The Dead Time describes the time interval between an outgoing LLC MOSFET turning off and the incoming LLC MOSFET turning on. Dead time control is critical for LLC operation. A certain amount of dead time is required to prevent shoot through. During the dead time, the HS node slews from one input rail to the other due to the inductive resonant current. In order to achieve zero voltage switching (ZVS) turn on, the dead time needs to be long enough for the resonant current to fully charge or discharge the HS node. But after the body diode starts conducting, the MOSFET should be turned on quickly. Too long of a turn on delay can result in reverse resonant current and lead to the loss of ZVS. Also, the voltage drop on the body diode is higher than that on the MOSFET channel. Optimized dead time can help to minimize the power loss.
The resonant current flowing through the HS node during the dead time depends on the LLC resonant tank design and varies by operating frequency and output/input voltage ratio. Therefore, the optimized dead time varies widely with LLC operating conditions. UCC25640x includes an adaptive dead time control to automatically find the optimized dead time across the entire operating range. It detects the change of slew rate of the HS node voltage. During a switching transition, the slew rate rises up first and then drops back to zero. A slew rate detector is used to detect the moment when the slew rate drops below a pre-defined threshold. A slew done event is only detected when the slew rate during dead time crosses the threshold and then drops back below the threshold. If the slew rate is lower than the threshold (i.e. minimal detectable slew rate) during the whole dead time period, no slew done will be detected. This is to prevent the mis-detection due to noise on the HS node voltage. If slew done is not detected, maximum dead time is used.
Because of the natural symmetric operation of LLC, only the dead time between high-side MOSFET turn off and low-side MOSFET turn on is determined by the slew rate detector. This dead time is copied and then applied to the dead time between low-side MOSFET turn off and high-side MOSFET turn on.