Signal Isolation Basics
Isolating signals is necessary to provide the following design-critical functions:
In order to ensure that true isolation has been achieved, it is important for the circuit designer to eliminate all possible coupling paths from one circuit (Circuit A in Figure 1) to the other that needs to be isolated (Circuit B in the illustration below). Hence, when isolating signals, it is equally important to isolate the power supplies. For a circuit designer, the challenge of isolating signals is really two-fold: to provide safe, reliable and accurate signal isolation as well as power isolation. There are multiple solutions available for signal isolation to suit the needs of designers – based on data rate capabilities, jitter restrictions, noise immunity concerns, high voltage capability, compliance with the various isolation component safety standards etc. However, for many applications where only a watt or so of isolated power is required, there have not been readily available or easily implementable solutions for power isolation.
Factory automation systems depend on efficient and reliable real time distributed networks to monitor and control complex manufacturing processes. A typical and simplified hierarchical structure used in these systems is shown in Figure 6. Human machine interface in the control room at the top is linked to an intermediary controller level and finally down to the physical layer where the sensors and actuators are situated as part of motor drive units or machines controlled by PLC’s (programmable logic controllers).
The physical layer connects the sensors and actuators in a process module and across the factory floor or plant. As shown above, a CAN-based bus communicates with the various motor control units while an RS-485-based bus (PROFIBUS) communicates with the various machines on the factory floor. These physical layers are used commonly in industrial automation because they are very robust even in a noisy environment and support the long distance, multi-point communication needed on a factory floor that may cover hundreds of square meters.
These buses have multiple nodes that connect to the bus through a CAN or an RS-485 transceiver. Isolating these interfaces is critical to protect against high voltages, high electromagnetic (EM) noise and large ground potential differences within the network.
The illustration below shows a detailed diagram of an RS-485 transceiver node that has been isolated from the processor. The isolated power solution is referred to as the isolated dc-dc converter block. Very few easy-to-deploy, high-performance solutions isolated power solutions are available to developers. Designers frequently have to design their own solutions from scratch to provide isolated power to the secondary side of the isolator and to the RS-485 transceiver on the isolated side.
The transceiver in the illustration below is a half-duplex device with receive and transmit lines connected together. It communicates with the RS-485 bus through differential I/Os labelled A and B in Figure 3. The transceiver provides the interface to the processor through its single-ended digital I/Os labelled Rx (receiver) and Tx (transmitter) and an EN (enable pin) signal that controls the transmitter.
The transceiver typically has two to four digital signals that require fast and accurate digital isolation and needs 0.5W to 1W of power, which has to be supplied by a dedicated isolated source with the following characteristics:
Solutions for industrial isolation
There are only a few products on the market that strike the right balance between compactness and the ability to deliver power and between minimizing emissions while maximizing efficiency.
Discrete solutions that use FET’s, controllers, single channel isolators (or optos) for feedback as well as other supporting BOM for power isolation are very common. Such solutions have to be designed from scratch and take specialized experience and skill and could take multiple iterations to get right.
Some solutions integrate digital isolation and the power transformer in a single IC package. These air core transformers have poor coupling coefficients and need to be driven at much high frequencies to deliver equivalent power. This results in a much higher emissions profile for EMI, which is a strong deterrent for many designers.
In addition, the power converter efficiency of such products is usually low, from 10-35%. In applications where space is at a premium, efficiency is a “don’t-care” and high emissions not a problem, these might work. But more often than not, such solutions are not compelling.
There are other solutions that integrate the signal isolators and the dc-dc converter and are designed to work with a discrete transformer. This approach is optimized for the highest efficiency and integration. These solutions are a total solution, are compact and can deliver up-to 2W of power at about 78 percent efficiency.
For example, Silicon Labs’ Si88xx isolation products combine quad digital isolators with a modified fly-back topology dc-dc converter with built-in secondary sensing feedback control. The Si88xx devices have been designed for very low emissions by employing dithering techniques.
Additional features include a soft start capability to avoid inrush currents on startup, cycle-by-cycle current limiting, thermal detection and shutdown for over-temperature events, and cycle skipping to reduce switching losses and thus boost efficiency at lighter loads.
Options for the Si88xx isolators are available for various voltage levels from 5 V to 24 V and for various combinations of digital isolation channels and their directionality. This solution leverages Silicon Labs’ proprietary signal isolation technology, with its signature low EMI profile, to provide high integration, high efficiency and very low EMI.
Figure 4 provides a simplified block diagram of an Si88xx isolator. In addition to the four high-speed digital isolation channels, the Si88xx device integrates a dc-dc controller and internal FET switches that modulate power to the external transformer. The output side incorporates feedback through an external resistor divider to provide excellent line and load regulation.
The dc-dc converter uses dithering techniques to minimize EMI peaks and a zero voltage switching (ZVS) scheme to minimize power loss when modulating power to the transformer. The device uses cycle skipping at light loads to minimize switching losses and boost efficiency. Multiple safety features include cycle-by-cycle current limiting, soft start to avoid inrush currents and thermal shutdown. The device also incorporates several user-programmable features such as soft start time control, a shutdown option for the dc-dc converter and switching frequency control to fine-tune the EMI profile.
In the application example above, the Si88xx is an ideal fit as shown in Figure 5 below. The isolated transformer is rated to 2.5kVrms and is designed to work with the Si88xx IC. By adding a few other components like resistors, diodes and capacitors, a complete power and signal isolation solution is available.
Elegant solutions that combine excellent digital isolation characteristics with high power conversion efficiency and extremely low EMI emissions are now available that make development easier for the digital designer. These are plug and play solutions that eliminate costly design time and iterations and take the guesswork completely out of the picture, ensuring first time success and the fastest time to market.
Check out the Si88X Isolator Evaluation Kit here.