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Understanding EMC Basics Part 1: 1Of 7 EM Field Theory and Three Types of EM Analysis Keith Armstrong

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Understanding EMC Basics Part 1: 1Of 7 EM Field Theory and Three Types of EM Analysis Keith Armstrong Understanding EMC Basics Part 1: 1of 7 EM Field Theory and Three Types of EM Analysis Keith Armstrong Understanding EMC Basics series Webinar #1 of 3, February 27, 2013 Webinar Sponsored by: EM field theory, and 3 types of EM analysis AR provides EMC solutions with our high power RF/Microwave Amplifiers to complete systems. EurIng Keith Armstrong CEng, FIET, Senior MIEEE, ACGI Learn more at www.arworld.us Presenter Contact Info Email: [email protected] Website: www.cherryclough.com Contents of Webinar #1 1. Electromagnetic fields, waves, and the importance Understanding EMC Basics of the return current path 2. Field theory, permittivity, permeability, wave impedance and velocity 3. Near-field and Far-field 4. Three types of EMC analysis (includes Skin Effect) 1 Electromagnetic fields and waves, and the importance of the return current path Send Return Electromagnetic (EM) fields Of course, a wave has different amplitudes along its path Every non-DC voltage/current is a wave of When a conductor is long enough propagating EM energy... – it cannot experience the same voltage or current, – guided by send and return current paths at the same time, over its whole length… ● and the insulators (dielectrics) that surround them (e.g. air) ● which is why high frequencies seem to behave so weirdly! EM waves spread out and create EM fields, (like ripples spreading out and making a pattern on a pool)... The ratio between wavelength () and conductor ● and we measure fields in terms of field strength dimension is very important – we can usually ignore “wave effects” when the Design for EMC is mostly about controlling fields dimension we are concerned with is < 1/100th of the – so that they are high where we want power or signals ● e.g. at 1GHz: – and low where we don’t want emissions or susceptibility < 3mm in air ( = 300mm); < 1.5mm in FR4 ( = 150mm) Understanding EMC Basics Part 1: 2of 7 EM Field Theory and Three Types of EM Analysis Keith Armstrong Importance of the return current path Understanding EMC Basics Electric and magnetic fields are the true nature of electrical and electronic power and signals – and they both depend on the physical routes taken by 2 the send and return currents Field theory , permittivity , A great deal of EMC design depends on permeability, wave impedance controlling the paths of the return currents and velocity All currents always flow in complete loops… – taking the path of least impedance – the path with the least area – i.e. the return current flows as close to its send path as it is allowed to We don’t need field theory Permeability (µ) and permittivity () – just a few concepts All media or materials have conductivity/resistivity Fluctuating voltages create Electric fields (E) (i.e. loss of EM energy, turned into heat), µ and ... -7 – which are measured in Volts/metre (V/m) – in vacuum (and air): µ0 = 410 Henries/metre… ● i.e. the vacuum can contain magnetic field energy Fluctuating currents create Magnetic fields (H) – And: = (1/36)10-9 Farads/metre – which are measured in Amps/metre (A/m) 0 ● i.e. the vacuum can also contain electric field energy EM waves have power (P) Other media and materials are characterised by – measured in Watts/square metre (W/m2) their relative permeability (µR) and permittivity (R) (i.e. the rate ate which energy passes through an area) – so their absolute permeability is: µ0µR and their absolute permittivity is: 0R Permeability (µ) and permittivity () µ and govern an EM wave’s impedance, continued... and it’s propagation velocity (e.g. wires, PCB traces) In conductors : µ and are what For the wave’s ‘far field’ impedance … causes them to have inductance (L) and capacitance (C)… Z = E/H = V/m A/m = (0R/0R) – so whenever there is a fluctuating voltage (V) there is Z = 377in air or vacuum always an associated current (I), and vice-versa Z = 377 (R/R) in a medium or material In insulators (e.g. PVC, FR4, air): µ and cause effects For the velocity of the wave’s propagation … similar to inductance and capacitance… v =1/ (0R0R) metres/second – so whenever there is a fluctuating electric field (E) 8 there is always an associated magnetic field (H), v = 3.10 m/s in air or vacuum (i.e. the speed of light) and vice-versa 8 v = 3.10 /(RR) m/s in a medium or material Understanding EMC Basics Part 1: 3of 7 EM Field Theory and Three Types of EM Analysis Keith Armstrong And the velocity of wave propagation (v) links frequency () to wavelength () Understanding EMC Basics v = In vacuum or air: v = c = 300 million metres/second 3 – 1/ ( ), equivalent to 3ns/metre, 3ps/millimetre 0 0 Near Field and Far Field But in media or materials with R and/or R >1.0, v is slower than c – so the wavelength () is shorter (for a given f ) ● e.g. for a printed-circuit board trace, v is approx. 50% of c ….so a is approx. 50% of what it would be in air NearNear--fieldfield and Far-Far-fieldfield An example of a nearnear--fieldfield field distribution Near fluctuating voltages or currents, E and H fields This shows the fields in one 3 plane at 5.32GHz, but the have complex patterns: field strengths vary as 1/r , simulator calculates all of the 2 frequencies in all of the three 1/r and 1/r dimensions ● where r is the radial distance from the source – because of stray capacitance and stray mutual inductance effects (i.e. E and H field coupling) But, far enough away, the fields become EM waves (E and H fields in the ratio of the wave impedance: Z)… This simulation is of a heatsink in free space – proximity to enclosure – and have simple ‘plane wave’ spherical distributions will have an effect with field strengths that vary as 1/r NearNear--fieldfield and farfar--fieldfield when the source’s largest dimension is << (for illustration only) NearNear--fieldfield and FarFar--fieldfield continued… Wave impedance 10,000 For sources with longest dimensions <<, Near (induction) field Far (radiation) field 5,000 the boundary between the near and far field regions (fields vary as 1/r3, 1/r2 and 1/r) EM fields 2,000 is: (also called plane waves, r = /2 1,000 which vary as 1/r) 500 377 But for sources with dimensions >, 200 The final value of the the near/far field boundary is: 100 wave impedance depends on the medium it is 50 propagating through r = 2D2/ 20 (e.g. 377 in vacuum / air) – where D is the largest dimension of the source 10 0.1 0.2 0.3 0.5 1 23 5 Radial distance (r) from source in units of /2 Understanding EMC Basics Part 1: 4of 7 EM Field Theory and Three Types of EM Analysis Keith Armstrong Understanding EMC Basics 4 Poll Questions Three types of EMC analysis (includes Skin Effect) EMC uses three types of analysis Resonances For conductor dimensions < /6 we can use All circuits have RF resonant modes ‘lumped circuit analysis’ methods (based on R, L, C) – where their currents or voltages experience resonant When conductor dimension is > /6 along one axis gain, called their ‘Q factor’... (i)(e.g. a wire) we must‘t use ‘transmission line’li’ analysis – Qs of 100 or more are common (i.e. gains of 40dB or more) But when conductors are > /6 in two or three As the voltage peaks, the current nulls, and vice-versa dimensions we must use ‘full-wave analysis’ (to maintain a constant energy as the wave propagates) – based on Maxwell’s Equations High levels of emissions (and poor immunity) ● only practical for very simple situations, tend to occur at resonances… or when using computers to do the analysis – so we often need to control them to achieve EMC Lumped analysis… Lumped analysis: everything has resistance (R), Resistance and Skin Effect inductance (L), and capacitance (C) DC currents travel through the whole cross- sectional area of a conductor – including all components, wires, cables, PCB tracks, connectors,,,, silicon metallisation, bond wires, etc – but AC currents are forced to flow close to the surface – also including their ‘stray’ or ‘parasitic’ Rs, Ls, and Cs This is known as the “skin effect” ● which can be intrinsic (e.g. the self-inductance of a wire lead) ● or extrinsic (e.g. stray C or L coupling due to proximity to So, high-frequency currents only penetrate weakly other objects) into the depth (thickness) of a conductor Resistance increases with f due to Skin Effect – increasing the resistance in their path Understanding EMC Basics Part 1: 5of 7 EM Field Theory and Three Types of EM Analysis Keith Armstrong Examples of crosscross--sectionalsectional current density in a copper sheet Resistance and Skin effect continued... One skin depth () is the depth into the conductor Vdc by which the current density has reduced to 1/e 1 = metres (f µ0 µR ) Example at d.c Uniform current density ● where = conductivity V ac For copper conductors: = 66/f ( f in Hz gives in millimetres) – e.g. at 160MHz = 0.005mm, so 0.05mm below the surface (10 skin depths) the current density is negligible Example of surface currents at 1MHz ( = 0.07mm) Graph of skin depth () Lumped analysis: Stray Inductance for copper, aluminium, and mild steel Skin depth (in mm) E.g. a thin wire has self-inductance of about 1µH 100 per metre (1nH per mm) ● this assumes its return current path is very far away 10 ● a close return path reduces the overall inductance experienced by the send/return current 1 Close proximity to ferromagnetic materials (e.g.
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