CURRENT ISSUE
Contests
Test your eq
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Issue #214 May 2008
Problem 1—The following figure shows the essential features of a modern power-factor controller, which is a specialized form of boost regulator.
What sort of current waveform would you expect to see in the inductor? How does this relate to the input line current?
Answer
Problem 2—Why is there a low-pass filter in the error feedback path? What should its cutoff frequency be?
Answer
Problem 3—Assuming a constant-current load for simplicity, describe the voltage and current waveforms in the output capacitor C2 in qualitative terms. How do they change, if you have a constant-power load, such as a secondary switching regulator?
Bonus Question—Note that with a 40-W load, the 120-VAC line is going to be supplying an RMS current of 300 mA, which has an average value of 270 mA. How do you account for the difference between this number and the 100-mA average current that the load sees? Answer
Problem 4—Getting back to the low-pass filter in the voltage feedback loop, what effect does this have on the output impedance of the PFC circuit, as seen by the load? Answer
Problem 5—What is the start-up transient for this circuit like, and what are the worst-case conditions for startup? Answer
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