There are two main methods for power factor correction (PFC): passive power factor correction and active power factor correction. Passive power factor correction (PFC) uses linear inductors and capacitors to improve power factor and reduce harmonic components. This method is simple, economical and can achieve good results in small power. However, in a large power supply, a large amount of energy must be stored and managed by this filter, and therefore large inductors and capacitors are needed, so that the volume and weight are larger and less economical. Moreover, the improvement of power factor and the suppression of harmonics can not achieve the ideal effect. Active power factor correction (APFC) is a so-called active current control method which can force the input current to follow the sinusoidal voltage variation of the power supply. The power factor correction has the advantages of small volume, light weight, power factor approaching 1, etc.
Function of NTC:
TC is a fine semiconductor electronic ceramic component made from manganese oxide and so on. The resistance value shows nonlinear variation with the temperature, and the resistance value decreases with the increase of temperature. By using this property, a negative temperature coefficient thermistor in series at the input end of the circuit increases the impedance of the circuit, which can effectively suppress the surge current generated by the surge voltage. When the circuit is in steady operation, the resistance value of the resistor becomes very small because of the NTC heat caused by the continuous working current in the circuit, and the effect on the circuit can be completely ignored.
The selection formula of NTC:
The above formula is explained as following:
1. Rt is the resistance of thermistor at T 1 temperature.
2. Rn is the nominal resistance of thermistor at normal temperature of Tn.
3. B is the material parameter, (commonly used range 2000K~6000K) .
4. exp is the index based on the natural number e (eke 2.71828).
5. Here T1 and Tn refer to K degrees, or Kelvin’s temperature, K = 273.15 degrees Celsius (absolute temperature).