The zero-voltage switching (ZVS) converters are widely used to improve the power converter’s efficiency. However, in those soft-switching topologies like LLC and triangular current mode (TCM) totem pole PFC, the device can lose ZVS depending on the load condition, inductor, magnetic parameters and control techniques, which affects the system efficiency. To insure ZVS, certain design margins or additional circuits are needed which sacrifices the converter performance and adds components.

To simplify the system design for soft-switching converters, the LMG3656R025 part integrates a zero-voltage detection (ZVD) circuit that provides a digital feedback signal to indicate if the device has achieved ZVS in the current switching cycle. The circuit diagram is shown in Circuit Diagram for Zero-Voltage Detection Circuit Block Diagram. When the IN pin signal goes high, the logic circuit checks if the device V_DS has reached below -1V to determine whether the device achieves zero voltage switching in the switching cycle. Once a ZVS is identified, a pulse-output with a width of T_{WD_ZVD} is sent out from the ZVD pin, after a delay time of T_{DL_ZVD} as indicated in ZVD Timing Specifications. Note that a certain third quadrant conduction time is required to allow the device detecting a zero-voltage switching, and T_{3rd_ZVD} is a function of the gate driver strength.

ZVD Function in a CCM Buck Converter shows the waveforms of the ZVD pin corresponding to a continuous conduction mode buck converter. These waveforms demonstrate how ZVD function works in both hard-switching and soft-switching conditions. For I_L in the waveforms in ZVD Function in a CCM Buck Converter load current going out of the switch node is positive. In CCM buck operation, the high-side device is the hard-switching device while the low-side device can achieve zero-voltage switching with a proper dead-time settings. In the first switching cycle when low-side device IN pin rises, the switch-node voltage V_DS drops below zero and stays in third quadrant conduction for a period of T₁. Since this third quadrant conduction time T₁ is larger than the detection time T_{3rd_ZVD} specified in Electrical Characteristics, a zero-voltage transition is identified and the ZVD pin outputs a pulse signal. The pulse width of the ZVD pulse is also defined in the electrical characteristic table as T_WD. In the second switching cycle, the device is turned on earlier, and the third quadrant conduction time T₂ is less than T_{3rd_ZVD}. Since T₂ is less than T_{3rd_ZVD}, the ZVD signal stays low, even though the device achieves ZVS. In the third switching cycle, the IN pin signal is advanced even earlier, and the device is in partial hard-switching. Accordingly, the ZVD output stays low when a ZVS transition is not acheived. Note the high side ZVD output stays low in this CCM buck operation as the high side device is always hard-switching turn-on.

The ZVD function facilitates the control in soft-switching topology. ZVD Function in a TCM TP PFC Converter illustrate the facilitation with the ZVD waveforms in a TCM totem pole PFC. This diagram shows the positive half line-cycle with V_IN greater than half of V_OUT. For I_L in the waveforms in ZVD Function in a TCM TP PFC Converter load current going into the switch node is defined as positive. In the first switching cycle, the load current builds enough negative current, and the low-side device achieves ZVS with a clear third quadrant conduction time beyond T_{3rd_DET}. Therefore, the ZVD pin outputs a pulse signal. The ZVD pulses are missing in the next two switching cycles because the third quadrant conduction time shortens in second cycle and the device loses ZVS in the third cycle.