Comparison of Fixed Switching Frequency and Variable Switching Frequency PFC Rectifiers

For convenience, the input current at fixed switching frequency is also represented by phase voltage, that is, M ‘=√ 3M instead of M, as shown in Figure 1-1

From the average value equation of current ia at constant switching frequency and variable switching frequency, the rated average input current waveform of constant switching frequency and variable switching frequency can be obtained, as shown in Figure 1-2.

Wherein, Figure (a) is the waveform of the rated input average current at constant switching frequency; Figure (b) is the waveform of the rated input average current when the switching frequency is variable.

The relationship curve between the harmonic distortion rate THDi of the low order harmonic component and the voltage gain M ‘can be obtained from the waveform shown in Figure 1-2, as shown in Figure 1-3.

From this curve, it can be seen that when M ‘is small (M’=UM/Um, that is, when the output voltage UM=Ud is small), variable switching frequency operation has a low harmonic distortion rate, which indicates that low harmonics are prevented.

When M “increases (i.e., Un=U, increases), the two curves gradually approach and tend to the curve with constant switching frequency.

Figure 1-4 shows the spectrum of the input current ia when M ‘=2.11 (the example described earlier).

Figure (a) is the frequency spectrum of input current ia at constant switching frequency; Figure (b) is the frequency spectrum of input current ia when the switching frequency is variable.

From this figure, it can be seen that the frequency spectrum harmonic content of input current ia is large when the switching frequency is constant, and it is concentrated around the switching frequency fs and its doubling frequency; When the switching frequency is variable, the frequency spectrum harmonic content of input current ia is small, and the harmonic extends backward along the frequency axis (horizontal axis).

The switching frequency fs variable control method with constant ton and variable toff (switching off time) has the same effect on reducing the harmonic distortion rate THDi of input current as the introduced harmonic injection PWM control method (injecting 6th harmonic, changing the switching frequency of duty cycle d, and variable ton).

For more information, please click here