How to Isolate Data and Power Using 3750 Volt RMS Digital Isolators

Automotive and industrial systems are operating at increasingly high voltages that can injure users and damage electronics. These systems also generate transients and noise that can interfere with local sensing and control circuitry.

While many suitable options for signal isolation exist, it’s hard to find a solution that helps you adequately balance the need for a compact form factor, efficiency, flexibility, and, of course, low cost. It may be time to take another look at digital isolators.

As their name suggests, digital isolators are ICs that provide a path for digital signals to travel within electrically challenging environments while maintaining signal integrity. They also provide the galvanic isolation needed to protect users and resist transients, noise, and ground loops, even in the presence of high voltages and high-power transients. The right device can greatly simplify your design task and streamline certification.

Let’s examine the isolation challenge and use digital isolator examples from Skyworks Solutions to see how effectively they address the key issues.

What is electrical isolation?

Electrical isolation blocks the transfer of direct current (DC) and unwanted alternating current (AC) signals between two sections of a system, while still supporting signal and power transfer between those sections. This isolation protects both humans and sensitive circuit elements from high voltage, prevents ground loops in communications networks, and improves noise and transient immunity.

We find isolation widely used in vehicular power converter applications for high-side motor drives, as well as in industrial applications to protect programmable logic controllers (PLCs), motor drives, and sensors.

Inside some digital isolator examples

To understand how digital isolators work, let’s go inside the Si86Px family (Figure 1) from Skyworks Solutions. These isolators have two sections: one for digital signal transfer and one for isolated power transfer.

Figure 1: The Si86Px family of digital isolators uses both capacitive (lower section) and inductive coupling (upper section). (Image source: Skyworks Solutions)

The digital signal section, shown in the lower portion of the figure, includes digital isolators that use capacitive coupling to provide galvanic isolation. Digital signal transfer uses simple on-off keying to modulate a radio frequency (RF) carrier with digital data, and the modulated signal is transmitted capacitively across the isolation gap. On the receiver side, the modulated RF carrier is demodulated to recover the original digital signal. This technique supports data rates up to 150 megabits per second (Mbits/s) and a maximum propagation delay of 15 nanoseconds (ns). It has a minimum common-mode transient immunity (CMTI) of 100 kilovolts per microsecond (kV/µs).

The upper section of the block diagram shows the isolated power transfer using a hysteretic control regulator. Power transfer is accomplished via magnetic coupling through an integrated air-core transformer.

Hysteretic voltage regulation is a power electronics control method that maintains a stable output using variable-frequency control and responds rapidly to changing load conditions. The regulator compares the output voltage with a reference and adjusts switching according to the deviation, using feedback transmitted across the isolation barrier. Hysteretic control offers simplicity and the ability to regulate power delivery under varying loads.

The Si86Px ICs support input supplies of 3.3 volts, 5 volts, or a continuous range from 3.3 to 5 volts, depending on the model. They provide isolated output voltages of 3.3 or 5 volts and peak output power as high as 0.5 watts, again depending on the model. The family has an isolation rating of 3750 volts RMS and is certified to UL, CSA, VDE, and CQC requirements.

Digital channel configurations

The Si86Px digital isolators are available in two and four-channel versions. The two-channel devices offer two forward channels (Figure 2, left) or one forward and one reverse channel (Figure 2, right). Both versions come in a 16-SOIC package.

Figure 2: Two-channel Si86Px devices are available with two forward channels (left) or one forward and one reverse channel (right). (Image source: Skyworks Solutions)

The Skyworks Solutions SI86P320BC-IS two-channel digital isolator has two forward channels. It accepts a 3.3 volt input for its integrated power converter and provides a 3.3 volt isolated output with up to 0.2 watts of output power.

The SI86P421BC-IS has one forward and one reverse channel. It accepts an input from 3.3 to 5 volts and provides a 3.3 volt isolated output with up to 0.4 watts of output power.

The four-channel devices provide four forward channels, three forward and one reverse channel, or two forward and two reverse channels (Figure 3). All three configurations come in a 20-SOIC package.

Figure 3: Four-channel Si86Px devices are available with four forward channels (left), three forward and one reverse channel (center), or two forward and two reverse channels (right). (Image source: Skyworks Solutions)

The SI86P540BC-IS has four forward channels, accepts a 5 volt input, and provides a selectable 3.3 or 5 volt isolated output with up to 0.5 watts of output power.

The SI86P441BC-IS provides three forward channels and one reverse channel. It accepts 3.3 to 5 volts and provides a 3.3 volt isolated output with up to 0.4 watts of output power.

The SI86P342BC-IS provides two forward and two reverse channels. It accepts a 3.3 volt input and provides a 3.3 volt isolated output with up to 0.2 watts of output power.

A typical application

A typical application for a four-channel digital isolator is to provide isolated communications between a microcontroller unit (MCU) and an analog-to-digital converter (ADC) using a serial peripheral interface (SPI) (Figure 4).

Figure 4: Shown is a schematic for an Si86Px41 isolator providing isolated power and four isolated signal channels between an MCU and an ADC. (Image source: Skyworks Solutions)

This application requires a four-channel isolator to handle the four SPI signal lines: chip select (CS), serial clock (SCLK), serial data in (SDI), and serial data out (SDO). A Skyworks Si86Px41x provides three forward channels and a single reverse channel for the SDO line. Depending on the selected model, its integrated power converter can supply up to 0.5 watts of isolated power. If the converter’s output voltage and power meet the ADC’s requirements, it can eliminate the need for a separate isolated power supply and external isolation transformer.

Conclusion

The Si86Px family integrates isolated power transfer with high-speed digital channels, making the devices suitable for compact, isolated data acquisition in industrial and automotive applications. By combining signal isolation and isolated power in a single package, these devices can simplify implementation while supporting reliable operation in electrically noisy environments.