Reinforced DC/DC transformer isolation

DC/DC converters offer galvanic (input to output) isolation of typically at least 1 kVDC that means the converter will withstand a test voltage of 1000 VDC for one 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 also permits positive-to-negative and negative-to-positive voltage conversion, allows many units to share a common information and power bus without concern that if one module fails that 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 1000 V 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 on a ferrite core.

A standard polyurethane enamelled round copper magnet wire might have a conductor diameter of 0.1 mm or less (remember: we are talking about low power 1 or 2 W converters here) and a polyurethane plastic film coating of 0.005 mm.

Yet, despite this extremely thin insulation coating, the minimum dielectric strength of the wire can easily excess 1000 VDC. If the primary and second windings are wound directly on top of one another without any additional insulation, the galvanic isolation would still be 1 + 1 kVDC = 2 kVDC.

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 1 kVDC 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).

Figure 1: Functional isolation transformer construction.

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 4 kVDC, 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 has defined the degree of separation required according to the working voltage of the transformer and three isolation classes: basic, supplementary and reinforced. 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 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.

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 a CTI (comparative tracking index) is defined.

Figure 2: Creepage and Clearance definitions.

CTI is a measure of the voltage that causes 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.

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 36 to 75 VDC input voltage requires a minimum isolation clearance of 0.7 mm to meet basic isolation and 2.4 mm to meet the criteria for reinforced isolation. For the creepage separation, the figures are 1.3 and 4.6 mm.

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 5 mm clearance separation, but a transformer operating from 12 VAC would need less than a third of the clearance to be also classed as reinforced.

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.

Considering that an industry standard low power DC/DC converter is in a DIP24 case with outside dimensions of around 32 x 20 x 10 mm, 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 4.6 mm would be unlikely to be able to fit into a case that is only 10 mm high.

Yet despite this seemingly impossible separation requirement, Recom engineers have finally developed a DIP24 sized DC/DC converter that meets all the requirements for reinforced isolation.

The standard converter on the right uses a bobbin transformer with functional isolation. The reinforced isolation converter on the left uses a new transformer construction that guarantees a minimum clearance separation of 2.4 mm.

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 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 <0x2192> magnetic field <0x2192> electric field is sharply reduced if there are large air gaps between the windings.

However, to meet the requirements for reinforced isolation, there have to be gaps and physical barriers between the input and output windings.

So although transformer designs can meet the separation requirements, they would not normally 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 3 W rated converter will consume 3.6 W at full load. The 600 mW 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 that limits the maximum operating temperature of the converter. If a converter is constructed with a lower efficiency transformer (say 75%) then the internal power dissipation increases to 1 W.

Figure 3: Reinforced isolation transformer construction.

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° 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 3.5 W of power with either 8 or 10 kVDC of reinforced isolation and the REC6-R8/R10 offers 6 W of power with either 8 or 10 kVDC of reinforced isolation.