Analysis of electrical fast transient burst anti-interference test model of medical electronic equipment

1. Introduction

With the wide application of high-speed circuits in the medical field, combined with the particularity of users in the medical field and the electromagnetic compatibility environment, the anti-interference of medical electronic equipment has attracted more and more attention. Therefore, the country has formulated the corresponding anti-interference level requirements, and verified whether the anti-interference level of medical electronic equipment meets the market requirements through the standard anti-interference test system. Pulse group anti-interference test is one of the most common and difficult anti-interference tests.

This paper combines the electrical fast transient pulse group anti-interference test system and medical electronic equipment, establishes the pulse group anti-interference test model according to the characteristics of medical electronic equipment, provides the system design methods and ideas for electronic engineers, and reduces the pulse group anti-interference by analyzing the path of interference current in the test model.

2. Pulse group generation mechanism

Once the contact points of the load circuit are opened or closed, electric breakdown may occur between the contact points. When the contact point is close and not fully contacted, the fault begins to occur. In the process of closing the contact point, the fault will continue to occur until the contact point is closed.

The energy stored in the inductive load of the power grid forms resonance with the distributed capacitance, and the peak high voltage continuously breaks down the relay. The contact gap of the contactor forms a discharge arc until the gap between the dynamic and static contact points becomes larger and cannot pass through. In this process, a series of high-voltage pulse sets, namely electric fast transient pulse sets, will be generated.

3. Burst immunity test system

In order to prevent the medical electronic equipment from being affected by the pulse group interference, the state stipulates that the medical electronic equipment needs to pass the pulse group interference immunity test. The medical electronic equipment should be placed on the metal coupling plate, and the interference generator is generated through the power line or a certain length of input and output cable. The interference is returned to the pulse group interference generator through the equivalent capacitor between the equipment and the metal coupling plate. An example is the case of L/N/PE cable injection interference. Usually, the DC high voltage generated by the high voltage source is charged by the pulse group interference generator. The plastic surgery generates the required pulse interference signal. The pulse interference signal is output to the L/N/PE of the equipment to be tested through the impedance matching resistance and the direct capacitance on the cable, and finally returns to the high voltage source through the metal coupling plate. Therefore, the pulse group interference generator and the metal coupling plate can be equivalent to the equivalent model in Figure 1.

4. Pulse group immunity test model of medical electronic equipment

4.1 Characteristics of medical electronic equipment

Medical electronic equipment usually has the length of the patient's cable. The product type is diversified. The anti-interference gate limit of the sensitive circuit is low. The patient cable is usually the longest part of the system. As an equivalent antenna, it can pick up and radiate noise. For example, the ECG monitor has a long patient cable. In addition, according to the needs of application scenarios, medical electronic equipment has a variety of product types, such as the combination of network power supply and floating ground equipment, dry battery power supply equipment, equipment with protective grounding, and equipment internal sheet metal.

4.2 Impulse group immunity test model

The pulse group anti-interference test model combines the characteristics of medical electronic equipment and the principle of the pulse group anti-interference test system, as shown in Figure 2:

C1~C pulse group generator coupling capacitance. L equipment ground wire equivalent inductance. C4. C5. C6 equivalent capacitance is the internal sheet metal of circuit board and equipment. C7. equivalent capacitance at both ends of floating ground circuit. C8. C9. equivalent capacitance between cable and metal coupling plate. C10 equivalent capacitance between internal sheet metal and metal coupling plate of equipment.

When using this model, there is no additional grounding equipment L, which can be considered as open circuit.

When there is no internal sheet metal equipment to use this model, delete C10 of this model and reduce C4.C5.C6 capacity value.

When using the device model without patient cable and input/output cable, C8. C9.

When the equipment model without floating ground application is used, C7 can be considered as short-circuited.

C1-C3. The capacitance value is the internal capacitance value of the impulse group anti-interference equipment. The floating ground equipment can be fixed according to the transformer. The capacitance value C7 of the equivalent capacitance and the jumper capacitance can be determined. The C8. C equivalent capacitance can be calculated according to the capacitance calculation formula between the conductor and the metal plane, and the other capacitance can be calculated according to the capacitance calculation formula between the two metal planes.

5. Application of medical electronic equipment pulse group immunity test model

The core idea of the design of pulse swarm anti-jamming filter is to control the flow direction of interference current and avoid passing through unexpected paths. According to the analysis of medical electronic equipment pulse group anti-interference test model, the main return path of interference current is shown in Figure 3:

The main interference energy should be guided as much as possible when designing the immunity of pulse group. First, return according to the red path, and then return according to the pink path to avoid returning according to the blue path. When the interference signal returns through the pink path, the distance between the sensitive equipment and the return path should be used to determine whether there is any adverse effect. According to the test model, the design idea is as follows:

1) When the equipment is well grounded and directly connected to the metal coupling plate, try to filter the interference through the ground wire to reduce the equivalent inductance L1, L according to the formula to reduce R=2 π fL. It can be seen that the sensing resistance value at high frequency will become smaller, and the ground wire provides a low resistance circuit for the interference current, so that the ground wire, i.e. the red line part in Figure 3, will not pass through the rear circuit to reduce the interference to the sensitive circuit at the beginning.

2) If the equipment has internal metal sheet metal, C it will exist in the equivalent capacitance between the metal sheet metal and the metal coupling plate. When the equivalent capacitance is large, R=1/(2 π fC) is formed by the formula. It can be seen that the capacitive reactance will become smaller, and then the interference will flow from this path to the metal coupling plate, back to the interference source, and avoid passing through the sensitive circuit.

3) When the equipment has no internal metal sheet metal, C10 in Figure 3 does not exist. C4.C5.C6 is directly connected with the metal coupling plate. At this time, the former circuit of C7 after AC/DC can be connected in series to form a common-mode inductance, increasing the impedance on the line, making the interference attenuate on the red path as much as possible, and making the interference pass through the digital circuit or analog circuit as little as possible.

4) If the device has patient cable, such as blood oxygen cable and ECG conductive connection cable. It picks up the radiated interference current from the space, and there is a large equivalent capacitance between it and the metal coupling plate, as shown in the figure C8. C9. Especially when the cable is long, the equivalent capacitance of C8. C9 will increase, so the capacitive reactance R will be very small, becoming a low impedance circuit of the interference current, so the interference must pass through the intermediate analog or digital circuit. If the interference path is blue in Figure 3, the device will be affected.

According to the analysis of the anti-interference test model of the pulse group of medical electronic equipment, the electronic engineer should analyze the interference path when carrying out the anti-interference design of the pulse group, and try to filter the interference in front of the sensitive circuit. The return of interference current can be guided by capacitance filtering, and the series inductance increases the attenuation of interference current. If it cannot avoid its flow through the intermediate sensitive circuit, the designer needs to increase the anti-interference gate limit of the sensitive current and make a good grounding design of the sensitive circuit.

6. Conclusion

Combined with the characteristics of medical electronic equipment and the electrical fast transient pulse group test environment, this paper proposes the electronic fast transient pulse group anti-interference test model for medical electronic equipment. This model is a simple and intuitive overall test model, which can provide the overall design concept for medical electronic equipment, reduce the design cost and design time.