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Balancing major elements of isolator for safety

30 Jan 2014  | David Krakauer

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Designers of industrial, medical, and other isolated systems used to have limited choices 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 three key elements of a digital isolator is important in choosing the right digital isolator. These elements are insulation material, their structure, and data transfer method.

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

Optocouplers, a traditional isolator, incur the greatest penalties. They consume high levels of power and limit data rates to below 1Mbit/s. More power efficient and higher speed optocouplers are available but impose a higher cost penalty.

Digital isolators were introduced over 10 years ago to reduce penalties associated with optocouplers. They use CMOS-based circuitry and offer significant cost and power savings while significantly improving data rates. 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.

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

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

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

Isolator structure
Digital isolators use transformers or capacitors to magnetically or capacitively couple data across an isolation barrier, compared to optocouplers that use light from LEDs.

Transformers pulse current through a coil, as shown in figure 1, to create a small, localized magnetic field that induces current in another coil. The current pulses are short, 1 ns, so the average current is low.

Figure 1: (a) Transformer with thick polyimide insulation where current pulses create magnetic fields to induce current on the secondary coil; (b) capacitor with thin SiO2 insulation using low-current electric fields to couple across isolation barrier.

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