What is maximum impulse voltage (VIMP)?
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Please note that all definitions and references are based on the latest standard IEC 60747-17
VIMP – Maximum Impulse Voltage
The highest instantaneous value of a voltage pulse with short time duration and of specified wave shape per IEC 60747-17 (VDE 0884-17) that an isolator can withstand in air.
From which certification/standard is this specification derived?
DIN EN IEC 60747-17 (VDE 0884-17)
How is it specified in TI datasheets?
How is it validated?
A standard surge test waveform is used to evaluate the surge isolation voltage. The test waveform has a front time of 1.2 µsec and time to half-value of 50 µsec. The test waveform can be seen in the figure below.
Prior to surge testing, all pins on the right and left side of the isolation barrier respectively are shorted to create two pin groups. The isolation unit is then stressed with 50 consecutive surge test pulses with the first 25 pulses in one polarity and the second 25 in the opposite polarity.
The impulse voltage test results in either a pass or fail with no intermediary values. To receive a pass status the unit must have a RIO ≥ 109 Ω. Where RIO is the measured isolation resistance between the left and right side at 500 V.
Why does this specification matter and how is it relevant to isolated systems?
If a sudden high voltage event occurs for a very short amount of time, such as a lightning strike, it is important that the isolator does not become damaged and is able to maintain the isolation barrier once the voltage returns to a lower value. The maximum impulse voltage helps define the maximum voltage that the device can tolerate without taking any damage.
How is reinforced isolation defined?
To achieve a reinforced isolation rating at the component level, the isolator must pass a greater than or equal to 10kV surge test. Since the VIOSM test condition is, VIOSM ≥ 1.3 x VIMP, an impulse voltage of roughly 7.7kV is required for reinforced isolation.
The same testing procedure is used for validating the maximum surge isolation voltage (VIOSM), with the only difference being that impulse voltage testing occurs in air instead of oil.