Select the right AC/DC power supply to meet unique medical requirements
OTS units: range of power levels, form factors It’s one thing to talk about certified, regulatory approved AC/DC power supplies for medical applications in the abstract, but looking at some of the versions available shows that meeting these mandates does not constrain use flexibility. Vendors offer different families of supplies with a range of voltage/ current ratings within each family, and so can meet nearly all project requirements. Some examples show the breadth of what is available in external adapters, open-frame modules, and enclosed units. Example #1 External desktop adapters such as the SDM65-UD series, include the 24 volt, 2.7 A SDM65-24-UD-P5 (Figure 7). This family of Class II supplies is often used for powering/ charging laptops and similar devices and offers a universal input range of 90 to 264 volts and 47 to
Example #3 Enclosed units such as the VMS- 450B series, include the VMS-450B- 24-CNF, a 450 watt supply that provides 24 volts at 18.8 A from inputs of 100 to 240 volts AC. The supply measures 127 × 86.6 × 50mm (approximately 5 × 3.4 × 2in.) and comes with a metal shield which allows for airflow while reducing EMI/RFI and providing some physical protection for both the supply and its users (Figure 11). Supplies in this series can deliver from 12 volts at 37.5 A up to 56 volts at 8 A. They also include PFC and a 12 V, 600 mA drive for a fan, plus an additional 5 V, 1 A auxiliary DC output that eliminates the need for a separate small supply in many applications.
Figure 11: The 450-watt VMS- 450B-24-CNF AC/DC supply series delivers 24 volts at 18.8 A and comes with a protective enclosure. Image source: CUI Inc.
Supplies in this family offer up to 550 watts of continuous power with outputs spanning 12 volts/42 A to 58 volts/9.5 A, with an industry standard 3 × 5 inch (in.) footprint and a low profile of 1.5in. (Figure 9). These supplies include power factor correction (PFC), a regulatory requirement at this power level, and have a standby power dissipation of less than 0.5 watts, along with an efficiency up to 92%. They operate over a -40°C to 70°C temperature range and include a separate 12 volt/0.5 A output for a local cooling fan. AC connection for this Class II unit is via the male connector on the supply’s circuit board using a two-wire cable terminated with a mating female connector. The datasheet includes thermal derating curves along with a useful mechanical drawing showing an arrangement for a cooling baseplate with mounting standoffs and screws (Figure 10).
Figure 9: The open-
frame VMS-550-48 delivers 48 volts at 11.5 A and has a standard footprint of 3 × 5in.
63Hz.
These nominal 65-watt units have outputs that cover 12 volts at 5 A to 5 volts at 1.36 A, are housed in a fully enclosed insulating package measuring about 120 × 60 × 36 millimeters (mm) and include a user-convenient “power-on” indicator LED. Supplies in this family operate from a user-supplied IEC320/C8 two-wire AC cord. The DC output comes with a 150 centimeter (cm) cord (16 or 18 gauge, depending on the supply’s output current), can be ordered with either of two polarity orientations, and any one of a number of common “barrel” plug terminations or stripped/tinned wires (Figure 8). Example #2 Open-frame (or tray) modules such as the VMS-550 series include the VMS-550-48, a 48 volt, 11.5 A unit.
Image source: CUI Inc.
precisely tailored to the application requirements and its priorities. There is no doubt that in some cases, the supply requirements are so unusual or unique that there is no commercial supply available, so “make” is the only course. Despite the feasibility of “make,” the arguments against it are strong: “make” comes with high design
and certification risk, plus lengthy time to market. In addition, the higher volume of supply vendors compared to “make” efforts results in a lower bill of materials (BOM) and assembly cost, so “make” is not even a cost saver, except perhaps at very low power levels (under about ten watts) where regulatory issues are less stringent.
Conclusion
AC/DC supplies for medical applications must meet a large set of complex, stringent regulatory standards and mandates covering basic and additional safety requirements. Supplies which meet all relevant standards are available in a wide range of power ratings and come in form factors including external “desktop” style, as well as “drop-in” for incorporation into an end product. By selecting one of these standard units, system designers are relieved of all the issues associated with supply design, certification, final approval, and manufacturing.
Figure 10: Mechanical drawing showing the dimensions and mounting arrangement for a suitable cooling plate for the VMS-550-48 power supply. Image source: CUI Inc.
AC/DC supplies for medical applications must meet a large set of complex, stringent regulatory standards and mandates covering basic and additional safety requirements.
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