Effective Design Techniques for Signal and Power Supply Isolation


Ashish Gokhale, Isolation Products Manager at Silicon Labs

Today’s electronics designers face a common set of goals: achieving higher throughput, higher resolution, more efficient systems and reduced time to market. In application areas such as industrial automation, medical electronics or telecommunication systems, it is often necessary to electrically isolate multiple signals to enable subsystems to share data or control signals without allowing noise or high voltages to interfere with system integrity and safety. Isolating signals is not sufficient to provide full isolation from noise and high voltages; the power supplies must be isolated too.

Designing an isolated power supply often requires specialized skills and experience. Without the right expertise, components and tools, it costs precious time and multiple iterations to get the design right. Fortunately, plug-and-play isolation solutions that combine excellent signal isolation and power isolation capabilities into one IC are now available to simplify the process of high-voltage power supply design.

Signal Isolation Basics

Isolating signals is necessary to provide the following design-critical functions:

Protection from high voltages: Isolation provides a dielectric barrier that acts as an insulator against high voltages in systems where higher power levels are required. 

Level translation: Enabling noise-free data transfer between circuits that operate at different voltage rails is a common challenge for electronics designers. Although there are many non-isolated level shifters available to circumvent this problem, using an isolator provides several solid advantages. Isolators are the most noise-free and robust solution, and they prevent parasitic paths that may inadvertently switch devices on or off. 

Noise elimination: Isolated products restrict the ground current (return path) of an electrical circuit to only one side of the barrier, enabling a noise-free environment for sensitive measurements on the other side. 

System Considerations

To ensure that effective isolation has been achieved, the designer must eliminate all possible coupling paths from one circuit (Circuit A in Figure 1) to another circuit that needs to be isolated (Circuit B in Figure 1). Hence, when isolating signals, it is equally important to isolate the power supplies. For a circuit designer, the challenge of isolating signals is two-fold: to provide safe, reliable and accurate signal isolation as well as power isolation. Multiple solutions are available for signal isolation to suit the needs of designers based on data rate capabilities, jitter restrictions, noise immunity concerns, high voltage capability and compliance with various isolation component safety standards. However, for many applications where only a watt or so of isolated power is required, very few easy-to-implement power isolation solutions have been available. 


Figure 1: Isolating signals and power is necessary for many designs

Application Example

Factory automation systems depend on efficient and reliable real-time distributed networks to monitor and control complex manufacturing processes. Figure 2 shows a typical simplified hierarchical structure used in these systems. Human machine interface (HMI) in the factory control room is linked to an intermediary controller level and also down to the physical layer where the sensors and actuators are situated as part of motor drive units or machines controlled by programmable logic controllers (PLCs).  


Figure 2: Example of factory automation system requiring isolation 

The physical layer connects the sensors and actuators in a process module and across the factory floor or plant. As shown in Figure 2, 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. 

Figure 3 shows a detailed diagram of an RS-485 transceiver node that has been isolated from the processor. Designers frequently have to design their own solutions from scratch using multiple discrete components to provide isolated power to the secondary side of the isolator and to the RS-485 transceiver on the isolated side.

The transceiver in Figure 3 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, accurate digital isolation as well as 0.5 W to 1 W of power. This power must be supplied by a dedicated isolated source with the following characteristics:
Compact solution: Depending on the particular application, space may be at a premium. In general, a smaller bill of materials (BOM) is always better for manufacturability, reliability and cost.
High efficiency: It is important to have a compact solution with high efficiency to minimize heat and maintain green energy standards. 

Low EMI: It is critical to keep the overall system noise to a minimum for sensitive measurements. To fine-tune the emissions spectrum to a specific use case, it is preferable to have a programmable frequency option that lets developers choose the switching frequency of the dc-dc converter. 

Safety features: In industrial environments where safety is a top concern, it is recommended that the device have a soft start option to avoid inrush currents, current limiting capability, and thermal detection and auto shutdown in case of excessive heat conditions. 
Multiple isolation channels: The isolated power solution must support multiple isolation channels with a minimum of 2.5 kV rms rated isolation capability for meeting safety standards. The isolator must have excellent signal integrity even in a noisy environment.

Solutions for Power and Signal Isolation

Only a few isolation products are currently available that strike the right balance between compactness and the ability to deliver power while minimizing emissions and maximizing efficiency.

Figure 3 shows a discrete solution that uses field-effect transistor (FET) power switches, a pulse-width modulation (PWM) controller, error amp and an optocoupler for feedback as well as other supporting BOM for power isolation. Such solutions are very common, however, these solutions must be designed from scratch, require specialized experience and skill, and may take multiple iterations to get right.


Figure 3: Isolating an RS-485 transceiver

Some solutions integrate digital isolation and the power transformer into a single IC package. These air core transformers have poor coupling coefficients and must be driven at much higher frequencies to deliver equivalent power. This approach results in a much higher emissions profile for electromagnetic interference (EMI), which poses an obstacle for many designers. In addition, the power converter efficiency of such solutions is usually low, from 10-35 percent. In applications where space is at a premium, efficiency is a “don’t-care” and high emissions are not a problem, these solutions might work. But more often than not, such solutions are not practical. 


Figure 4: High-speed digital isolator with integrated dc-dc converter

Other solutions are now available that integrate the signal isolators and dc-dc converter and are designed to work with a discrete transformer. Optimized for the highest efficiency and integration, these compact, single-chip solutions can deliver up to 2 W of power with approximately 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 lev-els 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 5: Si88xx solution used to isolate an RS-485 interface

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.

The Si88xx isolator is an ideal fit for the application example shown in Figure 5. Rated to 2.5 kV rms, the isolated transformer is designed to work seamlessly with the Si88xx IC. By adding a few other inexpensive components such as resistors, diodes and capacitors, the developer has a complete power and signal isolation solution. 

Summary

Elegant IC solutions that combine excellent digital isolation characteristics with high power conversion efficiency and extremely low EMI emissions are now available to simplify the development of isolated power suppliers. These plug-and-play isolation solutions eliminate costly design time and iterations and take the guesswork out of power supply design, ensuring first time success and faster time to market. 

More information: Silicon Labs    Source: Bodo's Power Systems, July 2015