How Should the Isolation Voltage for Opto-Isolation Devices Be Selected?

The isolation voltage for opto-isolation devices should be selected based on the voltage level of the operating environment, potential voltage fluctuations, and relevant safety standards. Generally, the isolation voltage is required to be at least 1.5 to 2 times the system's maximum operating voltage. In common industrial applications, it is recommended to select products with an isolation voltage ranging from 3.75 kV to over 5 kV.

I. The Core Functions of Isolation Voltage

Isolation Voltage (Viso) is a critical safety parameter for optocouplers; it refers to the maximum voltage that the device can continuously withstand between its input and output terminals without suffering dielectric breakdown. Its primary functions include:

Personal Safety Protection: Preventing fault voltages from the high-voltage side from intruding into low-voltage control circuits, thereby eliminating the risk of electric shock.

System Reliability Assurance: Suppressing common-mode interference, voltage surges, and ground loop noise to ensure stable signal transmission.

Equipment Protection: Blocking potential damage to sensitive components (such as MCUs and PLCs) caused by high-voltage circuits, thereby extending their operational lifespan.

II. How to Scientifically Select Isolation Voltage?

1. Matching Based on System Operating Voltage

Low-Voltage Signal Systems (<50V): Such as sensors and communication interfaces; optocouplers with an isolation voltage of 1.5 kV to 3.75 kV may be selected to strike a balance between response speed and cost.

Mains-Related Systems (220V/380V AC): Such as switching power supplies and variable frequency drives; devices with an isolation voltage of ≥ 3.75 kV must be selected. 5 kV or higher is recommended to effectively withstand lightning surges and switching transients.

High-Voltage Industrial Equipment (e.g., PV inverters, motor drives): It is recommended to use high-isolation-voltage devices of over 8 kV to meet safety regulatory requirements, such as those specified in IEC 60747-5-2.

2. Considering Transient Overvoltage and Safety Margins

In practical applications, transient disturbances-such as voltage spikes and Electrical Fast Transient (EFT) bursts-must be taken into account; therefore, the rated isolation voltage should include a sufficient safety margin.

Typically, an isolation voltage ≥ 2 times the system's peak voltage is selected. For example, in a 220V AC system where the peak voltage is approximately 311V, it is recommended to use an optocoupler with an isolation voltage of 3.75 kV or higher. 3. Adherence to Safety Certification Standards

Fields such as medical equipment and industrial control systems must comply with safety standards such as UL, IEC, and VDE, which impose specific requirements regarding isolation voltage:

IEC 60601 (Medical Equipment): Patient-contacting isolation sections require a minimum isolation voltage of ≥4kV RMS.

IEC 61010 (Measurement Equipment): Specifies minimum isolation voltage requirements based on overvoltage categories (CAT II/III).

When selecting components, priority should be given to models that have obtained the necessary certifications and provide corresponding test reports.

4. Reference Typical Device Parameters

Tip: For high-frequency or high-speed systems, it is also essential to consider parameters such as Common Mode Rejection Ratio (CMR) and propagation delay; avoid focusing solely on isolation voltage while overlooking dynamic performance.