How to Correctly Select Medical Power Supply

In today's society, with the continuous progress of science and technology, more and more modern medical devices have been rapidly developed, especially electronic instruments that are in direct contact with the human body. In addition to the increasingly high requirements for the performance of the instruments themselves, the consideration of human safety has also received more and more attention. For example: cardiac puncture monitors, ultrasound, maternal and child monitors, baby warmers, life monitors and other instruments that are in close contact with the human body. That is to say, when patients use the instruments, they must not cause electric shock or any other dangers to the human body. So, when choosing the most relevant medical power supply, what should we pay attention to?

Medical power supply selection

Medical electronics, unlike other electronics and power electronics that are positioned in the mass market and cost-conscious consumer electronics and other low-priced products, have many more rules to follow. If the designer is responsible for system power design, the first question to consider for the system power supply part is: buy or make the relevant solution?

Since the production volume of medical electronics is generally relatively low, designers must consider whether to buy or make their own. Medical electronics designers rarely consider designing their own offline power supplies. Because the investment required for this type of special design and testing does not match the final production scale; equipment manufacturers will find it difficult or impossible to amortize the investment made in the design phase with the production volume of the product. Therefore, it is more cost-effective to purchase power supplies directly from companies that already have the corresponding professional design capabilities and testing technology.

1. Price

In commercial application design, if quality is guaranteed, it is easy to shop around and directly choose the lowest-priced power supply product. At this time, the lowest-priced but "acceptable" product is often the winner, while the best product is not popular. This is not bad for disposable electronic products that will be discarded soon or do not need repair, but what are the risks if designers randomly choose such a power supply to use in medical systems? Medical electronics are of high value and need to complete some critical tasks. If a medical system fails, the consequences are not as simple as missing a game or taking the wrong bus. 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 specifications constitute a set of strict normative requirements. Power supplies used in such demanding application systems must meet strict specifications in terms of insulation measures to prevent patients and medical staff from electric shock. EMC is also a key issue, which includes how to reduce electromagnetic radiation and how to protect against electromagnetic radiation. Therefore, for the design of medical power supplies, the first priority must be product quality and reliability.

Often, designers confuse commercial power supplies with medical power supplies, and manufacturers that make various low-priced 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 cheapness will lead to terrible consequences. Therefore, designers need to understand the relevant regulations and laws. So, what problems are 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 also some other issues, such as whether the product is produced by a GMP-certified factory?

2. GMP qualification

The U.S. Food and Drug Administration (FDA) requires that medical products must be produced by factories that have obtained GMP qualifications (i.e., good manufacturing practices). This is a set of quality certification systems that manufacturers should be required to present in addition to the traditional ISO9000 certification to prove that their quality control procedures comply with GMP standards. Similarly, China also has strict controls on medical power supplies, such as the need to comply with a series of requirements such as CE and UL medical certifications.

GMP regulations require manufacturers to have component quality control procedures and relevant documentation. When choosing a medical power supply, you can ask the manufacturer if it has component quality control procedures? Are there quality data and test documents? You can ask what the procedures are and ask for relevant documents. Ask the manufacturer to produce documents that can prove the quality and reliability of its products, and reputable manufacturers will be happy to provide them. We found that many low-priced manufacturers are using power products without trademarks, manufacturers 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 change the power supply of the POS terminal and sell it as a medical power supply, then such a product will only make the user lose more than the gain. Therefore, for the selection of a good medical power supply, GMP qualification can correspondingly prove its product 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 for general-purpose power supplies. Medical power supplies also need to meet the minimum basic requirements of this standard. However, the international safety standard for medical power supplies is the more stringent IEC601-1 A2, and there are three versions depending on the region: 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 protection, and mechanical aspects, as well as creepage distance and electrical clearance, and test indicators such as high-voltage insulation. Medical power supplies must use appropriate design techniques to ensure stable operation when the input is abnormal and to 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 that flows to the ground through the protective ground conductor. In the absence of grounding, if there is a conductive path (such as the human body), this current can flow from the surface of the conductive or non-conductive parts to the ground. There is always an external current in the safety ground conductor. Generally, the upper limit of the leakage current of medical power supplies is one tenth of that of ordinary power supplies.

The leakage current indicators of all power supplies in the IEC601 standard are much stricter than those for non-medical power supplies. 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 shell through the patient). The IEC601 standard defines the maximum leakage current of the following three main types of equipment power supplies differently:

Class B: Equipment that does not come into contact with the patient's body, such as laser therapy devices

Class BF: Equipment that comes into contact with the patient's body, such as ultrasound, various monitors (including EGC equipment), and operating tables

Class CF: Equipment that is intended to come into contact with the patient's heart, such as cardiac puncture monitors.

It is often misunderstood that the leakage current specifications for these types of equipment are different. In fact, the permissible leakage current for these types of equipment is the same. The North American specification requirements are more stringent than the permissible leakage current specified in Europe's EN60601-1. For example, Europe allows 0.5mA, while the United States and Canada only allow 0.3mA. Therefore, medical device designers need to pay attention to which region their products will be sold.

BF or CF devices (commonly known as "human contact" devices) also require additional insulation measures to insulate the patient from the ground, signal ports and power output. This is to protect the patient in the event of an unexpected failure of the device and to keep the leakage current on the patient within the limits specified by the standard. This insulation can also be achieved through other parts of the final device, such as plastic probes or sleeves with sufficient insulation performance. However, in applications where the patient needs to be powered, one of the solutions is to use an AC/DC power supply that complies with the IEC601-1 standard to power one or more isolated DC/DC converters, that is, to add a second level of insulation protection. Therefore, the DC/DC converter must be carefully selected to ensure that the insulation requirements can be met.

Since medical devices are often directly connected to patients and form a conductive path through the skin or even subcutaneously, the leakage current must be as zero as possible, the insulation must be reliable, and there must be no sneak current.

4. Safety and Isolation

Safety and isolation are also a major difference between commercial power supplies and medical power supplies, which are related to the safety of electric shock for patients receiving treatment and medical personnel using equipment. Although human skin is a good insulator, once a very small AC current is applied to the heart, it may cause cardiac muscle fibrillation and neuromuscular damage. Therefore, for environmental equipment involving patients, any part that may come into contact with the patient must strictly limit its current to 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, all offline power supplies must meet the basic safety requirements of the EN60950 standard. In addition, medical power supplies that are close to patients must also comply with the IEC601-1 standard. In addition to meeting the above two requirements, equipment that contacts patients also needs to be protected by additional isolation barriers. In addition, when the mains power is interrupted, the hospital's backup generator will not be able to supply power until several seconds or minutes later. Therefore, many medical power supplies and equipment using such power supplies are relayed through UPS systems. As a result, the input waveform of the power supply may change and is no longer an ideal sine wave. Therefore, a medical transformer is also required to be connected to the front end of the power supply to further improve the safety level. Therefore, equipment directly used on patients must meet the highest insulation technical indicators of all these parameters.

5. EMI-RFI Radiation and Protection

Electromagnetic radiation and electromagnetic radiation protection of equipment are also important parameter standards for medical power supplies, involving surge and transient current strength, 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 therapeutic 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 the EN60601-1-2 standard, which is coordinated with many technical requirements related to EMC. In addition, medical power supplies must also meet IEC61000-4-2 ((electrostatic protection capability, required to reach 3kV), IEC61000-4-3 (radio frequency radiation protection capability, required to reach 3V/m), IEC61000-4-4 EFT (voltage transient withstand capability, required to reach 1kV), IEC61000-4-5 (mains surge current withstand capability, required to reach 1kV and 2kV), IEC61000-3-2 (mains line harmonic requirements), IEC61000-3-3 (power line flicker requirements), and EN55011 (Class A product or B product radiation limit) and other requirements. For this reason, power supplies that meet the IEC601-1 standard generally comply with the EN55022/11 Class A 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 complex filtering and shielding measures must be taken, which increases the size of the equipment and the cost. Guangzhou Goldensun's medical power modules have made a good balance in terms of product size, cost, performance, safety, etc.

There are other requirements involving specific application scenarios: for example, the system may be used in an emergency vehicle, which may cause voltage surges. For this, the power supply should at least comply with the IEC 68-2-29 standard; some equipment is portable and may be used in helicopters, which may cause random vibrations. 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, and a unit price of $5 for a batch of several hundred. The equipment using this power supply sold for tens of thousands of dollars. This surprisingly low-priced power supply seemed very good, and if it was suspiciously good, it was indeed so. After disassembling this power supply, it was found that it had only a single-sided printed circuit board, and the circuit board had no metallized through holes to save costs. The circuit board used a surface mounting structure and the component density was very high. The component spacing did not meet the creepage distance requirements and design guidelines stipulated by medical power supplies and relevant safety management agencies. The leakage current reached 900mA, far exceeding the specification limit. The creepage distance and electrical clearance voltage indicators did not reach the voltage used in general power supplies. The spacing between high-voltage areas was only 0.5 mm, and solder resist was used to improve the dielectric resistance of the points. The pin spacing of the power MOSFET should have been increased to increase the creepage distance, but this power supply was not treated in this way, probably because of manufacturing costs and the fact that doing so would take up more space. The aluminum heat sink was inserted into the circuit board through some claws and then soldered in place. If not handled correctly, the heat sink is prone to loosening. The MOSFET in the flyback circuit is a fullpak packaged device from no manufacturer, with a silicone jacket and an insulating tape added to the heat sink, which hinders the contact between the device and the heat sink. This may cause serious thermal stability problems during use, that is, when the MOSFET is overheated, the heat sink is still cold. This power supply just meets the commercial EMI-RFI requirements, but does not meet the medical power supply requirements.

There are other problems with this power supply, such as poor solder joints throughout the power supply. Even if it is assembled correctly, it may fail at any time during use. Although this power supply is sold as a medical power supply, it is not designed according to medical requirements. There are no relevant documents and technical indicators except for the input voltage, output voltage and current. In addition to the amazingly low price, the other performance requirements of this power supply are far from meeting the requirements of medical power supplies. Why do some people deliberately take so many risks just to reduce the cost of power supplies to such a level? After all, power supplies are 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, you must consider the relevant risks and purchase them from reputable manufacturers who can provide certification documents that comply with relevant specifications and standards and comply with quality and reliability standards and requirements.