Selection criteria and typical application solutions of AC-DC medical power supplies

The medical equipment industry is an emerging industry related to human life and health. In the past decade, it has maintained a high annual market growth rate in developed countries around the world, and is therefore known as a sunrise industry. With the improvement of medical standards, more and more advanced medical equipment is widely used in various medical occasions. As an important part of medical equipment, power supply has more stringent requirements than other types of power supply products. This article focuses on the special requirements for medical power supplies and the technical characteristics of Golden Sun's high-quality medical AC-DC products.

Special requirements for medical power supplies

At present, most medical equipment uses switching power supplies. With the development of electronic technology, switching power supplies have not only greatly reduced in size and weight, but also greatly reduced energy consumption and improved reliability. Medical diagnostic, measurement and treatment equipment using AC power may generate leakage current due to improper grounding and electrical insulation, exposing patients and medical personnel to potential dangers such as electric shock, burns, internal organ damage and arrhythmia.

Given the special use environment of medical equipment, medical power products have more stringent requirements in terms of safety and reliability. For example, they must meet the insulation and leakage current requirements of IEC60601-1 safety regulations, or related standards such as EN60601-1, UL60601-1, and CSA22.2 Section 601.1 M90. Table 1 summarizes the safety requirements for IEC 60601-1 medical equipment.

Table 1 IEC60601-1 safety standard requirements

On the other hand, electromagnetic radiation and electromagnetic radiation protection of medical equipment are important parameter standards for medical power supplies, involving surge and transient current strength, electrostatic discharge (ESD) level, and radio frequency interference (RFI) protection capabilities. 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 that matches the EMC-related technical requirements. 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.

In addition, there are some special requirements for specific applications. For example, the system may be used in an emergency vehicle, where voltage surges may occur. This power supply should at least comply with the IEC 68-2-29 standard. Some equipment is portable and may be used in helicopters, where random vibrations may occur. This power supply should comply with the MIL-STD-810E standard.

Features of Mornsun LD Series Products

Mornsun is committed to the development of ACDC power supplies for medical use. It currently has LD05 and LD10 series products. The product block diagram is shown in Figure 1. The product always takes high reliability as the goal during the design process, and this is reflected in every link of the design.

Figure 1 LD series product block diagram

First of all, the reliability of the product control chip plays a vital role in product reliability. Our products use PWM control chips from ON Semiconductor. Because the chip is very powerful, only a few external components are needed to form a complete switching power supply, which greatly simplifies the design of the product and greatly improves the reliability of the product in a sense. In addition, the chip has very low standby power consumption and can provide solutions that meet the US Energy Star and European Blue Angel standards. It can also meet the latest recommendations of the International Energy Agency (IEA) under no-load conditions. The short-circuit protection function of the chip can also detect short circuits by continuously monitoring the status of the feedback line and immediately reduce the output power, thereby protecting the entire power supply and subsequent circuits. Once the short circuit disappears, the controller resumes normal operation. This function also greatly improves the reliability of the product under abnormal conditions.

Secondly, we have also done a lot of useful research on the thermal design of module power supplies. MTBF is a measurement indicator of power supply reliability, and the most important factor affecting MTBF is the temperature of the components. Therefore, to improve the reliability of the power supply, it is crucial to maintain a low temperature. The simulation method helps to make some abstractions more intuitive, so we introduced the electrothermal model. The so-called electrothermal simulation is to use a heat-conducting electric circuit to simulate the heat dissipation circuit of electronic devices and use electrical parameters to simulate the transfer of heat. Specifically, the current in the circuit is used to simulate the heat in the thermal circuit (in the calculation, the dissipated power PD of the semiconductor device is often used to replace the heat Q), and the flow of current is used to represent the transfer of heat flow. The voltage U is equivalent to the temperature difference △T from the device temperature to the ambient temperature, and the resistance R is equivalent to the thermal resistance Rth. Of course, in addition to the simulated heat transfer, a large number of high and low temperature reliability tests are also used to verify and improve the thermal design of the product during the design and verification process.

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Third, strict requirements on the EMC characteristics of products to ensure the reliable operation of themselves and the system. People are paying more and more attention to the EMC of electronic products, and countries and organizations around the world have also made corresponding regulations on the relevant restrictions on electronic products. Typical standards include: Part 15 of the Rules and Regulations of the Federal Communications Commission (FCC) of the United States; the IEC61000 part of TC77 of the International Electrotechnical Commission; CISPR22 (Information Technology Equipment) of the International Special Committee on Radio Interference (CISPR); EN55022 (Information Technology Equipment) of the European Union; China's GB9254-1998 (Information Technology Equipment) is converted from CISPR22 of CISPR.

Generally, there are three conditions for the formation of electromagnetic interference: the source of electromagnetic interference (noise source), the path of electromagnetic interference (noise coupling and radiation), and the object that receives electromagnetic interference (interfered equipment). Therefore, the suppression of electromagnetic interference of power products should also start from these three aspects. The following means are used to solve the EMC problem of products: 1. Selection of EMI filter; 2. Optimization of input and output filter network design; 3. Application of buffer circuit; 4. Minimizing the high-frequency loop area as much as possible; 5. Optimizing ground wire design; 6. Application of shielding.

In addition to the high reliability guaranteed in the design, the LD series module power products also have the following features:

Small size and high power density. The LD series power supplies of Jinshengyang are very small, among which: the output rated power of the LD05 series power supply is 5W, the global universal voltage input, and the volume is 50.08*25.4*15.16mm. The output rated power of the LD10 series power supply is 10W, the global universal voltage input, and the volume is 53*28*19mm, as shown in Figure 2. This type of small power supply saves a lot of component space for small medical equipment, thus making it possible to miniaturize the equipment.

Figure 2 LD10 series product dimensions

Good safety performance. This series of products is strictly designed according to the requirements of medical power supply. In terms of safety design, the distance between input and output is more than 8mm, and the withstand voltage between input and output reaches 4000VAC/1min. The leakage current from input to output is also very small. Among them, the LD05 series has no internal safety capacitor design, making the leakage current almost zero; the LD10 series is also only less than 100μA. The LD05 and LD10 series power supplies have passed the EN60601 medical certification (certificate numbers are: E/CE2995090218023, E/CE3180090723209) and UL60950 safety certification (certificate numbers are: E235235-A17-UL-1 and E235235-A18-UL-1).

Good EMC performance. The product has passed the comprehensive EMC test of CESI Laboratory of China Fifth Electronics Research Institute and obtained the certificate. See Table 2 for details.

Table 2 Product EMC test standards and results

Complete application guide. To help customers use the product better, the LD series products provide detailed typical application circuits, as shown in Figure 3 (specific parameters can be found in the product technical manual). Customers can choose the appropriate application circuit according to their own needs.

Figure 3 Typical application circuit