“Medical grade DC/DC converters” is an often used, but more often abused, description of a class of converters that are increasingly in demand. What are actually the particular requirements that medical applications place on DC/DC converters?
DC/DC converters for Medical Applications "Medical grade DC/DC converters" is an often used, but more often abused, description of a class of converters that are increasingly in demand. What are actually the particular requirements that medical applications place on DC/DC converters? This seems a simple question which should have a simple answer, but this is unfortunately not the case. Probably only the end user can answer this question because the requirements and restrictions are as diverse as the medical applications in which they are used. However, if we look through the questions and answers that arise during the daily customer technical support and design-in work that RECOM Technical Support does, we can offer some guidelines: Isolation - the yardstick by which all things are measured The main requirement that almost all specifications for converters appropriate for use in medical applications have in common is the search for as safe as possible galvanic separation between the input and output, usually expressed as a high as possible isolation voltage. The ability of an isolated converter to withstand high voltages across its isolation barrier is dependent on the materials used to construct the converter, the physical separation between the input and output tracks on the internal PCB and the ability of the isolation transformer to withstand the electrical stress between input and output windings. Consequently, the two most essential points that decide the suitability of a DC/DC converter for medical applications are the transformer construction and the internal construction of the converter. This second point is important because the medical standards define creepages and clearances in air which do not translate 1:1 to the separations used in most DC/DC converters which are potted and filled with a homogenous epoxy mass which excludes air, therefore it is still acceptable to have internal separations which are smaller than the limits given by the standards. This important fact also helps bridge the gap between meeting the requirements of the standards and manufacturing a converter which is as small as possible to fit into the increasingly restricted space available on most circuit boards - if this was not so, the size of medical grade converters would be unacceptably large for many applications. Now to the essential part: the construction of the transformer in order to meet the requirements for high voltage isolation. Here, there are also several different solutions available. The simplest and mostly commonly applied solution is to use windings with as thick as possible coating of lacquer or with multiple coatings of lacquer to ensure adequate isolation. However, as the trend towards miniaturisation progresses, this solution is reaching its limits as input and output windings have to wound directly over each other to save space. Also, small diameter toroidal transformers, like the ones usually used in low power small size DC/DC converters, have a very tight bending radius which could lead to a thinning of the lacquer coating due to the mechanical stress placed on the wires during the winding process, which in turn could compromise the electrical isolation. Thus this method of construction reaches a limit of isolation strength of around 4kVDC applied for one second, which translates to a continuous working voltage of approximately 1.8kVDC. Picture 1: Simple DC/DC transformer construction for isolation of up to 4kVDC/1 Second Nevertheless, isolation voltages higher than 4kVDC/ 1 second are possible. For example Recom product series like the RP family with 5.2kVDC/1 second or the RxxPxx and RxxP2xx converters with 5.2kVDC/1 minute (which corresponds to about 6.5kVDC/1 second) and the RV-xxxx with 6kVDC/1 second all have small case sizes, but use a different technique to achieve these very high isolation figures. RECOM is one of the few manufacturers that have perfected the two chamber transformer system. Each winding is therefore wound separately on to the transformer and therefore the isolation is not wholly reliant on the thickness of the lacquer as there is in addition a physical separation between the wires. There are also two different techniques that can be used, either separation by means of separation-bridge (see Picture 2a) or the application of a pot system, in which one winding is wound directly around the toroidal kernel which is placed within a non-conductive ann... [download for more]
