Medical Grade Power Supply for Type BF Medical Applications

Introduction

Research has shown that even a small amount of current flow through the human heart can lead to serious consequences such as ventricular fibrillation. Medical patients’ weakened bodies are even more susceptible to electrical harm. Therefore, it is important for Medical Electronic Devices to undergo stringent electrical safety tests as outlined by IEC 60601-1 3th edition and its harmonized equivalents for different countries around the world. For medical grade power supplies, it is critical to ensure the leakage current and isolation levels can meet the specified limits of the corresponding Applied Parts type of the medical application.

Applied Parts (AP) are classified into three types: Body (B), Body Floating (BF), and Cardiac Floating (CF). Type CF is the most stringent classification, which is for Applied Parts that may come into contact with the heart, such as surgery equipment. Type B is the least stringent classification, which is for Applied Parts that are normally not conductive, such as hospital beds. For the purpose of this paper, we will focus on Type BF, which is used for Applied Parts that have long term or short term conductive contact with the patient. We are most interested in Type BF applications because Type BF is perhaps the most prevalent to the largest group of patients, covering Medical Electronic Devices from blood pressure monitors to non-cardiac surgical equipment. In this MEAN WELL white paper, we will discuss the leakage current and insulation requirements of power supply for Type BF medical application, as well as examining a case study of MEAN WELL’s medical grade power supply to demonstrate the considerations for medical compliance testing.

Leakage Current Considerations 

Leakage current sources can be categorized into Earth Leakage, Enclosure Leakage, and Patient Leakage. Patient Leakage is specified for each type of Applied Parts, but the patient leakage is only considered for the end system. The outputs of power supplies should never be connected directly to a patient as part of the Applied Parts, so only Earth and Enclosure Leakage currents are tested for compliance.

The leakage current limit for each source as defined by IEC 60601-1 3th edition is indicated in Table 1.

NC = Normal Condition        SFC = Single Fault Condition

In order to qualify a power supply for Type BF application per IEC 60601-1 3th edition, the Earth Leakage needs to be less than 5mA under normal conditions and cannot exceed 10mA under single fault conditions. Diagram A shows the circuit diagram for measuring Earth Leakage. The normal condition is defined as S1 closed and S5 normal or reversed. The single fault conditions are defined as S1 open and S5 normal or reversed.

Diagram A.

The Enclosure Leakage is also known as Touch Current, and Type BF applications for IEC 60601-1 3th edition requires the Enclosure Leakage current to be less than 100µA under normal conditions. Under single fault conditions, the leakage current cannot exceed 500µA. Diagram B shows the test circuit for Enclosure Leakage. The normal conditions are defined as S1 and S8 closed, and S5 normal or reversed. Single fault conditions are created by either S1 or S8 open, while S5 is terminated in a normal or reversed state.

Diagram B.

Isolation Level Considerations

In addition to leakage currents, isolation levels also need to be considered when qualifying a power supply for Type BF medical application. Table 2 shows the insulation requirements for I/P-O/P, I/P-FG, and O/P-FG.