## Materials and Methods
### Three-Point Fuzzy MPPT Algorithm of Variable Step Size
First, according to the data provided by the manufacturer, the initial maximum power point voltage is set at 80% of the open-circuit voltage, and the disturbance reduction value is increased. After collecting and comparing the changed power value, the disturbance direction is determined by the value of Flag_3P. To increase the threshold of the power amplification of the PV array, we used a variable step size control. Detecting \(\Delta\)P between any two points of A, B, and C, if the illumination intensity changes considerably, assuming that the amplitude exceeds \(\Delta\)P\({}_{L}\), \(\Delta\)P > \(\Delta\)P\({}_{L}\), the working point will not be changed, which not only avoids the mismatch between the front and back output power caused by an excessive change of the front output power, but also ensures the smooth operation of the system without blind oscillations. When the illumination intensity is stable and the \(\Delta\)P between the two points is large (but not \(\Delta\)P\({}_{L}\)), it can be considered that the current working point is far from the maximum power point on both sides of the power curve, and the maximum power point tracking can be carried out with a larger step to speed up the tracking speed. Assuming that the threshold of the power difference exceeds \(\Delta\)P\({}_{M}\), then, \(\Delta\)P\({}_{M}<\Delta\)P < \(\Delta\)P\({}_{L}\). When the power difference between the two points  
Figure 6: Judgment diagram of three point working mode. (**a**) Pattern 1; (**b**) Pattern 2; (**c**) Pattern 3.  
is small, it is considered that the current working point is near the maximum power point, and is disturbed by a small step disturbance. Assuming that the threshold of the power difference is \(\Delta\)P\({}_{\text{S}}\), then, \(\Delta\)P\({}_{\text{S}}\) < \(\Delta\)P < \(\Delta\)PM. On the basis of the above three-point variable step-size perturbation and observation method to determine the direction of disturbance, the duty cycle D is determined using the fuzzy MPPT control algorithm by comparing the detection power, thus driving the boost switch in order to achieve the efficient and fast tracking of the maximum power point of the solar panel. The control block diagram based on the fuzzy MPPT algorithm is shown in Figure 7.  
Figure 8 shows the simulation waveform of the output power of the photovoltaic array. The illumination intensity is 3000 W/m\({}^{2}\). It can be observed from the figure that the fuzzy control model can track the maximum power within 0.03 s, and the oscillation at the maximum power point is small. The local magnification of the model is shown in Figure 9. The power fluctuation is 2 W and the fluctuation coefficient is 0.1%. The large oscillation before the maximum power point indicates that a large step tracking is being carried out--the tracking speed is faster. It is obvious that the tracking speed and accuracy of the three-point variable step size fuzzy MPPT control algorithm are very high. In practice, the illumination was reduced from 3000 to 2000 W/m\({}^{2}\) at 0.1 s, and was increased from 2000 to 2500 W/m\({}^{2}\) at 0.2 s, to simulate the tracking effect under a change in the illumination intensity. The tracking effect was observed as shown in Figure 10. The results show that the fuzzy MPPT can track the new maximum power point quickly under disturbance. Figure 8: Power output curve of PV panel. Figure 7: Control block diagram based on fuzzy MPPT algorithm. Figure 9: Partial tracking of MPPT tracking power.