EMC dag voor Installateurs

Transcription

1 EMC dag voor Installateurs Picture or Drawing 20.7 x 8.6 cm Frits J.K. Buesink, Senior Researcher EMC frits.buesink@utwente.nl The Current Boundary a provision to split loops (and shut out noise sources) (Mains cord 1) Unit 1 Situation in detector: practice AM-radio (I/O cable 1-2) Unit 2 (Mains cord 2) I cm (noise current) Ground 1 Ground 2 loop closes through ground Short circuit(s) check reduce loop area Create one or more inner-loops Ground 1 I cm Ground 2 loop closes through ground 2 1

2 Install current boundaries at natural interfaces edge of PCB, cabinet wall, basement of a building; one boundary per unit! Right Wrong Drawbacks: Current follows long path over equipment Loop area cannot easily be minimized I cm I cm 3 Examples of current boundaries on equipment wide conductors and low-resistance transitions (be careful with paint)! Short Wide No Paint! protect all units with a current boundary! (and check any conductor that passes it) check DC resistance with a milli-ω meter: < 1 mω! 4 2

3 Use Current Boundary to protect existing pig-tail pig-tails can be acceptable as long as CM currents are kept away from it H-field lines Wide metal plate (Current Boundary) I cm EMC glands 5 What goes wrong on current boundaries mix of plastic and metal EMC glands signals errors Metal gland on plastic cable sheath Paint.? No cable shield no connection either?? 6 3

4 Separating cables with current boundaries classify cables into categories Category 1. Noisy (E) 2. Sensitive (I) 3. Indifferent (N) red = source = Emission green = sensitive = Immunity blue = indifferent = Neutral E I I CM N Model 7 Separating cables with current boundaries use Neutral conductor to reduce loop area; then insert current boundary E N Steps: I N 1. herken recognize kring loop 2. reduce looparea 3. add boundary 8 4

5 Separating cables with current boundaries neutral conductor in practical cases: never a wire, always a structure part Ι cm Emission Neutral cross section: twin wires! (CM-) Transfer impedance of combination of two relatively thin conductors is too high (radiates fields) (does not work for high frequencies) 9 Separating cables with current boundaries wide metal reduces fields i.e. the transfer-impedance of the cm-current loop Ι cm Emission Neutral advantage: proximity & skin effects Wide sheet metal ( cable guide ) is far superior to the previous situation. The common-mode transfer impedance is much lower. Skin effect helps. 10 5

6 Special form of current boundary: a cable guide Neutral conductor, usually structural part of existing installation Noisegenerator E-Cable (source) I CM (=noise) 50 Ω I CM (=noise) 50 Ω scope I-Cable (passive) I cm squeezes under cable (proximity effect) 11 Mutual induction in practice noise greatly reduced with wide return conductor (ground plane) source (50Ω) Wide ground plane is preferred return path for current! A E-cable (source) Ground Plane 50 Ω 50 Ω B scope I-cable(passive) 12 6

7 Mutual induction in practice proximity effect: return current concentrates under red wire source (50Ω) A E-cable (source) Ground Plane 50 Ω 50 Ω B scope I-cable (passive) I cm squeezes under cable (proximity effect) 13 Wide metal also features: the Skin Effect Lenz Law and the basis for shielding effects J 0 Current Source Eddy currents current density J 0 e J 0 δ d d = J 0 e d δ Ι δ = 1 π f σ µ f = frequency [Hz] σ = conductivity [S/m] µ = permeability [H/m] 0 d Induced Eddy currents oppose direction of external current (Lenz Law) 14 7

8 Proximity effect current concentrates under conductor, minimizing loop inductance Ι J(x) R=50Ω x Current concentrates under conductor (proximity effect) Field distribution can be measured with small sniffer probe 15 Separating cables with current boundaries use (Ground-) Plane to reduce loop area; then insert current boundaries E Plane could be metal mesh I N Steps: 1. recognize loop 2. cover loop with metal (ground-)plane 3. connect current boundaries to plane 16 8

9 Cables without Cable-Tray crosstalk through transfer-impedance 17 Cables over Cable-Tray, Large Separation 18 9

10 Cable Against Cable-Tray, Pig-Tail Ignored. 19 Cable Against Cable-Tray, Pig-Tail Treated Properly 20 10

11 Use available metal to short-out CM-currents ship s deck and walls can be used as groundplane(s) UNPROTECTED Good contact (< 1 mω) PROTECTED Good contact!? Important: keep cables near metal over their full length! (unless cables have sufficient shielding to go unprotected ) 21 Grounded Cabinet on Shock-Absorbers (Litz Wire) 22 11

12 Cabinet on Wide Grounding Bracket, Cables Float 23 Cabinet on Wide Grounding Bracket, Cables Improved 24 12

13 Either filter or shield entire cable when passing through shielding wall O.K. C L C EMC Filter E, H fields Ι cm 10 ma 1 V/m EMC Gland O.K. E, H fields reradiate (3-5 µa = RE limit) Not O.K. 25 Shielding Experiment Shielding a noisy interconnection using a metal tube (wave guide) Ι cm Generator Wire carrying modulated RF signal Battery DC cable Radio tuned to RF harmonic frequency Modulation Detected 26 13

14 Shielding Experiment Entering a conductor into tube couples out the noise again Generator Wire carrying modulated RF signal Battery DC cable Radio tuned to RF harmonic frequency Modulation Detected 27 Shielding Experiment Insulating generator case: battery cable now reradiates noise (antenna) Generator Ι cm Wire carrying modulated RF signal Battery DC cable Radio tuned to RF harmonic frequency Modulation Detected 28 14

15 Shielding Experiment Entering a shielded conductor into tube also couples out the noise again Generator grounding wire Wire carrying modulated RF signal Battery DC cable Radio tuned to RF harmonic frequency Modulation Detected 29 Shielding Experiment Grounding the shield with the wire does not solve the interference problem! Generator Wire carrying modulated RF signal Battery DC cable Radio tuned to RF harmonic frequency Modulation Detected 30 15

16 Shielding Experiment Cable shield must be grounded directly to the metal shield to stop the noise Generator Wire carrying modulated RF signal Battery DC cable Radio tuned to RF harmonic frequency Modulation Detected 31 Bad Grounding habits it is Inductance, not the milliohms that count! 32 16

17 Front Door / Back Door EMI Front Door: Via Intended Coupling; Back Door: Via Unintended Coupling 9 [MHz] out-of-band interference 100 [MHz] in-band interference Front Door Receiver [MHz] Back Door Ι CM Mains Cord 33 Shielding Experiment A filter in the inserted wire does not help if only grounded with a wire Generator Wire carrying modulated RF signal Battery DC cable Radio tuned to RF harmonic frequency Modulation Detected 34 17

18 Shielding Experiment only when the wide metal filter plate touches, the shielding works Generator Wire carrying modulated RF signal Battery DC cable Radio tuned to RF harmonic frequency 35 Filter Installation installation at least as important as the correct filter choice C. Filter mounted Plumbers Delight fashion (input and output completely separated by shielding) B. Filter mounted on wide metal strip A. Filter grounded with long wire 36 18

19 Filter Grounded with Long Wire some effect in the low frequency range; hardly any in higher frequencies 37 Filter Mounted on Wide Metal Strip better than the long wire; sensitive to cable routing (crosstalk) Wires pressed against ground plane Wires far from ground plane 38 19

20 Filter In and Output Separated by Shielding construction often called Plumbers Delight by radio amateurs Filter performance as specified by manufacturer (noise floor on analyser) 39 Systems Designers Heaven independent building blocks with abstract behaviour Object Oriented realise complex from simpler behaviour make assemblies independent solve undesired as low as possible System EMC Principal Laws No high frequencies Do not transport them Use adequate boundaries Software Modules Hardware Components 40 20

21 Good EMC behavior insensitive to standard used systems EMC requirements are set by the environment it is intended for Industry Ground Based Airborne At Sea Domestic Small Medium Large [Tests to cover] 41 EMC is achieved during the design process when problems appear during testing, we are too late! many M$$ Available Mitigation Options Requirements Bankruptcy Measures Repair/redesign Check bonding Cost of Modification 0 Concept Design Manufacture Test Operational phase in the lifecycle

22 Product Development/Program Support perform engineering & qualification tests 43 EMC approach through the Knowledge Cycle insert electro magnetic behavior up front Problem definition: desired behavior Test Validation/Verification Validated models Research/Analyses Development Support Behavioral Model Knowledge Transfer & Education 44 22

23 EMC Rules and Guidelines a lot of information on EMC engineering can be found on the internet 45 EMC Rules and Guidelines or: buy a book! ISBN

24 The End Relation of MIL-STD-461E tests to Phenomena survey of test identifiers CE102 RE101 RE102 RE103 CS101 CS114 CS116 RS101 RS103 RS105 Conducted Emissions, Power Leads, 10 khz to 10 MHz Radiated Emissions, Magnetic Field, 30 Hz to 100 khz Radiated Emissions, Electric Field, 10 khz to 18 GHz Radiated Emissions, Antenna Spurious and Harmonic Outputs, 10 khz 40 GHz Conducted Susceptibility, Power Leads, 30 Hz to 150 khz Conducted Susceptibility, Bulk Cable Injection, 10 khz to 200 MHz Conducted Susceptibility, Damped Sinusoidal Transients, 10 khz to 100 MHz Radiated Susceptibility, Magnetic Field 30 Hz to 100 khz Radiated Susceptibility, Electric Field, 2 MHz to 40 GHz Radiated Susceptibility, Transient Electromagnetic Field (NEMP) 48 24