How to Select and Apply Board-Mounted DC/DC Converters in Medical Systems

With competitive pressure on designers of medical system power supplies to increase power density, reduce footprint, and meet cost constraints—along with meeting extended temperature operation and high reliability requirements—it can be tempting to design their own custom-mounted DC/DC converter and fully optimize it for the application. Depending upon available resources, in many cases, this may not be the best option as not only is power supply design challenging, medical DC/DC converters are particularly so since they need to be certified to an array of user and operator safety requirements.

These requirements include; IEC/EN/ES 60601-1 3rd edition for 2 x means of patient protection (MOPP) safety, risk management according to ISO 14971, IPC-A-610 Level 3 criteria for Electronic assemblies, and electromagnetic compatibility (EMC) compliance according to IEC 60601-1-2 4th edition. In many instances, a 4:1 input range is needed to operate from various battery and vehicle supplies, and it must have high voltage isolation and low leakage current.

Instead, designers can select standard board-mounted DC/DC converters in a compact form factor that are certified to all the requirements detailed above, with the added advantage of a mean time between failures (MTBF) of over 1 million hours, as well as numerous control features including; remote on/off control, remote sense, and output voltage trim; plus protection for undervoltage input, short-circuit, overtemperature and overvoltage.

This article provides a review of the industry standards to be considered when specifying DC/DC converters for medical applications. It then introduces and discusses the application of an off-the-shelf 60-watt DC/DC converter from TRACO Power that meets all the industry standards for medical systems.

DC/DC converter considerations for medical systems

Industry standards for medical systems design are primarily concerned with safety; safety of patients, safety of equipment operators, and safety of the equipment to protect it from potentially damaging conditions. The concept of “means of protection” (MOP) is the key to understanding and achieving patient and operator safety. Various MOPs are defined as safety insulation, a creepage distance, an airgap, protective impedances, and a protective earth. At a minimum, medical devices must include one MOP to protect patients and operators from the risk of an electric shock should a fault occur.

IEC 60601 assigns different MOPs for patients and operators, resulting in specific requirements for MOPP and means of operator protection (MOOP), defined in terms of an isolation voltage, creepage distance, and insulation level (Table 1). MOPP requirements are more restrictive since patients may have less ability to protect themselves, and depending on the application, one or two MOPPs or MOOPs may be required by IEC 60601.

How to Select and Apply Board-Mounted DC/DC Converters in Medical SystemsTable 1: IEC 60601 requirements for MOPPs are more restrictive than for MOOPs. (Image source: TRACO Power)

The level of protection needed is dependent on the specific application. For example, body floating (BF) safety levels are required for applied parts (AP) that are electrically connected to the patient, such as ultrasound equipment and blood pressure monitors, and must be floating and separated from earth.

The use of an AC/DC power supply approved for 2 x MOPP safety is one approach to meeting IEC 60601, but it may not be the most cost-effective. Most “medically approved” AC/DC power supplies are not rated for 2 x MOPP and cannot be used in BF applications. In BF medical applications, part of the system used by the operator needs to meet the less-restrictive 2 x MOOP, while the AP section of the system must be rated for BF safety levels and meet 2 x MOPP. Combining an AC/DC power supply that meets 2 x MOOP with a DC/DC converter that meets the 2 x MOPP is usually the lowest cost solution (Figure 1). This approach can also be useful for medical devices that include battery backup power and need to comply with 2 x MOPP during an AC power failure.

How to Select and Apply Board-Mounted DC/DC Converters in Medical SystemsFigure 1: An AC/DC power supply with a 2 x MOOP rating can be combined with a 2 x MOPP rated DC/DC converter to arrive at a cost-effective solution for medical device designs. (Image source: TRACO Power)

Most off-the-shelf DC/DC converters have isolation ratings of only 500 to 1,600 volts direct current (Vdc) and cannot meet 2 x MOPP. Designers can turn to specialized DC/DC converters with up to 5,000 volts alternating current (Vac) isolation, double insulation, and 8 millimeter (mm) creepage that meet the 2 x MOPP requirements when used with medically approved AC/DC power supplies rated for 2 x MOOP.

In addition, standard DC/DC converters have not been subjected to a risk assessment as defined in ISO 14971, which defines the best practices for all life cycle stages of medical devices. This medical device directive also requires manufacturers of DC/DC converters and other medically approved devices to implement an ISO 13485-compliant quality management system.

Protecting system operation

Ensuring system operation is another requirement for medical devices. The printed circuit boards (pc boards) in medical devices, including DC/DC converters, must meet the requirements of IPC-A-610 Level 3, Class 3 for high performance products. Class 3 pc boards are expected to provide continuous performance, or performance on demand, with no equipment downtime. High levels of inspection and testing to stringent standards are required for these boards. Typical applications that use Class 3 pc boards include critical systems such as medical devices, life support systems, automotive systems, and military equipment.

Electromagnetic compatibility (EMC) requirements for medical designs are strict and have recently become even more demanding. IEC 60601-1-2:2014+A1:2020 applies to the safety and performance of medical devices and systems in the presence of electromagnetic disturbances. It also limits the electromagnetic disturbances emitted by medical devices and systems. In the latest edition, published in 2020, conducted emissions (CISPR 11) must be tested at minimum and maximum rated voltage, compared with the single voltage test used in the previous edition. Medical devices and systems such as DC/DC converters that were able to pass the single voltage test may fail when tested at minimum and maximum rated voltages. Other changes in the latest edition include:

  • Immunity test levels are now specified relative to the environment of intended use, and the location categories are harmonized with IEC 60601-1-11 such as professional healthcare facilities and equipment intended for use in residential and special environments.
  • Immunity tests and test levels are specified based on the ports in medical electrical equipment within a medical electrical system.
  • Additional tests have been included to ensure the safe operation of medical devices and systems when portable communications devices are used in closer proximity, compared with the tests limits specified in the previous edition.

Standard DC/DC converters for medical applications

When tasked with meeting the myriad of medical safety and performance requirements, designers can either spend time and resources developing their own converter and getting it through the qualification and certification processes or turn to the THM 60WI series from TRACO Power. These 60-watt DC/DC converters come in a 2.3 x 1.45-inch, quarter-brick plastic package (Figure 2). These converters feature a wide 4:1 input voltage range making them suitable for both AC-powered and battery-powered designs. They have 5,000 VAC reinforced isolation between the input and output, a leakage current of less than 4.5 microamperes (μA), are approved to IEC/EN/ES 60601-1 3rd edition for 2 x MOPP, IEC/EN/UL 62368-1, and have an ISO 14971 risk management file. Their design and production meet the quality management system requirements of ISO 13485. In addition to medical designs, the THM 60WI series are also suitable for use in transportation, industrial, and control and measurement applications.

How to Select and Apply Board-Mounted DC/DC Converters in Medical SystemsFigure 2: The THM 60WI series of 60-watt, quarter-brick medical qualified DC/DC converters are an off-the-shelf solution to the challenges of medical system design and standards qualification. (Image source: TRACO Power)

The THM 60WI series quarter-brick DC/DC converters comprise 12 models with input ranges of 9 to 36 VDC or 18 to 75 VDC, and single or dual outputs of 5.1, 12, 15, 24, ±12 volts, or ±15 VDC with up to 92% efficiency. For example, the model THM 60-2411WI has an input voltage range of 9 to 36 VDC, an output of 5.1 VDC at 12 A, and an efficiency of 90%. This series of 2 x MOPP compliant and BF rated DC/DC converters are suited for AP applications. They have a calculated MTBF of over 1 million hours (according to MIL-HDBK-217F, ground benign) and have a 5-year warranty. A summary of features include:

  • IEC 60601-1-2 4th edition EMC compliance
  • 5,000 Vac reinforced isolation with <4.5 µA leakage current
  • Remote sense; output voltage trim and remote on/off functions
  • Protection against undervoltage on the input, output short-circuit, overtemperature, and output overvoltage
  • An ambient operating temperature range of -40°C to +75°C that can be extended using an optional heatsink

Thermal design options

The THM 60WI series quarter-brick DC/DC converters are specified for an ambient temperature up to +75°C, with derating. For more demanding thermal environments, TRACO also offers the THM-HS1 heatsink with a thermal impedance of 4.71 Kelvin/watt (K/W) that significantly increases thermal dissipation under both natural convection and forced-air conditions. For example, when used with the THM 60-2411WI, the THM-HS1 extends the maximum full-load operating temperature from about 30°C to 60°C (with 20 linear feet per minute (LFM) of airflow), and from about 80°C to 90°C (with 500 LFM airflow) (Figure 3).

How to Select and Apply Board-Mounted DC/DC Converters in Medical SystemsFigure 3: Thermal derating for the THM 60-2411WI without a heatsink (left) and with an optional heatsink (right), shows how much the heatsink extends the maximum operating temperature for a given airflow. (Image source: TRACO Power)

EN 55032 compliance

Under EN 55032 in North America, any equipment primarily used in a residential environment must meet Class B limits. All other equipment must comply with Class A limits. TRACO offers suggested electromagnetic interference (EMI) filter implementations for both Class A and Class B environments (Figure 4).

How to Select and Apply Board-Mounted DC/DC Converters in Medical SystemsFigure 4: Filtering for dual output models for compliance with EN 55032 Class A limits (left), and suggested pc board layout (right). (Image source: TRACO Power)

The Class A filter consists of; C1, 100 microfarad (μF)/100-volt aluminum capacitor; C2, 2.2 μF/100-volt 1210 multilayer ceramic capacitor (MLCC); C3 and C4, 100 picofarad (pF) Y1 capacitors; L1, 285 microhenry (μH) common-mode choke (TCK-103 from TRACO Power).

The suggested Class B EMI filter for Class B is shown in Figure 5. It consists of; C1, 100 μF/100-volt aluminum capacitor; C2, C3, and C4, 2.2 μF/100 volt, 1210 MLCCs; C5 and C6, 47 pF Y1 capacitors; C7 and C8, 33 pF Y1 capacitors; L1 and L2, 285 μH common mode chokes (TCK-103) (Figure 5).

How to Select and Apply Board-Mounted DC/DC Converters in Medical SystemsFigure 5: Filtering for single output models for compliance with EN 55032 Class B limits (left), and suggested pc board layout (right). (Image source: TRACO Power)

Conclusion

Power supply design for medical applications is challenging, but designers can opt for off-the-shelf board-mounted DC/DC converters. Still, it’s important to choose carefully. As shown, the right DC/DC converter can help enhance all aspects of medical device safety, including safety for patients and equipment operators, and protecting the safety of the equipment from potentially damaging conditions. It can also support the use of a variety of powering architectures including AC mains and battery-powered solutions while providing overvoltage, short-circuit, and undervoltage protection.

Recommended reading

  1. Why and How to Use Synchronous Buck DC/DC Converters to Maximize Down-Conversion Efficiency
  2. How to Select and Apply the Right Components to Protect Medical Devices, Users, and Patients
  3. The Basics of Isolation Transformers and How to Select and Use Them