Engineers designing power supplies for medical applications have some unique challenges. They need to understand not only the application, but the stringent requirements of the medical standards. For global compliance, power supplies need to meet IEC 60601 “Medical Electrical Equipment, Part 1: General Requirements for Safety.” Regional and national variations such as the European EN 60601-1, the U.S. ANSI/AAMI ES 60601-1, the Canadian CAN/CSA C22.2 No. 601.1, and Japanese JIS T 0601-1 standards may also be required. Once you decide where the medical device will be used you can define what design will be needed for the power supply to comply with the regulations.
The IEC60601-1 third edition has added several new requirements that will at the same time help make the design process easier and more challenging. For example, it requires isolation between the ac input to the power supply, the internal high voltage stages, and the dc output. This isolation is necessary to prevent electric shock to the user or patient. To ensure the proper amount of isolation a double layer of insulation or reinforced insulation should be used in medical power supplies and then tested by subjecting it to a much higher voltage than it will typically face. For example, medical supplies that operate from a 240-Vac mains must withstand a dielectric test at 4 kVac for medical applications (compared to 3 kVac for industrial applications).
New terms and definitions were also introduced in the third edition of the IEC60601-1. For example, the measure for leakage current that is required to ensure patient and operator protection in contact with medical devices is now defined as “touch current.” Touch currents are the leakage paths from an enclosure that may be in contact with a patient or operator. The levels within the third edition of IEC 60601 are 100 μA for normal operation and 500 μA for a single fault condition.
A leakage test considers the total patient leakage current. The basis for the total patient leakage current test is to measure the leakage current when all applied parts required for the operation of the medical device are in contact with the patient. The test protects the patient for devices that may have multiple connections and leakage paths. The leakage current concept is used to measure (to or from) all the same type of applied parts. The term “applied part” refers to a part of the medical device that may come into physical contact with the patient during its normal operation.
Applied parts fall into three classifications: B (body), BF (body floating), and CF (cardiac floating) are used according to the nature of the medical device and the type of contact. Each classification must have a different protection level against electrical shock. Type B applied parts may be connected to earth ground, but Type BF and CF are separated from earth and are considered floating. Power supply isolation voltages vary according to the type rating. Each classification must have a different protection level against electrical shock (see Table 1).
|Type||Input to Output Isolation||Input to Ground Isolation||Output to Ground Isolation|
|B rated||4,000 Vac||1,500 Vac||500 Vac|
|BF/CF rated||4,000 Vac||1,500 Vac||1,500 Vac|
Table 1: Requirements for different medical classification.
Type B (body) is the least stringent classification, and is used for applied parts that are normally not conductive and can be immediately released from the patient. Examples would be LED operating lighting, medical lasers, MRI body scanners, hospital beds and phototherapy equipment.
Type BF (body floating) is generally used for applied parts that have conductive contact with the patient, or have medium or long term contact with the patient. Examples of this type of equipment are blood pressure monitors, incubators and ultrasound equipment.
Type CF (cardiac floating) is the most stringent classification, and is used for applied parts that may come into direct contact with the heart, such as dialysis machines.
The IEC60601-1 third edition also has a requirement for a Risk Management Process and record retention in compliance with the ISO14971 standard. The ISO 14971 2007 Medical Devices — Application of Risk Management to Medical Devices was developed in a joint ISO/IEC committee along with the third edition IEC60601 standards. The goal is to guide the engineer through a process that ensures hazards are identified and mitigated.
Terms you need to know
Additional terms from the IEC 60601 Third Edition include type tests for leakage, isolation, and creepage/clearance. The determination of the correct category and values is guided by the concept of something called “essential performance.” Essential performance identifies operating characteristics that can impact the safety of operators or patients. The purpose is to allow the manufacturer to identify the appropriate levels to ensure safe medical devices.
Another concept introduced in the new standard is “means of protection” (MOP), which describes the isolation protection between the electrically charged circuitry and any equipment that may come into contact with the device. The isolation protection includes the creepage/clearance distances, insulation, and protective earth connections. The means of protection is further separated into two categories: means of operator protection (MOOP) and means of patient protection (MOPP). The classifications provide protection for the patient who may be more vulnerable to the medical device in use.
The means of operator protection and patient protection require the following test voltages and creepage:
Operator protection (MOOP):
One layer of insulation at 240 Vac requires a test voltage of 1,500 Vac and 2.5-mm creepage. Double layer of insulation at 240 Vac requires a test voltage of 3,000 Vac and 5-mm creepage.
Patient protection (MOPP):
One layer of insulation at 240 Vac requires a test voltage of 1,500 Vac and 4-mm creepage. Double layer of insulation at 240 Vac requires a test voltage of 4,000 Vac and 8-mm creepage.
Passing the test
While medical regulations for power supplies are stringent they are not beyond the capability of supply manufacturers. The EFE300M series from TDK-Lambda is an example of a power supply that meets all the isolation requirements for the IEC60601-1 medical standard. Based on TDK-Lambda’s modified LLC topology these BF Rated supplies fit in 1U enclosures and feature 90 to 264-Vac input, an ORing FET, 350 to 400 W peak for 10 s and full digital control.
As you can see, it’s not a simple process to design a power supply to meet the requirements for medical equipment but the successful end result of choosing a power supply designed for the job will be a safe medical device that passes the tests for medical equipment – on the first try. And that saves you design time and cost.
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