Home  Semiconductor Information

Mar 3 2018

What is Electrical Isolation

 Warm hints: The word in this article is about 3000 words and  reading time is about 15 minutes.

Summary

This paper is mainly about what is electrical isolation and the electrical isolation in high-speed digital circuits.This application note outlines the necessity, implementation and characteristics of electrical isolation in high-speed digital circuits. The advantages and disadvantages of optical, magnetic (inductive) and electrical (capacitive) signal transmission over an isolation layer are discussed. This article casts special emphasis on the ISO72x series of digital Capacitor coupling technology used in isolators .

Article Core
What is electrical isolationPurposeLearn about electrical isolation in high-speed digital circuits
English nameElectrical isolationCategoryelectrical isolator,digital circuit
FunctionNoise reduction and noise protectionFeatureSignal Rate,Dielectric Breakdown Voltage,Transient Immunity,Reliability and etc.



Catalogs

Catalogs
I. Electrical Isolation Definition3.2 Inductance coupling3.6 automatic protectionIV.Electrical Isolation Conlusion
II.Electrical isolation Meaning3.3 Capacitive coupling3.7 power consumption
III.Circuit Isolator3.4 isolation performance3.8 reliability
3.1 Optical coupling technology3.5 transient immunity3.9 External magnetic field immunity




Introduction

I. Electrical Isolation Definition

Electrical isolation is separating the part with the non-ideal effects from the other parts. In electronic circuits, the dielectric is isolated by blocking the direct current (DC). How does the isolated circuit operate in a larger electrical system? The answer to this question is the subject of this application note.As the number of products introduced by Texas Instruments and other vendors continues to increase, the option to transmit isolated signals increases, complicating the designer's choice of product. This report describes the important features of the isolator and explains the differences and similarities between the products. After reviewing the need for circuit isolation, we discussed three methods of dielectric signal transmission and analog-to-digital isolators, described and compared examples for each type of digital isolator. In another words,electrical isolation is a method of corrosion control. Conductors are prone to corrosion from stray current that originates from dissimilar metals. Providing good isolation for these conductors manages the corrosion significantly.

Electrical isolation is achieved using a mechanical switch that isolates a section of a circuit from the main electrical power system as and when required.



Detail

II.Electrical isolation Meaning

The main reason for isolating the circuit is to protect the circuit from dangerous voltages and currents. In the medical application example of FIG1, even a small amount of AC current can cause fatal injuries and therefore it require an isolation layer to protect the patient. Isolation also protects sensitive circuits from the high voltages present in industrial applications. The industrial example in Figure 2 is just one high pressure measurement. Isolating the sensor from the actual high voltage makes it possible to measure low voltage circuits.

Medical application example

Figure 1:Medical application example

Industrial application example

Figure 2:Industrial application example

The principle of protection is to isolate high potential potentials that may appear in various systems or circuits. As shown in the cable application in Figure 3, where a long distance isolates one driver from the receiver. After such a long distance, the ground may be at different voltages. By isolating, a voltage difference is formed in the isolator rather than the sensitive circuit.

Ground voltage difference between devices

Figure 3: Ground voltage difference between devices


As shown in FIG. 4, the isolation interrupts the loop formed by the circuit path by high impedance relative to other circuit components. By interrupting the loop, the noise voltage appears on the isolation layer rather than on the receiver or the more sensitive components. The high level of the noise voltage can be coupled by an external current or voltage source (eg, an inductive motor and lightning).

Figure 4:Isolation interrupts the loop formed by the circuit path

III.Circuit Isolator

While allowing analog or digital signal transmission through electromagnetic or optical links, the circuit isolator blocks the low frequency currents between the circuits. Digital isolators transmit binary signals while analog isolators transmit continuous signals on the isolation layer. In analog and digital isolators, the operating and peak rated voltage and common mode transient immunity are important characteristics of this isolation layer. When isolating digital signals, these important characteristics of the isolation circuit are the input and output logic voltage levels, the signal rate, the data run length, and the autoprotective response.

Traditionally, transformers, capacitors, or photodiode transistors and discrete circuits were conditioned on input and output signals to meet specific needs. This method is effective, but it can not be transferred from one application to another application. Although this may preserve analog isolators, a new generation of digital isolators has emerged in the market that uses innovative circuitry to isolate standard digital signals at signal rates in excess of 100 Mbps DC. These universal digital isolators have their own advantages and disadvantages. The following sections describe a variety of different technologies and compare specific products to the new ISO72x family from TI.

This video introduces Digital Electrical Isolator for new learners

  • 3.1 Optical coupling technology

Optical coupling is the transmission of light over a transparent insulating layer (eg air gap) for the purpose of isolation. Figure 5 shows the main components of a digital isolator. The current driver uses a digital input and converts the signal to current to drive a light emitting diode (LED). The output buffer converts the photodetector's current output to a digital output.

Basic optical coupling mechanism

Figure 5: Basic optical coupling mechanism

The main advantage of optical coupling technology is that light has an inherent immunity to external electronic or magnetic fields and that optical coupling technology allows the use of constant information transmission. The disadvantages of optocouplers are mainly reflected in the speed limit, power consumption and LED aging.

The maximum signal rate of an optocoupler depends on the speed at which the LED can be turned on and off. From the current available products, the fastest optocoupler HCPL-0723, which can reach the signal rate of 50Mbps.

The current transfer ratio (CTR) from input to output is an important feature of optocouplers and LEDs typically require a 10mA input current for high-speed digital transmission. This ratio adjusts the current used to drive the LED and the current generated by the phototransistor. Over time, LEDs have become less efficient, requiring more current to produce the same level of brightness and the same level of phototransistor output current. In many digital isolators, the internal circuitry controls the LED drive current, and the user can not compensate for the falling CTR. The advantages of LEDs are diminished and the isolators no longer work as effectively as before.

  • 3.2 Inductance coupling

Inductive coupling technology uses a changing magnetic field between two coils to communicate on an isolation layer. The most common example is a transformer whose magnetic field depends on the coil configuration (turns per unit length) of the primary and secondary windings, the dielectric constant of the core, and the current amplitude. Figure 6 shows a transformer with a signal conditioning circuit module.

Inductance Isolation

Figure 6: Inductance Isolation

The advantages of inductive coupling are the possible common-mode differences and differential propagation characteristics. The careful design of the transformer allows the noise and signal frequency to overlap, but will present the noise high common-mode impedance and signal low differential impedance. Another advantage is that signal energy transfer efficiency can be nearly 100%, making low-power isolators possible.

The main disadvantage of inductive coupling technology is the magnetization of the external magnetic field (noise). Industrial applications usually require magnetic field isolatio. For example: motor control. Another disadvantage of digital transformer transmission is the data run length. A signal converter transmits the signal over a range of frequencies and amplitudes, and its distortion is acceptable. A data run length limit or clock encoding is required to keep the signal within the available transformer bandwidth. Universal digital isolators using inductive coupling require that signal processing jointly transmit and reconstruct digital signals along with the transmission of low frequency signals (1 or 0 long characters). Isoloop by NVE / Avago and iCoupler by ADI (Analog Devices, Inc.) use encoding capabilities and provide digital isolation solutions that support DC-100Mbps operation.

The ADUM1100 is an example of iCoupler technology from Analog Devices. The ADUM1100 uses a basic transformer to transmit information across an isolation layer. This Isoloop technique (eg, HCPL-0900) replaces the secondary coil with a resistor network such as shown in Figure 7. The resistor consists of a GMR (Giant Magneto-Resistive) material, which changes as the magnetic field acts. The circuit senses the change in resistance and meets its conditions for output. This technology was actually introduced to the market when the AC performance has improved, exceeding the performance of existing optocouplers. Now, with the recent introduction of more digital isolators from ADI and the introduction of TI's ISO72x family of devices, the performance of these Isoloop devices has been surpassed.

GMR structure

Figure 7: GMR structure

  • 3.3 Capacitive coupling

Capacitive coupling technology uses an ever-changing electric field to transmit information on the isolation layer. The material between each capacitor plate is a dielectric isolator and forms a barrier. The size of the plate, the spacing between the plates, and the dielectric material all determine the electrical performance.

Capacitive coupling

Figure 8: Capacitive coupling

The benefits of using a capacitive isolation layer are high efficiency in terms of size and energy transfer, and immunity to magnetic fields. The former makes it possible to integrate low-power and low-cost isolated circuits; the latter makes it possible to operate in saturated or high-density magnetic fields.

The disadvantage of capacitive coupling technology is that it has no differential signal and noise, and the signal shares the same transmission channel, which is different from the transformer. This requires that the frequency of the signal be significantly higher than the expected frequency of the noise so that the isolation capacitance exhibits a low impedance of the signal and a high impedance of the noise. With inductive coupling, the capacitive coupling can not transmit a steady state signal and requires clock encoded data.

  • 3.3.1 TI introduced the ISO72x--Electrical Isolation Test

TI introduced the ISO72x series of isolators using capacitive coupling technology. Capacitive coupling solutions using proven, low-cost manufacturing process, and the magnetic field has an inherent immunity.


In order to provide constant information transmission, ISO72x uses a high signal rate and low signal rate channel to communicate, as shown in Figure 9. The high signal rate channel is not encoded and it transmits data on the isolation layer after a single-ended-to-differential conversion. The low signal rate channel encodes the data in a pulse width modulated format and transmits data differentially across the isolation layer, ensuring accurate communication at constant conditions (long 1 and 0 characters).

Differential transmission of single-ended logic signals across the isolation layer allows the use of low-level signals and small coupling capacitances. This presents a high impedance to common-mode noise, and, through common-mode noise rejection at the receiver, provides excellent transient immunity, the major issue that needs to be addressed with signal-capacitance coupling.

ISO72x and ISO72xM structure diagram

Figure 9: ISO72x and ISO72xM structure diagram

  • 3.4 isolation performance

Three major criteria verify the need for isolation protection, respectively UL 1577, IEC 60747-5-2 and CSA. Although each standard is slightly different, it provides a standard for comparing isolation performance. Testing by IEC, UL and CSA confirmed the voltage beyond the dielectric breakdown between the input and the output. The use of these standards is very simple, because the test standards and isolation method has nothing to do. Figure 10 shows how the isolation test treats the isolator as a two-terminal device. Although the physical structure of each device is different, but the isolation test is measured in the dielectric breakdown voltage.

Isolation test

Figure 10:Isolation test

The ISO72x series of isolators are tested with UL 1577, IEC 60747-5-2, IEC 61010-1 and CSA. Table 1 shows the isolation performance of these five devices illustrating the three isolation technologies.

Device
The technology usedUL1577(VRMS)

IEC 60747-5-2,V IORM(VPeak)

IS0721Capacitive isolation2500560
ADuM1100Inductance isolation2500560
HCPL-0900Inductance isolation2500Now reviewed
HCPL-0721/HCPL-0723Light isolation3750560

Table 1:Isolation performance

All three tests, UL, CSA and IEC, tested the quality of the insulation. The UL and CSA tests are stress tests that test the dielectric breakdown voltage using the time set by the manufacturer. Dielectric breakdown was a symptom of a failure during this test. The IEC test uses a phenomenon known as partial discharge to detect voids in the dielectric. A large voltage is applied to the device, which is a function of the manufacturer's defined operating voltage and is then reduced to another voltage level, Vm. In this low voltage application, the device under test is monitored for ineffective partial discharges in the dielectrics. These inefficiencies cause the final breakdown of the entire dielectric.

  • 3.5 transient immunity

High slew rate (high frequency) transients can disrupt data transmission on one isolation layer. The isolation capacitor provides a path as shown in Figure 11, allowing transient events to pass through the isolation barrier and destroy the output waveform. A Faraday shield keeps this part of the displacement current in the optocoupler or inductive coupler away from the important output structure.

Isolator capacitance

Figure 11: Isolator capacitance

Faraday shielding is not a viable solution in capacitive coupling solutions. In addition to transients, Faraday shields also block the electric field used for data transmission. To provide transient immunity, the ISO72x family of capacitive isolators transmits only the fo signal (the data signal representing only the highest frequency energy). This allows a small coupling capacitor with a high frequency of noise frequency. Other noise comes from the differential technique that transmits data on the isolation layer. Figure 9 shows the four signals passing through the capacitive isolation layer; two contain low signal rate information and the other two contain high signal rate information. By using the difference technique, any remaining common-mode transients passing through the isolation layer can be seen in the true and compensated signals, and the differential receiver suppresses them. As shown in Table 2, the transient immunity of the ISO72x series is as high as any comparable device up to 25kV / us.

Device
The technology usedTransient immunity
IS0721Capacitor transient imunity25
ADuM1100Capacitor transient imunity25
HCPL-0900Capacitor transient imunity15
HCPL-0721/HCPL-0723Light transient imunity10
  • 3.6 automatic protection

Data line circuits and digital isolators need to pay attention to is the input signal loss output state. Input loss may occur when the cable is disconnected or the power is removed directly from the isolator input. Auto-protection refers to a deterministic or known output state at input loss. The ISO72x series uses a periodic pulse to determine if the input structure is energized and working. If the isolator output does not receive a pulse after 4us, the output is set to a high state. The ADUM11 ADUM1100 also integrates an auto-protection circuit in the IC's output section. Avago Technologies introduced optical solutions (HCPL-0721 and -0723) that did not mention automatic protection and the inductive GMR solution (HCPL-0900) explicitly described the uncertain nature of the output during power sequencing.

  • 3.7 power consumption

In addition to the efficiency of signal transmission on the isolation layer, the design of the input and output regulation circuitry is most relevant to power consumption. As shown in Table 3, the optocoupler consumes more power than the inductor or capacitor example.

Device

The technology used

Vcc1 and Vcc2

Icc1(mA)

Icc2(mA)

Power consumption(mW)

ISO721

Capacitive coupling

5

1

11

60

3.3

0.5

6

21.5

ADuM1100

Inductance coupling

5

0.8

0.06

4.3

3.3

0.3

0.04

1.2

HCPL-0900

Inductance coupling

5

0.018

6

30

3.3

0.01

4

13.2

HCPL-0721

Light coupling

Only 5

10

9

95

HCPL-0723

Light coupling

Only 5

10

17.5

137.5

Table 3:Quiescent supply current

  • 3.8 reliability

Mean before-failure time (MTTF) is the standard measure of semiconductor device reliability. For digital isolators, this measurement shows the reliability of the integrated circuit and the isolation mechanism. Table 4 shows the MTTF for a light, inductor, and capacitor digital isolator. ISO721 is very reliable compared to inductive and optical solutions.


Typical value, 60% confidence

Typical value, 90% confidence

Device

The technology used

Ambient temperature

MTTF

(hour/malfunction)

PIT

(malfunction/109hour)

MTTF

(hour/malfunction)

PIT

(malfunction/109hour)

ISO721

Capacitive coupling

125

1,246,889

802

504,408

1983

HCPL-0900

Light coupling

125

288,118

3471

114,654

8722

HCPL-0721

Light coupling

125

174,617

5727

69,487

14,39

Table 4:Reliability measurement

The ADUM1100 reliability datasheet does not explicitly state the MTTF, but it provides the result of a reliability test. Table 5 shows the ISO721 and ADUM1100 reliability test parameters.

Device

The coupling technique used

Junction temperature

Time(hour)

Number of samples

unqualified products

ISO721

Capacitive coupling

150<TJ<175

1000

344

0

ADuM1100

Capacitive coupling

150<TJ<175

500

231

0

Table  5:Raw reliability data

  • 3.9 External magnetic field immunity

Figure 12 compares the magnetic field immunity of the ADUM1100 and ISO72x (data for HCPL-0900 not found). Relatively speaking, although both examples have a certain degree of immunity to magnetic fields, the ISO72x provides greater margin. As mentioned earlier, optocoupler isolation circuitry has inherent magnetizing immunity to external magnetic fields.

Sensitivity to external magnetic fields

Figure 12: Sensitivity to external magnetic fields



Analysis

IV.Electrical Isolation Conlusion

Noise reduction and noise protection have made isolators widely used in electronic circuits where isolators interrupt the ground loop and isolate the ground voltage difference. Designers now have many options for digital signal isolation, including TI's ISO72x family of key features such as signal speed, dielectric breakdown voltage, transient immunity, power dissipation, magnetic field immunity and reliability Good performance in all aspects. Table 6 summarizes these characteristics of the examples discussed in this report.

Device

The technology used

Vcc(V)

Signal rate(Mbps)

UL1577(VRMS)

Transient immunity(kV/μs)

Power consumption(mW)

Magnetic field immunity

Reliability(MTTF),60% confidence(hour/malfunction)

ISO721

Capacitive coupling

3.3 or 5

150

2500

25

60

+

1.25M

ADuM1100

Inductance coupling

5

100

2500

25

4.3



3.3

50

1.2



HCPL-0900

Inductance coupling

5

100

2500

15

30


288k

3.3

13.2


HCPL-0721

Light coupling

5

25

3750

10

95

++

175k

HCPL-0723

Light coupling

5

50

137.5

++


Table  6:Different digital isolator parameters



Book Recommendation

  • High-Speed Digital Circuits: Stability, Instability and Chaos

overview of theory, models of devices and low frequency circuits, digital circuit theory including inductance, microstates, submicrostates, local time, and complexity of interconnects.

--Masakazu Shoji

  • Noise Coupling in System-on-Chip (Devices, Circuits, and Systems)

discusses a breakthrough substrate coupling analysis flow and modelling toolset, addressing the needs of the design community. 

--Thomas Noulis



Relevant information about "Electrical Isolation in High-Speed Digital Circuits"

About the article " Electrical Isolation in High-Speed Digital Circuits", If you have better ideas, don't hesitate to  write your thoughts in the following comment area. You also can find more articles about electronic semiconductor through Google search engine, or refer to the following related articles.


0 comment

Leave a Reply

Your email address will not be published.

 
 
   
Rating: