Medical Electronics


Figure 3: Comparing REC3.5 (Reinforced Isolation - right) and REC3 (Functional Isolation - left).


Using the information in this table, it is possible to decide exactly how “separate” the input and output windings must be for each application and isolation class.


Reinforced isolation


Considering that an industry standard low power DC/DC-converter is in a DIP24 case with outside dimensions of around 32mm


x 20mm x 10mm, it is not surprising that almost all DC/DC- converters are either Functional isolation or Basic isolation at best. A transformer with a creepage separation of over 4.6mm would be unlikely to be able to fit into a case that is only 10mm high. Yet despite this seemingly impossible separation requirement, engineers at Recom have been able to develop a DIP24 sized DC/DC- converter that meets all of the requirements


for Reinforced isolation. Note in Figure 3 the larger transformer and opto-coupler dimensions required for the higher isolation but the overall case size and pin-out remain the same The standard converter on the left uses a bobbin transformer with functional isolation. The reinforced isolation converter on the right uses a completely new


(patent pending) transformer construction that guarantees a minimum clearance separation of 2.4mm. The yellow coloured plastic film that can also be seen in the photograph is there to ensure the minimum creepage separation of 4.6mm. The transformer uses an internal construction with multiple layers of insulation and separation barriers to meet the requirements. Previous attempts to build a compact transformer with reinforced isolation have not met with success. The reason being that the efficiency of the transformer decreases if the electric and magnetic fields within the transformer are not physically close together. The transfer ratio of electric field –> magnetic field –> electric field is sharply reduced if there are large air gaps between the windings. However, in order to meet the requirements for reinforced isolation, there has to be gaps and physical barriers between the input and output windings. So although transformer designs similar to that shown above meet the separation requirements, they would not normally in be practical as a DC/DC-converter transformer because the conversion efficiency would be too low. A standard, functional isolation, DC/DC- converter has a typical power transfer efficiency of around 84%. This means that a 3W rated converter will consume 3.6W or power at full load. The 600mW difference between input power and output power is the internal power dissipation which manifests itself as heat. The converter runs warm. At high ambient


temperatures, it is the internal power dissipation which limits the maximum operating temperature of the converter. If a converter is constructed with a low efficiency transformer (say 75%) then the internal power dissipation increases to 1 watt. This will sharply reduce the maximum operating temperature. A typical DIP24 sized converter will have a maximum operating temperature of +85°C with 84% efficiency, but only +71°C with 75% efficiency. As the industrial temperature range for DC/DC- converters is up to 85°C, a converter with only 75% efficiency would be rejected by many industrial users. However, Recom has used a combination of techniques to develop a transformer and driver system that meets all of the requirements for reinforced isolation but also with higher efficiency. Thus the converters can deliver 20% more power with the same power efficiency as their functional isolation equivalents. Thus the REC3.5-R8/R10 offers 3.5W of power with either 8kVDC or 10kVDC of Reinforced isolation and the REC6-R8/R10 offers 6W of power with either 8kVDC or 10kVDC of Reinforced isolation.


RECOM Electronic | www.recom-international.com


Paul Cheeseman is Business Development Manager, Sales Northern Europe, RECOM Electronic


www.cieonline.co.uk


Components in Electronics


November 2011 33


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46