05 Feb

Isolating USB Connections

Isolating USB Block DiagramQuite simply, isolation is achieved by removing any direct electrical connection between two devices or portions of a circuit, but having them work as if they were electrically connected. Isolation is generally rated by how high a low current AC voltage can be set before there is arcing across the isolation barrier.

Some common isolation terminologies are:-

Isolation Rating
– This is the transient overvoltage that an isolation barrier can withstand. Usually expressed in terms of AC voltage, some common ratings are 500 Vrms at 1 minute for industrial equipment, 2500 Vrms (IEC 60950-1) at 1 minute for IT equipment and 5000 Vrms ( IEC 60601-1 ) at 1 minute for medical equipment.

Working Voltage
– This is the continuous voltage that can be applied across an isolation barrier for the life of the device without breaking down. Like the isolation rating, it usually expressed as an AC voltage.

Reinforced Isolation
– Usually required for medical systems, this the isolation rating equals two separate isolation barriers. In medical systems, this means the isolation barrier would have to withstand a short duration of 10,000 Vrms.

Creepage
– This is the shortest distance between two lines on a surface layer of a PCB, on either side of the isolation barrier.

Clearance
– This is the shortest air gap between two lines on a PCB.