In today's society, with the continuous advancement of science and technology, more and more modern medical devices have developed rapidly, especially the electronic devices that directly contact the human body. In addition to the higher and higher requirements for the performance of the device itself, Human safety considerations have also attracted more and more attention, such as: cardiac puncture monitors, ultrasound, maternal and child monitors, baby warmers, life monitors, and other instruments that are in close contact with the human body, which means that patients use Do not use the instrument to cause an electric shock or any other danger to the human body. So, what should we pay attention to when choosing the most relevant medical power supply?
Choice of medical power supply
Medical electronics, unlike other electronics and power electronics that are positioned in the mass market and cost-conscious consumer electronics and other low-priced product applications, medical electronics must comply with many more rules. If the designer is responsible for system power design, the first question to consider for the system power supply part is: buy or manufacture related solutions?
Since the output of medical electronics is generally relatively low, designers must consider the issue of purchase or self-made. Designers of medical electronics rarely consider designing off-line power supplies by themselves. Because the investment required for this kind of special design and testing does not match the final output scale; equipment manufacturers will find it difficult or impossible to share the investment in the design stage. Therefore, it is more cost-effective to directly purchase power supplies from companies that already have the corresponding professional design capabilities and testing technologies.
1. Price
In commercial application design, if the quality is guaranteed, it is easy for people to shop around and choose the lowest-priced power supply product. At this time, the lowest-priced but "decent" product is often the winner, while the best product is not popular. This is not bad for disposable electronic products that are quickly discarded or do not require repair, but what are the risks if the designer chooses such a power supply at random to use in the medical system? The value of medical electronics is very high, and some key tasks need to be completed. If the medical system fails, the consequences will not be as simple as missing a race or taking a wrong ride. The normal operation of medical equipment is a matter of life and death, especially the power supply of medical equipment, which must comply with relevant regulations on safety, leakage, EMI-RFI radiation and protection. These standards and related safety regulations constitute a set of strict normative requirements. The power supply used in such demanding application systems must comply with strict regulations on insulation measures to prevent patients and medical staff from being electrocuted. EMC is also a key issue, which includes how to reduce electromagnetic radiation and how to protect electromagnetic radiation. Therefore, the quality and reliability of the product must be the first choice for the design of medical power supplies.
Often, designers confuse commercial power supplies with medical power supplies, and manufacturers that manufacture various low-cost power supplies for the mass market may sell these commercial power supplies as medical power supplies without modification. In this regard, buyers must be careful, because choosing such power products for cheap will cause terrible consequences. Therefore, designers therefore need to understand relevant regulations and regulations. So, what are the issues involved in choosing a medical power supply? The power supply of medical equipment must comply with relevant regulations on safety, leakage, EMI-RFI radiation and protection. There are other questions, such as whether the products are produced by GMP certified factories?
2. GMP qualification
The U.S. Food and Drug Administration (FDA) requires that medical products must be manufactured in factories that have obtained GMP qualifications (ie, have good operating practices). This is a set of quality certification system that manufacturers should be required to produce in addition to the traditional ISO9000 certification to prove that their quality control procedures comply with GMP standards. Similarly, China also has strict control of medical power supply, for example, it must meet a series of requirements such as CE and UL medical certification.
GMP regulations stipulate that manufacturers must have parts quality control procedures and related documents. When choosing a medical power supply, does the manufacturer have a part quality control procedure? Are there quality data and test documents? You can ask what is the procedure and ask to produce relevant documents. Manufacturers are required to produce documents that can prove the quality and reliability of their products, and reputable manufacturers will be happy to provide them. We found that many low-priced manufacturers are using unbranded, unbranded power products or counterfeit power products, which brings many problems to the final medical electronic product OEM. If the manufacturer cannot produce the relevant certification documents, and they just sell the power supply of the POS terminal as a medical power supply after a rebrand, then such a product will only make the user lose more than the gain. Therefore, for the choice of a good medical power supply, GMP qualification can correspondingly prove its production and quality control, but is such a product a good product? No, we must also grasp the performance parameters and reliability of the product, that is, whether the product has passed some international general specifications.
EN60950 is an international safety standard applicable to general power supplies. Medical power supplies also need to meet the minimum basic requirements in this specification. However, the international safety standard for medical power supplies is more stringent IEC601-1 A2, and there are three versions according to different regions: EN60601-1 in Europe, UL2601-1 in the United States and CSA22.2 No 601.1 in Canada.
These specifications cover technical indicators for electric shock protection, fire prevention and machinery, as well as creepage distance and electrical clearance, as well as high-voltage insulation and other test indicators. The medical power supply must adopt appropriate design techniques to ensure that it can still work stably when the input is abnormal, and can work under certain special environmental conditions (such as oxygen and/or anesthetic gas). In these applications, fire protection is also an important issue.
3. Leakage current
Leakage current is the current flowing to the ground through the protective earth conductor. Without grounding, if there is a conductive path (such as the human body), the current can flow from the surface of the conductive or non-conductive part to the ground. There is always an external current in the safety grounding conductor. Generally, the upper limit of leakage current of a medical power supply is one-tenth of that of an ordinary power supply.
All power leakage current indicators of IEC601 standard are far stricter than non-medical power supplies. Among them, the technical indicators define several different and most critical leakage currents, such as: ground leakage current (flowing into the ground along the grounding body) and shell leakage current (flowing into the ground from the casing through the patient). The IEC601 standard has different definitions for the maximum leakage current of the following three main types of equipment power supplies:
Class B: Equipment that does not come into contact with the patient's body, such as laser treatment equipment.
BF category: equipment to be in contact with the patient's body, such as ultrasound, various monitors (including EGC equipment), and operating tables.
CF category: equipment to be in contact with the patient's heart, such as a cardiac puncture monitor.
People often misunderstand that these device types have different leakage current indicators. In fact, the allowable leakage current of these types of equipment is the same. The indicator requirements in North America are more stringent than the allowable leakage current specified in European EN60601-1. For example, Europe allows 0.5mA, while the United States and Canada only allow 0.3mA. Therefore, medical equipment designers need to pay attention to where their products will be sold.
And BF or CF equipment (commonly known as "human body" equipment) also requires additional insulation measures to insulate the patient from the ground, signal port and power output. This is to protect the patient in the event of an unexpected failure of the equipment, and to keep the leakage current on the patient within the limit specified by the standard. This insulation can also be achieved by other parts of the final device, such as plastic probes or sleeves with sufficient insulation properties. However, in applications where the patient needs to be energized, one of the solutions is to use an AC/DC power supply that meets the IEC601-1 standard to supply power to one or more isolated DC/DC converters, that is, add a second-level insulation protection. Therefore, the DC/DC converter must be carefully selected to ensure that the insulation requirements can be met.
Because medical equipment is often directly connected to the patient, and it will form a conductive path through the skin or even the subcutaneous connection, the leakage current must be as zero as possible, the insulation must be reliable, and there must be no sneak current.
4. Security and isolation
Safety and isolation are also a major difference between commercial power supply and medical power supply, which is related to the safety guarantee of electric shock for patients receiving treatment and medical staff using equipment. Although human skin is a good insulator, once a very small alternating current is applied to the heart, it may cause cardiac muscle fibrillation and neuromuscular damage. Therefore, when it comes to the patient's environmental equipment, the current of any part that may be in contact with the patient must be strictly limited within 40-70Hz.
The level of protection required for medical device applications is related to the proximity of the equipment to the patient. For medical system power supply, there are three safety levels for insulation and protection indicators. First of all, all offline power supplies must meet the basic safety requirements in the EN60950 standard. In addition, the medical power supply close to the patient must comply with the IEC601-1 standard. In addition to meeting the above two requirements, the equipment that contacts the patient needs to be protected by an additional isolation barrier. Also, when the mains power is interrupted, the hospital’s backup generators can take a few seconds or minutes to supply power. Therefore, many medical power supplies and equipment using this power supply are relayed through the UPS system. As a result, the input waveform of the power supply may change and it is no longer an ideal sine wave. Therefore, a medical transformer needs to be connected to the front of the power supply to further improve the safety level. Therefore, the equipment used directly on the patient must meet the highest insulation specifications for all these parameters.
5. EMI-RFI radiation and protection
The electromagnetic radiation and electromagnetic radiation protection of equipment is also an important parameter standard for medical power supplies, involving surge and transient current intensity, electrostatic discharge (ESD) level, and radio frequency interference (RFI) protection capabilities. For medical power supplies, these electrical indicators must be three times that of commercial products of the same level. Many medical applications involve RF treatment devices or non-invasive electronic surgical instruments, so the power supply must be able to resist interference and not be affected. Qualified medical power supplies should comply with EN60601-1-2 standards that match many technical requirements related to EMC. Not only that, the medical power supply must also meet IEC61000-4-2 ((electrostatic protection capability, required to reach 3kV), IEC61000-4-3 (RF radiation protection capability, required to reach 3V/m), IEC61000-4-4 EFT (voltage Transient tolerance, 1kV required), IEC61000-4- 5 (mains surge current tolerance, 1kV and 2kV required), IEC61000-3-2 (harmonic requirements for mains lines), IEC61000-3-3 (power line Flicker requirements), and EN55011 (Class A product or B product radiation limit) requirements. For this reason, power supplies that meet the IEC601-1 standard generally comply with EN55022/11A equipment specifications, rather than the more stringent Class B EMC specifications These devices can also be designed to comply with Class B EMC specifications, but more complicated filtering and shielding measures must be taken to increase the size of the device and increase the cost. The power supply of Shenzhen Longxingchen Power Supply Company is in product size, cost, and Performance, safety and other aspects have made a good balance.
There are other requirements related to specific applications: if the system may be used in emergency vehicles, voltage shocks will occur. For this, the power supply should at least comply with the IEC 68-2-29 standard; some devices are portable devices, It may be used on a helicopter, and random vibration will occur. For this, the power supply should comply with the MIL-STD-810E standard.
Case: A customer once used a low-cost "medical power supply" with a voltage of 5V and a current of 200mA. The unit price for several hundred batches was US$5. Equipment using this type of power supply costs tens of thousands of dollars. This surprisingly low-priced power supply seems to be very good, if it is too good to be doubted, it is true. After disassembling this power supply, it is found that it has only one single-sided printed circuit board, and in order to save costs, the circuit board has no metallized through holes. The circuit board uses a surface mount structure, and the component density is very high. The component spacing does not meet the creepage distance requirements and design criteria specified by medical power supplies and related safety management agencies. The leakage current reaches 900mA, far exceeding the specification limit. Creepage distance and clearance voltage indicators are not up to the voltage used in general power supplies. The distance between the high-voltage areas is only 0.5 mm, and solder resist is used to improve the resistance of the dot medium. The pin spacing of the power MOSFET should have been increased to increase the creepage distance, but this kind of power supply has not been treated in this way, probably because of the manufacturing cost and taking more space to do so. The aluminum heat sink is inserted into the circuit board through some claws, and then fixed by welding. Once handled incorrectly, the heat sink is prone to loosening. The MOSFET in the flyback circuit is a fullpak packaged device without a manufacturer. It has a silicone jacket and an insulating tape on the heat sink, which hinders the contact between the device and the heat sink, which may cause serious heat during use. Stability issues, that is, when the MOSFET has overheated, the heat sink is still cold. This kind of power supply just meets the commercial EMI-RFI requirements, but does not meet the requirements of medical power supplies.
This kind of power supply also has some other problems, such as the existence of false welding everywhere in the whole power supply. When in use, even if it is assembled correctly, it may fail at any time. Although this type of power supply is sold as a medical power supply, it is not originally designed for medical use. In addition to the input voltage and output voltage and current, there are no relevant documents and technical indicators. In addition to the surprisingly low price of this power supply, other performance requirements are far from the requirements of medical power supplies. Why do some people only want to reduce the cost of power supply to such a level, and deliberately take so many risks? After all, the power supply is the main source of risks and dangers such as high temperature, high voltage, noise, leakage current, and fire.
Therefore, when selecting and purchasing medical power supplies, the relevant risks must be considered. It must be purchased from a reputable manufacturer that can provide certification documents that comply with relevant standards and standards, and comply with quality and reliability standards and requirements.
Typical application of medical power supply
By understanding the various selection requirements of medical power supplies, we are talking about several typical applications of medical power supplies below:
1. Application of ECG detection circuit:
As shown in the figure, this circuit is a human body ECG detection circuit composed of ISO122 and INA115, etc. This circuit can fully meet the requirements of medical equipment, that is, low input impedance, low leakage current, high detection accuracy and human body safety. The output signal is composed of ISO122 is isolated and transmitted to the post-amplification circuit to realize the isolation and amplification of human body signal and output and power supply. The use of isolated DC-DC module for power supply can effectively suppress electromagnetic interference in the surrounding environment and eliminate ground loops.
2. Patient ECG monitoring circuit
As shown in the figure, this circuit is an application of 3652 in the patient ECG monitoring system. This circuit is a balanced input connection, and its common mode and differential mode input impedance are both
