What are the electromagnetic compatibility problems often encountered in medical equipment?

1. The distribution parameter of components

In medical equipment, the defects of the selected components themselves will also limit their performance. We know that all capacitors have parasitic series inductance, and the two constitute a series resonant circuit: while all wire-wound inductors have distributed capacitance between winding turns and layers, the two constitute a parallel resonant circuit, The resonant frequency of this circuit is much lower than expected. For example, the resonant frequency of many capacitors is lower than 100MNz; the resonant frequency of many wire wound inductors is lower than 20MHz, and the resonant frequency of transformers is even lower than 5MHz. Therefore, circuit designers must fully understand the actual performance of these components, regardless of the filter or decoupling components, they have the possibility of working in a resonant state, which will lead to crosstalk between the wire line and the line, this crosstalk Not only can it cause damage to the line, but also cause the signal transmission to become very bad.

2. Impedance discontinuity in signal transmission

With the wide application of high-speed digital circuits, the electromagnetic compatibility problem in printed circuit boards is becoming more and more important, and many PCB problems can be summarized as impedance discontinuities during signal transmission.

The problem of impedance discontinuity in signal transmission always revolves around the circulation circuit of the signal. Ideally, the signal flows out along a line and then quickly flows back along the ground return line. However, according to the law of minimum energy consumption in physics, current usually returns along the path of minimum energy. At this time, the return path of the signal often has impedance discontinuities. Once this happens, it will cause signal reflection and cause Distortion of the transmitted signal. In addition, discontinuities in the transmission line can also cause radiated emission problems.

3. The problem of signal and its return path

For the wiring of equipment, people often think that even using an extra ground return line will be a waste. Since the distance between most signal lines and the ground loop is too far, a ground return line should be used as a multi-following signal line, or even the grounding interrogation line of all signal lines. At this time, there are at least two problems with this ground return line. First of all, because the distance between this ground loop and most of the signal lines is relatively long, the loop antenna formed between the signal line and the ground loop is relatively large, and the loop antenna's external radiation emission and the loop antenna's interference with external charges The problem of receiving cannot be ignored; secondly, the common impedance problem of this signal line sharing the ground loop cannot be ignored, especially when the signal speed transmitted on the signal line is high and the edge of the signal is steep, the mutual interference of the common impedance Especially serious.

In practice, how many ground return lines are needed for the transmission line is appropriate, which is determined by the data transmission rate and the length of the transmission line. For high-speed transmission of a signal over 100MHz, it is recommended to use a signal to a grounded return line, and for low-speed transmission less than 10MHz, you can compromise the use of two (or more) signals to a grounded return line.

4. The problem of electrostatic discharge of the device casing

Electrostatic discharge is a problem that often occurs and causes headaches. Taking medical equipment with a plastic shell as an example, in order to reduce the electromagnetic emission of the plastic shell equipment and improve the anti-radio frequency interference capability of the plastic shell equipment, designers will conduct conductive spray coating on the plastic shell. In order to make the effect of conductive spraying obvious, it is usually required to spray all the gaps of the joints of the plastic shells in order to achieve the conduction continuity of the bonding, but this approach has created new electrostatic discharge contact points. In other words, while solving a problem, the designer has introduced a new and very difficult problem.

In order to solve the above problems, designers have three choices: redesign the circuit and internal layout to minimize the need for conductive spraying of the equipment; repaint the equipment casing to avoid discharges as much as possible, and use shielding materials carefully to make possible The discharge gap is protected.

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