UCC28780: UCC28780 control method

Hello Eleven,

Thank you for your interest in the UCC28780 ACF controller. I’ll try to answer your questions as best as I can, but I cannot release any proprietary details. Ifb controls the level of current sense threshold (Vcst) using different methods, depending on which mode the IC is operating in. Vcst determines the peak primary current of each switching cycle.
Ifb can range from 0uA at maximum output power to ~75uA at zero output power (stand-by).

1. In AAM, the peak current reference is a decreasing linear function of Ifb. When Ifb = 0uA, Vcst is clamp at 0.8V. As Ifb increases, Vcst decreases linearly to the level established by the voltage on the BUR input, Vbur. The threshold level where burst begins is Vcst(bur) = Vbur/4. At this transition point in to ABM, Ifb increases by ~20uA and begins to have a high ripple current content superimposed on it.
2. In ABM, the Vcst remains fixed at Vcst(bur) and the average level of Ifb remains relatively fixed, but the ripple content (called delta-Ifb) decreases as load decreases. An internal reference current called Iref is established to provide a threshold against which the ripple of Ifb is compared (as shown in Figure 41). This comparison determines when to enable the RUN signal and produce a burst of switching cycles. The number of cycles within a burst is adjusted to maintain a switching frequency between 25kHz and 34kHz.
3. In LPM, the peak current reference is once again a decreasing linear function of Ifb while the switching frequency is kept fixed at 25kHz. When the cycles in a burst is at the minimum number of 2 and decreasing load forces Ifb to increase further, the UCC28780 controller reduces the peak primary current amplitude as necessary to maintain regulation. Vcst decreases as Ifb increases until it is clamped at Vcst(min). If load decreases further, operation must enter the SBP mode.
4. In SBP, Ifb modulates an internal signal that controls the period of the burst frequency. Here, the peak primary current is at minimum level and the burst frequency is allowed to decrease below 25kHz through the audible frequency range as Ifb continues to increase. However, with minimal peak current, audible noise can be avoided. For Ifb > 75uA, there is no longer any effect.

I hope these answers are sufficient to help you understand the control methods of the UCC28780.

Regards,
Ulrich

Hello Eleven,

I'm sorry for my delayed reply.

For your new questions:
1. The higher ripple content occurs in ABM and this ripple is at the burst frequency, not the switching frequency. In AAM, the switching is continuous and the ripple content of Ifb is attenuated by the lower bandwidth of the feedback path. In ABM, the gaps between bursts create a low-frequency ripple that has a significant amplitude. This ripple is used for regulation during ABM.
2. I can't reveal how Iref is established; this is proprietary information to TI.
3. When in SBP mode, once Ifb becomes about 75uA the burst frequency is so low that VDD may droop to UVLO point before another burst is generated. However, "Survival mode" will intervene and generate sufficient pulses to keep VDD above 11V. Increasing Ifb beyond 75uA will not change this situation. Survival mode will dominate. If the output load is too low, Vout will begin climbing toward OVP, so a preload may be necessary to prevent this. The preload may be enough to avoid the Survival mode operation. Closed-loop feedback usually will not result in Ifb > 75uA, but if Ifb is forced higher, then open-loop operation will result.

Regards,
Ulrich