First, what is thermistor
Thermistors are also often referred to as NTC, and the higher the temperature, the lower the resistance, due to this feature is often used to detect temperature or to solve the current surge of switching power supplies.
Second, the application of thermistor in switching power supply
- Thermistors with negative temperature coefficients have a higher resistance value before power-up to limit the NTC flowing through the current and dissipating electrical energy heat after power-up, reducing their resistance to reduce losses on the NTC.
- This method is characterized by simplicity, but the problem is to limit the inrush current performance to the ambient temperature and the initial temperature of the NTC.
- Therefore, when the ambient temperature is high and the power-up time interval is very short, the effect of NTC in limiting the inrush current is greatly reduced.
- NTC is too high at low temperatures, and a resistor of the same value is usually connected in parallel next to the NTC to reduce the resistance in the loop, but the conventional NTC resistance can be reduced
Third, thermistor selection attention
- 1. Inrush current
- The switching power supply has been working continuously for a long time, and then suddenly the machine is switched on and off, and this inrush current will not be small
- Switching power supply input electrolytic capacitor capacitance, the highest input peak voltage, inrush current energy is generally given in the data sheet, if this energy is not given, the manual will recommend you which model of thermal can match how much input electrolytic capacitor This parameter is related to the size of the thermal resistance, if the manufacturer does not give the impact energy parameter and matching capacity value
- 2. Resistance value
- Meet the inrush current requirements, select the minimum resistance value, the maximum inrush current value, the customer will generally make a request, if not given, you can be determined according to the enterprise test standards, or according to the maximum operating current of the fuse or rectifier bridge as the limit of the current, Rntc_min = Vmax / Ilmit
- 3. Steady-state current
- For the calculated current value, you need to choose from the curve in the manual according to the actual temperature of the thermistor in operation, if the manual does not give the curve will definitely give a maximum steady-state current, if the manual does not give the curve will definitely give a maximum steady-state current, and then leave a certain margin on it, Imax = Po / (ηVmin) This value is less than the maximum steady-state current value in the manual, and leaves a certain margin
- Calculation of the resistance of the thermistor at a certain temperature tb, Rntc = 8Ω (the value of the resistance of the thermistor at room temperature)
- Bn=2800K (Bn is the material parameter (common range 2000K-6000K thermal teaching resistance data sheet will be given)
- Ta = 25 (room temperature unit degree Celsius)
- Tb = 25 (temperature of the resistor value to be calculated)
- T1 = (273 + Tb) K = 298 K (T1 and Tn are K degrees (i.e. Calvin) K degrees = 273.15 + degrees Celsius
- Tn=(273+T)K=298K度)
- Rt=Rntc·e^(Bn·( 1/T1-1/Tn))=8Ω
- Rt is the resistance value of the thermal teaching resistance at the tb temperature