What is Withstand Isolation Voltage (VISO) and Maximum Transient Isolation Voltage (VIOTM)?
Please note that all definitions and references are based on the latest standard IEC 60747-17
VISO – Withstand Isolation Voltage
VIOTM – Maximum Transient Isolation Voltage
Definitions:
VISO - Maximum AC root mean square (RMS) isolation voltage the isolator can handle without breakdown for one minute.
VIOTM - A peak isolation voltage assigned by the manufacturer, characterizing the specified withstand capability of its isolation against transient over voltages for one minute
From which certification/standard are these specifications derived?
The withstand isolation voltage is derived from UL 1577 and the maximum transient isolation voltage is derived from IEC 60747-17 (VDE 0884-17).
How are they specified in TI datasheets?
How are they validated?
The withstand isolation voltage is validated during qualification testing by applying a sinusoidal stress at the claimed withstand voltage value for 60 seconds. During production testing a sinusoidal stress of 1.2* VISO is applied for one second.
The maximum transient isolation voltage is validated with “Method a” testing and “Method b” testing for qualification and production testing respectively.
Why do these specifications matter and how are they relevant to isolated systems?
Events such as arcing or load changes on the power supply can result in voltages much greater than the expected working voltage to be seen across the isolation barrier. It is important that the isolator does not become damaged during these brief high voltage events. An isolator should be able to withstand the brief increase in voltage and then continue to operate normally once the expected working voltage is restored. The withstand voltage quantifies the maximum RMS voltage the isolator can support for 60 seconds, and the maximum transient isolation voltage quantifies the maximum peak voltage the isolator can support for 60 seconds.
Do VISO and VIOTM define reinforced isolation?
VISO and VIOTM do not directly define whether a device is reinforced. The UL 1577 standard does not distinguish a reinforced isolation rating for isolators. Reinforced isolation is defined in the component level digital isolator standard IEC 60747-17 as the insulation of hazardous-live-parts which provides a degree of protection against electric shock equivalent to double insulation. In order for an isolator to qualify as reinforced, a minimum of 10kV surge voltage is required
Other Notes:
The main difference between the withstand isolation voltage and the transient isolation voltage is the units used to define the specification. The withstand isolation voltage is defined as a RMS voltage value, while the maximum transient isolation voltage is defined as a peak value. For sinusoidal stresses, the isolation withstand voltage is typically the same as the maximum transient isolation voltage.
