4 Common Problems with Isolating Signals and Power

High voltage circuit designs require isolation to protect operators, communicate with low voltage circuits, and eliminate unwanted noise within the system. Digital isolators provide a simple and reliable method to achieve high voltage isolated communications in industrial and automotive applications.

Maintaining the integrity of the signal across the isolation barrier requires isolating all coupling paths between the primary and secondary sides of the circuit, including the power supply. While the secondary side of a digital isolator typically requires very little power, system designers often add additional power margin to power multiple devices.

In this article, I will share issues that often arise when isolating signal and power designs and provide a brief overview of the discrete and integrated devices available.

Question 1: Why should the power supply of the digital isolator be isolated?

The internal architecture of a digital isolator consists of two separate digital integrated circuits (ICs) on split lead frames with a high-voltage isolation dielectric barrier between them, as shown in Figure 1. Each IC requires separate power and ground for the primary and secondary sides of the device, with no physical connection between them. This requirement is independent of whether the device supports basic or reinforced isolation and applies to digital isolators as well as isolation devices with integrated interfaces.

Figure 1: The internal architecture of a digital isolator consists of a split lead frame, requiring separate primary and secondary power supplies

Question 2: What are the power supply requirements for digital isolators?

Before selecting a power topology for a digital isolator solution, it is important to determine the basic requirements of the power supply, including the input voltage range, output voltage, output power required on the secondary side, and the number of output rails. Other considerations for isolated power solutions compared to non-isolated power solutions include system insulation ratings, required creepage and clearance distances, and electromagnetic compatibility requirements such as electrostatic discharge and the emission performance of the system. Many of these requirements are specified in industry end-equipment standards. To learn more about insulation ratings and creepage and clearance distances for isolation systems, watch the TI Precision Labs Isolation video series.

The input and output signal voltages of digital isolators are usually dependent on the supply voltage applied to them, and usually have a direct relationship to the supply voltage (VCC) on the secondary side. Before finalizing the power input and output requirements, I recommend carefully reviewing the power requirements in the digital isolator data sheet. It is also a good idea to optimize the digital isolator for the logic level of the interface components. For example, when supplying 5V to a digital isolator connected to a microcontroller, choose a signal that also uses 5V or close to 5V logic levels on the secondary side.

Question 3: Can the secondary side power supply be used as an isolated power supply?

In some cases, two independent power rails in the system can be used as primary and secondary side supplies as long as the minimum requirements for the isolator logic levels are met. This includes supply voltage levels that match the input and output signal levels, with separate grounds provided for each. While an existing secondary side supply can be used, noise coupling and supply regulation often become an issue, and designers often choose to design an isolated supply that has been optimized for logic levels and system noise performance.

Question 4: What are the solutions for isolated power supply?

When designing isolated power for digital isolation circuits, there are many options available. Power solutions for digital isolators include flyback, H-bridge inductor-inductor-capacitor, push-pull, and integrated isolated data and power solutions.

Integrated isolated data and power solutions such as the ISOW7741 digital isolator with power, the ISOW1412 isolated RS-485 transceiver with power, or the ISOW1044 controller area network transceiver with power all feature an integrated DC/DC converter. These devices are designed to meet the International Special Committee on Radio Interference (CIPSR) 32 Class B limits and are significantly smaller than discrete design alternatives. In order to achieve high performance in the smallest possible footprint, the advantages of eliminating the need for transformers on the board, reducing board size, and simplifying certification are often trade-offs that cannot be ignored.

So while discrete solutions can improve efficiency and reduce radiated emissions in some cases, ultimately the benefits of space savings and simplified certification can lead to faster time to market.

To learn more about the benefits of these devices, read the application brief, “Low-Emission Signal Isolators with Integrated Power Meet CISPR 32 Requirements.”