# What's the LC filter arrangement on dual output DC/DC converters?

It's pretty hard to find a filter arrangement for a dual output converter, but I came across the following:-

Taken from here.

So for the bottom example, is the LC arrangement correct? I always thought that a -ve voltage flowed into the 0V terminal. That would imply that the lower filter should be the other way around, with the capacitor put adjacent to the converter, and the inductor away from it. That way the current flows firstly though the inductor and then past the capacitor, as it does in the +ve configuration above the 0V rail.

Am I wrong?

• @DKNguyen Well, an appropriate response might be to say whether the posted schematic is correct or not... – Paul Uszak Oct 15 at 22:14
• Your description of how you think things should work is so wrong that I don't know how to correct you because I'm unable to pinpoint the misconceptions to point them out. The "voltage flowing past the capacitor after the inductor" part makes particularly little sense. All I can really say is...you need to trace the CURRENT loops, voltage does not flow, and that nothing (not current, and definitely not voltage) flows "past" the capacitor before the inductor since the capacitor has 2 terminals and unless you are working with transmission lines, everything starts flow everywhere at the same time – DKNguyen Oct 15 at 22:14
• Point taken: That schematic is correct. The decoupling capacitors are closest to the load so that no inductor is in the way to slow down its ability to respond to the load's transient current demands. The inductor between the capacitor/load and converter helps prevent high frequency currents in the converter from reaching and contaminating the load. – DKNguyen Oct 15 at 22:27
• @DKNguyen Err, where are the decoupling capacitors? Isn't the LC circuit a single unit with a resonance at $\frac{1}{\sqrt{LC}}$? And as a low pass circuit, doesn't the inductor have to come before the capacitor? I've never seen them the other way around. Another misconception :-) ? – Paul Uszak Oct 17 at 21:39
• I am not sure what you mean by "doesn't the inductor have to become before the capacitor". Yes it does and still does in what I described and in your schematic. Unless you think that I mean the capacitor is decoupling the DC-DC converter. It's not. You decouple loads, not sources (unless it's a source/regulator that acts as the load for another source) Therefore, I am referring to whatever the converter is powering. They are both filter caps and bulk decoupling caps. – DKNguyen Oct 17 at 21:53