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Issue #214 May 2008
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 4 —Below the cutoff frequency of the filter, the PFC circuit behaves as a voltage source, with a correspondingly low output impedance. However, at frequencies above the cutoff frequency, it behaves more and more like a current source, with an output impedance that rises in frequency. This means that the PFC by itself is not going to provide good voltage regulation to highly-dynamic loads.
This can be mitigated by making C2 larger—its impedance drops with increasing frequency, compensating for the rising impedance of the PFC. Also, a secondary regulator can be used to provide tight voltage regulation to the load, in which case the voltage at the output of the PFC can be allowed to vary (within limits).
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