Datasheet

UCC29910A
www.ti.com SLUSAK8A – MAY 2011– REVISED JUNE 2011
The buck converter operates off a rectified sinusoid and there are periodic dead times when the input voltage is
lower than the output. During these times no power can be transferred to the output and the input current is
nominally zero. Figure 2 shows the line current, I
AC
, falling to zero when V
AC
is less than V
BULK
. The associated
conduction angle increases as the RMS line voltage increases and the current waveform changes from low line
to high line. The input current is skewed a little towards the beginning of the conduction cycle because V
BULK
is
at its lowest value at this time so conduction starts at a lower voltage than it finishes. This effect may be seen in
Figure 3 and Figure 4. These waveforms are taken from a 90-W buck PFC reference design, both meet the
harmonics requirements set out in EN61000-3-2 and their PF is greater than 90%.
Figure 3. 115 V, 60 Hz, Full Load, 0.5 A/div
Figure 4. 230 V, 50 Hz, Full Load, 0.5 A/div
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