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Post-switching supply linear regulators

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Speakerguy

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Hey guys,

General question here. I was watching an Analog Devices web seminar the other day and they talked about power supply filtering of a switching power supply followed by a standard linear regulator (LM7805). They mentioned that a choke in series with the voltage output of the 7805 (after the output capacitors) helped dramatically in their application.

Question: how do I go about selecting the value of the choke, and do I want to put any bulk electrolytics after it (pi filter)? The ripple rejection of linear regs seems to be pretty poor above 100KHz which is where most switchers operate.

Thanks!
 
Wouldn't you want a choke before your capacitor? THat extra inductance directly on the output of the regulator would filter noise...as well as any attempts to provide transient current! So yeah, I'd never place the inductor without having a capacitor after it. Whether or not there is a cap before the inductor (PI) is a different issue (ie. how worried am I? does the linear regulator need an output cap for stability?)

I'm not too technical about selecting chokes, but I find the fundamental frequency caused by the switcher. Then i decide how many odd harmonics I want to worry about (a total guesstimation 20? 50?100? 1000?). I sometimes look on wikipedia and math sites to see what a square wave approximation looks like with X harmonics and pick the number of harmonics where the square wave is "pretty much a square wave" in my mind without going too overboard (ie. that looks exactly like a square wave and that's how many harmonics I am going to worry about).

I then look at the capacitors I have picked- size and type. Then I go hunting around for impedance charts of similar capacitors to try and find the typical harmonic frequency for that size, type, and package of capacitor to find out roughly what frequencies it is best at. I adjust the size and repeat the process if I am not happy.

Then I start choosing the choke to either catch the frequencies that aren't caught by the caps, or I choose them to augment those that the cap is already working on. I do this by comparing the impedance charts of the inductors in the same series of products- that way you can more directly compare the tradeoff between effective frequency range and impedance over the effective frequency range of the inductor. Ideally you'd pick it soley based on the impedance, but more often than not you are limited by the actual effective frequency range of the inductor (ie. I settle for less impedance in exchange for actually covering the frequencies of interest since I can't find an inductor that has the current capacity, frqeuency range, and super high impedance in that range that I would prefer).
 
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I have no idea what you just said. Are you talking about using a choke before the linear regulator rather than after? I see no reason why that wouldn't work...but remember to account for the dropout voltage of the linear regulator.

Or you can also use an adjustable regulator and apply the feedback connections to after the inductor.
 
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