Electronics - Examples of filtering methods
Created 03-11-2015, Changed; 05-02-2024, 07-03-2024
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Electromagnetic compatibility is and has always been wrapped in some mystique. Rules that people don't know or understand but must adhere too. Star Point, Bonding everywhere, coils in earth bonding that electricians and plumbers do. This part of electronics is art and is comparable to mechanical design and considering where to put a rubber coupling (soft magnetic bead or common mode choke or C+RC damping) Or stiff coupling (thick straight with no right angles conductor) Or a metal spring (high Q inductor or Polypropylene capacitor)
If you are unsure, then put options into your design such as zero ohm link resistors that can be removed to all your PCB mounting points. It will work better if designed in and left out later, than left out and added later or replaced by capacitors.
Drawn using OrCAD Capture 16.6 - AL-0012-02B like the London Underground schematic diagram/map needs to be plain to see but also placing things in realistic geographical locations. It is worth taking time drawing and refining a circuit or block diagram. OrCAD capture has been the best tool for drawing circuits and diagrams for 35 years, I am using the free limited capacity versions of a number of CAD tools at home and a licensed version of CADSTAR 17 and 18.
This page shows more ideal solutions than this diagram above and how the function blocks could be implemented. Hopefully I have labelled enough to be clear, but unfortunately the diagrams look busy.
In general, to make a low powered instrument CE compliant is fairly straightforward, requiring minimal filtering but always good PCB design. But to do nothing will not only fail and be unreliable in an unpredictable way.
That is even if you do not bond everywhere you bond 0V to the casing near the i/o use common mode noise methods; differential amplifier with filtering, common mode choke or opto-isolation.
If you daisy-chain other circuit boards, only follow that strategy for one board distance from the next and within the enclosure and over a short distance.
If the PCB or module contains switch mode power supplies, motor drive, do make sure that it is screen bonded to the casing and use the common mode filtering as shown below. Or at least enclose it and filter the Inputs and Outputs.
Automotive industry set much higher standards for immunity and susceptibility, even so low power LED lighting units, with electronics and switch mode power supplies comply with just good PCB and circuit design, enclosed within a non-screened plastic enclosure.
It was not until I was 30 that my got electromagnetic design achievements became good. After this equipment I designed had to be tested and a colleague and I got know for bringing equipment to test which passed with no issue. That was until I paired down the filtering to nothing and then the equipment failed, which surprised the test house. I have spoken to other engineers trying to design with no filtering on a cost sensitive item - it just does not pay to do that I believe and the engineer I spoke to did not like the compromises.
Near Fields
It is not possible to measure emissions or susceptibility with sources and detectors close to a printed circuit board under test very accurately. But it can be useful to measure things with near field probes, and this will give a good indication of hot (radiating) components. I found that A near field magnetic loop and an analogue 30KHz - 30MHz valve voltmeter worked very well for me many years ago. But I only used it when EMC first became part of CE marking and learnt what I need to learn from the exercise.
To restate another point;
Small metal enclosures can produce very unpredictable results. This is again put down to near fields. I do not know the mechanism, it is related to aerial design theory, of this situation but warn of it.
And in any case, engineering is full of analogies to things we may assume a physicist understood. But they work, and sound engineering is built on them because they fit well.
To restate some of what I said in the mechanical design page; Contain high frequencies and susceptibility to near the source and point of susceptibility;
Therefore, use an embed microcontroller rather than a conventionally bussed microprocessor.
Bandwidth limit circuit stages with active and passive filtering if necessary.
Filtering on the PCB at the boundaries, e.g. connectors,
PCB with at least an 0V power plane,
PCB canning with the lid ultimately soldered all around,
The first three are important, 4 less so, and 5 is usually only necessary with high speed, power oscillating product designs.
Circuit below; A & C are optional interconnections B & D are PCB's or a single PCB;
The diagram above will work well with just block D the PCB with all functions and I/O connectors mounted on that PCB. Achieving good results with an insulated or metallised plastic enclosure. It also works fine with a single 0V connection to a metal enclosure near the I/O provided the filtering and screening is contained within the PCB. But many connections to 0V is most desirable, but can be bad in the case of a metal enclosure. I will show other ways below;
Put the OV or earth, input, output and power reasonably close together and any PCB close to where those enter the enclosure,
Place associated parts that are particularly sensitive the closest together,
Conversely keep power magnetics away from sensitive electronic circuits - distance first then add screening enclosures (for example PCB canning).
Use transmission lines strategy and ensure 0V planes are not fragmented as a PCB design strategy,
The 4 line common mode choke could be replaced by two 2 line common mode chokes. Select the highest inductance part with a leakage inductance that will not compromise the AC performance of the signal.
If it is necessary to put a cable between the PCB and the I/O connection, insert block B or D, but keep the filter near the inputs and outputs if possible, or at least connected by shortest distance to the PCB.
For Automotive applications which are much more stringent than is required for CE marking insert block A. The capacitors add nothing but do increase the margin for compliance.
If the enclosure is metallised there is no good reason for not to make many 0V connections to the metallised enclosure. The metallisation will be resistive and therefore low Q thereby any voltage differential will not cause significant current to flow between 0V connections.
Rout wiring away from magnetics.
Screened enclosures (such as a PCB can) should also be kept a little distant from magnetics.
General rule; Aluminium, copper and gold for high frequency, mild steel or mu metal for low frequency,
The power line chokes shown above are not low Q, soft magnetic types (that I have recommended generally). Placing a resistor in series or a resistor in parallel with the inductor would lower the Q of the inductor. I have not experimented with adding a resistor though I have placed pads to do that but using an easily available high Q inductor works fine.
Printed Circuit Board to board interconnection;
AL-0009-02A
Where there is no high speed high power switching, the filter on the right is sufficient. Without this minimal filter, the circuit is most likely to be susceptible to interference.
A plastic enclosure is usually sufficient, but I have shown a dashed connection for a screen bond.
Floating electronics - so that isolation to earth can be achieved conveniently, But;
My experience is that it does not work with very high impedance circuits, mains 50Hz is electrostatically coupled in to the circuit.
This strategy may be okay with lower impedances and where the sensitive circuit is very small and on a ground plane. But providing opto or transformer isolation is the only safe way.
In Low Voltage Directive systems, an approved type capacitor to the casing is permitted.
I would expect, but I have not tried it but to not just adding a capacitor but adding an C + R+C network between 0V and case earth would be better. As I have said though single point RF bond would be quite effective as is specified in such industrial standards, but it is not the best way. Such capacitive coupling of cause leaves high impedance sensitive circuits to low frequency electrostatic coupling. Conversely, such high frequency coupling to 0V will minimise the likelihood of coupling low frequency components modulated by high frequencies, and this is significant (I have concluded from other observations).
I have described 0V strategy but many approaches amount to bonding everywhere to 0V, Bonding one point to a 0V case and running 0V wires to every PCB or block, or daisy-chaining one to another, to another. You should recognise some of each in what I described, But not truly a star point strategy.
Star point is very important in protective earthing - using crimped wire, nut bolt and anti-shake washers just for the safety earthing and the bolt sole used for earthing but not construction. The Safety earth path made to every metal component and wired back to one common safety earth incoming point should be done. Then, for screening, applying many bonds to every metal component.
THE COMPONENT VALUES SHOWN ARE FAIRLY ARBITRARY AND DEPEND ON THE CIRCUMSTANCE.
RF design principles can give useful clues to how and why things work out well. A 50 ohm track on a double-sided PCB may require an inconveniently wide track width, but if the tack is very short this may not compromise the design significantly.
My blog for discussion - Trade-off for safety and EMC in mains voltage power supplies; Electronics - high-frequency metal-vapor-arc-lamp power supply
To discuss electronics, see; Blog page Electronics
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