On the switching power supply power factor accounting, in terms of accounting, is still in accordance with the definition of power factor, that is, active power ratio apparent power. The input end shall prevail. In practice, the manufacturers of switching power supply are using the measurement method, there is a special measurement of switching power supply parameters of the instrument, including its power because of the value. Due to historical reasons, the calculation formula of power factor in many people's mind is: PF=cosφ. In practice, this formula is established only in specific cases, provided that the load is an impure linear load. The true definition of power factor is: PF= active power/apparent power.
What is active power? Active power is the power consumed by the load in practice. The accounting method is: P active power =∫ U · I. This is, in practice, the average value of the product of the instantaneous voltage and the instantaneous current. The definition of apparent power is: P apparent =URMS·IRMS. Instead of calculating the active power, the current and voltage are measured separately. Now, where does the formula PF=cosφ come from? With respect to inductive loads such as communication motors (in practice the same is true for capacitive loads, although these are rare), the current and voltage waveforms are the same, but with a phase difference.
For the current I with a phase difference φ : can be decomposed into a weight in phase with the voltage, fluctuating by I·cosφ; A weight that is orthogonal to the voltage (90° off) and fluctuates by I·sinφ. The two weights are multiplied by the voltage and averaged. The average of the orthogonal weights multiplied by the voltage is zero, and the rest is P active power =P apparent at cosine φ. Here, the reactive current is: I reactive power =IRMS sine φ.
Regarding the load of switching power supply, there is no significant phase difference between the current waveform and the voltage waveform, but the current waveform is not a sine wave. This load is a typical nonlinear load in practice. As before, the average value of the non-sinusoidal current multiplied by the sinusoidal voltage can be calculated by dividing the current into weights in the same frequency and phase as the voltage, and weights in different frequencies or phases. There is basically no phase difference in the current waveform, but there is a lot of harmonic weight. Fourier transform of the current waveform, resulting in a series of harmonic weights. Meanwhile, as long as the average value of the product of the fundamental frequency component and the voltage is not zero, this part of the current is active current; And all the higher harmonic weight and voltage product mean is zero, so all the current of the higher harmonic current are reactive current, because they are not related to the practical consumption of power, the total value of reactive current is the square root of all the higher harmonic current squared.