Re: [SI-LIST] : Bulk Caps -> Inductor? -> Plane

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From: [email protected]
Date: Tue Apr 11 2000 - 14:35:12 PDT


It's not uncommon to see a post filter topology used in power supplies. In
fact, the most recent application notes from Power Integrations incorporate
post filters. They are essentially part of a low frequency (relative to the
SMPS frequency) output Pi filter. Any feedback that is used for output
regulation is generally taken upstream of the post filter to avoid the phase
lag the extra L & C would introduce. A typical post filter would use a 2.2
UH choke and a 220 UF capacitor. For your design I'm guessing, as the final
capacitor(s) to which the inductors would be connected are not shown.

For the circuit you illustrated, all outputs are provided with an inductor;
however, the inductor values are not present. Another option is that they
are simply provisions for ferrite beads (or surface-mount beads) to prevent
switching transient spikes from the SMPS reaching the downstream circuits
while not measurably impacting the normal DC delivery. If the supply outputs
are feeding other circuit boards via a distribution network, local capacitor
decoupling on those remote boards would be necessary for containment of high
frequencies anyway.


Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

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