In a real circuit, the transformer has leakage inductance and other parasitic inductance or capacitance as shown in Figure 3-2, which can affect the secondary current waveform.

Figure 3-2 Fly-Buck Converter Circuit Considering Parasitic

The Figure 3-3 shows the typical current waveform under different levels of leakage inductance(L_LK).

When L_LK is low, the i_sec ramps up quickly to charge up C_OUT2. With larger L_LK, i_sec rises linearly,resulting in larger negative peak current for i_pri. If the negative peak current of i_pri reaches the negative current limit of the device, the LS will be turned off and the charging to C_OUT2 will be terminated. Consequently this would result in less energy being transferred to the output and produce lower output voltage.

Therefore, the leakage inductance should be minimized and the maximum duty cycle must be chosen carefully to mitigate these issues. When the secondary output has no load, the turn on of LS can force a small current in the secondary side, and it would gradually charge up C_OUT2. Since there is no load to discharge C_OUT2, a net charge will be accumulated on C_OUT2 and raise the V_OUT2 remarkably. In order to prevent this from happening, a preload must be added to the secondary output to help removing the net charge on C_OUT2 so as to maintain the output voltage at the setting point.

Figure 3-3 Current Waveforms Affected by Leakage Inductance