A brief discussion on the insulation safety standards of driver chips

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A brief discussion on the insulation safety standards of driver chips [Copy link]

As we all know, each industry and each field has its own standard specifications that need to be followed, and generally puts forward various requirements for the use conditions that its products need to meet. For example, motor drives, photovoltaics, passenger cars, etc. have their own application specifications, and each component also has corresponding standard certification. The former belongs to application safety standards, while the latter belongs to product safety standards.

Why should we follow safety regulations?

As we all know, each industry and each field has its own standard specifications that need to be followed, and generally puts forward various requirements for the use conditions that its products need to meet. For example, motor drives, photovoltaics, passenger cars, etc. have their own application specifications, and each component also has corresponding standard certification. The former belongs to application safety standards, while the latter belongs to product safety standards.

For the application of electrical equipment and electronic components, when the human body touches a charged object, current will flow through the body, which is called electric shock. As for how much damage caused by electric shock, it is related to the size of the current generated and the time of bearing. Severe cases can cause disability or even death. Even if the voltage is the same, the equivalent resistance of the human body will be affected by factors such as humidity, shoes and socks, or environmental dust content, resulting in different currents. In order to protect the lives of people who come into contact with it, corresponding insulation protection is often required for voltages higher than 24V. For example, the familiar international standard IEC60664 has made various insulation rules and requirements for equipment in low-voltage systems below 2000 meters above sea level and rated voltage below 1000 V.

Including the electrical clearance and creepage distance of the equipment, and also includes electrical test methods related to insulation coordination. Today we will talk about the insulation safety standards of driver chips. Infineon's isolated driver chips have fully entered the certification era. Most of the new products come with various standard certifications. The certificates can be downloaded from the web links of related products, or you can ask for help from local Infineon product technical support.

Safety regulations for power semiconductor driver chips

From a system perspective, UL, VDE and IEC standards are often used. From the evolution history of the three in the figure below, it can be found that there is no change in UL's certification standards for driver chips, and there is no obvious change in the certification IEC60747-5-5 and VDE0884-5 for optocoupler driver products. Only the certification standards for magnetic isolation and capacitive isolation driver products have been greatly upgraded in 20 years, and the requirements for products have become more stringent. From the original VDE0884-10 to VDE0884-11, it was proposed for the first time that the life of the driver device must meet the predicted requirements. After the IEC Standards Conference in 2020, IEC60744-17 was launched, which is also for magnetic isolation and capacitive isolation driver products. It is based on VDE0884-11 as a template, but is slightly different in the partial discharge test, which will be mentioned later in this article.

The UL insulation standard is relatively simple, and generally gives the voltage value that needs to be withstood under 1 minute and 1 second conditions. For example, Infineon's 1ED3321 product meets the requirement of VISO of 5700V under UL1577 (file number E311313). Generally, electronic products exported to the United States need UL certification.

VDE certification is a certification issued by the German Association of Electrical Engineers. It is one of the most experienced certification bodies in Europe and enjoys a high reputation in the world. The IEC standard is the standard set by the International Electrotechnical Commission. The International Electrotechnical Commission (IEC) was established in 1906 and is the world's earliest non-governmental international electrotechnical standardization organization. The authority of the IEC standard is recognized worldwide. The standards of these two systems have detailed grade descriptions of insulation, so it is necessary to explain the various insulation terms here.

Basic insulation

The purpose of basic insulation is to provide a basic protection against electric shock to avoid the risk of electric shock.

Supplementary insulation: Insulation added to basic insulation to provide another layer of insulation when the basic insulation fails.

Double insulation: Double insulation is an anti-electric shock measure consisting of basic insulation and supplementary insulation, where basic insulation and supplementary insulation are independent of each other.

Reinforced insulation

A single protective measure for preventing electric shock that is equivalent to double insulation. A single protective system can be a single insulating material or can be composed of several layers (several) of tightly connected single-element insulators. However, unlike double insulation, it is not easy to be divided into basic insulation and supplementary insulation. It may be an integrally formed insulator.

Let's take a simple and vivid example. For example, to prevent the new coronavirus, wearing one medical mask is basic insulation, wearing two is double insulation, and the second mask is additional insulation, so you can also directly wear an N95 mask for enhanced insulation.

Moreover, when we say that a device meets the requirements of reinforced insulation, there is a voltage limit, which means at what voltage the device meets the requirements of reinforced insulation. The voltage concepts used are VIORM, VIOSM and VIOTM, which are defined as follows:

● VIORM: Maximum rated repetitive peak isolation voltage. For example, for a 1700V IGBT device, if the driver chip meets the reinforced insulation requirements, VIORM >1700V.

● VIOSM: Maximum surge isolation voltage (in VDE0884-11 standard, there are two test conditions, one in air and one in oil). For a specific VIOSM driver chip product, it must pass 1.6 times of this value to meet the requirements of reinforced isolation.

● VIOTM: Maximum rated transient isolation voltage Maximum transient isolation voltage (usually refers to non-repeatable). For devices, this also includes the insulation considerations of the driver chip electrical gap and the internal binding wires. In general, how much transient isolation voltage is required in actual use is related to the working bus voltage and the overvoltage level requirements under the operating conditions. You can refer to the following table. For example, if the system voltage is 600V, if the basic insulation of overvoltage level III is to be met, the transient isolation voltage is 6000V; if the reinforced insulation of overvoltage level III is to be met, just choose a higher level, either down or to the right, that is, 8000V. One thing to note is that interpolation is not allowed, so it can be higher. For example, when SiC MOS is used at a bus voltage of 800V, we need to look at the horizontal column of 1000V. If the basic insulation of Level II is required, then the VIOTM of 6000V can be selected. If reinforced insulation is required, the VIOTM must reach 8000V; and if the basic insulation of Level III is required to reach 8000V, then for reinforced insulation, the VIOTM must reach 12000V. The VIOTM of Infineon's driver chip 1ED3321 is 8000V, so in applications that meet Level II overvoltage, this device meets the reinforced insulation requirements of up to 1000V!

TDDB - Time Dependent Dielectric Breakdown Phenomenon

So, will the isolation voltage, which is related to personal safety, decrease after a long time of use? How do we evaluate it? As we have said above, unlike the previous VDE0884-10, the VDE0884-11 standard also adds a test model for the life of the insulating medium. It is the TDDB (Time Dependent Dielectric Breakdown) test. Because in actual use, the voltage stress level that the device can withstand will be reduced over time due to the influence of the insulating material, the chip body size and internal structure, and the long-term use temperature. The figure below is a relationship between the maximum operating voltage (that is, the VIORM mentioned above) and the use time. The vertical axis is the operating time when the failure occurs, where the black thick oblique line represents the failure rate of 1ppm limit. It can be seen that if it meets 1ppm, the higher the operating voltage, the shorter the time to failure.

In this standard, VIORM is used as the life evaluation point. If it is reinforced insulation, a margin factor of 1.5 times is required and a life of 30 years is required, while the basic insulation factor is 1.2 and 24 years. For example, a driver chip claims to meet the VDE0884-11 standard with a VIORM of 1700V reinforced insulation, which corresponds to a life of 30 years under a VREF of 2550V. So in actual use, if your system has a higher repetitive peak voltage, the isolated driver chip can still provide a certain degree of insulation, but the service life will be affected. In some applications, when the isolated driver meets the functional isolation requirements, it can be used at higher voltages. Please consult the device supplier for specific usage conditions.

Taking Infineon's isolated driver chip 1ED3321 as an example, the voltage parameters that comply with the VDE0884-11 standard are shown in the following table. In fact, all Infineon driver chip products that comply with VDE0884-11 pass the same tests.

Partial Discharge

In addition, simply looking at how much insulation voltage can be applied is not enough. In high-voltage applications, partial discharge is an important cause of insulation breakdown. Under high voltage and strong field strength, if the insulation medium is not good, such as a little air gap, then partial discharge will occur at these weak points, causing the insulation strength to drop quickly, which is not what we want to happen. Partial discharge testing is one of the important means to test insulation reliability. Since partial discharge measurement is a non-destructive test, it is becoming more and more popular. According to the requirements of the VDE0884-11 standard, all devices that need to meet various insulation requirements will undergo relevant insulation voltage and partial discharge tests to screen qualified chips. The two tests are required to be combined as shown in the figure below, where a is a type test, and b and c are routine experiments. Take one of the two. Infineon uses the test method in Figure b to ensure the insulation of all devices.

Taking Infineon's isolated driver chip 1ED3321 as an example, the partial discharge test voltage value is given in the specification as follows:

Compared with previous standards or the current optocoupler driver product standard IEC60747-5, VDE0884-11 and the similar IEC60747-17 are more stringent, especially the relatively higher requirements of partial discharge testing, which helps to detect defective products and ensure reliable insulation of products after they are shipped and put into use.

in conclusion

In short, whether insulation is needed, whether basic insulation or reinforced insulation is needed, and how many volts of insulation capacity are required are all related to the standard requirements of each application field. You can also tell the device supplier about your needs and let them help you choose suitable products that meet certain standards.