SLUSC60B December 2017 – October 2019 UCC28064A
Under certain conditions (such as inrush, brownout-recovery, and output over-load) the PFC power stage sees large currents. It is critical that the power devices be protected from switching during these conditions.
The conventional current-sensing method uses a shunt resistor in series with each MOSFET source leg to sense the converter currents, resulting in multiple ground points and high power dissipation. Furthermore, since no current information is available when the MOSFETs are off, the source-resistor current-sensing method results in repeated turn-on of the MOSFETs during overcurrent (OC) conditions. Consequently, the converter may temporarily operate in continuous conduction mode (CCM) and may experience failures induced by excessive reverse-recovery currents in the boost diodes or other abnormal stresses.
The UCC28064A uses a single resistor to continuously sense the combined total inductor (input) current. This way, turn-on of the MOSFETs is completely avoided when the inductor currents are excessive. The gate drive to the MOSFETs is inhibited until total inductor current drops to near zero, precluding reverse-recovery-induced failures (these failures are most likely to occur when the AC-line recovers from a brownout condition).
The nominal OC threshold voltage during two-phase operation is -200 mV, which helps minimize losses. This threshold is automatically reduced to -166 mV during single-phase operation, either by detection of a phase failure or because COMP is below PHB.
An OC condition immediately turns off both gate-drive outputs, but does not trigger a soft-start and does not modify the error amplifier operation. The overcurrent condition is cleared when the total inductor current-sense voltage falls below the OC-clear threshold (–15 mV).
Following an overcurrent condition, both MOSFETs are turned on simultaneously once the input current drops to near zero. Because the two phase currents are temporarily operating in-phase, the current-sense resistance should be chosen so that OC protection is not triggered with twice the maximum current peak value of either phase to allow quick return to normal operation after an overcurrent event. Automatic phase-shift control will re-establish interleaving within a few switching cycles.