There are many interferences in medical equipment control systems. Many interferences will enter the control system through the power supply system and cause serious harm to the control system. The quality of the power supply system will directly affect the control system. When designing the power supply system, the anti-interference performance and the redundant design of the power supply system when the control system does not allow power outage should be considered.
1 Power supply method of power system
1.1 Split-phase power supply method
Since a lot of interference is introduced by power lines, the equipment that generates greater interference should be separated from the control system in the configuration of the power supply system, and powered by different lines. It is best to use shielded cables to draw out the power supply directly from the power distribution room. As shown in Figure 1
1.2 Separate power supply mode for measurement and control devices and power equipment
The controlled equipment in the computer control system, such as AC motors, converters, solenoid valves, heaters, etc., use AC power supplies with large capacities and large changes in various loads, which have a great impact on the system and cause serious interference. With unsymmetrical loads, the neutral point will shift. The AC low-voltage power supply used by the measurement and control device has a small capacity, but requires the voltage to be as stable as possible and the interference to be as small as possible. Therefore, it is not easy to combine the two power supplies into one power supply. The following two methods can be used to supply power.
(1) Separate power supply from distribution box. When the measurement and control devices are few and centralized, special cables can be laid directly from the main distribution box of the hospital or branch building to distribute power to the electronic control power distribution box. This distribution box is dedicated to powering the measurement and control system and cannot carry any power load. The power load should be powered from the power distribution box, that is, power supply from the power distribution box to the control system and other electronic equipment should be avoided, as shown in Figure 2.
(2) The power transformer supplies power separately. When there are many electronic control devices, a dedicated electronic control device transformer can be equipped, and it is best to use a high-voltage bus for power supply. Because the interference caused by the frequent operation of a large number of low-voltage power loads will be greatly attenuated after being isolated and transmitted by the power grid, the noise of the higher-voltage busbar is relatively less than that of the low-voltage power grid. As shown in Figure 3.
Regardless of AC or DC power supply, attention must be paid to the hierarchical setting and capacity of the air switch to prevent override tripping from causing a power outage in a larger area.
1.3 Power supply capacity
In order to enable the measurement and control device to adapt to a wide range of load changes and prevent internal interference caused by the power supply, the power supply of the entire machine must have a large reserve and have good dynamic characteristics. Of course, if the power supply capacity is increased too much, it will inevitably lead to excessive size and increased cost. Generally, a margin of 0.5 to 1 times should be selected.
2 Isolation technology of power supply system
2.1 Isolation of AC power supply system
Since there is a large amount of harmonics, lightning surges, high-frequency interference and other noise in the AC power grid, suppression measures should be taken for control devices and electronic and electrical equipment powered by AC power. Using a power isolation transformer can effectively suppress noise interference from the AC power supply. However, ordinary transformers cannot fully play the role of anti-interference. This is because even if the primary winding and secondary winding are insulated, it can prevent the noise voltage and current on the primary side from being directly transmitted to the secondary side, which has an isolation effect. . However, due to the existence of distributed capacitance (between windings and cores, between windings, between layer turns and between leads), noise in the AC grid will be coupled to the secondary side through distributed capacitance. In order to suppress noise, a shielding layer must be added between the windings, which can effectively suppress noise, eliminate interference, and improve the anti-interference performance of the equipment. Figure 4(a-b) shows the distributed capacitance of the isolation transformer without shielding layer and with shielding layer. In Figure 4a, the isolation transformer does not add a shielding layer. C12 is the distributed capacitance between the primary side and the secondary side. Under the action of the common mode voltage u1C, the common mode noise voltage coupled to the secondary winding is u2C, and C2E is The capacitance of the secondary side to ground, then it can be seen from the figure that the common mode noise voltage u2C of the secondary side is:
u2C ＝u1CC12 ／(C12 ＋C2E)
In Figure 4b, the isolation transformer is added with a shielding layer, where C10 and C20 respectively represent the distributed capacitance of the primary side and the secondary side to the shielding layer, ZE is the impedance of the shielding layer to ground, and C2E is the capacitance of the secondary side to ground, then It can be seen from the figure that the common mode noise voltage u2C on the secondary side is:
u2C=〔u1CZE/（ZE ＋ 1/jω C10）〕〔C2E/（C20 ＋C2E)〕
Since C2 is the ground impedance of the shielding layer, in the low frequency range, ZE<<(1/j ω C10), so u2C → 0. It can be seen that the common mode noise voltage passing through the isolation transformer is greatly weakened after taking shielding measures.
The power transformer is the main component of the power supply. In order to suppress interference in the power grid, an isolation transformer is generally used, and the transformer capacity should be about 1.2 to 1.5 times larger than actual needs. During use, the shielding layer of the transformer should be well grounded, and the secondary coil connecting wire should use twisted pairs to reduce interference between power lines. For the controller power supply of the control system, if conditions permit, a filter can also be added before the isolation transformer. At this time, both the primary and secondary connection lines of the transformer must use twisted pairs, as shown in Figure 5. In this way, the interference signal can be greatly weakened after filtering and isolation, which enhances the reliability of the system.
The power supply system of large medical equipment control systems can adopt the following method. The controller and I/O system are powered by their own isolation transformers and are separated from the main circuit power supply. When a certain part of the power supply fails, it will not affect other parts. For example, when the input and output power supplies are interrupted, the controller can still continue to provide power, which improves the reliability of the system, as shown in Figure 6. When the power supply quality is not guaranteed (not a long-term power outage), the controller can use UPS uninterruptible power supply, that is, change the shielded transformer in front of the controller to a UPS uninterruptible regulated power supply. For some important equipment (medical equipment, etc.), in order to improve the reliability of the system, the AC power supply circuit can use a dual-channel power supply system.
Figure 7 shows a comprehensive solution for AC power supply anti-interference. In order to isolate the measurement and control system from the power supply grid, eliminate coupling caused by common resistance, reduce the impact of load fluctuations, and for safety, a 1:1 isolation transformer is often added before the power transformer and low-pass filter.
At present, foreign countries have successfully developed an isolation transformer (NCT for short) that specifically suppresses noise. This is a multi-layer shielded transformer with shielding layers on the windings and the entire transformer. The structure, core material, shape and coil position of this type of transformer are quite special. It can cut off the cross-link between high-frequency noise leakage flux and winding, so that differential mode noise is not easily induced to the secondary side. Therefore, this kind of transformer can It can cut off common mode noise voltage and differential mode noise voltage, making it an ideal isolation transformer.
2.2 Isolation of DC power supply system
The method of isolating DC power supply is to use a DC-DC converter. Figure 8(a) shows the power supply circuit using the DC-DC converter to the unit isolated by the photoelectric isolator, and the power supply of the input loop and output loop of the photoelectric isolator. The system power supply has been isolated, which can better improve the system's ability to suppress electromagnetic interference.
When the control device and the internal subsystems of the electrical equipment need to be isolated from each other, their respective DC power supplies should also be isolated from each other. The isolation method is shown in Figure 8(b).
3 Power supply redundancy design technology
3.1 Dual AC power supply redundancy design
In order to improve the reliability of the power supply system, it is best to use dual-channel redundant power supply technology in the AC power supply system. The two power supplies are drawn from different substations. When one power supply line fails, it must be able to automatically switch to the other power supply line. Figure 9 shows the typical structure of a dual-channel redundant power supply system. The protection circuit mainly includes under-voltage protection, switching interlocking and other methods.
3.2 Redundant design using UPS
Uninterruptible power supply UPS is an effective protection device for computers. Although the UPS has high reliability, UPS failures may occur due to changes in power supply conditions, aging of the UPS's own electrical devices, premature failure of individual components, etc. Since the control system is the heart of the entire equipment system, in order to ensure its stable and high-reliability operation, dual-machine hot backup, that is, redundant technology, can be used to connect the output end of the backup machine UPS to the "bypass power supply" input of the host UPS terminal, and the AC power input terminals of two UPSs can be connected to the same mains power supply.
During normal operation, the host UPS provides load power. When an internal failure occurs in the host, the static switch at the output end of the host UPS will automatically switch to bypass, and the output end of the backup UPS provides the power required by the load. When the abnormal condition is eliminated, the static switch will automatically transfer from the bypass backup UPS to the inverter output of the host UPS, and the host UPS will continue to provide power to the load. The switching of the static switch has strict circuit control to ensure that no power outage occurs during switching. The above principle shows that during a power outage, if one UPS fails, the other one can still supply power; during maintenance, the UPS function is still maintained; the life of both UPSs is extended, and the structure of the hot backup machine can ensure that the load equipment will not be damaged when the mains power is outage. Power outage due to host failure to ensure that load equipment does not cause data loss, equipment damage, system crash, etc.
3.3 Dual DC power supply redundancy
The reliability of the DC power supply system can be improved by connecting two DC power supplies in parallel through diodes, as shown in Figure 10. The system can continue to operate when a DC power supply fails. At this time, attention should be paid to the selection of two independent diodes with very close conduction voltages; otherwise, it will not be possible to deal with a diode failure, and it will also cause uneven loads on the two power supplies.
4 Conclusion
The power supply design technology of medical equipment control systems plays an important role in system reliability design. Good power supply technology can effectively suppress external interference to the system. In actual design, only based on the specific characteristics of the application system and the specific conditions of the application environment, the system requirements are comprehensively considered, and reliable power supply technology can be flexibly selected as a whole to improve the reliability of the control system.
