How to choose an isolator in system design

    Recently, electronic engineer netizens have often raised the question that in system design, a very confusing issue is how to choose a reasonable and applicable isolator. Not long ago, this website reporter interviewed Zhou Xiaoqi, a senior application engineer of digital isolator products at ADI. He gave electronic engineers a good answer to questions such as the role of isolators in circuits and what steps engineers should pay attention to when selecting digital isolators.

Why use an isolator?

    Isolators are mainly used to isolate the electrical connection between high-voltage (dangerous circuit) systems and low-voltage (safe circuit) systems to protect the safety of users and circuit systems, and to isolate the connection between sensitive circuits (such as high-precision detection circuits) and noise sources (such as high-power switching power supplies) to reduce noise interference.

    Zhou Xiaoqi said that there are four main structures of isolators: one is traditional photoelectric coupling; the second is integrated transformer (magnetic coupling); the third is integrated capacitive coupling; and the fourth is discrete transformer coupling. The most commonly used is the optocoupler, which generally consists of three parts: light emission, light reception, and signal amplification. The input electrical signal drives the light-emitting diode (LED) to emit light of a certain wavelength, which is received by the photodetector to generate photocurrent, and then further amplified and output. Its main advantage is that it transmits signals in one direction, the input and output ends are completely electrically isolated, it has strong anti-interference ability, long service life, and high transmission efficiency. It is widely used in level conversion, signal isolation, inter-stage isolation, switching circuits, long-distance signal transmission, pulse amplification, solid-state relays (SSR), instrumentation, communication equipment, and microcomputer interfaces.

    As the speed of data transmission continues to increase in recent years, traditional optocouplers have also exposed some shortcomings. For example, the entire circuit is large in size and has low integration. In addition, the optocoupler itself has the disadvantages of easy wear, slow speed (generally the data rate is less than 1Mbps), and high power consumption. In particular, the performance is extremely unstable during temperature and aging changes, which limits its application. These problems are particularly prominent in industrial applications. In order to overcome the many shortcomings of optoelectronic isolation technology, many semiconductor companies have begun to develop non-luminous isolator solutions. Zhou Xiaoqi told reporters that ADI was the first company to launch a new digital isolator solution, and in 2001 it took the lead in launching standard digital isolation products based on its proprietary iCoupler magnetic coupling isolation technology.

New digital isolators

    Zhou Xiaoqi introduced that iCoupler technology is based on chip-sized transformers, and each iCoupler channel consists of two parts: CMOS interface circuit and chip-level transformer. The core of the iCoupler isolation transformer is the planar transformer that can cross the isolation barrier and transmit and receive signals. They can not only provide isolation, but also eliminate the shortcomings of photoelectric conversion in optocouplers, including excessive power consumption, large timing errors and limited data rates. In fact, since there is no need for external circuits to drive LEDs, the power consumption of iCoupler digital isolators is only 1/10 to 1/50 of that of optocouplers.

    iCoupler isolation transformers can achieve thousands of volts (5000V) of isolation. The key to its high withstand voltage lies in the top and bottom coils of the sending and receiving transformers. A 20 μm thick polyimide material is used as the isolation layer, and four transformers are integrated on a 4 mm×4 mm chip (Figure 1). Zhou Xiaoqi said that the advantages of this new digital isolator are mainly reflected in: First, the transmission speed is faster, much higher than traditional optocouplers, up to 150 Mbps. Second, the static/dynamic current is smaller. Third, the life is longer, and the life can even reach 50 years within the range of high voltage resistance and breakdown resistance. Fourth, other functions under standard CMOS process can be integrated. Fifth, isolated power supply can be realized inside the chip, and it has a smaller volume and thickness.

    The working principle of the iCoupler isolator is that when the input end of the digital isolator receives the rising edge or falling edge of the signal, it will encode these signals into double pulses or single pulses and transmit them to the other side through the integrated transformer. At the same time, the decoder will restore these pulse signals to the same waveform as the input signal. At the same time, the iCoupler digital isolator also integrates a watchdog circuit. When the signal does not change for a long time, the update circuit on the input side will also periodically send a corresponding pulse signal to the output end. If the watchdog circuit on the output end does not receive any signal for a long time, it will be considered that the input end has entered an abnormal state. At this time, the output signal on the output end will return to the preset default value (Figure 2).

    Zhou Xiaoqi said that since 2001, ADI launched the first iCoupler isolator product, and now there are 10 series and more than 100 types, which is the most complete type of isolator products in the industry, including standard digital isolators, digital isolators using iso Power, digital isolators with integrated PWM controllers and transformer drivers, USB 2.0 compatible isolators, isolated gate drivers, isolated I2C digital isolators, isolated RS-485 transceivers, isolated RS-232 devices and isolated Σ-Δ ADCs, etc. It can be said that you can find a suitable iCoupler product to meet your design needs for almost any application.

Application Cases

    During the interview, Zhou Xiaoqi also specifically cited two successful cases using iCoupler digital isolators.

    First, in 2012, ADI provided its highly integrated iCoupler digital isolation solution for the PowerSCOE system of Siemens Aerospace Business Group. As an advanced satellite power protection system, it can shut down several kilowatts of power in the satellite within microseconds when an abnormality occurs in the satellite. Siemens Aerospace Business Group also took other technologies into consideration when designing the system, such as smart grid power monitoring, but since each channel contains auxiliary analog inputs and digital and analog outputs, the biggest challenge is to ensure that the 100 channels are isolated from each other. There are more than 100 analog and digital I/Os on the PCB. Faced with such a high PCB density, traditional optocoupler and DC/DC converter solutions are powerless. In the end, the excellent performance of ADI's highly integrated iCoupler digital isolator won the trust of Siemens and once again verified the leading edge of iCoupler technology.

    Second, the customer used ADI's iCoupler and isoPOWER technologies with digital isolators with integrated isolated power supplies to design a very miniaturized USB interface industrial-grade sensor series. This is also due to ADI's iCoupler series products. The appearance of this series of sensors is very compact (it is impossible to achieve such a small volume using discrete solutions) and the power consumption is very low (it can be powered directly by USB without external power). This design makes the device extremely small while ensuring sufficient isolation strength to protect the safety of on-site workers.

Digital Isolator Selection Steps

    Zhou Xiaoqi gave electronic engineers some suggestions on how to choose a reasonable and applicable digital isolator in the design:

1. To understand the isolation strength requirements in your application:

A. What is the working voltage in the actual application? What are the grid conditions? (220V civil grid/industrial grid)
B. What is the actual application environment? (Altitude/humidity conditions/temperature conditions)
C. What is the isolation protection level adopted in this application? What type of protection object is it? (Functional isolation/basic safety isolation/enhanced safety isolation)
D. Which specific international safety standard should this application comply with?

2. Understand your requirements for the isolator's operating speed and interface:

A. What is the highest speed in this application?
B. How many channels are required in one package?
C. Timing requirements in this application, such as signal delay requirements.
D. Whether other functions need to be integrated (I2C/USB/RS-485/integrated isolated power supply).

3. Price (price per channel).

4. After selecting a digital isolator, you still need to carefully consider the high voltage safety requirements when laying out the PCB.

Future technical challenges for isolators

    What key technical issues will be solved in the future technological development of isolators? Zhou Xiaoqi believes that the following issues need to be solved: 1. Higher isolation voltage requirements; 2. Higher transmission speeds; 3. More diverse interfaces and integrated functions; 4. Longer life, smaller size, and more advanced packaging technology.