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Issue #214 May 2008
Problem 5 —What is the start-up transient for this circuit like, and what are the worst-case conditions for startup?
Answer 5 —Regardless of anything that the controller might or might not do, the AC line will charge the output capacitor C2 to the peak line voltage on the first half-cycle. There’s a direct path through the diodes and the coil that permits this to happen.
The worst case occurs when the supply is switched on at the peak of the input waveform with a discharged tank capacitor. The charging will occur with a quick burst of current that will also store significant energy in the inductor, potentially causing a voltage overshoot.
How bad is this? Well, based solely on the relative energy storage capacities of the coil versus the capacitor (the latter should be 100× or more times the former), there really isn’t much risk here. The overshoot should only be a few percent of the peak line voltage at most. Also, saturation of the inductor itself will help limit these effects as well.
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