How to select a digital isolator? You have to know that!

Ⅰ. What is a digital isolator?

A digital isolator is a circuit device that can realize the isolation function, and is mainly used to protect and isolate the mutual influence between various circuits. It can convert the digital signal from the input to optical or magnetic signal, process it in the isolation layer, and finally output the new signal to the output.

Ⅱ. The characteristics of the digital isolator

Features of digital isolators include the following:

1. Signal isolation: Digital isolators can isolate signals between different signal sources to prevent interference and cross-effects.

2. Electrical isolation (the most important feature): To achieve electrical isolation between the input and output circuits, thereby preventing the propagation of current, interference, and faults.

3. Low power consumption: Modern digital isolator designs focus on low power consumption to reduce system energy consumption.

4. Wide bandwidth: Many digital isolators work in a wide bandwidth range, suitable for high-speed data transmission.

5. Small size: Digital isolators usually use integrated circuit technology, small size, suitable for high-density integration.

6. High precision: The design of digital isolators usually pursues high precision and low distortion to ensure the accuracy of signal transmission.

7. High isolation performance: Digital isolators usually have high isolation voltage and isolation level to ensure reliable isolation effect.

8. Anti-interference: Digital isolators usually have better anti-electromagnetic interference and anti-noise performance, which improves the stability of the system.

Ⅲ. Classification of digital isolators

According to the mode of transmission, it can be divided into:

1. Electromagnetic isolator

Inductive coupling technology uses a changing magnetic field between two coils to communicate across an isolation barrier. The most common example is a transformer. The magnitude of the magnetic field of a transformer is related to the coil structure (turns/unit length) of the primary and secondary windings, the dielectric constant of the magnetic core, and the current amplitude.

2. Optically coupled digital isolator:

An optocoupler is an optical transmission technology that transmits electrical signals from two circuits into another circuit. It uses a special transparent insulating barrier (eg: air gap) for isolation purposes.

An optocoupler generally consists of three parts: light emission, signal amplification, and light reception. In the optocoupler, the electrical signal at the input terminal enters the light-emitting diode (LED) through the input electrode (optocoupler), and light of a certain wavelength is added to both ends of the LED. When a certain voltage is applied to the input terminal, a light emitting diode (LED) emits light of a certain wavelength. The photodetector is the most important part of the photocoupler, which can amplify the photocurrent signal generated at this wavelength, and then output it after further amplification. This completes the electrical-optical-electrical conversion, thereby playing the role of input, output, and isolation.

An optocoupler's signaling rate depends on how quickly the LED can be turned on and off. From the products currently available, the fastest optocoupler is the HCPL-0723, which can reach a signal rate of 50Mbps.

The current transfer ratio (CTR) from input to output is an important characteristic of an optocoupler, which indicates the transfer of current from the input to the output of the LED from its supply pin to the device. For example, when an LED is used for high-speed digital transmission, it typically requires an input current of 10 mA. This ratio typically regulates the current used to drive the LED and the current generated by the phototransistor. Over time, LEDs become less efficient and require more current to produce the same level of brightness with the same level of phototransistor output current. In many digital isolators, internal circuitry controls the LED drive current, and there is no way for the user to compensate for the gradually decreasing CTR. When an LED is used as a brightness level, it will have a low CTR, and in other cases it will have a high CTR. However, LEDs have become more efficient and effective over time. Isolators are no longer as effective as they used to be.

① Advantages: 

Not only is light inherently immune to external electrons or magnetic fields, but optical coupling techniques allow the use of constant information transfer. 

② Disadvantages: 

Affected by temperature changes and light attenuation, there is a certain response time.

3. Capacitively coupled digital isolator

Capacitive coupling technology uses a constantly changing electric field across the isolation barrier to transmit information. The material between the capacitor plates is a dielectric spacer and forms the isolation layer. The plate size, the spacing between the plates, and the dielectric material all determine the electrical performance.

① Advantages: 

High efficiency in terms of size and energy transfer, and immunity to magnetic fields.

② Disadvantages: 

It has no differential signals and noise, and the signals share the same transmission channel, which is different from transformers. This requires that the signal frequency be much higher than the expected frequency of the noise, so that the isolation barrier capacitance presents a low impedance for the signal, and a high impedance for the noise.

Ⅳ. Application Scenarios of digital isolators

According to the research of the market research organization IHS, the isolator market is growing steadily every year, reaching an output value of about 1.6 billion US dollars in 2016, among which the demand for isolation components driven by automotive, communication, photovoltaic and industrial applications has grown the most violent.

1. Industrial automation

In industrial automation, the working signal and communication transmission voltage of the programmable logic controller (PLC) are both 24V, while the core electronic components of the system are basically 5V. At this time, isolation devices are needed to ensure the safety of 24V-5V; It is a higher high-voltage and low-voltage transmission; for motor drives, the distance between the control board and the motor is often very far, and a long communication cable connection is required. The cable will form a loop with the reference level ground wire, which will cause noise. It needs to be cut off by isolation Ground loops to eliminate noise interference; for building automation, industrial transportation, etc., bus isolation technology is also required.

There are many high-voltage equipment in the factory. The isolation scheme can not only protect the electronic system and personnel, but also suppress noise to ensure the reliability and stability of the equipment. As industrial automation applications increase performance requirements for related equipment (high safety, high anti-interference and long life), built-in isolation technology has also begun to change. Traditional isolation uses optical coupling technology, which is gradually difficult to meet industrial automation due to its inherent characteristics. Design requirements, including poor reliability, poor timing characteristics, common-mode transient immunity (CMTI) without margin, and low energy efficiency, have prompted the industry to begin to shift their attention to new CMOS capacitively coupled digital isolator solutions.

The CMOS digital isolator adopts the CMOS design of on-off keying modulation technology, and uses differential capacitive coupling, which can achieve high reliability and stability, and will not change with temperature, operating voltage and product life. In addition, it also has excellent timing characteristics, including low propagation delay, low propagation delay skew and so on. More importantly, CMOS digital isolators have high common-mode transient immunity (CMTI) and require less power than optocouplers, thus reducing overall system power consumption.

According to IHS statistics, the global industrial automation equipment market size is estimated to be about 202.2 billion U.S. dollars in 2017, and about 218.2 billion U.S. dollars in 2019, with a compound growth rate of about 3.9% from 2017 to 2019. As the global industry 4.0 continues to advance, the global industrial automation equipment market will grow steadily.

In China, according to CCID Consulting's forecast, the scale of China's industrial control market will be about 205.3 billion yuan in 2019, and will be about 260 billion yuan by 2021, with a compound growth rate of about 12% from 2019 to 2021. the

With the continuous growth of the market size of industrial automation equipment, digital isolators will gradually replace optocouplers due to their excellent performance, and their market size will continue to increase.

2. New energy vehicles

Complex road conditions and noise during high-speed operation have greatly interfered with the control system of new energy vehicles. These have led to stricter safety requirements for automotive applications, and higher requirements for isolators used in new energy vehicles. To meet ever-tightening quality and reliability requirements, automotive system designers are turning to digital isolators instead of optocouplers to provide safety isolation for hybrid electric vehicle (HEV) battery monitoring and power conversion applications. Unlike optocouplers, digital isolators are based on standard wafer CMOS semiconductor processes that have a good reputation for use in automotive systems.

The application scenarios of digital isolators in electric vehicles include on-board charging, power management, charging piles, communication buses, and system monitoring. Taking an EV system as an example, there are many modules that use isolation, such as: OBC (On-Board Charger), Battery Management System (BMS), CAN bus, DC-DC Converter, Traction Inverter, and Heating/Cooling. Among them, the on-board charger (OBC) is used to convert 120V or 240V AC from a wall socket or wall charger to DC voltage to charge the car battery.

DC-DC converters are used to convert DC voltage from one voltage domain to another in order to power various auxiliary systems. The function of galvanic isolation is required here, and the high-voltage switch on the primary side of the auxiliary power module (APM) transformer requires an isolated gate driver.

According to IHS data, a market research organization, the global market size of OBC and DC-DC for electric vehicles in 2018 is about 7 billion yuan. The DC market size will reach 100 billion yuan. The number of digital isolators used in automotive OBCs and DC-DC converters will continue to increase.

Expanding to the entire field of new energy vehicle electronics, according to IHS data, the global sales of new energy vehicles in 2018 was about 2.018 million, and it is predicted that the global sales of new energy vehicles will reach 20.74 million in 2025, and the CAGR of global new energy vehicle sales in 2018-2025 About 39.49%. IHS predicts that the global automotive electronics market will grow from US$207 billion in 2017 to US$240 billion in 2020 in the next three years, with a three-year compound growth rate of 5.05%. The increasing demand for new energy vehicles and the replacement of optocouplers by digital isolators will greatly promote the market demand for digital isolators.

3. Photovoltaic inverter

Solar panels convert sunlight to DC voltage, which must be converted to high-voltage AC power to minimize line losses and enable longer distances for power transmission. A PV solar inverter performs this DC to AC conversion and is the most critical component of any photovoltaic power generation system. Taking the common photovoltaic inverter—single-phase photovoltaic inverter as an example, the pulse width modulated (PWM) full-bridge converter voltage switches synthesize a discrete (albeit noisy) 60 Hz current at the output of the full bridge waveform. The high-frequency noise components are filtered inductively to produce a medium-amplitude 60 Hz sine wave. The filtered waveform then passes through an output transformer, which performs three functions: first, it further smoothes the AC waveform; second, it corrects the voltage amplitude to meet specified grid requirements, and third, it connects the inverter's DC input to the high-voltage AC grid for galvanic isolation.

PV systems are expected to operate reliably at full rated output for at least 25 years, and exposure of PV inverters to hot or cold temperatures for 25 years will cause components used in the inverter to be out of service. Obviously, components such as optocouplers, which provide galvanic isolation, have no chance of "going far". The LED brightness of the optocoupler gradually fades, glows, and stops functioning. Solutions for these delicate components include replacing electrolytic capacitors with high-value film capacitors (higher reliability, but higher cost). The best long-term solution is to turn off the optocoupler, which favors modern CMOS isolation components.

CMOS process technology has high reliability, cost-effectiveness, high-speed operation, small feature size, low operating power and operating stability, voltage and temperature extremes, and many other advantages. Additionally, unlike the gallium arsenide (GaAs) process technology used in optocouplers, CMOS fabricated devices have no inherent wear-out mechanisms. The underlying CMOS isolation cells are capacitive, fully differential, and highly optimized for tight timing performance, low power operation, and high immunity to data errors induced by external fields and fast common-mode transients. In fact, for the first time, the advantages brought about by CMOS process technology combined with proprietary silicon product designs have resulted in reliable "near-ideal" isolation devices. These devices feature a higher level of overall functional integration, higher reliability (60+ year isolation shield life), continuous operation from 40°C to +125°C up to VDD, and performance, power consumption, board space savings and ease of use.

According to IHS survey data, driven by the downstream demand of solar modules, wide bandgap semiconductors - silicon carbide (SiC) and gallium nitride (GaN) will lead the solar inverter isolator market to reach 1.4 billion US dollars in 2020.

Separately, global demand for replacement inverters could grow by nearly 40% to 8.7GW in 2020, as deployments are fueled by a large and expanding installed base of aging solar PV installations, IHS Markit reported. According to Yole's forecast, the global PV inverter market is expected to reach USD 9.672 billion by 2023. The scale of China's photovoltaic inverter market will start to recover strongly in 2020 after shock adjustments, and will return to the level of 9.28 billion yuan around 2025. The increase in the market size of photovoltaic inverters will drive the increase in the market size of digital isolators.

PV systems are expected to operate reliably at full rated output for at least 25 years, and exposure of PV inverters to hot or cold temperatures for 25 years will cause components used in the inverter to be out of service. Obviously, components such as optocouplers, which provide galvanic isolation, have no chance of "going far". The LED brightness of the optocoupler gradually fades, glows, and stops functioning. Solutions for these delicate components include replacing electrolytic capacitors with high-value film capacitors (higher reliability, but higher cost). The best long-term solution is to turn off the optocoupler, which favors modern CMOS isolation components.

CMOS process technology has high reliability, cost-effectiveness, high-speed operation, small feature size, low operating power and operating stability, voltage and temperature extremes, and many other advantages. Additionally, unlike the gallium arsenide (GaAs) process technology used in optocouplers, CMOS fabricated devices have no inherent wear-out mechanisms. The underlying CMOS isolation cells are capacitive, fully differential, and highly optimized for tight timing performance, low power operation, and high immunity to data errors induced by external fields and fast common-mode transients. In fact, for the first time, the advantages brought about by CMOS process technology combined with proprietary silicon product designs have resulted in reliable "near-ideal" isolation devices. These devices feature a higher level of overall functional integration, higher reliability (60+ year isolation shield life), continuous operation from 40°C to +125°C up to VDD, and performance, power consumption, board space savings and ease of use.

According to IHS survey data, driven by the downstream demand of solar modules, wide bandgap semiconductors - silicon carbide (SiC) and gallium nitride (GaN) will lead the solar inverter isolator market to reach 1.4 billion US dollars in 2020.

Separately, global demand for replacement inverters could grow by nearly 40% to 8.7GW in 2020, as deployments are fueled by a large and expanding installed base of aging solar PV installations, IHS Markit reported. According to Yole’s forecast, the global PV inverter market is expected to reach USD 9.672 billion by 2023. After shock adjustments, China’s PV inverter market will start to recover strongly in 2020 and return to RMB 9.28 billion around 2025 level. The increase in the market size of photovoltaic inverters will drive the increase in the market size of digital isolators.

Ⅴ. How to choose a digital isolator?

As digital isolators are becoming more and more popular in industrial, automotive and other fields, designers are faced with a large number of parts to choose from. How can they choose the most suitable parts for the entire system? To meet this challenge, many digital isolators are designed according to specific system requirements and application requirements, requiring designers to classify countless technical indicators and functions to ensure that the selected device meets the system requirements. If the wrong device is selected, it will seriously affect the design of the entire system, making it unable to meet the specified standards, or unable to provide a reliable solution for the entire system at a lower cost. Following are the steps for digital isolator selection.

Step 1: Know your isolation code requirements

The first step is to understand the isolation specification requirements for the system. While the demands sometimes seem endless, there are a few key factors engineers can start with when making a selection.

1. Operating voltage (VIOWM): What is the constant voltage that the isolation barrier needs to withstand during the service life of the product?

2. Surge isolation rating (VIOSM): Is reinforced isolation required? An isolator capable of withstanding >10 kV surge pulses is required.

3. Data rate: What data rate do you need for your communication interface? Are you running low-speed UART speeds or high-speed ≥100-Mbps data protocols?

4. Power consumption: Is overall system power consumption a critical specification for your application (for example, 4 to 20 mA loop powered or battery powered system)?

5. Isolation withstand voltage (VISO): Is basic isolation and ≤3,000 VRMS sufficient for your design? Or is the design requirement requiring ≥5,000 VRMS? This specification is usually set by the regulatory requirements of the system and means that the isolator can withstand voltage breakdown for at least 60 seconds.

6. Creepage/clearance: Is the creepage/clearance sufficient for a 4mm package, or does your system standard call for a specification of 8mm or higher?

7. Common Mode Transient Immunity (CMTI): Can the isolator be used in noisy environments such as motor drives or solar inverters (where data integrity is critical and any bit error can be dangerous short-circuit event)? If so, then a high CMTI rating is critical for your digital isolator.

Step 2: Select the appropriate package

After narrowing down the specification requirements for a digital isolator, the next step is to choose a package. Packaging can make a big difference in terms of isolation because the size and characteristics of the package directly affect the high voltage performance of the device.

Some of the same high voltage requirements discussed above may come into play when selecting the correct package. Larger packages with greater creepage and clearance distances will allow higher isolation voltage specifications to be used. If you are evaluating a system in this package, consider this option to help save board space and cost.

It is important to note that when choosing a smaller package, the device can meet the system regulatory requirements you need at the same time. For example, in the discussion above, one might find that the same voltage levels apply to smaller package types such as FWP. However, in a real design, you may need a higher level or a more demanding isolation voltage specification.

Typically, you will consider the following when selecting the appropriate package for each communication interface:

How many isolation channels are required for the communication interface? Higher channel counts will determine which package type you can use.

Step 3: Determine the number of channels and configuration

Once all the above options are confirmed, you can move on to the next step. Next, we'll consider some other options. First, we need to determine how many isolated channels your signal requires and the direction each signal is sent. This will help determine the number of channels required, as well as the channel configuration.

Second, we need to consider whether you prefer to design a default output state (or a fail-safe state). This can help us determine which predefined state (high or low) the output pin will be in when the input channel of the digital isolator is not powered or the pin is floating. This will help determine how to calibrate the digital isolator to ensure it is working on the right track.

Third, we need to consider whether you would like to use a fail-safe state (or default output state) in your digital isolator. 

Finally, we need to consider whether we are willing to use options 1 and 2. This may vary based on your design preferences. If you choose to use option 3, it may work with both the default high and low outputs.

Step 4: Assess available equipment

Get ready to put your digital isolator knowledge into practice.

Ⅵ. Precautions for digital isolator selection

Both feature set and isolation performance are important factors when selecting a digital isolator. In terms of functionality, we have to consider the number and configuration of channels. In terms of isolation performance, we need to understand the isolation level required by the system. Transient immunity and electromagnetic radiation distribution are other considerations related to the isolation structure. For the isolation level, we need to consider the packaging options considering the system environment.

The first challenge for designers is to select the correct digital isolator for each correct application. Once the appropriate equipment has been identified and designed, system designers can proceed to system evaluation in the manner they typically design. In addition, these digital isolators must comply with the appropriate safety standards required by terminal safety agencies such as UL, CSA, VDE, and CQC.

These safety agencies use their component safety standards to determine and specify a safety component's one-minute withstand voltage rating, typically 2.5kVrms, 3.75kVrms, or 5kVrms, or its lifetime operating safe voltage, typically between 125Vrms and 1000Vrms. For surge protection, some devices can go up to 10kVpk.

For basic and reinforced insulation end systems requiring up to 250Vrms working insulation, the two most common creepage and clearance requirements are 3.2mm and 6.4mm respectively. These CMOS digital isolators enable designers to create isolated circuits that are less expensive, smaller in size, higher in performance, lower in power consumption, and more reliable.

Ⅶ. The prospect of digital isolator

As more and more industrial and communication equipment become intelligent, providing more high-power system control and feedback information, thereby achieving higher efficiency and reducing system cost, the demand for isolators in today's market is also increasing.

Although the market demand for digital isolators has grown tremendously, the price of digital isolators has not increased significantly because the prices of isolator components have remained stable. Of course, when it comes to isolator components, there has been price pressure over the past few years. However, as current digital isolator solutions continue to add new features such as low power modes, safety capabilities, and solutions with more compact designs and packages, digital isolator solutions will continue to increase their value Keep prices low for a long time.

In terms of optocouplers, Avago, Vishay, Toshiba, Panasonic, NEC, and Taiwan Guanxi, Baihong, etc. are all leaders in the industry, and Avago has a dominant position in the market. The isolator market is dominated by manufacturers such as ADI, NVE, TI, and Silicon Labs.

In terms of low power consumption, take Avago’s ultra-low power consumption optocoupler products ACPL-M61L/061L/064L/W61L/K64L as an example, they save 90% power than current standard optocouplers. These optocouplers can use unique integrated circuit design and thick insulating layer materials, which can greatly save power consumption without affecting isolation and insulation performance. Target markets include communication interfaces such as RS485, CANBus and I2C, and microprocessor system interfaces, and digital isolation for analog-to-digital conversion applications such as A/D and D/A.