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UCC28730: UCC28730-Valley Switching

Part Number: UCC28730

Hi. I was using UCC28730 PSR flyback controller and I am happened to see that UCC28730 is using valley switching. Depending on my knowledge a DCM flyback converter has zero current at the end of energy transfer period in which the power switch is off. After all the energy transferred to the secondary some time passes until the primary mosfet is switched again and during that time secondary current reaches zero. Even though the secondary current is zero when the mosfet is switched, why is valley switching needed or chosen in this controller? Did the designers think that there are some switching losses ? Can you please explain the reason why valley switching is used? And lastly, if there is soft switching purpose here, where is that energy spent ? Thanks from now. 

Best regards.