Medical Electronics
Figure 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)
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
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
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.
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 per cent 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 per cent. 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).
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).
The Class A filter consists of; C1, 100 microfarad (μF)/100-volt aluminium 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 aluminium 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).
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
Figure 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)
www.cieonline.co.uk Components in Electronics December/January 2023 15
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