7.4.1 Adaptive ZVS Control with Auto-Tuning

Figure 19 shows the simplified block diagram explaining the ZVS control of UCC28780. A high-voltage sensing network provides the replica of the switch node voltage waveform (V_SW) with a limited “visible” voltage range that the SWS pin can handle. The ZVS discriminator identifies the ZVS condition and determines the adjustment direction for the on-time of PWMH (t_DM) by detecting if V_SW reaches a predetermined ZVS threshold, V_TH(SWS), within t_Z, where t_Z is the targeted zero voltage transition time of V_SW controlled by the PWMH-to-PWML dead-time optimizer.

In Figure 19, V_SW of the current switching cycle in the dashed line has not reached V_TH(SWS) after t_Z expires. The ZVS discriminator sends a TUNE signal to increase t_DM for the next switching cycle in the solid line, such that the negative magnetizing current (I_M-) can be increased to bring V_SW down to a lower level in the same t_Z. After a few switching cycles, the t_DM optimizer settles and locks into ZVS operation of the low-side switch (Q_L). In steady-state, there is a fine adjustment on t_DM, which is the least significant bit (LSB) of the ZVS tuning loop. This small change of t_DM in each switching cycle is too small to significantly move the ZVS condition away from the desired operating point. Figure 20 demonstrates how fast the ZVS control can lock into ZVS operation. Before the ZVS loop is settled, UCC28780 starts in a valley-switching mode as t_DM is not long enough to create sufficient I_M-. Within 15 switching cycles, the ZVS tuning loop settles and begins toggling t_DM with an LSB.

Figure 19. Block Diagram of Adaptive ZVS Control

Figure 20. Auto-Tuning Process of Adaptive ZVS Control