Digital isolator-high voltage enhanced isolation: Safety regulations, standard definitions and test methods
Understanding the definition of high-voltage isolation parameters, their relevance to practical applications, and the methods used to test them, system engineers can choose the appropriate isolator according to the design requirements.
The design involving high voltage and high voltage isolation is very complex.
How much isolation does my system need?
What system-level isolation standards apply to my product or final equipment? Are there component-level standards that help me compare isolators and choose the one that best suits my system-level needs? Which parameters or indicators should I compare-there seem to be many? What test processes do isolated components go through to support the parameters in their data sheets? Most importantly, how can I ensure that a system is built to ensure the reliable operation of the entire product life cycle? These are the problems faced by many system engineers dealing with high-voltage and high-voltage isolation.
- Maximum transient isolation voltage (VIOTM) and isolation withstand voltage (VISO)
- Maximum repeatable peak voltage (VIORM) and operating voltage (VIOWM)
- Maximum surge isolation voltage (VIOSM)
- Comparative Leakage trace Index (or Relative leakage trace Index) Comparative Tracking Index (CTI)
What is a digital isolation chip & digital isolator?
In recent years, digital isolation chips have been increasingly trusted by engineers in industrial, medical, automotive and other fields. Digital isolation chips are gradually replacing traditional optocoupler devices because of their small size, high integration, low power consumption, and high communication speed. The digital isolation chip is the core device of the high-voltage safety involved in the system, so it needs to be strictly screened when leaving the factory.
Isolation is a means of preventing direct current and unwanted AC current from passing between the two parts of the system, while allowing signal and power transmission between the two parts. Electronic devices and semiconductor ICS used for isolation are called isolators. For various reasons, modern electrical systems need to be isolated. Some examples are to prevent electric shock to manual operators, prevent damage to expensive processors, ASICs, or FPGAs in high-voltage systems, break ground loops in communication networks, and communicate with high-end equipment in motor drives or power converter systems. Examples of applications that require isolation include industrial automation systems, motor drives, medical equipment, solar inverters, power supplies, and hybrid electric vehicles (HEV).
Functional isolation and enhanced isolation of digital isolators
When isolation is used to enable the normal operation of the system, but not necessarily as a shockproof barrier, it is calledFunctional isolation。 If isolation provides sufficient protection against electric shock, as long as the insulating barrier is intact, it is calledBasic isolation。 Safety regulations require basic isolation to be supplemented by a secondary isolation barrier for redundancy, so that even if the first barrier fails, the additional barrier can provide shock protection. This is called double isolation. In order to make the system compact and cost-effective, it is best to have only one level of isolation, with the required electrical strength, reliability, and two levels of basic isolation shock protection. This is calledEnhanced isolation。 The high-voltage isolation performance of the isolator is quantified by parameters at the component level, such as the maximum repeated peak voltage (VIORM), the operating voltage (VIOWM), the maximum transient isolation voltage (VIOTM), the isolation withstand voltage (VISO), the maximum surge isolation voltage (VIOSM), and the comparison tracking index(CTI) and others. These parameters represent the isolator's ability to handle high-pressure stresses of different amplitudes and transient contours, and can be directly mapped to actual operating conditions. The definition and test methods of these parameters are described in component-level standards (such as IEC 60747-5-5, VDE 0884-10, and UL1577). The test methods of basic and enhanced isolators are slightly different, while the latter are more stringent. VDE 0884-10 is particularly suitable for magnetic and capacitive pairs or isolators. When isolators are used in practical applications, the system and final equipment standards also require certain minimum values of these isolation parameters based on the system line voltage and whether basic isolation or enhanced isolation is required.
- IEC 61800-5-1 (safety standard for adjustable speed electrical drives)、
- IEC 60664-1 (insulation coordination of equipment in low-voltage systems) and
- IEC 61010-1 (safety standard for measurement, control and laboratory equipment)
Is an example of system and final equipment standards. This document discusses in detail the definition of the above-mentioned high-voltage isolation parameters and their relevance to actual system scenarios, and introduces how to test and certify these parameters. This understanding is essential to compare the performance of competing isolation solutions, determine whether the isolator meets the system-level isolation requirements, determine whether the isolator can be used to enhance isolation, and determine the long-term reliability of the isolator. TI's ISO7842 is a powerful electromagnetic compatibility (EMC), high-speed, high common-mode transient immunity (CMTI), four-channel enhanced digital isolator. It uses capacitance-based isolation with silicon dioxide (SiO2) as the dielectric.
The device uses advanced processing technology, precise packaging technology and innovative circuit design to provide industry-leading high voltage and electrical performance.
This document discusses the test procedures and results of high voltage testing on ISO7842. The test results show that the device has excellent high voltage performance and reliability, enabling system engineers to confidently solve the most difficult isolation problems.
Maximum transient isolation voltage and isolation withstand voltage
Both the maximum transient isolation voltage (VIOTM) and the isolation withstand voltage (VISO) are designed to quantify the isolator's ability to handle high voltages through the isolation barrier in a very short period of time. During normal operation, the stress voltage on the isolation barrier is limited by the maximum system line voltage. However, unintentional interference in the system (for example, noise on the power supply caused by an arc or load change) may temporarily cause the voltage on the isolator to be several times the line voltage. The isolator should be able to handle these transient overvoltage without damage.
VIOTM is defined by IEC 60747-5-5 and VDE 0884-10 as the peak transient voltage that the isolator can handle without decomposition. During the certification process, VIOTM's isolator is tested for 60 seconds, and then a 10-second partial discharge test is performed at 1.6 times VIORM (see the next section for the definition of VIORM). This is called the method A test.
During the manufacturing process, VIOTM emphasized each device of VIOTM for one second, and then performed a partial discharge test at 1.875 times VIORM for one second. This is called the method B1 test. Partial discharge is a partial discharge inside the insulating material, indicating the integrity of the insulation. relatedFor more detailed information about the method A and method B1 test configuration files, please refer to the appendix at the end of this article.
The value of VIOTM can also be used to determine compliance with system-level standards (such as IEC 60664-1), which requires the insulation barrier to tolerate a certain level of temporary overvoltage, depending on the system voltage, which can withstand 5 seconds. For example, isolators with VIOTM greater than 6222 Vpk (4400 Vrms) comply with IEC 60664-1's temporary Vpk overvoltage standard for line voltages up to 1000 Vrms. VISO is defined according to UL 1577 as the rms value of the 60-second voltage that the isolator can handle. During the certification period, the sinusoidal stress of VISO is applied for 60 seconds for testing. In production, VISO performs tests by emphasizing that each device is 1.2 times VISO for one second. For sinusoidal stress, VIOTM and VISO are equivalent.
Test example：
TI tests its digital isolators to meet UL, IEC, and VDE requirements. In order to test VIOTM or VISO, you can use HT9464M high voltage isolation test & partial discharge test system, orUse HT9464 high voltage isolation test system。 This device can apply the required transient overvoltage profile according to method A and method B1, and measure partial discharge. This test is done by connecting all the pins on the first and second sides, and then applying a voltage to the isolation barrier (Figure 1)To execute。
TI's ISO7842 complies with VISO per UL 5700 Vrms, and VIOTM per VDE0884-10 and IEC 60747-5-5 is 8000 Vpk. Vpk This is based on method A to test more than five fabs, as well as a total of more than 2,000 devices. In addition, each ISO7842 device will be tested in production according to method B1, and the stress voltage is greater than 6840 Vrms to meet UL requirements. These levels of VISO and VIOTM are the highest levels provided by any isolator in the industry in a standard 16-pin SOIC package.
It must be noted that ISO7842 can easily meet the requirements of 4400 Vrms, that is, according to IEC 60664-1, for line voltages up to 1000 Vrms, temporary overvoltage required for enhanced isolation. .
Maximum repeatable peak voltage and operating voltage
Both the maximum repeated peak voltage (VIORM) and the operating voltage (VIOWM) are designed to quantify the isolator's ability to continuously and daily handle the high voltage across its barrier throughout its life cycle.
For example, an isolator used to provide gate control for a high-side IGBT in a motor drive system will see a periodic trapezoidal potential difference on its isolation barrier because the auxiliary side of the isolator is referenced and moves up and down between the high-voltage DC rails. This trapezoidal stress exists whenever the motor is running. VIORM and VIOWM are defined in IEC 60747-5-5 and VDE 0884-10. VIORM is defined as the maximum repeated peak voltage that the isolator can withstand, while VIOWM is defined as the maximum rms or equivalent DC voltage that the isolator can withstand over a specified long period of time. For sinusoidal stress voltage, VIORM and VIOWM are equivalent. These two values are specified by the isolator manufacturer based on the manufacturer's tests.
VDE 0884-10 Ed 1.0 and IEC 60747-5 check VIOWM and VIORM through a partial discharge test, which checks for partial discharge in the insulating layer, indicating that the insulating layer is degraded. Part of the discharge test is performed together with the VIOTM test, using method A test during the certification period, and method B1 during the production test. The upcoming VDE 0884-10 Ed 2.0 also includes additional requirements for VIORM and VIOWM. In order to meet this new and upcoming standard, the manufacturer of the enhanced isolator must provide accelerated stress test data to the certification body to prove that the isolator can handle 1.2 times VIOWM/VIORM within more than 37.5 years. During the accelerated stress test, the isolator is subjected to varying degrees of high voltage, much higher than its expected operating voltage, and the corresponding failure time is recorded. Then, the voltage and time curves are extrapolated to make life prediction at the expected operating voltage. For isolators that use silica (SiO2) as an insulating material, the relationship between failure time and stress voltage is exponential. Therefore, the expected failure time log decreases linearly when voltage stress is applied. Therefore, VDE 0884-10 Ed 2.0 requires SiO2-based isolators to use the same relationship as curve-fitting accelerated test data.
figure2 Shows the test settings used to perform the accelerated stress lifetime test. All terminals on one side of the isolator are shorted together, and all terminals on the second side of the isolator are shorted together. The required high voltage, 60 Hz sine wave, is applied between one side and two sides, using a high voltage source (such asAR 7715 high voltage insulation tester) Stress the isolation barrier. Apply stress voltages continuously until the impedance from one side to both sides drops below 4 MΩ. At each voltage point, batches of at least 32 devices were stressed. Device failure times fit well with the Weibull distribution, and statistical analysis was used to find the failure times corresponding to <1 ppm failure rates. This time is then plotted on a voltage versus failure time plot. This process is repeated at different voltage points to generate the entire voltage versus time-to-failure curve. When this curve was extrapolated to be greater than 37.5 years and further de-estimated by a factor of 1.2, the value of VIOWM/VIORM was given. For a more complete understanding of accelerated stress testing and related extrapolations, refer to the VDE 0884-10 Ed 2.0 standard. Accelerated stress test at high temperature (150°C) and room temperature (25°C) VDE 0884-10 Ed 2.0 requires that the VIORM and VIOWM values from the accelerated stress test provide greater confidence in the long-term reliability of an isolator that is subjected to continuously applied high voltage. The same cannot be said for the partial discharge tests specified in IEC 60747-5-5 and VDE 0884-10 Ed 1.0, since there is no established relationship between long-term withstand capability and partial discharge.
Figure 3It shows the life expectancy forecast of ISO7842, which is based on accelerated stress testing of isolation barriers used by five different wafer fields and a total of more than 2,000 devices. The shaded area indicates the safe operating area (SOA) of this device. Please note that the actual test data is deliberately not displayed in the figure. SOA includes a 1.2 de-rating factor required by the standard, and is based on a more conservative statistical extrapolation than required by the standard. SOA can be used to estimate the life expectancy of any given operating voltage.
The <<1 ppm line indicates that less than one in a million devices is expected to be outside the SOA. The SOA curve in Figure 3As shown, Ti's ISO7842 can withstand 2121 Vpk VIORM and 1500 Vrms VIOWM for more than 40 years. These levels of VIORM and VIOWM are the highest levels provided by any isolator in the industry in a standard 16-pin SOIC package.
Maximum surge isolation voltage
The maximum surge isolation voltage (VIOSM) quantifies the isolator's ability to withstand very high voltage pulses of a specific transient profile. The surge test configuration file is shown in Figure 4 As shown。 Due to direct or indirect lightning strikes, failures, and short-circuit events, surge voltages may be caused in the device. According to IEC 60747-5-5 and VDE 0884-10, isolators that claim to have a specific VIOSM must pass a surge test with a peak voltage of 1.3 times VIOSM for basic isolation, while the VIOSM used for enhanced isolation is 1.6 times. Only when the component level passes a surge test higher than 10 kV can it be called an isolator at the component level. The pass level of surge testing is also used to determine standards that meet system-level standards (such as IEC 61800-5-1), which require a certain surge capability of a given system voltage.
For example, for equipment directly connected to a power supply (called Class III) that operates at a line voltage of 600 Vrms, IEC 61800-5-1 requires a minimum surge capacity of 8000 V to enhance isolation.
Please note that passing the surge test at a level greater than 10 kV has been widely used as the gold standard for enhanced isolation, although the system-level standard allows systems with lower line voltages to reduce the surge capability value.
figure5 Shows the settings used to test the surge performance on ISO7842. The isolator is configured as a double-ended device by shorting all left-hand pins to one group and all right-hand pins to another group.
use MIG1203 or MIG2403 （EMCPARTNERMIG1203 and MIG2403 are discontinued，Alternative model INS-1250-x series products, Contact EUTTEST for details for more information) The surge generator applies a surge voltage to the isolation barrier, depending on the required test voltage.
The test is performed by applying 50 pulses to each of the positive and negative polarity of the rated stress voltage. After the surge test, the partial discharge test, insulation impedance test and full-function production test of each method B1 of the device are carried out. If the device successfully passes all these tests after a surge voltage is applied, it is deemed to have passed the surge test. In order to avoid arcing in the air, this test is performed in dielectric oil. ISO7842 is based on the testing of more than 5 wafer positions and a total of more than 2,000 devices, and passed the surge voltage test at a voltage greater than 12,800 V. Since this exceeds 10 kV, it meets the limitations of the enhanced isolator. Based on the scaling factor of 1.6 required for enhanced isolation, VIOSM is rated at 8000 V. Passing the 12800 V surge test also means that the device meets the surge standard for enhanced isolation of line voltages up to 1000 Vrms for devices directly connected to the main power supply, according to IEC 61800-5-1.
Compared to the leakage trace index
Comparative Leakage Trace index (or Relative leakage trace Index) Comparative Tracking Index (CTI): The highest voltage value on the surface of the material that can withstand 50 drops of electrolyte (0.1% aqueous ammonium chloride solution) without forming traces of leakage, in V. It is clearly proposed in the IEC 60112 standardTest method。 The test method is: on the circuit board of the bare substrate, at two points 4mm apart, pierce the plate with a force of 100g in a direction of 60°. The angle between the tips of the two electrodes and the horizontal plane is 30°. After the panel is pierced, 0.1% sodium chloride solution is continuously dropped between the two points we selected, one drop every 30s, and an alternating voltage of 100V600V is introduced. You can try a voltage of 300V first and control the current to 1A, because there is a sodium chloride solution on the noodles at this time, and there is resistance, so it will heat up, and the heat will evaporate the solution, and then continue to drip into the sodium chloride solution until 50 drops, at this time observe whether the plate itself is leaking electricity. Once the insulating sheet has a leakage current of more than 0.1A and the duration exceeds more than 0.5s, it will be recorded as a fault. At this time, the buzzer will generally be called, and the test instrument will automatically record the number of drops of sodium chloride solution that has been dropped at this time. The number of drops. If not, continue to increase the voltage until a failure is generated, and record it, and the voltage that finally makes it fail is its CTI data.
When the isolator is placed on the motherboard as part of the final device, in addition to its internal isolation parameters, the mold compound used in its package is also very important. This is because when a high voltage is applied to the isolator, the discharge on or near the surface of the package may cause local deterioration of the mold compound, resulting in a partial conduction path from one side of the isolator to the other. This phenomenon is called tracking. The material's ability to withstand tracking is quantified by the comparative tracking index (CTI).
IEC 60664-1 divides the material into four material groups according to the CTI value of the material：
| Material group I： | 600 V |
| Material Group II： | 400 V |
| Material group IIIA： | 175 V |
| Material group IIIb： | 100 V |
CTI plays an important role in determining the minimum creep on the surface of the isolator or the shortest distance from the pin on one side of the isolator to the pin on the other. According to the degree of pollution present in the system environment, a given operating voltage requires minimum creep. Using molds with higher CTI, smaller packages can be used and circuit board space can be saved. For example, according to IEC 60664-1, an 8 mm creep package using a CTI-I mold compound can withstand operating voltages of up to 1600 Vrms, while the same package using a CTI-IIIa mold compound can only withstand 800 Vrms.
ISO7842 adopts CTI-I mold compound. This means that it can actually enable an operating voltage of 1500 Vrms at the actual enable system level and is equipped with a standard 8 mm creep SOIC-16 package. In contrast, competing isolators that use CTI-IIIa mold compounds in the same package can only achieve an operating voltage of 800 Vrms at the system level, even though they may claim a higher value of VIORM/VIOWM at the component level.
Insulation penetration distance
Distance through insulation (DTI): The minimum straight distance between two metal parts separated by additional insulation or reinforced insulation in the tool.
The distance of the insulation penetration distance (DTI) is the minimum distance between the two voltage domains in the isolator inside the isolation package. Many final equipment standards (such as IEC 60601-1 (Medical Electrical equipment standard)) specify the minimum required distance achieved by insulation.
However, these standards have provisions that allow thinner insulating layers, as long as they pass certain tests. These tests are a subset of the types of tests required by VDE 0884-10. Historically, a higher DTI is based on the direct isolation performance indication of the insulating material used. However, since the new generation of magnetic and capacitive isolators uses higher-quality insulating materials, a much smaller DTI can obtain very high isolation performance.
The minimum internal DTI of ISO7842 is 21 µm, and the typical DTI is 25 µm. However, at 800 V/µm, the decomposition strength of the dielectric material SiO2 used is very high. The dielectric quality used is the reason for the excellent high voltage performance of the device. The device complies with the enhanced isolation type test standard of VDE 0DE 0884-10, which proves that a DTI of 25 µm for a material with a decomposition strength of 800 V/µm is not a µm problem.
Table 1. ISO7842 Performance Summary
| Serial number | attribute | standard | value |
| 1 | Viso | UL1577 | 5700Vrms |
| 2 | Viotm | VDE 0884-10 Ed1.0=2.0 | 8000Vpk |
| 3 | Viorm | VDE 0884-10 Ed1.0=2.0 | 2121 Vpk (>40 years) |
| 4 | Viowm | VDE 0884-10 Ed1.0=2.0 | 1500 Vrms (>40 years) |
| 5 | Viosm | VDE 0884-10 Ed1.0=2.0 | 8000 V (surge test pass level >12.8 kV） |
| 6 | CTI | IEC 60664-1 | CTI>600 material group: I |
| 7 | Dti | NA | 21 microns (min) / 25 microns (type) Note: The subdivision field of SiO2 is 800 V/µm µm |
Maximum transient isolation voltage (VIOTM) and isolation withstand voltage (VISO)
Maximum repeatable peak voltage (VIORM) and operating voltage (VIOWM)
Maximum surge isolation voltage (VIOSM)
Comparative Leakage trace Index (or Relative leakage trace Index) Comparative Tracking Index (CTI)
Distance through insulation (DTI)
notes：
1. ISO7842 also complies with the VISO, VIOTM, VIORM, and VIOWM values mentioned in Table 1 (IEC 60747-5-5). However, ISO7842 will not be certified by IEC 60747-5-5 because the standard is specific to optocouplers, not capacitive couplers.
2. VDE 0884 Ed 2.0 (coming soon) is a revised version of VDE 0884 Ed 1.0. Compared with IEC 60747-5-5 and VDE 0884 Ed 1.0 for VIOWM and VIORM, it has stricter constraints and additional requirements.
conclusion
The high-voltage isolation performance of the isolator is quantified by different parameters, which represent the isolator's ability to handle high-voltage stresses of different amplitudes and transient contours. Various component-level standards define these parameters and methods for testing them. This white paper discusses in detail the definition of these parameters, their relevance to actual system scenarios, and describes how to test and certify these parameters.
The test results of TI ISO7842 enhanced digital isolator are introduced, which are performed in accordance with standard procedures. The device meets the transient overvoltage and surge requirements of enhanced isolation at the component and system levels, and achieves reliable operation for many years at continuous high operating voltages. The test results show that the device marks a major leap forward in TI's capacitive high-voltage isolation function, while providing industry-leading high-voltage performance.
quote
1. IEC 60747-5-5 Ed 1.1, Semiconductor devices • Discrete devices • Part 5-5: Photoelectric devices • optocouplers, May 2013
2. DIN V VDE V 0884-10 Ed 1.0, Semiconductor devices-magnetic and capacitive connectors for safety isolation, December 2006
3. UL 1577 Ed 4.0, safety standard for optical isolators, May 2000
4. IEC 61800-5-1 Ed 2.0, adjustable speed electric drive system, safety requirements, electrical, thermal and energy, July 2007
5. IEC 60644-1 Ed 2.0, Insulation coordination of equipment in low-voltage systems, principles, requirements and tests, April 2007
6. IEC 61010-1 Ed 3.0, Safety requirements for electrical equipment used in measurement, control and laboratory use, general requirements, June 2010
7. ISO7842 product folder
8. ISO7841 product folder
9. ISO7821 product folder
10. Enhanced isolation for unparalleled performance
11. Sarangan Varawan, understanding electromagnetic compliance testing in digital isolators，
Texas Instruments White Paper, November 2014
Appendix
Maximum transient isolation voltage (VIOTM)
Maximum repeated peak voltage (VIORM)
Maximum transient isolation voltage (VIOTM)
Maximum repeated peak voltage (VIORM)
Important: The products and services of Texas Instruments (TI) and its subsidiaries described here are subject to TI's standard terms and conditions of sale. Customers are advised to obtain the most up-to-date and complete information about TI products and services before placing an order. TI is not responsible for application assistance, customer application or product design, software performance, or patent infringement. The release of information about any other company's products or services does not constitute an approval, guarantee, or endorsement by TI.
author：
Anant S Kamath ,Systems Engineer, Isolation, Interface Group Texas Instruments.
Kannan Soundarapandian, General Manager, Motor Drivers Texas Instruments.
Reference product：
HT9460 and HT9464 high voltage isolation test system
INS-1250-7K5 and INS-1250-30K pulse insulation test EMCpartner
- Creation date: October 22, 2020
- Revision date: July 2, 2024