Reinforced DC/DC transformer isolation

DC/DC converters offer galvanic (input to output) isolation of typically at least 1kVDC. This means that a converter will withstand a test voltage of 1kVDC for 1s placed across the input and output pins without the insulation across the transformer breaking down. This has many uses, including: the galvanic isolation breaks ground loops and therefore removes signal noise from circuits; it allows information to be transmitted between two independent circuits by remotely powering one circuit from another; and, most importantly, acts a safety barrier to prevent electric shock and to avoid the possibility of excessive current flow that could cause overheating or start a fire.

Although 1kV isolation sounds impressive, the transformer construction is very simple. A typical low-power DC/DC converter will use an internal toroidal or bobbin-type transformer consisting of primary and secondary windings of magnet-wire wound upon a ferrite core. A standard polyurethane enameled round copper magnet wire might have a conductor diameter of 0.1mm or less and a polyurethane plastic film coating of only 0.005mm. Yet despite this extremely thin insulation coating, the minimum dielectric strength of the wire can easily exceed 1kVDC. If the primary and secondary windings are wound directly on top of one another without any additional insulation, the galvanic isolation would still be 1kVDC + 1kVDC = 2kVDC. So, even if the insulation on one winding fails or contains pin-hole defects or is scratched during assembly, the insulation on the other winding can still withstand the full 1kVDC test voltage.

This means that the input and output windings can be wound directly on top of one another without compromising the electrical isolation—even taking into account that the insulation on one winding or the other might be defective. This class of isolation is called Operational or Functional isolation.

However, although a transformer with functional isolation is reliable and safe for most industrial and commercial applications, for safety critical applications or for isolation ratings higher than 4kVDC, it is not permitted or desirable to wind the input and output windings directly on top of one another. They must be separated. But by how much?

Underwriters Laboratories Inc. (UL) has defined the degree of separation required according to the working voltage of the transformer and three isolation classes: Basic, Supplementary and Reinforced isolation. The physical separation is further subdivided into Creepage and Clearance.

The definitions given for Basic, Supplementary and Reinforced isolation are unclear. Basic is defined as “Insulation sufficient to provide basic insulation against electric shock”; Supplementary is defined as “Supplementary insulation applied in addition to basic insulation to ensure protection from electric shock if the basic insulation fails”; and Reinforced is defined as “A single insulation system that provides a degree of protection against electric shock equivalent to double insulation (which is in turn defined as insulation comprising of both basic and supplementary insulation)”.

When considering the transformers used in DC/DC converters, many of these definitions are recursive. When does a transformer design have basic or just functional isolation? Does adding a strip of plastic tape between the windings make a functional isolation transformer a supplementary isolated transformer? Does adding two layers of plastic tape make it a reinforced isolated transformer?

In practice, these formal definitions of the isolation class of a transformer are only useful when used in conjunction with the requirements for creepage and clearance. Creepage is the shortest distance between two points measured by following the surface (tracking distance). Clearance is the shortest distance between two points measured point to point (arcing distance).

TRANSFORMER ISOLATION CLASS
Using these definitions of Creepage and Clearance, UL has defined the minimum separations required to meet the three classes of isolation.

From Table 1, we can see that a DC/DC converter for telecoms applications with 36VDC to 75VDC input voltage requires a minimum isolation clearance of 0.7mm to meet Basic isolation and 2.4mm to meet the criteria for Reinforced isolation. For the creepage separation, the figures are 1.3mm and 4.6mm respectively.


Table 1: Isolation Class definitions (from UL 60950 2nd Ed., Table XVI).


At higher operating voltages, the creepage and clearance requirements are higher for the same class of isolation. Thus a Reinforced isolation mains transformer must have at least 5mm clearance separation, but a transformer operating from 12VAC would need less than a third of the clearance to be also classed as Reinforced.

REINFORCED ISOLATION
Considering that an industry standard low power DC/DC converter is in a DIP24 case with outside dimensions of around 32x20x10mm, almost all DC/DC converters have 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, Recom engineers have finally developed a DIP24 sized DC/DC converter that meets all of the requirements for Reinforced isolation.

Previous attempts to build a compact transformer with reinforced isolation have not met with much 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.

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 percent 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.