Boost Converter Input Filter - Page 1
Forum section: electronics / beginners

author: Nogtail
author_rank: Regular Contributor
timestamp: April 22, 2020, 07:06:01 am
content: I've got a boost converter in a design I'm working on that drives a bunch of white LEDs. It's most likely going to be the highest current device on the board, and I assume it will probably spit out a bunch of noise on its inputs. I want to filter the input of the converter to reduce the amount of noise that gets back to the ground and power planes. I've looked at some existing designs, and most use what looks like a pi filter with an inductor or ferrite bead in series with the positive input. A few designs also have an inductor or bead in series with the ground connection.Inductor (L1) on only positive supply:Inductor on positive supply (L1) and ground (L2):Instinctively I'd expect that having an extra inductor in series with ground would provide more filtering, but I tried to simulate a basic boost converter in LTspice and I couldn't see any difference between the two. I haven't really got a clue what I'm doing when it comes to simulation so it's definitely possible I cocked something up.What's the benefits/drawbacks of having an extra inductor in series with ground?

author: Siwastaja
author_rank: Super Contributor
timestamp: April 22, 2020, 08:22:46 am
content: Verify that you indeed have a noise problem at the boost input, before thinking about filtering it.Because the boost has an inductor in its input, and the inductor current is a triangle wave, boost is very gentle for the input side. There is a lot of noise at the output, though. (This is opposite to buck converter, which is harsh on the input.)The tiny amount of input noise comes through the inductor by its inter-winding capacitance. This is very high-frequency; if you need to do anything about it, consider ferrite bead in your pi filter. Be careful when selecting; you need to use ferrite beads that do not saturate, and saturation current is almost always unspecified, so you need to guess, for example you could guess that a 0805 ferrite bead rated at 2A maximum could maybe perhaps saturate at 200mA, or so.The best option, however, is to minimize the noise to begin with, so look at the boost converter design and layout first (now a three-terminal black box, it seems). If the boost is done properly, it's very likely you won't need extra filtering, unless this is a part of some very high precision analog measurement thing.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 22, 2020, 12:17:29 pm
content: If first case inductor has twice inductance as second circuit then it works almost the same (for differential signal).So, we usually put inductor only in one wire if there is such possibility (because it is easier to place one part but not two).We place inductor in ground wire if there is such a need - for example, if we need to separate grounds. Or there isn't such large inductor in one case (and there is a possibility to separate ground).

author: jackthomson43
author_rank: Contributor
timestamp: April 22, 2020, 01:10:25 pm
content: Inductor should be with Power side, not with ground. So, the first one is better. You should also have a look at thisBoost Converter.

author: 16bitanalogue
author_rank: Regular Contributor
timestamp: April 22, 2020, 04:52:07 pm
content: Quote from: Siwastaja on April 22, 2020, 08:22:46 amCorrect. Both buck and boost topologies will have discontinuous currents; e.g., where the inductor current can fall to zero. With a buck converter this occurs on the input side which is why the input cap is critical in layout. It should be sized appropriately and placed as close to the input pin(s) as possible to eliminate trace and board parasitic inductance. We are trying to eliminate those nasty di/dt's on the input loop. The buck converter's output current is constant , but will have ripple.The opposite is true of a boost converter. The input current is constant and the output current is discontinuous and it is the output cap that is most critical.QuoteAbsolutely I agree. Minimize the loop inductance of the most critical path (input loop for Buck and output loop for Boost). TI is really good at layout examples for their DCDC converters - one could even follow suit and layout their boards to match their EVBs.

author: max_torque
author_rank: Super Contributor
timestamp: April 22, 2020, 08:31:05 pm
content: The inductor in the ground side is there because real switching circuits have unbalanced parasitic elements, a bit of extra inductance in a lead, a bit of capacitance between two copper layers that result in a lot of extra, mostly higher frequency "noise" that occurs on all the potentials in the circuit, ie on power AND ground. When you simulate a switching converter, unless you EXPLICITLY model those parasitics, then your model won't generate any of the noise that occurs in a real circuit. In LTspice for example, if you just apply "gnd" to all parts of your circuit, that link actually clamps all those nodes together to zero volts with zero impedance, so you can't ever see any voltage difference between them.This is the "art" of simulation, to make sure your simulation has a suitable level of fidelity to the real circuit....