Have Your Cake and Eat It, Too: Overcoming Conflicting Isolation and EMC Standards (.PDF Download)

Designers of power systems often must satisfy sets of seemingly incompatible goals. Small size. High power. High efficiency. Low emissions. And, of course, low cost. Add a requirement that your small, high-power, efficient, quiet, inexpensive design also include isolation, too, and the harried designer might start to feel like there’s too much cake—sorry, functionality—to fit into too little a mouth—er, space.

An isolated design prevents the flow of both direct current (dc) and unwanted alternating current (AC) between different sections of a system while still allowing signal and power transfer between them. Applications employ isolation for many purposes: to protect human operators and low-voltage circuitry from high voltages; to improve noise immunity; and to reduce the effects of ground differences between communicating subsystems. Some examples of isolated systems include:

• Test and measurement: In a data-acquisition system, it’s often necessary to electrically isolate the signals from the system controller to prevent high common-mode voltages from reaching low-voltage circuits, or to eliminate ground loops between field-side signals and the system controller.
• Industrial control: Programmable logic controllers (PLCs) are widely used in industrial control to provide the interface between sensors, actuators, and the factory network. Isolation allows for robust communication to distant nodes that may be connected to a different ground potential.
• Medical equipment: Isolation protects the patient when hooked up to diagnostic equipment. During an ECG, for example, the technician connects multiple leads to check the heart’s electrical activity. Isolation protects the patient from ac line voltage and high-voltage transients from other equipment.

Designing an isolated system requires learning a particular set of definitions, standards, and test methodologies. Go here to learn more.