Datasheet

Over-Current Protection
Phase Fail Protection
Distortion Reduction
UCC28061
www.ti.com .............................................................................................................................................................. SLUS837A – JUNE 2008 – REVISED JULY 2009
Under certain conditions (such as inrush, brownout recovery, and output overload), 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 the MOSFET source to sense the
converter current, 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 requires
repeated turn-ons of the MOSFETs during over-current conditions. As a result, the converter may temporarily
operate in continuous current mode (CCM) and experience failures induced by excessive reverse recovery
currents in the boost diode.
The UCC28061 uses a single resistor to continuously sense the total inductor (input) current. This way, turn-on of
the MOSFETs is completely avoided when the inductor currents are excessive. The 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).
Following an over-current condition, both MOSFETs are turned on in phase when the input current drops to near
0. Because two phase currents are temporarily operating in phase, set the over-current protection threshold to
more than twice of each phase maximum current ripple value in order to allow a return to normal operation after
an over-current event.
The UCC28061 detects failure of one phase by monitoring the sequence of ZCD pulses. During normal
two-phase operation, if one ZCD input remains idle for longer than approximately 14 ms while the other ZCD
input switches normally, PWMCNTL goes high, indicating that the power stage is not operating correctly. During
normal single-phase operation, phase failure is not monitored. On the UCC28061, phase failure is not monitored
if COMP is below approximately 222 mV.
Because of the resonance between the capacitance present across the drain-source of the switching MOSFET
and the boost inductor, conventional transition mode power factor correction circuits may not be able to absorb
power from the input line when the input voltage is around 0 V. This limitation results in waveform distortion and
increased harmonic distortion. To reduce line current distortion to the lowest possible level, the UCC28061
increases switching MOSFET on-time when input voltage is around 0 V to increase the power absorption and
compensate for this effect.
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