What destroyed the Hindenburg can ruin your equipment
In the 1400’s European forts were using static control procedures and devices to prevent static discharge from igniting gun powder. Paper mills in the late 1800’s in the U.S. used various techniques to dissipate static electricity from the paper web during the drying process[i]. The graph below illustrates the level of static charge and its subsequent developed voltage of various materials as well as the effect of humidity on the ability to generate and retain charge.
Graph 1: ESD Voltage Level vs. Relative Humidity
Creating of electrostatic charge by contact and separation of materials is known as “triboelectric charging,” derived from the Greek word tribo meaning “to rub.” Once a material is charged and does not have a path to conduct the charge, its considered static. Static electricity is measured in coulombs. The charge “q” is determined by an object’s capacitance “C” and the voltage potential on the object “V”, therefore, q=CV.
Table 1. Static voltage level of materials
The amount of charge developed, electrical potential (voltage) and length of time the charge is retained is a complex matter of material properties, contact area, speed of separation between the materials, relative humidity as well as other factors. As shown in the table and chart, significantly high voltages can be generated and energy stored on an object’s surface. The amount of charge stored and the voltage combine to create a hazard for electronic components and circuitry. How devices fail due to an electrostatic discharge varies, but generally fall into two categories, catastrophic and latent although circuit transient malfunction is also possible.
When a device ceases for function, caused by junction breakdown, metal melting, or oxide failure for example, this is considered a catastrophic failure. Latent defects are more difficult to identify. A device may be partially damaged due to an ESD event, but continue to function, albeit at a reduced operating life. Latent defects are very difficult to find and can show up as early life failures in the field. Manufacturers make great effort to mitigate possible ESD damage during the manufacturing process and product designers incorporate circuitry to offer some level or protection to ESD.
The most common cause of electrostatic damage is the direct transfer of charge from the human body or charged material to the ESD sensitive device.
Today, as microelectronic devices have become faster and smaller, their sensitivity to ESD damage has increased. This is why it is essential for the industry to have in place mitigation methods and test methods to ensure adequate protection from electrostatic discharge. Hence, electronic equipment is tested to withstand ESD to a certain degree.
To understand the methods and standards for ESD testing and protection, it’s helpful to know what ESD model is appropriate for your application.
MIL-STD-1686[ii] classifies ESD sensitive (ESDS) parts into three models:
· Human Body Model (HBM)
· Machine Model (MM)
· Charger Device Model (CDM)
Table 2. ESD class and voltage range. NOTE: The Classes may be divided into subclasses. HBM, MM and CDM voltage levels do not correlate with each other.
The human body model was developed over a century ago to simulate sparking in mines and ammunitions storage areas. The model was refined by IBM in the 1960 and 70’s and legend has it they actually found brave volunteers who allowed themselves to be charged to a high voltage and then discharged through a 1 ohm impedance. The studies resulted in a simple resistor-capacitor network to simulate the discharge from a human body. The Human Body Model has since been standardized as a JEDEC standard (JESD22-A114D)
Generally accepted by the commercial and industrial electronics industry, the test methods are spelled out in IEC61000-4-2, Electrostatic discharge and follow the HBM.
The IEC 61000-4-2 standard defines four standard levels of ESD protection, using two different testing methodologies, contact and air discharge. Contact discharge involves discharging an ESD pulse directly from the ESD test gun that is touching the device under test. This is the preferred method of testing. However, the standard provides for an alternate test methodology known as air discharge for cases where contact discharge testing is not possible. In the air discharge test, the ESD test gun is brought close to the device under test until a discharge occurs. Although this is an alternate method, it is not intended to imply that the test severity is equivalent between the test methods. It is recommended to test using both methods.
The following table is a composite of the test levels and the guidelines for selecting the test levels based on environments and use conditions:
The ESD threat is divided into four threat levels depending on material and ambient humidity. Threat level 1 is considered the least severe while threat level 4 is the most severe.
- Levels 1 & 2 are reserved for equipment which is installed in a controlled environment and in the presence of anti-static materials.
- Level 3 is used for equipment which is sparsely, but not continuously handled
- Level 4 is required for any equipment which is continuously handled
The behavior of the device being tested is classified into four categories:
- Normal performance within the specification limit
- Temporary degradation or loss of function or performance which is self-recoverable
- Temporary degradation or loss of function or performance which requires operator intervention or system reset
- Degradation or loss of function which is not recoverable due to damage or equipment or software or loss of dat.
Equipment is not to become dangerous or unsafe as a result of the application of the tests defined in the standard.
The acceptable response to an ESD event and test voltage level needs to be determined when specifying equipment and power supplies. Check the product datasheet to ensure you are getting the appropriate level of protection for your application.
EN61000-4-2 power-supply considerations
Internal type power supplies are meant to be handled only during the manufacturing process, as parts are installed in end equipment. Therefore the assumption might be that the power supply is to be designed for and tested to level 3. However, as internal power supplies are increasingly being designed into portable devices – such as home healthcare equipment – consideration should be given to higher level of immunity. Power supplies meeting the level 4 test parameters will provide, to the end system designer, a more robust power supply potentially allowing easier system compliance to level 4.
External power supplies, however, are commonly handled frequently, and therefore it would be beneficial for engineers choosing an external power supply for use with their system to opt for a power supply compliant with level 4 test levels.
From both a technical and marketing perspective, it is recommended that new products be designed and tested to the level 4 requirements.
An engineering analysis should be performed to ensure that there would be no significant cost adder in order for the power supply design to be compliant with the more stringent standard. In addition, a review of available test equipment should be conducted to ensure that a significant capital equipment expenditure is not necessary.
From a power supply design perspective, designing a product to comply with the IEC61000-4-2 ESD requirements can be a challenge at the higher discharge voltages. This becomes even more challenging with a Class II AC input (two wire, no earth ground conductor). When the ESD discharge is applied to the output or signal pin, the voltage is developed across various isolating barriers and capacitors. This occurs because the AC mains are virtually grounded at some point so applied ESD voltage appears between the point where the charge is applied and earth ground. Without careful consideration of the various discharge paths within the power supply, unexpected arcing and damage to the power supply can occur. Testing to the standard and providing a test report is some assurance that the product will perform well within the confines of the specification.
In summary, electrostatic discharge is an inevitable event electronic equipment will be subjected too. The severity is a function of many environmental parameters potentially rendering the equipment inoperable. Standards are well established to help quantify the level of immunity to ESD. Understanding these standards and the appropriate level of immunity for your equipment will provide a path to reliable long product life.
About the author:Lorenzo Cividino is Director Field Technical Support at SL Power Electronics
[i] Fundamentals of Electrostatic discharge, Part One, An introduction to ESD. ESD Association, Rome, NY.
[ii] MIL-HDBK-263B Electrostatic Discharge Control Handbook for Protection of Electrical and Electronics Parts, assemblies and equipment.