cranage emc and safety group - product safety testing, ce marking, low voltage directive or lvd

INTRODUCTION

In our experience, many items of equipment were designed with little consideration of EMC despite, in some cases, having a history of satisfactory operation in the field.

Good design practice usually results in intra-system compatibility and problems of this nature are invariably solved in the design process. EMC problems pertaining to the CE mark, i.e. Inter-system compatibility, may not become apparent until the equipment is submitted for testing, or at worst, is already in production with the customer threatening legal action! At this point it is usually a costly experience with high commercial pressures demanding instant action.

Extra effort is needed to ensure that the product will meet the legal requirements for EMC and this usually places a different perspective on design particularly if specifications for ingress protection and low voltage safety are to be maintained.

THE EMC AUDIT

A self-administered method for identifying key factors which influence EMC.

The purpose of this check list is substantially four-fold: (1) to be able to develop a consistent approach in product design which will lead to achieving electro-magnetic compatibility first time; (2) to be able to obtain an insight into the thinking and experiences of others working in the same field; (3) not to have to re-think or re-write what has perhaps been done many times before; and (4) to ensure that no vital issue or important aspect of design is overlooked.

Cranages' check list is offered as a starting point. To be fully effective, the list needs to be further expanded to meet the needs of the auditor and users should not assume that there are no other factors which might influence EMC performance. The auditor must decide what is relevant and consider all factors which might affect EMC.

ANALYSING THE BENEFITS

It must be very obvious that unless some attention to EMC is paid in the design process, it will be difficult if not impossible to achieve a pass result. A careful and on-going consideration to EMC in the design process is far more likely to result in a product that will cost less, be more reliable and get placed on the market faster.

1) MECHANICAL DESIGN:-

1.1 Shielding effectiveness of covers, panels, ventilation slots for radio-frequency (RF) and electrostatic discharge (ESD) protection;

1.2 Effective use of shielding material for electric and magnetic fields at high and low frequencies;

1.3 Use of shielding components including mesh, gasket and conductive coatings;

1.4 Joints in shields, use of screws, rivets and general fabrication for ensuring optimum reflection and absorption loss leading to high shielding effectiveness;

1.5 Ventilation slots, viewing panels and apertures.

2) ELECTRONIC DESIGN:-

2.1 Application of logic family, microprocessors and clock oscillators for minimising electromagnetic emissions;

2.2 Prospective susceptibility of analogue and digital circuits to continuous wave and transient impulses caused by RF, ESD, and induced overvoltages and currents;

2.3 Use of low ESR/ESL decoupling capacitors for achieving RF stability in power supplies, switching and signal processing circuits;

2.4 Supply rail filtering, bypassing and suppression to minimise electromagnetic radiation and reduce RF coupling onto mains supply;

2.5 Use of ground-breaking devices, e.g. Opto-isolators, common-mode chokes, capacitors, balancing networks, differential amplifiers and isolation transformers.

3) CABLING AND INTERCONNECTIONS:-

3.1 Cable type, length, input/output segregation and position for minimising electromagnetic coupling effects;

3.2 Connector quality and termination for effective shielding;

3.3 Identification of transmission lines, impedance matching, and effective use of ground return paths in unscreened cables for minimising common mode currents;

3.4 Selection of shielding materials, braid/solid conductors for optimising electric/magnetic field protection and use of multiple/selective shielding;

3.5 Earth loop mitigation including grounding and bonding of panels, coaxial cables, power and signal conductors;

3.6 Location and specification of ferrite's and filters.

4) PRINTED CIRCUIT BOARD LAYOUT:-

4.1 Track length, width and routing to electronic components including transient suppressors, filters, operational amplifiers, logic switching circuits, bus and voltage regulators;

4.2 Use of guard rings for ESD protection;

4.3 Analogue and digital circuit partitioning;

4.4 Location of high speed/fast rise time logic, peripheral drivers and receivers;

4.5 Identification of H-field loops in signal, supply and ground conducting paths;

4.6 Use of transmission lines in high frequency analogue and digital circuits;

4.7 Effective supply and ground distribution, bus strips, planes and grid geometries;

4.8 Positioning of components with respect to connectors.