Bench Talk

Digital Isolation in a Nutshell

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Digital, or galvanic, isolation prevents current from flowing between two parts of a system while still allowing the required signal and power to pass. It is an important function that protects people and equipment from dangerous voltages and prevents noise in sensitive circuits by eliminating ground loops.

Think of digital isolation like a contactless payment terminal. There is no physical connection between the card and the terminal, but the magnetic field generated by the terminal passes power to the payment card, and the system reads and transmits the payment details.

Digital isolation is often found in industrial products like power converters, industrial automation systems, and electric vehicles. For example, in an electric vehicle charger, digital isolation ensures that the high-voltage charging circuit is safely isolated from the low-voltage control circuitry and operator panel.

Basic Isolation Technologies

There are three basic digital isolation technologies: optical, magnetic, and capacitive.

In optical isolation, an LED passes data across the isolation barrier in the form of light pulses. These are received by a phototransistor (also called an optocoupler), which converts the pulses into electrical signals.

In magnetic isolation, a magnetic field passes data and power across the isolation barrier, similar to how a transformer works. While transformers are usually used to step up or down the voltages, isolation transformers feature a 1:1 ratio, keeping the voltage the same. These types of transformers are used mostly in safety applications.

In capacitive isolation, data is passed across a capacitor using digital encoding to charge and discharge the capacitor at high speed. Where capacitors are used for isolation from power supply circuits, they may carry special ratings to indicate risks for the system or the user in case of a short circuit. For example, Class-X capacitors, which are connected between the AC line and AC neutral, reduce the risk of electric shock in a short circuit but may still have a fire risk. Class-Y capacitors, connected between the AC line and the chassis, pose little fire risk but may create an electric shock.

Where shock hazard safety is required, designers should integrate optical or magnetic isolation instead of capacitive because when connected in any AC circuit in a series configuration, there is always a current flow through the capacitor.

Conclusion

Digital isolation is used where two or more electric circuits must communicate, but their grounds may be at different potentials. It effectively breaks ground loops by preventing unwanted current from flowing between two units sharing a ground conductor. Digital isolation is also used for safety, preventing accidental electric shocks.