Surge Testing of Digital Isolators

Many applications require the isolation of hazardous voltages to comply with international safety standards. To ensure the safety of equipment and operators, these standards often require isolation components (such as digital isolators or optocouplers) to withstand high voltage surges of more than 10 kV (peak). Therefore, testing isolator surge performance is an essential part of developing safe and reliable devices.

Standards published by the International Electrotechnical Commission (IEC) and VDE (Verband der Elektrotechnik) define the use of isolation technology at the system and component level in medical, industrial, consumer, and automotive systems. To ensure the safety of personnel and equipment during high voltage surges, these standards specify different surge ratings based on the isolation level required for the specific application.

There are three common isolation levels: functional isolation, basic isolation, and reinforced isolation. Functional isolation has only a few safety requirements because it is generally only used in applications where the isolation of the ground reference voltage is required to ensure the circuit can operate properly. As can be seen, safety and surge performance are not the main considerations for functional isolation.

However, safety is the main consideration for basic and reinforced isolation, so surge levels are key to determining the quality of isolation. Basic isolation protects end-equipment users from electric shock, while reinforced isolation is a separate isolation system that provides the same protection as two redundant single or basic isolation systems. Medical and industrial applications typically require reinforced isolation to protect patients and end users from lethal electric shock. The VDE reinforced isolation standard for digital isolators is VDE 0884-10, which specifies a minimum surge voltage (VIOSM) rating of 10 kV, and also specifies working voltage (VIORM) and withstand voltage (VISO). The surge voltage

rating of a digital isolator specifies the ability to withstand a surge after being subjected to continuous short-term high-voltage pulses. Figure 1 shows the timing characteristics of a surge waveform that complies with IEC 61000-4-5.

 
Figure 1. Surge voltage waveform

For testing, place the device on a test board and short all pins across the isolation barrier (see Figure 2). Connect a high-voltage pulse generator to one end of the isolation barrier through a 1000Ω/1000 pF network. Connect the generator loop to the other end of the isolation barrier. Connect a 100 kΩ, 2.5 W resistor across the isolation barrier to discharge the circuit after each pulse. Monitor the pulses with an oscilloscope with a 1000:1 high-voltage probe. Set the discharge gun to the lowest voltage specified in the test plan and the oscilloscope to single-shot trigger. Apply 10 pulses at this voltage level and monitor each pulse with the oscilloscope. A break in the isolation barrier can be seen by a sudden drop in pulse amplitude (to 50% in less than 50 μs). If the part can withstand 10 pulses, increase the discharge gun voltage and apply another 10 pulses. Continue until the isolation barrier fails or until the maximum test voltage is reached.

 
Figure 2. Surge test setup

Whether or not to pass this test depends primarily on the thickness of the insulation (also known as the distance to isolation, or DTI) and the quality of the insulation material. Applied electric fields tend to concentrate at defect points within the insulator, so lower defect densities generally result in higher breakdown ratings. Thicker materials are more resistant to breakdown because the field strength is inversely proportional to the distance between the conductors at either end of the insulator.

Optocouplers can generally pass the 10 kV surge test because the insulator is thick (typically 400 μm), which reduces the effect of insulation quality on the breakdown characteristics. Simply put, the insulation is thick, so high-quality materials are not required to pass the 10 kV test. Transformer-based isolators use a high-quality 20 μm to 32 μm polyimide layer stored in a clean room environment. Because the defect levels of this material are much lower than the injection-molded epoxy used in optocouplers, a much thinner insulation layer can still meet the 10 kV requirement. Capacitive isolators also use a high-quality insulation layer, in this case silicon dioxide (SiO2) deposited during the wafer manufacturing process. Silicon dioxide has a high dielectric strength, but generally cannot be deposited very thickly without causing mechanical stress in the film. Thicker SiO2 also reduces capacitance, which in turn results in a decrease in the coupling efficiency of the isolation barrier. For this reason, capacitive isolators generally cannot pass the 10 kV surge test and therefore cannot pass the VDE's reinforced isolation certification.

In reinforced isolation applications where protection of personnel and equipment is required, 10 kV surge protection is required. Surge testing is a critical step in determining the safety level of isolation components in such applications. Analog Devices offers a wide range of iCoupler and isoPower products that are fully capable of meeting this need.