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Power-Factor-Slide23

Let’s review the calculated requirements for average, RMS, and peak diode current stress at some common power levels for PFC. The nominal worst case operating condition is specified as 85 VAC, nominal efficiency target at low line of 88%, and design for full load ripple current of 30% of average peak. There is a large difference between average/rms levels, and peak current levels but because the forward voltage and dissipation is strongly dependent on the peak current levels and nature of crest to average and duty cycle, this peak level is of considerable concern. Though average current even at low line is not that high, peak current exhibits a crest factor of > 7:1 over average, and 6:1 for RMS current. Dynamic properties will be determined by behavior at peak current levels. The thermal stability of forward losses and switching losses can become a serious issue.

PTM Published on: 2011-09-02