SLUSCM5A August 2017 – February 2018 UCC24612
Right after SR turn-on, for some topologies, such as Flyback, the SR starts to conduct with its maximum current. Due to the parasitic ringing, SR sensed voltage drop may trip the turn-off threshold and prematurely turn off the SR. This is largely caused by the ringing due to the package inductance of the SR MOSFET. The ringing voltage can be managed through appropriate snubbing and use of low package inductance MOSFETs. To further improve the noise immunity, UCC24612-1 blanks the turn-off comparator with a fixed 360-ns (540 ns for UCC24612-2) minimum on-time timer. The SR needs to conduct a minimum of 360 ns (540 ns for UCC24612-2) regardless of its turn-off comparator state. The minimum on-time is short enough to allow the UCC24612 to be used at up to 1 MHz switching frequency (1 MHz for UCC24612-1 and 800kHz for UCC24612-2), while still maintaining good noise immunity. Due to the different applications, the minimum on-time is set up differently for UCC24612-1 and UCC24612-2. For UCC24612-1, the minimum on-time is 360 ns and for UCC24612-2, the minimum on-time is 540 ns.