1. EMC solutions in medical equipment
With the further improvement of the modernization of medical instruments and equipment, various operating power equipment is related to each other and affects each other through electromagnetic conduction, electromagnetic induction and electromagnetic radiation. Under certain conditions, it will cause damage to the operating equipment and personnel. Interference, influence and harm.
1.1 Ways of interference
Interference is divided into differential mode interference, common mode interference and series mode interference. Differential mode interference is also called normal mode interference, transverse mode interference or symmetrical interference. It refers to the interference superimposed on the line voltage sine wave, which is the interference between current-carrying conductors. Such as overvoltage and undervoltage of the power grid, transient mutations, spikes, etc. Common mode interference is also called longitudinal mode interference, asymmetric interference and grounding interference. It refers to the interference generated between the power grid and the neutral line. It is the interference between the current-carrying conductor and the ground. It is coupled into the circuit by radiation or interference. Coming. Such as spike interference, radio frequency interference, steady-state voltage between neutral and ground, etc. Serial mode interference refers to interference caused by external magnetic fields and electric fields.
1.2 Types of interference
All electronic equipment produced now contains electromagnetic interference filter circuits. Similarly, all switching power supplies have internal electromagnetic interference filters. However, in some environments, the electromagnetic interference filters of these electronic devices require auxiliary filters to meet more stringent electrical noise controls or to protect the devices from excessive external noise sources.
The types of power interference include voltage drop, power loss, frequency offset, electrical noise, surge, harmonic distortion, and transient.
1.3 The impact of interference on medical equipment
Electrocardiographs, monitors, ultrasound diagnostics, acupuncture and electrotherapeutics or silver needles that directly contact the human body, especially those that detect human bioelectric signals, because the signals are very weak, and if they are interfered, they will Superimposing a kind of distortion similar to certain lesions on the detection results such as waveforms, graphics, and images can cause misdiagnosis, and at the same time cause micro-electric shocks, and in severe cases, life-threatening. If it is a medical instrument with a computer system, when the peak interference amplitude in the common mode interference reaches 2V～50V for a few microseconds, it can cause computer logic errors, loss, etc.
2. Common methods to suppress interference
2.1 Grounding
Before expounding on grounding, it is necessary to clarify the basic concepts of ground wire and change, protective grounding and protective grounding. That is: the ground wire refers to the metal connection wire that connects the earth to the earth, and the neutral wire is the working wire provided by the power department of our country; the protective grounding is to connect the metal casing of the instrument and equipment to the ground wire. When the casing is charged due to interference, The current flows into the earth along the ground wire to achieve the purpose of protecting the safety of people and equipment. The protective zero connection is to connect the metal shell of the instrument and equipment with the neutral wire of the power supply. In the event of a short circuit, the fuse is immediately blown to achieve the purpose of cutting off the power supply.
2.1.1 Signal grounding of equipment
①Floating ground insulates the "zero" potential of the circuit or the "zero" potential of the equipment from the common grounding system, or the common wire that may cause circulation, that is, not grounding, so that this "zero" potential is relative to the zero potential of the earth. Floating "zero" potential. Commonly used methods are transformer isolation and photoelectric coupling isolation. The advantage of floating is strong anti-interference ability, but the disadvantage is static accumulation. When the charge accumulates to a certain extent, the potential difference between the equipment ground and the common ground may cause violent electrostatic discharge, which becomes a destructive source of disturbance. The solution is to bridge the bleeder resistance between the floating ground and the public ground, and the size of the resistance should not affect the leakage current of the equipment.
②Single-point grounding is called single-point grounding when all points that need to be grounded in single-point grounding circuits and equipment are connected to a defined point where only one physical point is the ground reference point. If single-point grounding is used for a system, each device must have its own single-point grounding point, and then the ground of each device is connected to the only designated reference grounding point in the system. The disadvantage is that when the operating frequency of the system is very high, there is a certain reactance effect, which causes poor grounding effect.
③Multi-point grounding Multi-point grounding means that all points in the equipment that need to be grounded are directly connected to the ground plane closest to it. The advantage is simple, small high-frequency standing wave. The disadvantage is that the amount of maintenance is relatively large.
④ Mixed grounding sets the length of single-point and multi-point grounding. Connect the points that need to be grounded nearby to the ground plane directly or to the points that need high-frequency grounding, connect to the ground plane through bypass capacitors, and use the other points. Single point grounding. Single-point grounding should be used when the wavelength of the circulating signal is lower than 0.05λ, and multi-point grounding should be used if the length of the grounding wire exceeds 0.05λ.
2.2 Shield
In order to effectively suppress the electromagnetic interference of the radiated electromagnetic energy inside and outside the equipment propagating through space, the usual measure is shielding. Specifically, there are three types of electric field, magnetic field, and electromagnetic field shielding. Practice has proved that: for instruments and equipment with computer systems, the method of shielding the computer host has a good suppression effect on electromagnetic interference and electrostatic interference. Different shielding methods and materials have different effects.
2.2.1 Electric field shielding
The mutual induction between objects with different potentials in equipment can be regarded as the voltage distribution between distributed capacitances. In order to reduce the interference of the interference source to the induced object, the usual measures are: increase the distance between the interference source and the induced object, reduce the distributed capacitance; make the induced object close to the ground plate as much as possible, and increase its capacitance to the ground ; Add a metal shield between the two. The shielding layer must be a good conductive conductor, with sufficient strength, and good grounding. For example, electrocardiographs, monitors, acupuncture and electrotherapy devices or silver needles that directly contact the human body should be kept away from the radiation of ultrashort wave therapy machines, high-frequency electrosurgical units, X-ray machines, CT, MRI and all medical equipment that can radiate electromagnetic waves. In the area, the importance of the high-voltage cable shielding layer of the X-ray machine.
2.2.2 Magnetic field shielding
Magnetic field shielding refers to the shielding of DC or low frequency magnetic fields. The shielding principle is to use the shielding body's high magnetic permeability and low magnetic resistance characteristics to the magnetic shunt effect on the magnetic flux, thereby weakening the magnetic field inside the shielding body. In order to reduce the magnetic resistance of the shield, the material used must be a material with high permeability and a certain thickness. The shielded object should be placed in the center of the shielding body as much as possible, paying attention to the gap. Ventilation holes must be distributed along the direction of the magnetic field, and electromagnetic shielding is one of the main measures of electromagnetic compatibility technology. That is, a measure to close the electromagnetic interference source with metal shielding material to make the external electromagnetic field strength lower than the allowable value; or to seal the electromagnetic sensitive circuit with metal shielding material to make the internal electromagnetic field strength lower than the allowable value Measures.
2.2.3 Electromagnetic field shielding
The circuit includes an inverting device coupled to one turn in the secondary winding for generating a phase opposite to the phase of a voltage signal induced from the secondary winding to the anode; an oscillating device for oscillating from the The voltage signal output from the output node of the inverter device and the oscillated signal is matched in level with the high voltage; and an electromagnetic field generating device for applying a voltage signal output from the output node of the oscillation device , Generating an electromagnetic field in response to the voltage signal substantially surrounding the periphery of the front portion of the kinescope, and canceling and shielding the electromagnetic field generated from the anode. As a result, the circuit can be applied to cathode ray tubes of various sizes at a lower cost, thereby improving production efficiency.
3. Technology to suppress interference
3.1 dedicated line
In order to suppress the mutual interference between equipment and equipment, the easiest way is to use the split-phase power supply system. Namely: In the three-wire power supply line, one phase is identified as the power supply for sensitive equipment; one phase is used as the power supply for external equipment; and the other phase is used as the power supply for commonly used test instruments or other auxiliary equipment. This measure is often applied to the power supply system of large-scale medical equipment.
It is worth noting that in the modern medical electronic equipment system, due to the use of non-linear loads in the distribution line, the existence of harmonic currents in the line, and the zero-sequence component harmonics cannot cancel each other in the neutral line, but overlap, so Too fine a neutral line will increase the line impedance and interference will also increase.
3.2 Transient interference suppressor
3.2.1 Gas discharge tube
commonly known as lightning protection. The advantages are high insulation resistance, small parasitic capacitance, and strong surge absorption capability. The disadvantage is that the response speed to the surge voltage is low.
3.2.2 Metal Oxide Varistor
The main parameters of varistor are nominal voltage and current capacity. When in use, the voltage selection of the varistor should consider the possible fluctuating voltage of the protected circuit, which is generally 1.2 to 1.4 times. If it is an AC circuit, also pay attention to the relationship between the effective value of the voltage and the peak value. For example, when 220V, the nominal voltage of the varistor should be 220×1.4×1.4=430V. The former is due to the high-speed performance of the varistor when it absorbs transient interference. The longer the lead, the greater the induced voltage, and the latter is due to the inherent capacitance of the varistor.
3.2.3 Silicon transient voltage absorption diode
TVS tube is also called transient voltage suppression circuit. When the transient voltage protection diode is impacted by the reverse transient high energy, at a speed of 1×10-12s, the high impedance between its two poles becomes low impedance, and it absorbs up to several kilowatts of surge power, so that the The voltage clamp is located at a predetermined value, which effectively protects the sensitive components of the electronic circuit. Specifically, it is divided into one-way and two-way. The main parameters are breakdown voltage, leakage current and capacitance. It is characterized by fast response time, high surge absorption capacity, high transient power, low leakage current, easy control of the clamping voltage, no damage limit and small size. It is widely used in static electricity of medical equipment, transient voltage generated when inductive load is switched, and overvoltage protection caused by lightning strike.
3.2.4 Solid discharge tube
The characteristics of solid discharge tube are fast response speed, large current absorption, stable operating voltage and long service life. Its working principle is: when the external interference is lower than the trigger voltage, the discharge tube is in the cut-off state; when the interference voltage exceeds the trigger voltage, the discharge tube works in the negative resistance zone. At this time, the current is extremely large, causing the interference energy to transfer. As the interference decreases, the current through the discharge tube drops. When the interference current is lower than the sustaining current, the discharge tube returns from the low resistance area to the high resistance area to complete the first discharge process.
3.3 power line filter
The power line filter is installed between the power supply and the electronic equipment, and mainly plays a role in suppressing the parasitic electromagnetic interference in the power transmission and improving the reliability of the equipment. Commonly used single-stage circuit composed of passive centralized parameters. As shown in Figure 1. In the figure, Cx is a differential mode capacitor, which attenuates differential mode interference. It is tens to hundreds of nF in 220V AC power supply, and the withstand voltage is 250VAC. Cy is a common mode capacitor, which attenuates common mode interference. Generally take 1nf~4.7nf, withstand voltage 3~6KVDC. L1 and L2 are common-mode inductors, and their inductance is related to the size of the passing current, which has a good filtering effect on the common-mode current. Multiple inductors connected in series: For filters with higher requirements, a large inductor can be decomposed into a larger inductor and several small inductors with different inductances. Connecting these inductors in series can expand the bandwidth of the inductor. But the price paid is volume and cost. In addition, pay attention to the same problem as the parallel connection of the capacitor, that is, the introduction of an additional series resonance point. The impedance of the inductor at the resonance point is very small.
Measures to improve filter performance: One is to use a filter with ground wire inductance. This can suppress interference on the ground wire. The second is to use multi-stage filters. The third is the combined use of filters and absorption devices. The fourth is to use new soft magnetic materials. Fifth, add lossy components.
3.4 isolation transformer
The principle of isolation transformer is the same as that of ordinary transformer. All use the principle of electromagnetic induction. The isolation transformer generally refers to a 1:1 transformer. Because the secondary is not connected to the ground. There is no potential difference between any secondary line and ground. Safe to use. Often used for maintenance power supply.
3.4.1 Ordinary isolation transformer
Ordinary isolation transformer does not have a shielding layer between the primary and the secondary. It solves the problem of common ground through electrical isolation between input and output. The advantage is that it has a certain inhibitory effect on common mode interference, and its size can be estimated by the ratio of the distributed capacitance between the primary and the secondary and the distributed capacitance of the device to the ground. Usually the distributed capacitance between primary and secondary is several hundred Pf, and the distributed capacitance of equipment to ground is several to tens of nF, so the attenuation value of common mode interference is about 10-20 times. The disadvantage is that the suppression effect of common mode interference decreases with increasing frequency due to the distributed capacitance between windings.
3.4.2 Isolating transformer with shielding layer
Add a shielding layer between the primary and secondary transformers, and connect the shielding layer to the ground reliably, which can not only obtain better suppression of common mode interference, but also use the shielding layer to suppress differential mode interference. The specific method is to connect the transformer shielding layer to the neutral end of the primary. For example, for 50HZ power frequency, due to the high capacitive reactance formed by the primary and the shielding layer, it can still be transmitted to the secondary through the transformer effect without being attenuated. For common mode interference with higher frequency, due to the smaller capacitive reactance between the primary and the shielding layer, this part of the interference directly returns to the grid through the distributed capacitance and the connection between the shielding layer and the primary neutral end, and enters the secondary loop.
3.4.3 Super isolation transformer
Isolation transformer is a safety power supply, generally used for machine maintenance for protection, lightning protection, and filtering. The isolation transformer is a 1/1 transformer. The primary is single-phase 220V, and the secondary is also single-phase 220V. Or the primary three-phase 380V, and the secondary three-phase 380V. First of all, we usually use one line of AC power voltage to connect to the ground, and there is a 220V potential difference between the other line and the ground. Human contact can cause electric shock. However, the secondary of the isolation transformer is not connected to the earth, and there is no potential difference between any two wires of it and the earth. People will not get an electric shock if they touch any line, so it is safer. Secondly, the output end of the isolation transformer is completely "open circuit" isolated from the input end, so that the input end of the transformer (the power supply voltage supplied by the power grid) can be effectively filtered. So as to provide pure power supply voltage for electrical equipment
3.5 AC voltage stabilizer
Since the mains supply voltage is unstable due to various reasons, especially in some power supply places, the voltage fluctuates greatly, which affects the normal operation of electrical equipment and may cause damage to the electrical equipment. The AC voltage stabilizer is a kind of Use voltage stabilizing equipment whose working voltage is basically stable.
The role of the AC voltage stabilizer is to stabilize the output voltage within the allowable range when the input voltage and load current change. The commonly used AC power supplies include ferromagnetic resonance, parameter adjustment type, servo type, step adjustment width, super isolation, switch type, uninterrupted and purified AC power supplies.
3.5.1 Ferromagnetic resonance AC power supply
An electronic device that can provide stable AC power to the load. Also known as AC voltage regulator. For the parameters and quality indicators of AC power supply, please refer to DC power supply. Various electronic devices require a relatively stable AC power supply, especially when computer technology is applied to various fields, the direct power supply from the AC grid without any measures is no longer sufficient.
The working principle is to realize the adjustment by changing the saturation of the inductor, and making the inductor and the capacitor resonate. When the input voltage is too high or too low due to some factors, the output voltage can be automatically adjusted with the input voltage, so that the output voltage remains stable. The advantage is that the circuit is simple, the output impedance is high, and the
