When the converter operates in burst mode, during the off period of the secondary side synchronous rectifiers, there is large parasitic ringing (DCM ring) caused by the transformer magnetizing inductance and the switch node capacitance. During the first few ringing cycles of the off period, there is a good chance that the SR MOSFET drain voltage will resonate below the SR controller turn-on threshold. The SR MOSFET could be falsely turned on at these instances, which could introduce extra power loss and EMI noise.

In UCC24624, a fixed 650-ns off-time blanking period is implemented. After the SR is turned off, and after its drain voltage rises above 1.5 V, the SR won't turn on again for at least the off-time blanking time, regardless of its drain to source voltage. Additional adaptive turn-on delay is also implemented to further enhance the noise immunity capability during burst mode operation.