Isolators are devices that minimize direct current (DC) and unwanted transient currents between two systems or circuits, while allowing data and power transmission between the two. In most applications, in addition to allowing the system to function properly, isolators also act as a barrier against high voltage. For example, in the motor drive system shown in Figure 1, the isolated insulated gate bipolar transistor (IGBT) gate drivers level shift low-voltage signals from the control module to IGBT gate-drive controls referenced to the inverter outputs. At the same time, they also form a protective barrier between the high voltage (DC bus, inverter outputs, and input power lines) and the control module, which may have human accessible connectors and interfaces.

In high-voltage applications, failure of the isolation barrier can result in a potential hazard to human operators, or cause damage to sensitive control circuitry leading to further system malfunction. Therefore, it is important to understand what may cause the isolator to fail, both under normal and fault conditions. You also need to know the nature of the failure in each case in order to check if additional measures are required to prevent an electrical hazard.

In this paper, we discuss two possible failure modes of isolators. The first is when the voltage across the isolation barrier exceeds the isolator’s rated limits. The second is when circuits or components integrated in the isolator close to the isolation barrier are damaged by a combination of high voltage and high current. Potentially this can cause damage to the isolation barrier. In our analysis, we consider the latest reinforced isolation technology from TI and traditional optocouplers as examples. We show that while all isolators “fail short” for the first failure mode, TI isolators reduce the likelihood of failure because of higher isolation performance. We also show through analysis and test results that TI reinforced isolators “fail open” for the second failure mode.

Figure 1 Simplified block diagram of an AC motor drive.