Pi filter, any need for a flyback diode - Page 1
Forum section: electronics / beginners

author: Infraviolet
author_rank: Super Contributor
timestamp: September 27, 2021, 10:55:15 pm
content: Just been wondering about Pi filters, a pi (greek letter) shaped topology of 2 capacitors and an inductor to heavily suppress noise and oscillations.signal--.--inductor--.--sig out| |= = caps| |gnd gndIf a large indcutor is used is a flyback diode often going to be needed for protection when the "signal" line, potentially delivering a power supply you want to make stable against transients, gets turned off? or If whatever sig out is leading to suddenly reduces its current demand?Thanks

author: Benta
author_rank: Super Contributor
timestamp: September 27, 2021, 11:26:27 pm
content: A PI filter is purely analogue, and diodes or other such elements are not desirable. The PI filter is simply a second-order filter that also provides impedance matching on input and output.The same can be done with a T filter.PI and T filters are mostly low pass.

author: David Hess
author_rank: Super Contributor
timestamp: September 28, 2021, 12:42:05 am
content: Pi filters are also used to replace decoupling capacitors where extra isolation in either direction is required.I have not seen a diode installed across the inductor to limit overshoot, but I may have seen a TVS installed after the pi decoupling network. Commonly pi filters for decoupling use either a lossy inductor or a low value of resistance in parallel with the inductor to limit the Q. Below is an example from the Tektronix 7A18A vertical amplifier where the "LR" part numbers indicate that the inductors were wound on top of the resistors.Pi filters can also be found replacing the output capacitor of switching power supplies to reduce ripple as shown in the second example below.I have sometimes retrofitted decoupling capacitors with CLC networks to improve isolation between different circuit blocks.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 28, 2021, 05:24:31 am
content: Somewhat of a misapplication / misunderstanding: flyback diodes are used when the source impedance changes wildly.For example, a saturated switch into a solenoid coil. When on, the impedance is extremely low, so current rises to whatever it does (limited by load resistance), and voltage is very well defined.When off, the impedance is extremely high, so current falls to zero, and voltage is very poorly defined. Namely, if the load has substantial inductance, and it was charged to a notable current before turn-off, then that current has to find somewhere to go. If there's no load current or charge, then there's not much wrong, though the voltage is still poorly defined.A poorly defined voltage also means the node is sensitive to voltage noise, say as might be induced (capacitive coupled) from other nearby wires switching. And if you have some sensing circuitry watching that node, maybe it can get confused by that noise. This is fairly hypothetical, but there are actually applications where this is used: some solenoid drivers offer load-open and load-short detection, so they need to measure something about the voltage and current across the load.A corollary is, the peak current under flyback is never more than the maximum load current while on. No need for big fat diodes, at least unless you also needed a big fat transistor to do the first part.Whereas, when we use an electronic filter in the most common case, we mean for well defined,fixedimpedances at either end. So in this case, there isn't such a thing as flyback at all.Now, in the power supply, we might use a CLC filter, and have a load resistance that can change rapidly. So we do need to worry about that. The source impedance likely does not, as it'll have a nice big capacitor on there, or something.We can use a step-change load to solve for the behavior. As it happens, the output impedance has a maximum of around Zo = sqrt(L/C) at the frequency Fo = 1 / (2 pi sqrt(LC)). If the power supply end has an extremely low impedance (as is often the case with lots of low-ESR ceramic or electrolytic capacitors), the peak impedance can be Q times higher than this, where Q is the Q factor of the L and load-side C. We typically choose an electrolytic with ESR ~ Zo to dampen this (bringing Q <= 1), and then our peak voltage above baseline, dV, will be dI * Zo (for a step change in current dI).And we can then solve for the filter component values, in terms of desired output ripple voltage, required load current (as ripple or maximum load step), and attenuation at frequency/ies of interest.Tim