Enclosure Design for EMC
Most equipment manufactures choo to design custom housings for their electronics products in order to differentiate them in the market place.Usually enclosure design is driven by the two beasts of beauty and cost,with EMC requirements an irritating afterthought.
The ideal enclosure from an EMC point of view is the perfect Faraday cage.Take the PCBs that make up a typical product,wrap them in a amless monolith of copper without apertures and bingo!You’ve fixed it.Unfortunately this solution is impractical.
The art of good enclosure design is therefore to get“as clo as is necessary”to the Faraday cage,without sacrificing the aesthetic appeal of the product or significantly increasing its cost. For most designers“As clo as is necessary”is usually defined as meeting standard or type specific requirements.This usually involves at least:
(i)Containing Radiated Emissions
(ii)Providing Radiated Immunity
(iii)Providing ESD Immunity
(iv)Providing Fast Transient Immunity
In designing a compliant enclosure the designer needs to look at the following:
(i)Mechanical components-their material composition and coatings
(ii)Electrically bonding the enclosure components
(iii)Electrically bonding the internal hardware and its interfaces to the enclosure
(iv)Dealing with apertures in the enclosure
(v)Screening and partitioning inside the enclosure
(vi)Earthing the product
Component Material and Coatings梦见蛇缠身
Generally the enclosure designer will work with components made from folded steel or aluminium alloy or bulk extruded alloy and possibly vacuum formed aluminium or plastic. Where the volumes of finished product justify the tooling expen,the designer might also u injection moulded plastic or r
ubber or die cast aluminium.
Often housings made entirely of conductive metal components form poor EMC enclosures simply becau the individual components have been painted or coated in a non-conductive material.
There are however a wide range of affordable conductive coatings available.For example: =For aluminium components-alochrome plating rather than non-conductive anodising.
=For steel components-zinc galvanising or passivated zinc plating(zinc and chromate passivated)for corrosion resistance.Where aesthetic appeal is important-bright nickel or chromium plating.Note:Zintec is a sheet steel with conductive zinc coating and can be ud as an alternative to plating where corrosion at the cut edges of a component is not an issue.
=For plastic components-internal metalisation or conductive carbon coating.Partially conductive carbon loaded plastics can be ud but often the strength and flexibility of a loaded plastic is inferior to an unloaded component.
Forming a Faraday Cage from Enclosure Components
If the various components of a product housing are not properly electrically connected to each other
to form an effective overall shield(Faraday Cage)an enclosure becomes an EMC accident waiting to happen.
When designing the fixing and bonding points there is one simple aim for EMC.That is“to minimi electrical apertures and incidental antennas.”It is important to note that where you have an electrical aperture you invariably create an antenna.Consider Figure1where a top plate is both fixed and bonded to a folded metal box.
Figure1-Folded Metal Example
The distance between adjacent points along one edge is A and the distances from end points and box corner are B and C.Assuming the are the only points of electrical contact then we have created electrical apertures of length A,B and C respectively(even though there may be no physical aperture).The metalwork between the points are antennas,both in the folded box and the top plate.The electromagnetic properties of the antennas and the frequencies they will radiate is complicated by many physical factors.We can u an approximate rule of thumb that A will radiate at=2A and B and C atλ=4B andλ=4C respectively.From this we determine a radiating frequency f r=C/λwhere C is the speed of light.The first design aim is to ensure that this frequency f r is much greater than the highest frequency radiated significantly by the enclod electronics.The cond that f r is much greater than the highest frequency to which the electronics are susceptible.Or by default f r is the upper limit t by applicable standards(usually1GHz).
Wave-guide Below Cutoff
For1GHz the fixing paration would be less than15cm which may be excessive for larger enclosures.In this ca we can extend the fold distance d to create a wave-guide operating below its
cutoff frequency[1].With this arrangement little EM radiation will pass in either direction provided that:g<λ/4.The depth of fold gives the shielding effectiveness(SE): SE≈27d/g(dB).
The only caveat when using this technique is that neither component must‘carry’a signal. Therefore the designer must be careful what circuitry is bonded(or coupled)to which mechanical component.
Bonding at Fastening Points
In most cas the designer,wishing to save cost,will consider bonding at the fastening points of the enclosure.The previously described simple techniques allow the designer to create a radiation tight bonded enclosure by connection only at the enclosure fastening points. Nevertheless where additional bonding is required it can be achieved in a number of ways: (i)Direct contact of conductive faces.
(ii)Contact via conductive fingers,clips or gaskets.
(iii)Indirect contact via capacitive coupling.
The first technique is inexpensive and requires only that the component surfaces are conductive and in good electrical contact.Where a good electrical contact can’t be guarantied due to mechanical tole我心中的诸葛亮
rances or flexing and distortion additional measures must be taken.An inexpensive method is to press small point dimples into one of the mating faces which themlves act as point contacts.Other more costly methods include fitting conductive gaskets or beryllium-copper/phosphor-bronze contact strips.
Electrically Bonding Painted Components
Even if all enclosure components are metallic some will inevitably be painted or powder coated.Masking off contact areas prior to painting is a frequently ud technique for creating electrical bonding points.There are problems with the technique especially if masking is ud near contact edges where the masked off area may become visible giving the finished product a poor finished appearance.Painted steel components that have expod masked off areas should first be plated to ensure the expod area doesn’t corrode.As steel starts to rust its electrical surface conductivity is quickly lost as tho of us who own older cars will readily testify.Note that both the plating and masking process add cost to the finished enclosure. Another bonding technique for painted sheet metal is to u press fit captive nuts and washers (Figure2).
Figure2-Bonding Painted Metal
The captive nuts commonly available have a knurled ction that bites into a pre-drilled hole in the sheet metal component.When they are presd any residual paint is displaced and a good electrical contact is formed.It ems to this author that captive washers of the same design are impossible to get hold of!Often a crimp washer is ud instead,but the danger is that it accidentally might not be fitted during production.Even if fitted it is not always guaranteed that a crimp washer will bite through a non-conductive coating.This is particularly true of durable powder coatings and stove enamelling.
Many designs also u earth cables to bond metal components attached via screw eyelets. This makes n from a product safety point of view,for example:ensuring the lid of a housing is earthed even when the lid is open.This sort of bonding does not have a good EMC performance,as even rela
tively short wiring runs become highly reactive at radio frequencies. Electrically Bonding Metalid Plastic Components
Captive nuts and threaded sleeves presd into moulded plastic enclosure components are the most frequently ud form of mechanical fastening.They cannot usually be relied upon to provide a good electrical bonding through the life of a product.This is becau they will often move slightly within their presd locations and will as a result break the direct contact with the thin metalisation layer.
For this reason metalid plastic parts are often bonded to the ground layer of the internal electronics(PCB)via conductive spring loaded fingers soldered directly to the PCB.The press against the housing component when the unit is asmbled.
This author has also en products where coiled springs have been ud to the same effect. Coiled springs should be avoided from an EMC point of view as they are naturally highly reactive and give high impedance at radio frequencies.
Capacitive Fold Coupling
Capacitive fold coupling can be ud to great effect to enhance the EMC performance of a metal enc
losure at the top end of the RF spectrum.It can be a cost effective alternative to conductive fingers or contact strips.For example,consider Figure3.
学历史的意义Figure3-Capacitive Fold Coupling
This is a common scenario where a metal housing has a slot cut in it to facilitate a removable card.The locking nuts on the card’s front panel are ud for both fixing and electrical bonding. We still have an aperture with an associated antenna of length L.We may judge from our rule of thumb that the antenna is likely to radiate unacceptably.We can reduce this radiation, damping the antenna by including a distributed capacitor along both edges of antenna.This is done by introducing folds in t
he sheet metal composing the card’s front panel and the housing front panel.
We will assume that both front panels are painted with a non-conductive finish.In this example the non-conductive finish helps us,becau it will usually have a greater permittivity than free space.
The capacitance(C)of each folded coupling(when the card is in place)is given in terms of the surface area of the folds(A)and distance between each plate(d):
C=εrε0A/d
八卦顺序εr is the relative permittivity of the insulating material andε0the permittivity of free space (8.85x10-12Fm-1).Each capacitor has an impedance Zc that decreas with increasing frequency:如梦曾梦
Zc=1/jωC
In this example the impedance of both capacitors combines to slug the antenna with impedance equal to Zc/2.The trick is to get the surface area and paration of the folds such that the slugging impedance Zc/2is much less than the impedance of the antenna at fr.
This author has ud this approach with some effect in the past.It should be noted however that there are potential pitfalls:
(i)If the antenna being slugged is predominantly inductive,then a resonant circuit is
being created which will create a radiator tuned to peak at a resonant frequency.
This can be a problem if the resonant frequency is an undesirable one.
(ii)The capacitance generated varies greatly with only small variations in the plate paration.
植物观察日记表格(iii)In reality wave-guides with low characteristic impedance are being created and not ideal capacitors.
Man Enclosure Interface
EMC problems are compounded where electronics meets man.The designer of housings for equipment incorporating displays and keyboards will have to create apertures in his enclosure for them.The apertures will allow radiation both in and out.Worst still for the machine,the man is a potentially lethal source of electrostatic discharges(ESD).
There are a number of techniques to defend the machine against the EMC problems caud by the annoying apertures:
(i)Fit a well bonded conductive plane immediately behind the keyboard or display of
a size equal to or larger than the aperture.This will,to an extent,confine aperture雨大的成语
radiation to that emitted by the display or keyboard.
(ii)For noisy displays(such as electro-luminescent and other high voltage technologies)cover the aperture with a e-through conductive gauze immediately
below or embedded in the viewing screen.Again this should be well bonded to the
enclosure毛衣怎么洗
(iii)For membrane keypads pick a technology that doesn’t break down in the face of an 8kV ESD gun or otherwi be prepared to fit some rious protection to the
electronics that the keypad connects to.
RF De-tuning
It's worth noting that metallic enclosures affect the performance of electronics circuits placed in them.
This is most noticeable with high frequency oscillators where earthed sheet metal planes
