TIDA-01623: The difference between active flyback and QR flyback

Hi xia

the spike on Vds caused by leakage inductance will be absorbed in clamp capacitor in ACF, and and will be recycled to secondary eventually. so the spike will not be seen on Vds compared with QR flyback, 

for ZVS, since QR will turn on at the vally of natural ring at Vds, so it might not be able to achieve ZVS at certain conditions, such as High line input, but for ACF, since the bidirectional switch on high side implemented, it will provide a path to generate a negative magnetizing current to discharge Vds Coss before main low side switch on, thus ZVS can be achieved. 

BR

Max

Hi, Mr Wang,

    Thanks for your reply, but I still have questions.

    The QRF also has a RCD snubber. When the low side switch turns off, some energies will be absorbed by the clamp capacitor, and some energies will go to the junction capacitor of the low side switch and produce the spike. I saw that the ACF also has the same structure, but the waveform of VSW is very clean. 

    In the QRF waveforms, the current of the magnetic inductor also has a negative part, but it still can't achieve ZVS.

Xia