Current Management for EMC
By Ken Webb, Technical Consultancy Manager, TUV Product Service and Tim Williams, Consultant, Elmac Services
EMC Control measures in product and system design are usually thought of in classical terms as applying some well-known techniques: filtering, shielding, grounding and circuit layout. But it is helpful to understand the underlying principles of these methods, and these principles can be summed up in two words: current management.
Current management incorporates two complementary approaches (fig 1). Firstly to improve immunity, it means managing unwanted or interfering currents caused by:
- external RF coupling into cables and enclosure
- ESD events on cables and enclosure
- surge and fast transient events coupling onto cables
Design solutions which address this include a low impedance grounding scheme and enclosure construction, and low transfer impedance cable screens, properly terminated.
Secondly, managing wanted currents due to the circuit operation, to improve control of RF emissions, by:
- using balanced circuit configurations
- keeping circuit loop areas as small as possible
- minimising di/dt and dv/dt by using the slowest switching or clock speeds that will do the job
- controlling stray and intentional capacitive coupling between the circuit and the enclosure
- maintaining a low impedance circuit grounding scheme
Figure 1
Both of these approaches demand that you know the complete current paths taken by the wanted and unwanted signals. EMC design then becomes an exercise in ensuring that the whole of these paths are controlled. This idea is often foreign to circuit and mechanical designers: it is easy to use the “ground” symbol on circuit diagrams to show a circuit reference whilst forgetting that the return current must flow in whatever structure is hidden behind that symbol. And a metal chassis or case will be designed purely for its mechanical properties, with no thought for the fact that it also carries ground currents at interfering frequencies. Once these ideas are accepted, the design process becomes clearer
Return Paths
The actual path taken by return currents is not always obvious. Consider a circuit consisting of a source and load, interconnected by a cable, each side of which is grounded (fig 2). The return current in this case could flow either via the cable (Iret1) or via the ground (Iret2).
Figure 2
In practice the return currents will divide according to the relative impedances of the two paths. You might think that the ground path has the lower impedance – after all there is usually a lot more conductor cross-section – but at high frequencies the cable path’s impedance is dominated by the coupling due to mutual inductance with the signal path. Close coupling means a lower effective impedance, because the magnetic flux due to Isig tends to cancel that due to Iret1. The significance of this is that signal currents can be separated from the ground path by proper cable construction even when both ends are earthed; thus, interference pollution from one path to the other (in either direction) is minimised.
This effect is most pronounced with coaxial cable structures where the mutual coupling is almost unity, but it is readily observed even with ordinary cables such as twisted pair. It dominates at frequencies where circuit inductive impedance is greater than resistance.
For typical small cable pairs where the mutual inductance is around 0.2µH/m and the resistance is around 0.1W/m, this break frequency is about 80kHz.
ESD: an example
An instance of how to think in terms of current management is given by ESD protection (fig 3). An electrostatic discharge can occur to any exposed part of the equipment, including conductive components separated from the outside of a non-conductive enclosure by a suitably short creepage path. Common trouble spots are keyboards and controls, external cables and accessible metalwork. A discharge to a nearby conductive object can also induce currents within the equipment.
Figure 3
From any of the potential points of discharge, the possible routes to ground that the discharge current can take are widespread. Some of them will include part of the PCB ground layout, via stray capacitance, external equipment or exposed circuitry such as switches and connectors. When the discharge current flows through the PCB it will create induced transient ground differentials and these will be capable of causing circuit malfunction, particularly with digital circuits which are susceptible to brief spikes. Even paths which do not include the circuit will cause intense transient magnetic fields which themselves will couple with the circuit’s conductors.
The discharge current will take the route of least inductance. If the case is well bonded to ground then this will be the natural sink point. If it is not, or if it is non-conductive, then other routes may be via the connecting cables. The discharge edge has an extremely fast risetime (sub-nanosecond) and so stray capacitive coupling is essentially transparent to it, whilst even short ground connectors of a few nH will present a significant impedance: for instance, a transient current pulse with a di/dt (rate of rise) of 1010A/s will create a voltage differential of 100V across a ground inductance of 10nH. Apertures in a conductive enclosure will also result in intense local magnetic fields which will couple to the internal circuits.
Interested in learning more?
TUV Product Service has developed a two-day training course which explains the fundamental aspects of EMC that underpin the design of all well-engineered products, and provides the necessary information for planning for EMC test. Understanding and applying this concept is fundamental to a successful and cost-effective design solution. For more information on the training course and dates for 2004 contact Audrey Anderson on +44 (0)1329 443325 or by email: aanderson@tuvps.co.uk.
For further information contact:
TUV Product Service, Segensworth Road, Fareham, Hampshire, PO15 5RH. Tel: +44 (0)1329 443300, Fax: +44 (0)1329 443421, email: info@tuvps.co.uk, web: http://www.tuvps.co.uk/.
Tim Williams, Consultant, Elmac Services, Chichester. For more information visit http://www.elmac.co.uk/.
Tim is also part of the EMC UK consortium who are organising the associated conference at EMCUK2004.
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