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Techniques and Solutions for USB Power and Data Isolation

By Doug Peters

Contributed By Digi-Key's North American Editors

Introduced in 1996, the universal serial bus (USB) has become the leading method in which to connect peripherals to PCs. With USB data rates increasing over the past 24 years from 1.5 megabits per second (Mbits/s) to over 20 gigabits per second (Gbits/s), test and measurement manufacturers in particular have taken notice and have gone to market with USB-based test equipment. Hobbyists have also taken advantage of the ubiquity of USB and have developed many of their own unique measurement tools.

However, there is a potential danger lurking when using or designing USB-based equipment connected to a PC’s USB port. While a device under test (DUT) may be powered from a floating power supply, once it’s plugged into an earth-grounded PC, ground loops may come into play. As a result, severe ground potential differentials may be generated that can cause circuit damage, or worse, personal injury.

To eradicate ground loop connections, both power and data communication paths need to be galvanically isolated from the PC’s USB earth ground. There are several options for isolating data communications depending on the data rate and protocol. In addition, multiple isolation strategies can be deployed including capacitive, optical, and electromagnetic.

This article defines galvanic isolation before describing many of the different USB isolation technologies and the pros and cons of each. It will then introduce real-world isolation solutions from Texas Instruments, Würth Electronik, ON Semiconductor, and Analog Devices, and show how to apply them effectively.

What is galvanic isolation?

At its core, galvanic isolation prevents current flow or conduction between two or more separate electrical circuits, while still allowing energy and/or information to pass between them.

For purposes of simplification, this article will focus on two separate circuits, referred to as the primary side and secondary side. The primary circuit is USB powered and shares bidirectional data flow with a host PC. The region separating the circuits is called an isolation barrier and is selected in order to withstand breakdown voltages of hundreds to thousands of volts. Typically, air, silicon dioxide (SiO2), polyimide, or other non-conductive material separates the two circuits (Figure 1).

Diagram of galvanic isolation between the USB input on the primary side and secondary sideFigure 1: Shown is an example of galvanic isolation between the USB input on the primary side of the circuit and the secondary side. The isolation barrier needs to withstand voltages of hundreds to thousands of volts. (Image source: Digi-Key Electronics)

Isolated data transfer

As defined above, galvanic isolation allows for data or information transfer between the separated electrical circuits. But how can this be achieved without some type of conductive material between the circuits? There are several practical solutions to this problem including optical, capacitive and electromagnetic technologies. There are advantages and disadvantages to each of these approaches as discussed below. For the designer, consideration of data rates, electrostatic discharge (ESD), interference, and power requirements all come into play when deciding on which strategy to use.

Optical: One of the most well-known approaches to isolation is the optical isolator or optoisolator (or optocoupler). Isolation is achieved through the use of a light emitting diode (LED) on the primary side of the isolation barrier and a photo sensitive transistor on the secondary side. The ON Semiconductor FOD817 is a good example of an optoisolator (Figure 2). Data is transmitted using light pulses over the isolation barrier from the LED, which is received by the photo transistor in an open collector configuration. When the LED is on, the photodiode will generate a current flow in the secondary circuit.

Given that light is used for data transfer, the optoisolator is not susceptible to electromagnetic interference (EMI). On the negative side, data transfer rates can be slow because the data rate is a function of the switching speed of the LED. Also, optoisolators tend to have a shorter lifespan compared to other technologies due to LED degradation over time.

Diagram of optoisolator - the LED emits light pulses through the isolation barrierFigure 2: Optoisolator - the LED emits light pulses through the isolation barrier which are received by the photodiode and generate current flow in the secondary circuit. (Image source: ON Semiconductor)

The FOD817 is a single-channel device rated up to 5 kilovolts (kV) rms AC for one minute. It comprises a gallium arsenide (GaAs) infrared (IR) LED driving a silicon phototransistor. Applications can include power regulators and digital logic inputs.

Electromagnetic isolation: This is perhaps the oldest technological approach to circuit isolation. The fundamentals of electromagnetic induction are used to transfer data (and power, as discussed later) between two coils. This approach has been significantly enhanced over time by companies like Analog Devices with its iCoupler technology. The iCoupler technology embeds the transformer coils within an integrated circuit and uses a polyimide substrate for the isolation barrier.

Electromagnetic approaches to isolation are more susceptible to magnetic field interference than optoisolators, and they generate their own potential EMI which may need to be addressed in the product design stage. However, the advantages are higher data rates of 100 Mbits/s or more and low power consumption.

The ADuM1250 from Analog Devices provides an example of this type of technology (Figure 3). Targeting bidirectional I2C data isolation applications such as hot-swap applications, the device features a data rate of up to 1 Mbit/s and is rated at 2500 volts rms for one minute per UL 1577. It draws 2.8 milliamperes (mA) of input current (IDD1) on the primary side and 2.7 mA of current on the secondary side (IDD2) at a 5 volt supply voltage (VDD1 and VDD2). Note that each I2C channel (clock and data lines) in the ADuM1250 requires two embedded transformers to achieve bidirectionality.

Typically, data is transmitted between the transformer coils using an edge transition schema. Short, one nanosecond pulses are used to identify leading and trailing edges of the data signal. Encoding and decoding hardware is also built into the device.

Diagram of Analog Devices ADuM1250 dual I2C IsolatorFigure 3: On the ADuM1250 dual I2C Isolator, each I2C line requires two distinct transformers to achieve bidirectional data and clock transfer. (Image source: Analog Devices, Inc.)

Capacitive isolation: Capacitive isolation is achieved, as its name implies, through the use of capacitors (Figure 4). Due to the characteristics of capacitive technology, DC voltage is blocked by the capacitor, while AC voltage is allowed to flow freely.

Diagram of capacitive isolation leverages the capacitive characteristic of blocking DC signals Figure 4: Capacitive isolation leverages the capacitive characteristic of blocking DC signals and allowing AC signals to flow across the isolation barrier. (Image source: Texas Instruments)

By using a high-frequency carrier (AC) for data transfer across the capacitor, information can be passed using a modulation schema such as on-off keying (OOK). The presence of a high-frequency carrier might constitute a digital output of zero (LOW), and absence of the carrier would signify a one (HIGH) (Figure 5).

Diagram of on-off keying (OOK) schema uses a high-frequency carrier (AC) signalFigure 5: An on-off keying (OOK) schema uses the presence or absence of a high-frequency carrier (AC) signal delivered through the isolation barrier to transfer a logic level HIGH or LOW signal. (Image source: Texas Instruments)

Like magnetic isolation, the advantages of capacitive isolation are high data transfer rates (100 Mbits/s or higher) and low power consumption. Disadvantages include greater susceptibility to electric field interference.

A great example of capacitive isolation technology is Texas Instruments’ ISO7742 quad-channel digital isolator with isolation up to 5000 volts rms . The device comes in multiple configurations depending on the required direction of data flow. It has a data rate of 100 Mbits/s and consumes 1.5 mA per channel. Applications for the ISO7742 include medical equipment, power supplies and industrial automation.

USB power isolation

Paying close attention to the datasheets of isolation components, designers will quickly realize that each side of the isolation component requires separate power sources: one for the primary side and one for the secondary side (VCC1 and VCC2), each with their respective ground reference to maintain the isolation barrier.

If the design under consideration has separate power sources, USB 5 volts on the primary side and a separate battery plus ground to power the secondary, then all is satisfactory. However, if the product is designed for a single source, say only a USB 5 volt input, then how is the secondary isolation voltage supply provided? A DC-DC converter (or transformer driver) and an isolation transformer provide the solution. The DC-DC converter can be used to step the voltage up or down, while the transformer provides the galvanic isolation.

An example of such an isolated power supply is shown in Figure 6 using a Texas Instruments SN6505 driver combined with a Würth Elektronik 750315371 isolation transformer (2500 volts rms isolation). Using the USB standard of 5 volts and 500 mA input to the SN6505 typically provides more than enough power to drive secondary-side isolation circuits for data transfer, as well as possibly other circuitry such as sensors. The two diodes on the secondary circuit side provide rectification on the output. Many designs add a low dropout (LDO) voltage regulator on the secondary for cleaner voltage regulation.

Diagram of Texas Instruments SN6505 transformer driver combined with a Würth Elektronik 750315371 isolation transformerFigure 6: The Texas Instruments SN6505 transformer driver combined with a Würth Elektronik 750315371 isolation transformer provides an isolated power path to drive secondary-side circuity. (Image source: Texas Instruments)

An additional criterion which might become important for the designer: printed circuit board (PCB) available space. Using separate components for power and data isolation can consume valuable real estate on a board. The good news is that there are devices which combine both power and data transfer isolation into a single package. An example of such a topology is Analog Devices’ ADuM5240 dual-channel digital isolator (Figure 7).

Diagram of Analog Devices’ ADuM5240 dual-channel digital isolatorFigure 7. Analog Devices’ ADuM5240 dual-channel digital isolator combines both power and data isolation in one device to save space. (Image source: Analog Devices)

The ADuM5240 uses transformer-based magnetic isolation for both power and data transmission in a single package to reduce overall pc board area requirements. The ADuM5240 provides isolation of 2500 volts rms for 1 minute per UL 1577, and a data rate of up to 1 Mbit/s.

Upstream USB data isolation

All of the examples shown above assume isolation between the primary and secondary circuit. In cases where there already exists a peripheral designed without data isolation hardware, designers can do the isolation at the USB interface (i.e.: at the cable). This effectively pushes the data isolation upstream between the USB host and the USB peripheral (Figure 8).

Diagram of moving USB data isolation upstream, between the USB host and USB peripheralFigure 8: If there already exists a peripheral designed without data isolation hardware, designers can still provide protection by moving USB data isolation upstream, between the USB host and USB peripheral. (Image source: Digi-Key Electronics)

To implement this approach, designers can use Analog Device’s ADuM4160 with isolation rated at 5000 volts rms for 1 minute. This solution uses the same iCoupler technology discussed above, but isolation is targeted at the USB data interface (D+ and D-) (Figure 9). Additional applications for the ADum4160 include isolated USB hubs and medical devices.

Diagram of Analog Devices ADuM4160Figure 9: The Analog Devices ADuM4160 provides a USB data line (D+, D-) isolation solution that can be useful where it’s necessary to provide isolation at the USB host-to-peripheral cable connection. (Image source: Analog Devices)

Design considerations for isolation

How does a designer choose the best isolation technology? As mentioned above, multiple factors come into play for selecting the right technology for the job at hand. Table 1 shows a few of those design criteria across the different types of isolation technologies. As with any design, careful consideration should be taken to fully understand the components being used. There is no substitute for thoroughly reviewing datasheets and prototyping with selected components.

Table of key factors to consider when choosing an isolation approachTable 1: There are some key factors to consider when choosing an isolation approach, but it’s critical that designers carefully study the datasheet and prototype with the selected components. (Data source: Digi-Key Electronics)

In addition to those defined in Table 1, other factors must be considered when developing USB-based isolated peripherals. For instance, the total power budget required for the secondary circuity must be calculated. Sufficient power must be transferred from the primary side to the isolated secondary circuity to deliver all of the necessary power for not only the isolation components, but also any other devices such as sensors, LEDs, and logic components.

Also, as mentioned above, if using an electromagnetic isolation solution, potential EMI generated from the transformer(s) must be accounted for in emissions testing and/or EMI impact on other circuitry.

Conclusion

USB continues to grow in data transfer rates and power source delivery capabilities. However, when designing products with USB power and/or data interfacing, it is prudent to keep galvanic isolation of data and power circuitry top of mind.

To achieve galvanic isolation, designers can choose between optical, capacitive, and electromagnetic approaches after factoring multiple criteria, including data transfer rates and EMI, as well as power and board space requirements. Regardless of which is chosen, there are many solutions available to help designers ensure both the integrity of the circuit and the safety of the designer and end user.

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

About this author

Doug Peters

Doug Peters is the Founder of Bluebird Labs, LLC in Eden Prairie, MN. He has a B.S. degree in Electrical Engineering from Northeastern University in Boston, MA and an M.S. certificate in Applied Statistics, from Penn State University. He worked for 10 years at GE in Telematics and worked at NeXT computer as a systems engineer many, many years ago. You can reach him at dpeters@bluebird-labs.com.

About this publisher

Digi-Key's North American Editors