ZCS protection is a necessary function for LLC converters to avoid crossing over into the capacitive region of operation. In the capacitive region, the MOSFETs experience severe reverse recovery which can lead to damage to the LLC power stage. In addition, the gain vs frequency relationship inverts in the capacitive region and can cause the converter to completely lose regulation of the power stage.

The goal of the ZCS protection is to make sure that the MOSFET can be turned off before the current inverts thereby eliminating possibility of a hard reverse recovery of the MOSFET's body diode. This can increase the reliability of the power stage. The minimum turn off current is set at a threshold which can increase the chances of achieving ZVS or close to ZVS switching for switches under this condition.

Couple with dead time engine which looks at both the slew done signal and the IPOL signal, we can make sure that opposite MOSFET turns-on at the valley point of the Vds voltage, providing lower turn-on losses.

When operation nears the inductive/capacitive boundary, the resonant current decreases before the gate turn is turned off. If the ISNS waveform is less than the V_{ISNS_ZCS} threshold, the gate pulse is terminated early instead of waiting for the VCR waveform to cross the VTH or VTL boundary. This early termination scheme is capable of leaving enough resonant current at the gate turn-off edge to drive the ZVS transition during the dead-time.

The ISNS signal is fed to two ZCS comparators. As there is a dedicated comparator for when the HO is on and for when the LO is on, the other comparator output is ignored.

The shape of the resonant current well below the resonant frequency poses some challenge for detecting the correct falling edge of the resonant current waveform. The UCC25660x implements additional logic to make sure that the correct falling edge of the ISNS signal is detected to avoid false tripping.

To improve robustness against noise, the ISNS ZCS comparators are blanked at the rising edge of HO or LO gate. The same blanking time t_lebis used for both the VCR comparators and the ISNS ZCS comparators.

When a ZCS event is detected, internally the Soft Start ramp voltage is slowly reduced. When the internal soft start ramps down, the switching frequency is also forced to increase further, forcing the system out of Capacitive region.

In the event of a persistent ZCS condition, the controller ceases switching action and move to the fault state. A configurable time-out is used to declare a persistent ZCS fault condition.