SLYY081B March 2018 – January 2024 AMC1305M25 , AMC1311 , AMC1311-Q1 , ISO1042 , ISO1042-Q1 , ISO5851 , ISO7741 , ISO7841 , ISOM8710 , UCC20225-Q1 , UCC20225A-Q1 , UCC21520 , UCC21540 , UCC23513 , UCC5390
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.
An example of an isolator configuration is shown in Figure 2. The isolator has two sets of pins. One set is on side 1 and the other set is on side 2. In normal operation, the pins on side 1 are all low voltage with respect to GND1; and the pins on side 2 are all low voltage with respect to GND2. The power dissipated in the isolator depends on voltages and currents applied to the isolator’s pins. In normal operation, power is maintained below the maximum limits specified in the isolator’s data sheet. Very high voltage can exist between GND1 and GND2.This voltage appears across the isolator’s internal isolation barrier.
Isolators are designed to withstand high voltage transient profiles of different magnitude and duration. Correspondingly, the isolation performance of the isolator is quantified by several parameters. The 60-second isolation withstand voltage (VISO and VIOTM) indicates tolerance to short duration overvoltage caused on the system supply lines by switching loads or faults. Repetitive peak or working voltage (VIOWM and VIORM) is the voltage that the isolator can withstand on a continuous basis throughout its operating lifetime. Surge withstand voltage (VIOSM) represents tolerance to a particular transient profile (1.2/50 μs – see IEC 61000-4-5) that represents voltages induced on the power supply lines during direct and indirect lightning strikes.
For each parameter, the limit is set by the voltage value that causes the isolation barrier to break, creating a short circuit from one side of the isolator to another. These parameters are indicated in the isolator’s datasheet and reflect the ability of the isolator to handle high voltage without damage. A detailed discussion of these parameters is presented in reference [1]. At the system level, for example for the motor drive system shown in Figure 1, the incoming supply lines do experience the different over-voltage profiles mentioned above. With one end of the isolated gate-driver galvanically connected to the AC lines, and the other end referenced to earth, it is clear that the isolation barrier in the gate-driver directly faces these stresses.
Figure 3 and Figure 4 depicts these high-voltage stresses appearing across the isolation barrier, for example, using an optocoupler and a series capacitor reinforced isolator from TI. As the stress voltage increases beyond the isolators’ safety limiting values, the isolation barrier can degrade, creating a low resistance path between sides 1 and 2. In the case of optocouplers, the isolation barrier is a combination of silicone and insulating tape; whereas in the case of isolators from TI, it is a series combination of two high voltage SiO2 capacitors. In each case, since the rated limits are obtained by failing the isolation barrier, by definition, both isolators “fail short.”
Depending on the application, end-equipment standards determine the specifications for isolators being used in the system. These standards ensure that isolators are stronger than the voltage stress levels they are likely to encounter across the isolation barrier in a realistic use case. For example, the IEC 61800-5-1 sets the requirements for isolators used in AC motor drive applications. Choosing an isolator that complies with the end-equipment standard minimizes the risk of breakdown of the isolation barrier through overvoltage during actual operation. However, if the isolator exceeds standard requirements, the risk can be reduced further.
Reinforced isolation devices from TI described in this paper use SiO2 as the insulation material, which has a much higher breakdown strength (500 V/μm) than those used by competing solutions. For example, silicone and mold compound used in traditional optocouplers have breakdown strengths of 30 V/μm to 50 V/μm. TI reinforced isolators are also built using a precision semiconductor fabrication process, leading to a tight control on dimensions and spacing. Due to these two factors, TI devices have very high isolation performance for a given package. They effectively address temporary over voltages and surges as well as continuous high-voltage operation for many years. For example, TI Isolators in 16-SOIC packages have 50 percent higher working voltage than similar competing solutions. Further details regarding isolation performance of TI isolators can be found in the corresponding product data sheets, and in reference [1].
For a given application, TI reinforced isolation devices can provide crucial margin, even beyond the requirements mandated by the end equipment standards, which minimizes the likelihood of failure mode 1. A discussion of the IEC 61800-5-1 standard and a performance comparison of TI isolators against the requirements of this standard are provided in reference [2].
Under abnormal or fault events, it is possible that the voltage or currents on one side of the isolator can be very high with respect to the ground on the same side (see Figure 5). One example of this is a short circuit event on a low-impedance output pin. Another example is a short circuit of any pin to a high-voltage DC bus line resulting in electrical breakdown. These are high-power events since high voltages and high currents are simultaneously present.
When these events occur, electrical over stress (EOS) or internal heating can cause the isolation barrier to degrade. For example, if the optocoupler in Figure 6 has a high power event on side 2, it can cause heating or EOS on the detector die. This damage can easily extend into the insulation material, which can degrade isolation performance. It is fair to assume that the insulation is not completely destroyed, but at the same time it is difficult to quantify exactly how much insulation is left.
Looking at Figure 7 for a series capacitor-based isolator, a high-voltage/high-power event on side 2 could damage the right die, and along with it the isolation capacitor that is part of the right die. However, due to the interceding mold compound, the damage does not extend to the left die nor to the isolation capacitor placed on that die. This maintains isolation while preserving roughly half of the original insulation. For instance, if the original isolator is rated for reinforced isolation, after the high-power event, it can be expected to retain full isolation rating of one capacitor. Therefore, while the isolator “fails open,” the “basic insulation” is still maintained.
One way to prevent failure mode 2 is to ensure through external means, for example through current limited power supplies, that even under fault events the heat dissipated inside the isolator is limited to a certain safe limit. This limit is specified through “safety limiting values” for current and power in isolator data sheets, below which the isolation performance remains intact.
However, such current limits are not always feasible to implement. Going back to Figure 1, if the IGBT (1) suffers from a collector to gate breakdown, the high voltage of the DC bus appears at the gate-driver output pin and causes electrical overstress on circuitry connected to that pin. There is no easy way to prevent this from happening at the system level. In such scenarios, the “fail open” behavior of
TI reinforced isolators greatly enhances the system’s electrical safety.