Use Integrated Digital Isolators to Protect Industrial Communications Networks

Reliable industrial communication is critical for smooth plant operation and the effective application of Industrial Internet of Things (IIoT) principles. Much of this communication takes place over local networks capable of multidrop communication links and long-haul data transfer. These use proven technology such as an RS-422/RS-485 interface supporting higher-level protocols such as Profibus, Interbus, or Modbus. Still, these networks are prone to disruption.

For example, an electrostatic discharge (ESD) to a cabinet connected to an industrial network can drive the system’s common-mode voltage above 20 volts, well in excess of the 12 volts operational maximum specified in the RS-422/RS-485 standard. Even a particularly robust RS-422/RS-485 transceiver is likely to produce corrupt data—or fail entirely—when exposed to a voltage spike of such magnitude. By isolating sensitive transceivers from signal and power voltage spikes, engineers can mitigate these risks. However, conventional isolation techniques using transformers or optocouplers introduce their own trade-offs including increased solution size, cost, complexity, and throughput limitations.

A new approach to digital isolation, based on chip-scale transformers, has made possible RS-422/RS-485 transceivers that integrate both isolated DC-DC regulators and a three-channel signal isolator on a single chip. The devices enable engineers to build more compact, simpler, and less expensive digitally isolated industrial communications systems.

This article discusses the problem of isolation and different approaches to addressing it. It then describes advances in planar transformers that enable higher integration of digital isolation technology. For a practical example, the article introduces two highly integrated, isolated RS-422/RS-485 transceiver solutions from Analog Devices and how to apply them.

Conventional approaches to power and signal isolation

Power isolation in conventional systems is typically achieved using a transformer (Figure 1), but this technique does have some drawbacks including:

• A transformer is typically more expensive and larger than the equivalent inductor used in a non-isolated power supply, so isolated devices are less compact and costlier.
• A transformer is less efficient than an inductor.
• Because the isolation barrier prevents the power supply’s output from being directly sensed and tightly controlled, its regulation and transient performance is inferior to non-isolated devices.
• Smaller, non-isolated DC-DC converters can be placed close to the load to reduce transmission line effects and boost efficiency.
• Because the transformer is typically a custom-manufactured device, no two devices provide exactly the same output.

Figure 1: An isolated DC-DC power supply (bottom) uses a transformer in place of the inductor of the non-isolated version. This increases size and cost and lowers efficiency. (Image source: DigiKey)

The conventional method to implement an isolated signal barrier is to use an optocoupler. The most basic type of optocoupler employs an LED and a phototransistor enclosed in a lightproof package, but other versions are available. The LED switches on and off to represent digital information, and the phototransistor—a light-sensitive bipolar device—reacts by altering the flow of current between its emitter and collector.

Signal isolation using an optocoupler is simple and effective, but it does have some disadvantages. These include:

• The LED’s power requirements are relatively high and the optocoupler’s LED must be on whenever the input signal is high. That can be inefficient.
• Optocouplers often stop working without warning due to failure of the LED.
• Propagation delays limit throughput.
• Because the optocoupler’s input and output are not driven by logic gates, connection between the device and the rest of a digital system is more complex.
• It is difficult to integrate multiple optocoupler channels into a single package.

In addition to these challenges, conventional isolation requires separate power and signal isolation components because the bulky transformer does not lend itself to integration on the same device as the optocoupler.

Shrinking digital isolation

Digital isolation offers a solution to the challenges imposed by conventional isolation by not using expensive, bulky transformers and optocouplers that feature limited throughput. Together with longevity and high throughput, the technology offers lower power consumption and a more compact solution.

However, digital isolation still tends to add cost and complexity because the components are relatively expensive, and separate devices are needed for isolated power and signal functionality (in addition to the network’s transceivers) in order to meet isolation standards.

But recent advances in technology, materials and miniaturization has led to much higher levels of integration and isolation performance such that the need for an external DC-DC isolation block has now been eliminated. These digital isolation solutions reduce the cost, complexity and space requirements.

Examples of enhanced digital isolation technology are Analog Devices’ iCoupler and isoPower digital signal and power isolation technologies. isoPower employs a secondary-side controller architecture with isolated pulse-width modulation (PWM) feedback. Power is supplied to an oscillating circuit that switches current into a chip-scale, planar transformer which in turn transfers power to the secondary side where it is rectified and regulated to 3.3 volts (Figure 2).

Figure 2: iCoupler and isoPower use chip-scale planar transformers that eliminate the need for off-chip power and signal isolation blocks. (Image source: Analog Devices)

A feedback loop using an isolated data channel modulates the oscillator circuit to control the power being sent to the secondary side. By adding feedback, higher power is possible, and efficiency and regulation are significantly improved. The chip-scale transformer provides excellent common-mode transient immunity of up to 100 kilovolt/microsecond (kV/μs).

iCoupler also employs chip-scale transformer windings to couple digital signals magnetically. This type of digital isolation reduces power consumption by an order of magnitude compared with an optocoupler. The technique is based on encoding the rising and falling edges of the input signals into double or single current pulses that drive the primary winding. This in turn creates a small, localized magnetic field that induces current in the secondary winding. The current pulses are around 1 nanosecond (ns) in duration, so the average current is modest. The pulses are decoded back into rising/falling edges on the secondary side (Figure 3).

Figure 3: iCoupler encodes the rising and falling edges of input signals into current pulses to drive the primary winding and induce current in the secondary winding. The pulses are then decoded back into rising/falling edges. (Image source: Analog Devices)

Isolated industrial network solutions

Commercial transceivers with integrated iCoupler and isoPower signal and power isolation on a single chip are now available. Analog Devices’ digitally isolated ADM2682EBRIZ and ADM2687EBRIZ RS-422/RS-485 transceivers offer a compact, simple, and inexpensive digital isolation solution with low power consumption.

The ADM2682EBRIZ features a data rate of 16 megabits per second (Mbits/s), while the ADM2687EBRIZ can manage 500 kilobits per second (kbits/s). The devices are fully integrated 5 kilovolts (kV)_rms signal and power isolated data transceivers with ±15 kV ESD protection, and are suitable for high-speed communication on multidrop industrial systems. The transceivers include an integrated 5 kV_rms isolated DC-DC power supply, eliminating the requirement for an external DC-DC regulator.

Incorporated into each chip is a three-channel isolator, a three-state differential line driver, a differential input receiver and an isolated DC-DC converter (Figure 4). The ADM2682EBRIZ and ADM2687EBRIZ are powered from a 3.3 volt or 5 volt supply. Features include current limiting and thermal shutdown to protect against output short circuits and situations where bus contention may cause excessive power dissipation. They are specified for operation over the industrial temperature range of -40˚C to +85˚C.

Figure 4: Analog Devices’ ADM2682EBRIZ and ADM2687EBRIZ transceivers integrate a three-channel isolator, a three-state differential line driver, a differential input receiver and an isolated DC-DC converter into a single package. (Image source: Analog Devices)

These RS-422/RS-485 transceivers are certified in accordance with UL1577, a specification for optical, capacitive and inductive isolators. The specification requires protection and insulation for up to 5 kV for up to one minute, and 25 kV per microsecond (kV/μs) transient immunity between the controller ground and RS-422/RS-485 signal lines.

Managing EMI of digital isolation devices

While digital isolation addresses the design challenges of conventional isolation, it does introduce a key one of its own; the technique’s use of oscillator circuits and current pulses increases the chance of electromagnetic interference (EMI).

For example, Analog Devices’ isolated power technology uses oscillator circuits that switch current into the transformer at a frequency between 180 and 300 megahertz (MHz). The rectifier circuit on the secondary side doubles this frequency during rectification. The resultant operating frequency is three orders of magnitude higher than a standard DC-DC converter, and noise generated by the device in the 30 MHz to 1 gigahertz (GHz) can cause problematic EMI.

There are two potential sources of EMI in four-layer pc boards with RS-422/RS-485 transceivers using iCoupler and isoPower: edge emissions and input-to-output dipole emissions. Edge emissions are generated where differential noise from many sources meets the edge of the board, leaking out of a plane-to-plane space which acts as a wave guide. Input-to-output dipole radiation is generated by driving a current source across a gap between ground planes – precisely the function of an isolated power supply (Figure 5).

Figure 5: Input-to-output dipole radiation is generated by driving a current source across a gap between ground planes. (Image source: Analog Devices)

Designers can use the following techniques to reduce these emissions:

• Input-to-output ground plane stitching capacitance
• Load control
• Edge guarding
• Interplane capacitive bypass

By placing a stitching capacitor in proximity to the signal across any splits in the pc board ground plane, the designer eliminates any differential currents and voltages between conductive planes of the pc board that could generate electrical noise. There are three techniques used to form stitching capacitance: A safety rated capacitor applied across the barrier; a floating metal plane spanning the gap between the isolated and non-isolated sides on an interior layer, or extending the ground and power planes on an interior layer into the isolation gap of the pc board to form a capacitor.

The designer can reduce incidence of EMI by operating the isoPower device under as light a load as possible. Light loads reduce the ON time of the oscillator which in turn lowers the amount of noise the device generates.

Edge guarding using a solid conductive edge treatment on a pc board is possible, but expensive. A cheaper solution that works well for edge guarding is to treat the edges of the board with a guard ring structure laced together by vias. There are two goals in creating edge guarding. The first is to reflect cylindrical emissions from vias back into the interplane space, not allowing it to escape from the edge. The second is to shield any edge currents flowing on internal planes due to noise or large currents.

Interplane capacitive bypass is a technique intended to reduce both the conducted and radiated emissions of the board by improving the bypass integrity at high frequencies. It can be implemented by using a thin core layer for the power and ground planes. These tightly coupled planes provide an interplane capacitance layer that supplements any board-mounted bypass capacitors.

Evaluating isolated industrial communication systems

Analog Devices offers evaluation boards for the ADM2682EBRIZ and ADM2687EBRIZ RS-422/RS-485 transceivers. Specifically, the EVAL-ADM2682EEBZ evaluation board for the ADM2682E and the EVAL-ADM2687EEBZ evaluation board for the ADM2687E.

The boards enable easy evaluation of the signal and power isolated RS-422/RS-485 transceivers. Screw terminal blocks provide convenient connections for the power and signal connections, and the evaluation boards are easily configured through jumper connections.

The evaluation boards can be used in half- or full-duplex configurations. A 120 ohm (Ω) termination resistor (R_T) is fitted to the receiver inputs. The driver and receiver are enabled and disabled by jumper connections. Test points are included on the power and signal lines on both sides of the isolation barrier. The LK1-4 links can be reconfigured to enable/disable functions or switch inputs and outputs from test points to terminal blocks. When LK5 and 6 links are both connected, the board is configured for half-duplex operation, and when both are open the board is configured for full-duplex operation (Figure 6).

Figure 6: Basic operational set-up of Analog Devices’ evaluation board for testing digitally isolated RS-422/RS-485 transceivers. The LK1-4 links can be reconfigured to enable/disable functions or switch inputs and outputs from test points to terminal blocks. LK5 and 6 links determine full or half-duplex operation. (Image source: Analog Devices)

Not only do the evaluation boards allow the designer to test out industrial communications systems based on the ADM2682EBRIZ and ADM2687EBRIZ RS-422/RS-485 transceivers, but they are also designed following the above techniques to reduce EMI generated by the high frequency switching elements used to transfer signals and power through the transceivers.

A full-duplex circuit implementation of the ADM2682E/2587E is shown in Figure 7. Up to 256 transceivers can be connected to the bus. Placement of R_T depends upon the location of the node and the network configuration. In general, to minimize reflections, terminate the line at the receiving end in its characteristic impedance and keep stub lengths to a minimum.

Figure 7: Up to 256 transceivers can be connected to the RS-485/RS-422 bus. Designers need to be careful to place R_T at the receiving end, which is both ends when in half-duplex mode (full-duplex mode is shown here). (Image source: Analog Devices)

For half-duplex operation, both ends of the line must be terminated because either end can be a receiving end.

Conclusion

Industrial communications systems are at risk from signal and power voltage spikes.  Engineers can eliminate these risks using digital isolation techniques, but conventional solutions that include discrete isolation components introduce cost, complexity and space trade-offs.

As shown, new approaches that advance the state-of-the-art in planar transformers have enabled the integration of digital isolation onto transceivers, such as those for RS-422/RS-485 networks, lowering cost and reducing space.