Corrective measures for electrostatic discharge testing in medical devices

Electrostatic discharge may affect the differentiation of patients in clinical application scenarios, and even damage medical devices. Therefore, the electrostatic discharge immunity (ESD) test belongs to the immunity test report in the electromagnetic compatibility specification. In the specific electromagnetic compatibility test, the one-time pass rate of electrostatic discharge test is very low, and most goods need to be rectified. This paper briefly introduces the electrostatic interference, the causes and effects of electrostatic interference, and takes the optometer as an example to analyze the causes of electrostatic failure and take reasonable improvement measures.

1. Brief introduction of electrostatic interference

With the rapid development of science and technology, electronic equipment has been widely used in various fields, resulting in the complex electromagnetic environment around, and the magnetic field effect caused by electrostatic discharge has also become a key issue that must be considered. Electrostatic discharge test is used to simulate the discharge phenomenon that may occur in the daily application of electronic products. In this case, the instantaneous discharge current will produce a short but high intensity magnetic field, which will lead to the use of computers and other electronic products. Intelligent terminals and medical electronic products will cause misoperation, and even immediately lead to equipment circuit penetration damage. In the rapid development stage of electronic industry, integrated circuits have been widely used. Its typical characteristics are that the intermediate circuit of electronic components is reduced, the compression level is reduced, and the wiring area is reduced. However, this highly integrated design significantly reduces the ability of electronic products to resist electrostatic interference. The discharge current and magnetic field caused by electrostatic interference are directly related to the service life of high-density components. In order to solve this problem, various high insulation materials are used in integrated circuits, but the characteristics of insulation products lead to daily assembly. It is easier to generate static electricity in adjustment and use [1]. During the registration inspection of its medical devices and equipment, the effectiveness of the equipment must be tested according to relevant requirements. Electrostatic discharge test is an important stage to detect the performance changes of electronic products.

The damage of electrostatic discharge conditions to electronic components of medical devices has the following characteristics: secret, except for electrostatic discharge conditions, it is not easy to be directly perceived by the body. Cumulative. When electronic components are subject to electrostatic interference, the characteristics may remain unchanged. However, if the equipment is subject to electrostatic discharge for many times during use, the cumulative effect will cause serious hidden danger to the equipment and increase the sensitivity. Randomness. Medical electrical equipment may be subject to static interference during use, adjustment and use. There are various paths to generate static electricity. Static electricity is generally released instantaneously, which is difficult to protect immediately. Electrostatic discharge may cause harm to medical electrical equipment: generating interference signals, resulting in equipment failure and operation errors, and reducing the accuracy of diagnosis and treatment; Accelerate the aging of components and reduce the reliability and service life of equipment; Penetrate integrated circuits and precision electronic components to improve the safety of diagnosis and treatment machines.

2. Causes of electrostatic discharge

Electrostatic discharge is very common in life. It is easy to generate in dry environment. Electrostatic discharge can be caused by various reasons. The accumulation of electric charge makes the potential difference between objects, and the touch or approach process will produce electric charge flicker, resulting in electrostatic discharge [2]. Friction electrification: matter is composed of atoms, and the basic structure of molecule is proton, neutron and electron. Under normal circumstances, the total number of positive protons and negative electrons is the same, the positive and negative charges are balanced, and the object is not charged. However, the change of natural conditions will make the positive and negative charges of molecules unbalanced. The above friction is essentially a process of continuous contact and separation. Induction electrification: electrostatic discharge conditions are not all caused by friction, but also induced electrostatic discharge. The electric field of a charged body will lead to the separation of charges near the object. As long as the object is electrified, it will cause an electric field around it, and the near object will be inductively electrified.

3. Effect of electrostatic discharge on active medical devices

The charge movement will affect the electronic components of the active medical device, and even lead to the static failure of the active medical device. When electrostatic discharge occurs, the electrostatic discharge will cause a large number of charges to shift rapidly, causing the high-voltage current to pass through the electronic components instantaneously, damaging the insulation layer, and immediately affecting the internal circuit of the electronic components. Through the serious consequences of the active medical device, there are two situations: ① permanent damage, according to the immediate discharge, the electronic equipment on the device is damaged, such as the display power switch penetration caused by the instantaneous pressure of electrostatic discharge; ② The change of magnetic field is caused by immediate or indirect discharge, resulting in equipment misoperation, and the detection results can not meet the basic characteristics.

In the clinical environment, active medical devices are being refined, integrated, and electronic informationized. With the development of chips, the generation of electrostatic discharge will cause serious harm to the normal startup of active medical devices, and even cause the destruction of electrostatic damage. This is not acceptable clinically, it will affect the diagnosis and treatment of patients, and even threaten the personal safety of medical staff and patients. Active medical devices should take corresponding measures to prevent unpredictable damage caused by electrostatic elimination, which requires all two and three types of active medical devices should be tested according to electromagnetic compatibility electrostatic immunity.

4. Rectification case of electrostatic discharge experiment of medical electrical equipment

Take the problems encountered by a certain optometer in the electrostatic discharge test and the relevant solutions as an example, as detailed below.

Problem description: The optometer of some ophthalmic equipment carries out the electrostatic discharge material test according to the relevant requirements and test methods of the medical device industry specification YY0505-2012 and the national standard GB-T17626.2-2012. During the test, the touch discharge ± 6kV. level coupling plate ± 6kV. gas discharge ± 8kV test conducted by the optometer did not meet the requirements, affecting the basic characteristics and delaying the diagnosis. The main performance is camera disconnection The mouse is invalid and its display screen is blurred. It cannot work normally. It must be restarted manually to restore the operation status.

The root cause of camera disconnection: first analyze the camera disconnection according to the experimental phenomenon. The camera is an electrostatic sensitive part. When the structure contacts and discharges, ± 6kV is easy to be disconnected. From the overall structure analysis, the camera USB line is connected to the internal host interface. After reinsertion, the camera can be powered on again to recover. After careful observation of the USB line path, it is found that the USB line part is in contact with the structural metal frame, and the metal frame is grounded. During contact discharge, the sensitive camera is disconnected according to the crosstalk of the contact part within the USB line at the construction site.

Improvement measure 1: The camera channel USB net set magnetic ring blocks electromagnetic interference from harming the camera, and the net set magnetic ring blocks electromagnetic interference from harming the camera USB to avoid grounding the metal frame as much as possible.

The root cause of mouse invalidity: the horizontal coupling board of the device is indirectly discharged at ± 6kV. The mouse is easy to be disconnected, and cannot be repaired after being inserted into the mouse interface. The internal host is disconnected from the USB adapter board, and can be repaired after being powered on again. It can be inferred that the USB adapter board at the bottom of the shell is jammed due to electrostatic coupling. Through the analysis of the circuit diagram of the adapter plate, it is found that the resistance between the structural floor and the floor is too small, which is easy to be connected in series from the metal bottom shell to the adapter plate, resulting in extremely high total voltage and endangering the characteristics of the adapter plate.

Improvement measure 2: change the release resistance value and select the resistance with larger resistance value.

The root cause of the lightning screen: the display of the gas discharge system is ± 8kV, and the display occurs. The electrostatic charge is connected with the internal circuit through the gap, which affects the display abnormality. The reason is that the electrostatic charge has not been successfully conducted in time according to the ground wire, which affects the working characteristics.

Improvement measure 3: expand the display grounding area, improve the display grounding effect, and successfully conduct the electrostatic charge.

Conclusion: After the above rectification, the optometer will conduct electrostatic discharge detection again, including touch discharge ± 6kV. level coupling plate ± 6kV. gas discharge ± 8kV. During the experiment, the equipment will no longer have camera disconnection. The mouse is invalid, the display screen is blurred, and there are no other adverse experimental conditions. The rectification standard also proves that the rectification opinions are effective and reasonable, and the production process can achieve continuous production.

In short, since the implementation of EMC standards, the basic rate of EMC electrostatic discharge test for active medical devices is very low. It is a process of integrating theory with practice and applying theory to solve practical problems to improve the comprehensive anti-interference characteristics of electrostatic discharge. This paper introduces the cause of electrostatic discharge and its impact on medical devices, and discusses the provisions of EMC electrostatic discharge test standard and its electrostatic rectification measures. On the premise of introducing the electrostatic discharge test, several rectification ideas are put forward, the EMC technical standards are introduced in the product development stage, and the planning is carried out. When producing medical devices, the impact of electrostatic discharge should be considered to avoid the risk of EMC from the source.