How to Choose Medical Power Supplies

As is well known, every industry has specific hard indicators for power supplies, and the medical industry is no exception. For a power supply to be used in medical equipment, it must first meet the relevant medical certification standards.

1. EMC Requirements

The conduction and radiation of medical equipment need to meet EN55011, with limits similar to those for EN50552 ITE equipment. Given that medical systems must meet high safety and isolation standards, while also meeting conduction and radiation requirements, this poses a significant challenge to power supply design. Designers must ensure high isolation while minimizing electromagnetic interference (EMI), which is a very difficult task. As a result, many medical devices have to compromise, only requiring EMI performance to meet the CLASS A level.

2. Safety Distance and Voltage Withstand

Medical standards differ from conventional certifications. To ensure safety and reliability, the standards explicitly define the system's required safety distance and voltage withstand specifications. These requirements vary based on the application and the people involved, and are divided into MOPP (patient protection) and MOOP (operator protection) according to the protection method. The devices are further categorized into Class B, BF, and CF based on the type of equipment. To ensure module versatility, Medical Power Supply engineers generally design the power supply to meet the most stringent requirements. A basic example is as follows:

① L, N to PE: 1500VAC isolation, electrical clearance 2.5mm, creepage distance 4.0mm.

② Transformer primary to secondary: 4000VAC, electrical clearance 5mm, creepage distance 8mm.

③ Output to PE: 500VDC (floating), electrical clearance 2.5mm, creepage distance 4mm.

3. Ultra-Low Leakage Current Requirements

Low leakage current is a core indicator for medical power supplies. The IEC standard requires that under normal operation, medical devices tested at 264VAC, 50Hz must have leakage current ≤ 500uA. However, medical devices typically have more than one isolated power source. Some large CT machines even use five or more power sources. Therefore, for power supply manufacturers, it is best to ensure that the leakage current of a single power module is ≤ 100uA. For DC-DC power supplies in direct contact with the human body, stricter isolation requirements are needed, especially for CF-class devices that come in contact with the heart. Medical certification clearly mandates leakage current ≤ 10uA.

4. Grounding Continuity Requirements:

The reason medical power supplies have strict requirements for this indicator is that a high ground resistance can easily cause significant ground potential differences between equipment powered by different sources. This could pose a risk to critically ill patients. Therefore, grounding continuity must undergo rigorous testing. IEC requires the resistance from PE to any accessible metal part of the shell to be < 0.1Ω.