Why You Need USB Isolation for Industrial I/O – 8500901
USB Isolation and Industrial I/O
The USB port has become the most popular method for connecting virtually anything to a computer; from simple flash drives to complex industrial I/O equipment. Most computers built in the last five to ten years have at least one USB port; with newer computers having upwards of six or more. For most home users, having an isolated USB connection is not an issue. However, isolating a USB connection can be a necessity for applications such as; industrial/remote I/O, data acquisition, IT or medical equipment. This paper examines the advantages of using USB; what isolation means, types of isolation, and when USB connections should be isolated.
What is Isolation?
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 still electrically connected. Isolation is generally rated by how high a low-current AC voltage can be set before there’s arcing across the isolation barrier. Some common isolation terminologies are:
- Isolation Rating: 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
- 5000 Vrms (IEC 60601-1) at 1 minute for medical equipment
- Working Voltage: Continuous voltage that can be applied across an isolation barrier, for the life of the device, without breaking down. Like the isolation rating, it’s usually expressed as an AC voltage.
- Reinforced Isolation: Usually required for medical systems. This 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: The shortest distance between two lines on a surface layer of a PCB; on either side of the isolation barrier.
- Clearance: 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 is being isolated and the bandwidth of the signals being isolated; as well as whether or not power must be transferred between the two sides of the isolation barrier. Some of the common methods are:
- Optoisolators: Combine an 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: Use magnetic coupling. Additionally, 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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