Pi Filter for Power Supply output - Page 1
Forum section: electronics / beginners

author: BuzzB
author_rank: Contributor
timestamp: February 02, 2021, 05:57:50 pm
content: I've got a project where I have a 12V power source. The 12 volt supply splits... (a) powering a few motors through PWM driven mosfets, and (b) into a buck converter module that outputs 5 volts for my microcontroller which is sending the pwm to a couple of BJTs which are in turn controlling the mosfets.Ok, so when I am driving the motors I see PWM noise on the 5 volts coming from the buck converter module. I figure I can filter it. Through trial and error, I've found I can clean it up well will a simple LC filter between the output of the buck converter and the microcontroller.I thought that instead of guessing values I'd actually use an online calculator to figure out the best components for a Pi filter at a given cutoff frequency. Now here is the question...These filter calculators want to know the input and output impedance. I have no idea how to calculate that. What kind of impedance would the output of the buck converter want to see? What kind of input impedance does the microcontroller module present to the converter?Is it as simple as...Buck converter can output 5V and 3A. 5V/3A = 1.6 ohms?Microcontroller might consume 150ma and 5V. 5V/.150A = 33 ohms?... or does that have nothing to do with it?Thanks,Buzz

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 02, 2021, 06:44:03 pm
content: Close!When working with LC networks, you'll find these pop up a lot:Fo = 1 / (2 pi sqrt(L C))Zo = sqrt(L/C)In fact, this is all the calculator is really doing, starting from tabulated values that give the desired response (Butterworth, etc.) then scaling them proportionally for the desired cutoff and impedance.So what is the impedance? Well, it's the ratio of voltage to current, at a given frequency. For example, the ratio of output voltage (supply ripple) to load current.For this filter, the impedance peaks around Fc (the cutoff frequency -- a special case of Fo), and drops either side. At low frequencies, the series inductor has low impedance, so if the source impedance is low, so is the output impedance. At high frequencies, the output side capacitor has low impedance shunting the output.So we really only have to worry about frequencies near Fc.How near? And what is the impedance at and around Fc, is it exactly Zo, or something else?When properly terminated (source and load are resistors R = Zo), the resistance is, well, about that. You need to provide adequate termination, to get the expected frequency response.If source and load are weird impedances (very high or low), then a damping network should be provided. Use a shunting R+C, or a series R||L, on one or both sides.For PS filters, the most common ingredients are electrolytic capacitors. They have appreciable ESR, and are cheap capacitance. You can easily afford to just throw enough either side of an inductor to get a well damped network. For equal capacitors C with ESR, choose C >= 1 / (2 pi Zo Fc), ESR = Zo (within say a factor of 2 or so), and L = (ESR)^2 / C.By the way, what is an inductor?Just a piece of wire, right? Well, it's wrapped around a bit, but, how much does that really matter?None at all, as it happens. Semantically speaking. Wires still have stray inductance, in fact about 10nH/cm in typical sizes and applications.So, depending on how you've got everything wired up, given this knowledge, you may suddenly realize that, what you are measuring, isn't actually going to be addressed by the filter at all, but is between -- what you thought was -- ground.Or in other words: when is ground not ground? At AC, when it's made of wires, for sure.So, you may find simply adding an LC filter anywhere, has little effect. Or filters low frequency ripple, but you're left with persistent sharp squigglies that show up almost everywhere, even when probing with the scope probe shorted to its ground clip (and in turn touching circuit "ground"). Well, speaking of wires, what's that ground clip hanging off of? Oh... So, in this way you can probe common mode (often, ground-ground, but actually the generalization of that idea) noise and see what's, well, common with the scope (least noise) perhaps.You may also find it's more effective to filter the noise at its source. You can PWM filter to the motors, so the switching noise is confined to the transistors.Also, if you're using a cheap China module for 5V, you may find it's noisy for the same reasons. Best fix is probably just to put it on a ground plane, or inside a metal box, and filter the terminals at one common point, where they leave that plane/enclosure.Tim

author: BuzzB
author_rank: Contributor
timestamp: February 02, 2021, 07:31:43 pm
content: Awesome response! Thanks!