Current Waveforms of CCM and DCM Flyback Converters

From the basic working principle of the flyback converter, we can know that: during the period ton when the main switch tube is turned on, the transformer stores energy; during the period toff when the main switch tube is turned off, the transformer releases magnetic energy and outputs it to the load. When a switching cycle Ts ends, if the energy stored in the transformer is not completely released to zero, the flyback converter can be considered to be operating in CCM mode. Its characteristic is that current flows alternately through the primary winding and secondary winding of the transformer during a switching cycle. Conversely, at the end of the switching cycle, the energy stored in the transformer is completely released, and the flyback converter operates in DCM mode. Its characteristic is that during toff, there is a period of time when the input and output currents of the converter are both zero.   　　When the CCM flyback converter operates in CCM and DCM, the waveforms of the switching current ir and the load current ir are shown in Figures (a) and (b), respectively. If the peak value of iv is Ip, then the peak value of is I＇p, 
             

    

　　Where n is the turns ratio of the primary and secondary windings of the transformer.
 

　　Figure 1 Input and output current waveforms of a flyback converter
 

　　Figure 1 shows the steady-state current waveform of a flyback converter (note: not a flyback PFC converter), so the current peak is constant. In fact, for a flyback PFC converter, in order to achieve a power factor close to 1, the current peak of each switching cycle changes with the input voltage μ waveform.

　　The input and output current waveforms of the CCM flyback converter are close to rectangles as shown in Figure 1 (a), but the amplitude (wave amplitude) rises or falls at a certain slope. The input and output current waveforms of the DCM flyback converter are triangular waves as shown in Figure 1 (b). Moreover, during t∈［（Du＋Du2）Ts,Ts］, the load current and the switch current are both zero. The duty cycle Du＝ton/Ts, DU2TS is the conduction duration of the output diode during the toff time.