A specific control waveform in the variable frequency DCM is shown in FIG. 12. The dimming processing module 701 fixes the peak current reference Icomp to be a peak reference corresponding to the first current threshold Ith1, the switching time calculation module 704 calculates the switching period Ts according to the input voltage, the output voltage, the dimming current signal Idim and the peak current reference (i.e., the ripple current reference Ipp). The timer module 703 sets the maximum counter value according to the switching period Ts. The switching time calculation module 704 calculates the on time Ton of the switch according to the input voltage, the output voltage, and the peak current reference (i.e., the ripple current reference Ipp). The second compare value of the timer module 703 is set according to the off time Toff of the switch, i.e., by subtracting the on time Ton from the fixed switching period Ts. As shown in FIG. 12, when the counter value is lower than the second compare value, an output of the timer module 703 is in a low level, and when the counter value is higher than the second compare value, the output is in a high level. Moreover, when the inductor current is equal to the peak current reference Icomp, the timer module 703 is cleared, and the output is changed to a low level, and the timer module 703 starts counting again from zero to control the next cycle. The whole control cycle repeats. Accordingly, the control signal formed of a plurality of pulses is output, such that the power conversion unit 10 operates in a discontinuous conduction mode. A specific waveform of the inductor current is shown in FIG. 12.
