The dead-time between PWMH falling edge and PWML rising edge (t_Z) serves as the wait time for V_SW transition from its high level down to the target ZVS point. Since the optimal t_Z varies with V_BULK, the internal dead-time optimizer automatically extends t_Z as V_BULK is less than the highest voltage of the input bulk capacitor (V_BULK(MAX)). The circulating energy for ZVS can be further reduced, obtaining higher efficiency at low line versus a fixed dead-time over a wide line voltage range. A resistor on the RTZ pin (R_RTZ) programs the minimum t_Z (t_Z(MIN)) at V_BULK(MAX), which is the sum of the propagation delay of the high-side driver (t_D(DR)) and the minimum resonant transition time of V_SW falling edge (t_LC(MIN)).

Equation 9.

where K_RTZ is equal to 11.2×10¹¹ (unit: F^-1) for V_SET = 0 V, and 5.6×10¹¹(unit: F^-1) for V_SET = 5 V.

Figure 8-14 RTZ Setting for the Falling-edge Transition of V_SW

As illustrated in Figure 8-14, when PWMH turns off Q_H after t_D(DR) delay, the negative magnetizing current (i_M-) becomes an initial condition of the resonant tank formed by magnetizing inductance (L_M) and the switch-node capacitance (C_SW). C_SW is the total capacitive loading on the switch-node, including all junction capacitance (C_OSS) of switching devices, stray capacitance of the boot-strap diode, intra-winding capacitance of the transformer, the snubber capacitor, and parasitic capacitance of the PCB traces between switch-node and ground. Unlike a conventional valley-switching flyback converter, the resonance of an active clamp flyback converter at high line does not begin at the peak of the sinusoidal trajectory. The transition time of V_SW takes less than half of the resonant period. The following t_LC(MIN) expression quantifies the transition time for R_RTZ calculation, where an arccosine term represents the initial angle at the beginning of resonance. As an example, the value of π minus the arccosine term at V_BULK(MAX) of 375 V, V_O of 20 V, and N_PS of 5 is around 0.585π, which is close to one quarter of the resonant period.

Equation 10.