Datasheet
ZCD
CT C
R
ZCD
CT C
R1
R2
UCC28063
www.ti.com
SLUSAO7 –SEPTEMBER 2011
Distortion Reduction
Due to the parasitic resonance between the drain-source capacitance of the switching MOSFET and the boost
inductor, conventional transition-mode PFC circuits may not be able to absorb power from the input line when the
input voltage is near zero. This limitation increases total harmonic distortion as a result of ac-line current
waveform distortion in the form of flat spots. To help reduce line-current distortion, the UCC28063 increases
switching MOSFET on-time when the input voltage is near 0 V to improve the power absorption capability and
compensate for this effect.
Figure 12 in the Typical Characteristics section shows the increase in on-time with respect to VINAC voltage.
Excessive filtering of the VINAC signal will nullify this function.
Zero-Current Detection and Valley Switching
In transition-mode PFC circuits, the MOSFET turns on when the boost inductor current reaches zero. Because of
the resonance between the boost inductor and the parasitic capacitance at the MOSFET drain node, part of the
energy stored in the MOSFET junction capacitor can be recovered, reducing switching losses. Furthermore,
when the rectified input voltage is less than half of the output voltage, all the energy stored in the MOSFET
junction capacitor can be recovered and zero-voltage switching (ZVS) can be realized. By adding an appropriate
delay, the MOSFET can be turned on at the valley of its resonating drain voltage (valley-switching). In this way,
the energy recovery can be maximized and switching loss is minimized.
The optimal time delay is generally derived empirically, but a good starting point is a value equal to 25% of the
resonant period of the drain circuit. The delay can be realized by a simple RC filter, as shown in Figure 25, but
the delay time increases slightly as the input voltage nears the output voltage. Because the ZCD pin is internally
clamped, a more accurate delay can also be realized by using the circuit shown in Figure 26.
Figure 25. Simple RC Delay Circuit
Figure 26. More Accurate Time Delay Circuit
Copyright © 2011, Texas Instruments Incorporated Submit Documentation Feedback 21
Product Folder Link(s): UCC28063