Research on Power Factor Measurement Circuit of High Frequency Switching Power Supply Based on Matlab

0 Introduction

The power factor of a high-frequency switching power supply is a very important parameter, which directly determines whether the product meets the general harmonic standards and measures the quality of the product. In order to reduce harmonics and improve the power factor, high-frequency switching power supplies generally use power factor correction circuits to improve the current waveform. In order to understand the power factor value of the high-frequency switching power supply in the design stage and facilitate the optimization of the power factor correction circuit parameters, it is necessary to measure the power factor. Based on Matlab simulation software, this paper designs and gives two power factor measurement circuits. These two circuits are used to simulate and measure the power factor of the RC sinusoidal circuit and verify the calculation; and these two simulation measurement circuits are applied to the power factor simulation measurement of the three-phase high-power constant current charging power supply, and finally their reliability is verified through experiments.

1 Definition of power factor
Power factor is used to measure the ratio of input active power to input apparent power, and is expressed as follows: the higher the power factor, the greater the proportion of input active power; when the power factor is 1, all input power is absorbed as active power.

In a sinusoidal system, P is all the active power done by the fundamental component, and S is all the apparent power of the fundamental component. However, in a non-sinusoidal system, P and S are not fundamental components, but the work done by the DC components and harmonic components of all voltages and currents. The definition of power factor can be expressed as follows:

Among them, Uk and Ik are the effective values ​​of the kth harmonic voltage and current respectively.

When there is no loss on the input side (i.e. the input voltage waveform is not distorted), equation (1) can be simplified to equation (2):

2 Two methods of power factor simulation measurement

According to formula (1), the first power factor measurement circuit can be designed, as shown in Figure 1. Among them, u and i are phase voltage and phase current.

In Figure 2, three sets of measurement models shown in Figure 1 are used to form a three-phase system power factor simulation model, and P=PA+PB+PC and S=UA·IA+UB·IB+UC·IC are substituted into the calculation to finally obtain the power factor value.

According to formula (2), the second power factor measurement circuit can be designed, as shown in Figure 3. Among them, u and i are phase voltage and phase current.

Among them, f(μ)=1／sqrt(μ*μ+1), K=pi／180. This method first obtains the THD value, then calculates the fundamental coefficient μ through f(μ), then extracts the angle of input voltage and current to calculate the displacement coefficient λ, and finally multiplies μ and λ to obtain PF.

In FIG4 , three sets of measurement models shown in FIG3 are used to form a three-phase system power factor simulation model. The output values ​​are obtained respectively and then averaged to finally obtain the power factor value.

3 Computational Verification

In order to verify the correctness of the two measurement circuits, the two measurement circuits are used to measure the power factor of the RC sinusoidal circuit, as shown in Figure 5.

In Figure 5, the power frequency is 220V/50Hz, R=5.1kΩ, C=1μF. At this time, the equivalent impedance can be calculated as:

In a linear circuit, the fundamental coefficient μ is 1, so the power factor PF is 0.848.

The measurement circuit is used to measure the power factor of the circuit in FIG5, and the power factors shown in FIG6 and FIG7 can be obtained respectively. FIG6 is the power factor measured by the first measurement method; FIG7 is the power factor measured by the second measurement method (the horizontal axis is the time t axis).

From Figure 6 and Figure 7, we can see that the power factors measured by the two measurement circuits are both 0.848. The measurement results are consistent with the calculation results.

4 Experimental Verification

Taking a 42kJ/S digital high-frequency high-voltage constant-current charging power supply without power factor correction as the experimental object, the power factors obtained by these two simulation measurement methods are shown in Figures 8 and 9. Figure 8 is the power factor measured by the first measurement method; Figure 9 is the power factor measured by the second measurement method (the horizontal axis is the time t axis):

The power factor values ​​measured by IDEAL 61-806 power analyzer are shown in Table 1. The waveform is shown in Figure 10:

It can be seen that in the three-phase high-power power supply system, the numerical error between the power factor measured by simulation and the power factor value actually measured by the power analyzer and the simulated measurement after stabilization (shown in the box) is very small. The error is mainly caused by the line inductance and its distributed parameters in the actual circuit.

5 Conclusion

From the calculation results and actual measurement results, we can see that, except for the unstable state at the beginning of the system simulation, the measurement results of the two power simulation measurement circuits are consistent with the actual calculation results and the actual measurement results. Therefore, the reliability of the two measurement circuits can be determined. These two measurement circuits can be used to measure the power factor of a single-phase or three-phase system based on Matlab simulation, and the measurement results have high accuracy.