When it comes to medical equipment, nothing is more important than the safety of patients and health care personnel. From diagnostic tools such as ultrasound devices to home health equipment like dialysis machines, human safety is top priority. To ensure that devices and equipment are not hazardous, strict standards are in place to help guarantee global compliance.

Fig. 1 – This is an example of a 100-watt AC/DC power supply for medical applications, featuring a universal input range and 2MOPP isolation with Class I and II protection.
For power supplies, one of the most important is IEC 60601-1, Medical Electrical Equipment, Part I: General Requirements for Basic Safety and Essential Performance. This standard covers essential safety-related specifications and values, such as isolation voltage, leakage current, and creepage/clearance distances that must be met to protect people from electrical shock.

In addition to safety concerns, medical equipment designers also must consider a host of other factors when choosing the best power supply for the application. Some of these include input range, output voltage and power, standby power, temperature and altitude constraints, and product warranties. Understanding how various power supplies compare to one another in terms of each of these factors will enable equipment designers to make the right design decision for the project at hand.

Safety First

Because safety is such an important—and highly regulated—aspect of medical equipment design, it pays to have an understanding of the main terms and requirements within the third edition of IEC 60601-1. To get an idea of what the standard covers, one can simply review a list of typical tests for medical electrical systems. Some of these include testing for leakage current, grounding impedance, isolation voltage, and electromagnetic compatibility. Compared to industrial power supplies, the levels required for medical power supplies are much stricter. As an example of what IEC 60601-1 codifies, isolation is required between the AC input, internal high-voltage stages, and DC output in order to prevent electrical shock to the operator or patient. To ensure correct and sufficient isolation, either double insulation or reinforced insulation should be used in medical power supplies instead of a protective earth (Class II isolation). Class I electrical equipment only calls for basic insulation and uses a protective earth to avoid electrical shock, and is suitable for certain equipment.

Other terms within IEC 60601-1 to become familiar with include the means of protection used, describing the isolation protection between the electrical circuits and equipment that may contact the device. Isolation protection includes creepage/clearance distances, insulation, and protective earths. Subcategories for this term include MOOP (means of operator protection) and MOPP (means of patient protection). (See Figure 1)

Fig. 2 – Under the IEC 60601-1 safety standard, proper creepage and clearance distances help power supplies achieve sufficient isolation protection between the electrical circuits and equipment that may contact the device.
The standard sets the following criteria for medical power supplies: For MOOP, one layer of insulation at 240 VAC requires test voltage of 1500 VAC and 2.5mm creepage, while double insulation at 240 VAC requires test voltage of 3000 VAC and 5mm creepage. For MOPP, one layer of insulation at 240 VAC requires test voltage of 1500 VAC and 4mm creepage, while double insulation at 240 VAC requires test voltage of 4000 VAC and 8mm creepage. (See Figure 2)

Leakage current, or touch current, is another issue covered within IEC 60601-1. Touch currents are defined as the leakage paths from an enclosure that may contact a patient or operator. Because medical patients are often in a weak state, even a small amount of leakage current can have an adverse health effect. The standard specifies maximum levels of 100 μA for normal operation and 500 μA for a single fault condition. Closely related to the leakage current concept is the test that measures it: The total patient leakage current test measures the leakage current when all “applied parts” required to operate the medical device are in contact with the patient.

Applied part means the part of the medical device that may contact the patient during normal operation and includes three classes: B (body/least stringent), BF (body floating/more stringent than B, less than CF), and CF (cardiac floating/most stringent/ in direct contact with heart). For example, B-rated parts such as hospital beds require 4000 VAC input to output isolation, 1500 VAC input to ground isolation and 500 VAC output to ground isolation. In contrast, BF/CF-rated parts require 4000 VAC input to output isolation, 1500 VAC input to ground isolation and 1500 VAC output to ground isolation. An example of a Type BF part is a blood pressure monitor, whereas a CF example is a dialysis machine. Be sure that your medical power supply adheres to the values spelled out in IEC 60601-1. Not all power supplies meet these criteria. (See Figure 3)