Technology Comparisons: Distance Through Insulation
This freedom from the need for proximity gives optocouplers a tremendous advantage over other isolation techniques when evaluating the critical parameter – DTI. As shown in Figure 1, the DTI of optocouplers can be one or more orders of magnitude greater than that of other isolation techniques. The typical magnetic isolation device, for example, is built on monolithic CMOS IC material with a thin layer of spin-on polyimide as insulation. Its DTI can be as low as 17 μm. Similarly, capacitors for both capacitive and RF isolation use layers of silicon dioxide (SiO2) as thin as 8 μm. In contrast, optocouplers can typically have insulation thicknesses of 80 μm to 1000 μm, even when housed in smaller SO5 and SO16 surface-mount packages.
DTI is an important element in isolator design for many reasons. The thinner the insulation layer, for instance, the greater the electrostatic stress on the insulator both during ESD and surge events as well as normal operation at its working voltage. The thick insulation layer of optocouplers thus helps reduce stress on the insulator, providing optocouplers with higher reliability and greater lifetime. DTI is also important in the insulation safety rating. Solid insulation needs to be 400 μm or greater in thickness and thin sheet insulation must be at least two layers deep to achieve reinforced status. Some optocouplers, for example, have three layers of insulation with a total DTI of 400μm while other isolation techniques typically offer only one thin layer.
Technology Comparisons: Common-Mode Noise Immunity
In some optocouplers vendors have integrated a low-cost Faraday shield to decouple input side from output side. Additionally, they employ a unique package design to minimize input-to-output capacitance, thus protecting the optocoupler from common-mode noise. The immunity can be demonstrated by applying a high-voltage pulse between the output ground reference and the input-supply ground reference of an isolator device. The optocoupler’s output shows that it is unaffected upon application of 45kV/µs high voltage transient (shown in Fig 2, left). On the other hand streams of data were lost on an RF-based isolator when a low voltage transient of only 4kV/µs is applied.
Technology Comparisons: EMI
In evaluating the EMI performance of isolation devices developers need to explore two aspects: immunity from radiated EMI in the environment and the amount of EMI the device itself generates. To test for susceptibility to the kinds of EMI commonly found in industrial environments, discharge a high-current spike through a coil centered on the isolation device. This will generate a wideband noise burst with both electric and magnetic components. As can be seen in Figure 3, optocouplers continued performing properly with EMI spikes as high as
15 A/30 ns. Magnetic isolation devices, however, failed at levels as low as 2.8 A/30 ns.
The measurement of radiated EMI uses a near-field probe and spectrum analyzer to measure the signal produced by the isolation device.
All of these devices are tested with similar test conditions such as similar inputs signals, same test boards and environment.
The test result shows optocouplers radiate very low emission compared to other isolation devices.
Technology Comparisons: High-Voltage Surge Immunity
The integrity of the barrier itself, which is essential to the safety of equipment and users, must prove resistant to voltage surges and electrostatic discharges (ESD). To test this, in accordance with IEC standard 60747-5-5, apply a 10 kV spike across the isolation barrier. The voltage at which breakdown occurs provides a reliable measure of the isolation barrier’s resilience. As can be seen in Figure 5, optocouplers are able to tolerate voltage surges in excess of 20 kV while other isolation technologies fail between 4 kV and 10 kV.
Optocouplers provide the highest levels of protection and reliability in electrical system. They also generate the least EMI and are most resistant to EMI of all the isolation technologies. Similarly, they are the most resistant to damage or disruption by high-voltage transients. Optocouplers also have the only well-defined safety specification that allows them to receive a reinforced rating for safety critical applications. Thus, when designing safe systems, optocouplers are the best choice for safety and reliable isolation in electrical systems.