will limit the one cycle maximum current rating of the diode bridge rectifier to 50% of its rating to ensure adequate surge protection. Several additional calculations must be made to determine the estimated resistance value required at the point in time of the maximum current surge. RTI provides for a maximum AC voltage rating of 265V RMS on most thermistor. (Reference the Specifications) If the desired maximum inrush current is less than 100 Amps (50% of the diode bridge with a peak current rating of 200 Amps), then solving for R would produce a value of 2.65 ohms. If the MAX Operating Temperature is other than 25°C then the zero power resistance value must be calculated using the NTC
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