Referring to FIG. 2, when switching from a light load to a heavy load, it can be seen from the figure that the LTH mode is adopted when the output voltage is less than Vomin. If PID adjustment is adopted, which is as shown in the thick dotted line, the voltage will drop still after the output voltage drops to Vomin, and the dynamic recovery time is also very long. When adopting the LTH mode, the LTH mode is immediately adopted when the output voltage is less than Vomin. Because the energy of the mode is greater than the energy during full load generally, the output voltage starts rising immediately and will not drop any longer; before the output voltage rises to a stable value, this is the fast dynamic method; when the output voltage is the same as the stable voltage, the size of the output load can be obtained through the slope, so that the energy of the working mode jumping out of the LTH mode can be close to the power consumption of the load, to eliminate subsequent resonance introduced by unmatched energy, which is as shown in the full line; it can be seen that after jumping out of the LTH mode, if the working state starts from full load, the input energy thereof is too large, which introduces voltage resonance, as shown in the fine dotted line.