Medical Electronics
Reinforced isolation
Paul Cheeseman looks at how a combination of techniques have helped to develop a transformer and driver system that is able to meet all of the requirements for reinforced isolation but at the same time offers higher efficiency
D
C/DC converters offer galvanic (input to output) isolation of typically 1kVDC. This means that the
converter will withstand a test voltage of 1000VDC for 1 second placed across the input and output pins without the insulation across the transformer breaking down.
This feature of DC/DC converters has many uses: 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, it permits positive-to- negative and negative-to-positive voltage conversion.
In addition it allows many units to share a common information and power bus without concern that if one module fails it will pull down the entire network and, most importantly acts as 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 one thousand volts 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 enamelled round copper magnet wire might have a conductor diameter of 0.1mm or less, on a low power 1W converter, 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 1000VDC. If the primary and secondary windings are
32 November 2011
wound directly on top of one another without any additional insulation, the galvanic isolation would be 1kVDC + 1kVDC = 2kVDC.
So, even if the insulation on one winding fails 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 (see Figure 1).
This class of isolation is called Operational or Functional Isolation. A transformer with functional isolation is reliable and safe for most industrial and
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.
Basic, supplementary and reinforced isolation 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 then 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.
Figure 1: Functional isolation transformer construction
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?
Components in Electronics
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). If the creepage distance is very small, it is usual to use the clearance separation for both measurements. In this way it is similar to the way that CTI (Comparative Tracking Index) is defined. CTI is a measure of the voltage that causes
Figure 2: Creepage and Clearance definitions
isolation failure either by tracking (a partially conductive path along or through the surface of an insulating material) or flashover (a spark across an air gap). Similarly, at very small creepage dimensions, the isolation failure could occur either via tracking or flashover, so in these situations creepage = clearance.
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. 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.
www.cieonline.co.uk
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