SLUSD12A October 2017 – February 2018 UCC28780
PRODUCTION DATA.
Refer to the PDF data sheet for device specific package drawings
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 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)}).
where K_{TZ} is equal to 11.2×10^{11} (unit: F^{-1}) for V_{SET }= 0 V, and 5.6×10^{11}(unit: F^{-1}) for V_{SET }= 5 V. As illustrated in Figure 18, after 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 resonance 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 resonance beginning. 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 resonance period.