[FAQ] What is Maximum Working Isolation Voltage (VIOWM) and Maximum Repetitive Peak Isolation Voltage (VIORM)?

Please note that all definitions and references are based on the latest standard IEC 60747-17

V_IOWM – Maximum Working Isolation Voltage

V_IORM – Maximum Repetitive Peak Isolation Voltage

Definitions:

V_IOWM - An RMS or equivalent DC voltage assigned by the manufacturer, characterizing the specified long term withstand capability of its isolation.

V_IORM - A peak voltage assigned by the manufacturer characterizing the specified withstand capability of its isolation against repetitive peak voltages. It includes all repetitive transient voltages but excludes all non-repetitive transient voltages.

From which certification/standard are these specifications derived?

DIN EN IEC 60747-17 (VDE 0884-17)

How are they specified in TI datasheets?

How are they validated?

The working isolation and maximum repetitive peak isolation voltages are validated using a partial discharge test, which verifies there is no degradation of the insulation from the absence of localized discharges within the insulation. Since a partial discharge test is included in both Method a and Method b tests, the working isolation and maximum repetitive peak isolation voltage can be validated during qualification and production testing.

End of life testing and accelerated stress testing is also conducted to verify the working and maximum repetitive peak isolation voltages. Accelerated stress testing, often referred to as time dependent dielectric breakdown (TDDB), involves the following:

1. Various levels of voltages greater than the claimed working isolation voltage are applied to the device until the isolation barrier breaks down
2. The time until breakdown is recorded
3. The recorded timepoints are extrapolated into a voltage vs time plot to find the predicted lifetime of the isolator at the claimed working voltage
4. The predicted lifetime is compared to the requirement for the claimed isolation level of the device

Why do these specifications matter and how are they relevant to isolated systems?

Certain applications will have high voltage levels applied across the isolation barrier frequently and/or continuously over the course of the product’s lifetime. The maximum working voltage quantifies the maximum RMS or DC voltage that the isolator can safely support over the course of normal operation. The maximum repetitive isolation peak voltage quantifies the maximum periodic voltage the isolator can safely support over the course of normal operation. It is important for the isolation barrier to have a certified working and maximum repetitive peak isolation voltage greater than or equal to the voltage seen across the barrier to ensure that the barrier does not fail within the device’s lifetime.

How is reinforced isolation defined?

For a reinforced isolation rating, accelerated stress testing must prove that the isolator can withstand a voltage of 1.2 x V_IOWM for over 30 years.

Other notes:

The main difference between the working isolation voltage and the maximum repetitive peak voltage is the units used to define the specification. The maximum repetitive peak voltage is defined as a peak value, while the working isolation voltage is defined as a RMS voltage value. Under sinusoidal stresses the working isolation will be the same as the maximum repetitive peak isolation voltage.