[FAQ] What is the difference between UL1577 VISO Rating and Test Voltage?

For devices that provide galvanic isolation, like digital isolators, optocouplers, or opto-emulators, there are many different specifications that might not always be shared across every single device.  This is sometimes due to a difference in isolation technology (capacitive vs optical). It could also be due to a difference in certification, or lack thereof (VDE certified vs non-VDE  certified). However, despite these differences, there is one spec that seems to be common across all isolation devices: V_ISO . When comparing isolation performance between products, it is important to understand the difference between the voltage at which the device is tested in production and the actual V_ISO rating that the device is certified for.

The V_ISO spec, defined by the UL1577 standard, is the maximum rms voltage the isolator can withstand without dielectric breakdown for 60 seconds. This spec is mainly used to indicate if the isolator can survive unintentional disturbances in the system. For example, noise on the supplies caused by arcing or load changes, could briefly cause the voltage across the isolator to be several times the line voltage. For a device to be certified for a specific V_ISO rating per UL1577, two things need to happen: 1.) certification testing and 2.) production testing. During certification testing, a set of at least 6 devices are stressed for 60 seconds with the same rms voltage as the intended V_ISO rating. During production testing, every single device has to be stressed for 1 second where the rms voltage is 120% of the rated V_ISO voltage. For example, if a device has a V_ISO rating of 3000 Vrms, the device is stressed at 3000 Vrms for 60 seconds during certification and 3600 Vrms for 1 second during production.

It is important to understand the differences between the certification (also referred to as qualification) test voltage and the production test voltage because some device datasheets can be misleading. For example, an optocoupler might be advertised as 5300 Vrms device, but hide that fact that 5300 Vrms is the 1 second production test voltage, not the 60 second voltage the device is actually certified and rated for (see below). If this optocoupler were to be used in a system that requires a V_ISO rating of 5 kVrms, it would cause system level reliability issues.

On the other hand, TI’s isolation devices, such as ISOM811x, clearly state in the insulation characteristics table how we spec V_ISO by showing the differences between the production test voltage and qualification test voltage (see below).

In conclusion, it is important to make sure you are interpreting the device spec correctly when deciding which device to use. When it comes to V_ISO rating, make sure you are looking at the 60 s certification rating and not the 1 second production test voltage.