Why You Need USB Isolation for Industrial IO Industry Technology Paper
USB Isolation and Industrial I/O
What is Isolation?
Quite 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.
Methods of Isolation
There are several methods of isolation. The method used depends largely on what exactly is being isolated, the bandwidth of any signals being isolated and whether or not power must be transferred between the two sides of the isolation barrier. Some of the common methods are:
- Optoisolators – These work by combining a LED and a phototransistor in the same package. As the LED emits light, the phototransistor will change states, these are well suited for isolating digital signals, but cannot transfer power across the barrier.
- Transformers – These work by using magnetic coupling. Transformers are very good for passing power and fast switching signals across the isolation barrier.
- Differential Capacitor Coupling (DCC)– This method makes use of a capacitors ability to pass AC signals while blocking DC. DCC can be used for high speed digital data.
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