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Hello and welcome to TI's Precision Labs. This Precision Labs video will provide an introduction to power solutions for digital isolators. In previous videos, we've introduced the digital isolators and the need for separated isolated power. Without isolated power supplies, the benefits of isolation are lost, as the isolation barrier would simply be shortened.

Isolated power subsystems require careful design for overall system performance to be optimized. Temperature rise from poor power transfer efficiency, data corruption due to emissions, and incorrect bypass capacitors or transformer selections can cause undesirable effects on the output signals. There are many ways to design an isolated power supply. But for this video, we will address some of the most common, discrete, and integrated options for isolated power supply designs specifically for your digital isolator needs.

This video will answer the following questions. Why do digital isolators need isolated power? What are the most common methods for designing isolated power? How do I design a discrete isolated power solution? And how do I design an integrated isolated power solution?

Digital isolators require a separate power supply on primary and secondary sides of the device as each side must have power for both the internal [? die ?] with no physical connection between the two. This requirement is independent of whether a device supports basic or reinforced isolation and applies to digital isolators as well as isolated devices with integrated interfaces.

The input and output signal voltages of a digital isolator are dependent on supply voltages VCC 1 and VCC 2. The exact relationship to VCC will vary from device to device. To ensure that the output of the digital isolator is optimized for the logic levels of interfacing component, it is recommended to keep supplies similar to the isolated power supply voltage.

When using a digital isolator powered with 5 volts interfaced to an MCU, it is important that the MCU signals are also operating at 5 volt logic levels. There are several options to generate power for a digital isolator. For this video, we will focus on the two common scenarios for each discrete and integrated solutions.

Design for discrete isolated power-- in some cases, two separate supply rails may already be available within the system, and these could be used for primary and secondary side power as long as some minimal requirements of the isolator logic are met, including power supply voltage levels match the input and output signal levels, and each offers a separate ground.

As mentioned earlier, while the supply voltages on each side of the isolator can be different, for example 2.25 volts on the primary and 5 volts on the secondary, the signal levels must be close to the supply and value as additional output of the isolator is directly related to the supply voltage. One additional challenge in using available supply rails is that often noise and supply regulation can become an issue.

When the voltage are vastly different or in cases where no secondary side power is readily available, one of the most common multichip or discrete solutions is to use a transformer and transformer driver to generate the secondary side power from the primary side. This solution can be designed with or without an LDO. The other solution is an integrated IC solution that includes the digital signal isolator and integrated transformers as well as an on-chip LDO like the one shown here.

Before we review the design guidelines for this isolated power solution, let's take a moment to discuss some transformer driver basics. What is a transformer driver? A transformer driver IC is an oscillator and gate-drive circuit. By scaling the transformer turns ratio to step up or down with different voltage configurations, transformer drivers provide an easy way to control the output voltage from very low voltages to as high as 24 volts or more.

Transformer drivers use a push-pull topology, which can benefit low-noise applications. This is because the topology uses two low-side switches that are on in alternate phases of a clock to transfer power continuously across the center-tapped isolation transformer.

The continuous power transfer results in a much lower peak current compared to other typologies and in lower emissions and higher efficiency. The symmetric drive also prevents transformer saturation at the output of the device as long as the load is balanced. This can be accomplished by using transformers with equal turns above and below the center taps.

In some cases, higher performance IC transformer drivers include functions for adjustable frequency with external or internal clocks, integrated current limits, fault detection, Under Voltage Lockout, or UVLO, thermal shutdown, and internal power MOSFETs. Regardless of whether your design requires the simplest driver or one with an improved feature set, the critical steps for design remain largely the same-- based around input supply voltage, desired output voltage, and output current.

Here are some design guidelines for discrete isolated power. Before you can determine the transformer for your design, you will need to determine the required supply current needed on the secondary side of the transformer. In many cases, this current will supply the secondary side of the power supply for an isolated component, which would be powered by the circuit, as well as the LDO.

You will additionally want to plan for some residual current to keep the LDO from dropping out of regulation. A general rule of 10% added current should address the residual current that may be needed. You will want to choose a device capable of providing the desired output current and accurate output voltage and minimal output voltage noise. It is advised to pay close attention to minimum dropout voltages. This will need to be taken into account to prevent the regulator from dropping out of line regulation and disrupting your isolated signal path or creating unwanted errors.

If you are considering a high-drive regulator, be aware that the high-drive regulators also usually have higher dropout voltages, which reduces the converter's overall efficiency. Next, choose the transformer and transformer drivers to create your isolated power supply.

You can start by identifying the turns ratio needed for your transformer given your target output voltage and input voltage. The ratio of the primary to secondary side number of turns equals the ratio of input voltage versus output voltage. This will give you the ratio needed to scale your output power according to your goals.

When selecting a transformer driver, there are many to choose from. Two options available are selecting a device with an internal clock or an external clock. Recall that some transformer drivers have internal clocks that can be used or the option to connect to an external clock to modify the switching frequency.

The last step in your design decision is the selection of rectifier diodes. A low-cost Schottky diode, in many cases, will work, but be sure that the diodes are rated for the output current expected and that you have also included over-temperature behavior. For temperatures above 85 degrees C, many Schottky diodes reverse linkage current increases to the thousands of microamps or milliamps range. This is too high for the system to operate properly recalling that the voltage and current balance must be maintained in push-pull power supplies.

If your system will operate in high temperatures, you may want to consider low-leakage Schottky diodes to avoid this issue. A comparison of typical reverse current over temperature versus low reverse current over temperature curves shows the dramatic difference that the thermals can make. You have now designed your discrete isolated power supply.

To avoid any surprises, let's review a few of the most common challenges designers encounter with discrete isolated power solutions. If you're isolated power solution will be used with an isolated interface, it is critical to keep a clean supply line from your secondary side power.

Noise on the supply line will definitely show up on the bus, noise on the supplies from under or over temperature regulation issues. Pay special attention to supplies stability over temperature. This example shows ripple that may suddenly appear when operating under lower output currents over temperature.

The transformer driver shown in this video can drive push pull converters with high output voltages of up to 30 volts or bipolar outputs of up to plus/minus 15 volts. Using commercially available center-tap transformers with their rather low turns ratios of 0.8 to 5 requires different rectified typologies to achieve high output voltages. Pay attention to loading conditions as loads greater than roughly 5 microfarads can require additional soft-start circuitry or hydride transformers.

If generating a higher voltage on the secondary side, consider that the primary side will be sinking or sourcing higher currents as well, and this may be a consideration you will need to continue. Now that we have a general idea of how to design a discrete isolated power supply for the isolated system, we will discuss the integrated IC alternatives and some of the design trade-offs that can help to determine which solution will best fit your use case.

Returning to our system diagram, we see the discrete solution for providing signal and power isolation. For designs prioritizing the smallest possible board size, integrated signal and power ICs combine the DC-to-DC converter, transformer, LDO, and digital isolator into a single package. Data channels are isolated by capacitive isolation, and chip-scale transformers are used for power isolation. But what are the challenges of this solution?

With the integrated power solution, the internal transformer ratios are fixed and typically designed for lower output currents in order to keep thermal profiles as low as possible. Efficiency for an integrated solution would range 20% to 30% lower than a discrete solution. A key aspect of interest for the integrated signal and power solution is emissions.

The use of low inductance micro-transformers requires the use of high-frequency switching, which results in higher radiated emissions compared to discrete solutions. CISPR22 is the European emissions standard that differentiates between Class A and Class B equipment, and it gives figures for conducted and radiated emissions for each class. While the integrated device shown here meets the CISPR22 Class B limit line regardless of supply, you can see in this example that emissions with a 3.3 volt input supply are significantly lower.

Choosing an integrated solution like this does require additional focus on board and system-level techniques to optimize for performance targets. The use of interlaying stitching capacitants filters, and common mode chokes to further reduce radiated emissions at the system level. You can learn more about low-emission system design for integrated signal and power solutions at www.ti.com/isolation.

Despite some of the challenges of integrated solutions, the benefit of eliminating the need for a transformer on the board and the reduction in board size and improved ease of certifications are considered worthwhile trade-offs to achieve a high-performance design in the smallest possible footprint.

This completes our introduction to isolated signal and power supply design considerations. We discussed that all digital isolators require isolated power with separate supply lines and grounds for primary and secondary sides of the isolator. Connecting grounds or supplies would effectively short the isolation barrier.

But there are two primary choices for isolating power, discrete solutions including transformer, transformer driver, and LDOs, or integrated solutions incorporating all components into a single chip, that clean supplies and consideration for over temperature variance of components will help to minimize issues with noise and efficiency, and that integrated signal and power solutions require special board and layout considerations to maximize efficiency and minimize emissions.

Thank you for your time. Please continue watching to take the online quiz. Why is isolated power required for digital isolator solutions? All digital isolators require isolated power with separate supplies and grounds for primary and secondary sides of the isolator. Connecting grounds or supplies would effectively short the isolation barrier, therefore, all digital isolator solutions require isolated power.

True or false, When designing a discrete isolated power supply, a transformer driver is required? False. Transformer drivers are oscillator and gate-drive circuits that simplify power supply design by providing an easy and reliable method to control the output voltage from very low voltages to higher voltages of 24 volts or more. LDOs help to provide a consistent isolated power and prevent line regulation errors. Both are common solutions for isolated supplies, but they are not required.

True or false, isolated integrated signal and power solutions are larger than standard digital isolators? False. Both digital isolators and isolators with integrated power use standard pinout packages. They do require different layout practices to optimize performance for emissions.

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