Digital isolation rivals optocouplers in terms of power, size and performance

For years, designers of industrial, medical and other isolated systems had limited options when implementing safety isolation: the only reasonable choice was the optocoupler. Today, digital isolators offer advantages in performance, size, cost, power efficiency and integration.

Understanding the nature and interdependence of the three key elements of a digital isolator is important in choosing the right digital isolator. These elements are: the insulation material: the structure: and the data transfer method.

Designers incorporate isolation either because of safety regulations or to reduce noise from such features as ground loops. Galvanic isolation ensures data transfer without an electrical connection or leakage path that might create a safety hazard. Yet isolation imposes a number of constraints, such as delays, power consumption, cost and size. A digital isolator's goal is, therefore, to meet safety requirements while minimising incurred penalties.

Optocouplers, a traditional isolation approach, incur the greatest number of penalties, consuming high levels of power and limiting data rates to less than 1Mbit/s. While more power efficient and higher speed optocouplers are available, these impose a higher cost penalty.

Digital isolators were introduced more than a decade ago to reduce penalties associated with optocouplers. These use cmos based circuitry and offer significant cost and power savings, while improving data rates significantly. They are defined by the elements noted above: insulating material determines inherent isolation capability and is selected to ensure compliance to safety standards; structure and data transfer method are chosen to overcome the cited penalties. All three elements must work together to balance design targets, but the one target that cannot be compromised and 'balanced' is the ability to meet safety regulations.

Digital isolators use foundry cmos processes and are limited to materials commonly used in foundries. Non standard materials complicate production, resulting in poor manufacturability and higher costs. Common insulating materials include polymers such as polyimide (PI), which can be spun on as a thin film, and silicon dioxide (SiO2). Both have well known insulating properties and have been used in standard semiconductor processing for years. Polymers have been the basis for many optocouplers, giving them an established history as a high voltage insulator.

Safety standards typically specify a one minute voltage withstand rating (typically 2.5kV rms to 5kV rms) and working voltage (typically 125V rms to 400V rms). Some standards also specify shorter duration, higher voltage (for example, 10kV peak for 50µs) as part of certification for reinforced insulation. Polymer/polyimide-based isolators yield the best isolation properties.

Polyimide based digital isolators are similar to optocouplers and exceed lifetime at typical working voltages. SiO2 based isolators, however, provide weaker protection against surges, preventing their use in medical and other applications.

The inherent stress of each film is also different. Polyimide has lower stress than SiO2 and can be increased in thickness as needed. The thickness of SiO2 and, therefore its isolation capability, is limited; stress beyond 15 µm may result in cracked wafers during processing or delamination over the life of the isolator. Polyimide based digital isolators, however, use isolation layers as thick as 26µm