25 October 2011
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.
Digital isolators use transformers or capacitors to couple data magnetically or capacitively across an isolation barrier, compared to optocouplers that use light from leds.
Transformers pulse current through a coil (see fig 1) to create a small, localised magnetic field that induces current in another coil. The current pulses are short – of the order of 1ns – so the average current is low.
Transformers are also differential and provide excellent common mode transient immunity: often, as high as 100kV/µs, compared to a typical optocoupler performance of about 15kV/µs. Magnetic coupling also has a weaker dependence on the distance between the transformer coils compared with the dependence for capacitive coupling on the distance between plates. This allows for thicker insulation between transformer coils, resulting in higher isolation capability. Combined with low stress polyimide films, high levels of isolation may be achieved for transformers using polyimide, rather than capacitors using SiO2.
Capacitors are also single ended and have higher susceptibility to common mode transients. Differential pairs of capacitors can compensate, but this increases size and cost.
One benefit of capacitors is that they use low currents to create the coupling electric field. This becomes noticeable at data rates in excess of 25Mbit/s.
Data transmission methods
Optocouplers use light from leds to transmit data across an isolation barrier: the led turns on for a logic HIGH and off for logic LOW. While the led is on, the optocoupler burns power, making optocouplers a poor choice wherever power consumption is a concern. Most optocouplers leave the signal conditioning at the input and/or output to the designer, which is not always the easiest to implement.
Digital isolators use more advanced circuitry to encode and decode data, allowing for more rapid data transmission and the ability to handle complex bidirectional interfaces, such as usb and i2c.
One method encodes rising and falling edges as double or single pulses that drive a transformer (see fig 2). These pulses are decoded back into rising/falling edges on the secondary side. This reduces power consumption by up to two orders of magnitude, compared to optocouplers because power is not applied continuously. Refresh circuits can be included to regularly update the dc level.
Another method uses rf modulated signals in much the same way that optocouplers use light; logic HIGH signal results in continuous rf transmission. This consumes more power than the pulsed method because logic HIGH signals burn power continuously.
Differential techniques may also be employed for common mode rejection; however, these are best used with differential elements such as transformers.
Choosing the right combination
Digital isolators offer significant, often compelling, advantages over optocouplers in terms of size, speed, power consumption, ease of use and reliability. Within the class of digital isolators, different combinations of insulating material, structure and data transfer method distinguish different products, making some more or less suitable to particular applications.
As noted above, polymer based materials offer the most robust isolation capability; this material can be used in almost all applications, but the most stringent requirements, such as those in the healthcare and heavy industrial equipment sectors, will gain the most advantage. To achieve the most robust isolation, polyimide thickness may be increased beyond what is reasonable for capacitors; therefore, capacitor based isolation may be best suited for functional isolation where safety isolation is not required.
In those cases, transformer based isolation may make the most sense, especially when combined with a differential data transfer method that takes full advantage of the differential nature of transformers.
David Krakauer is a product line manager with Analog Devices.
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