Enhanced DC/DC Isolation Transformer

DC/DC converters provide voltage isolation (input to output) of typically at least 1kVDC. This means that such converters can withstand a 1000VDC voltage test applied to the input and output pins for 1 second without damaging the transformer insulation.

This feature of DC/DC converters has many uses: electrical isolation interrupts ground loops, thereby eliminating signal noise from the circuit , supports information transmission between two independent circuits by remotely separated power supplies , supports conversion from positive to negative voltage and negative to positive voltage, and allows multiple unit components to share a common information and power bus without worrying about the failure of a unit affecting and damaging the entire network. Most importantly, this isolation can act as a safety barrier to prevent electric shock and avoid excessive current from causing overheating or fire accidents.

Although 1KVDC of isolation may sound very high, the transformer construction is very simple. A typical low power DC/DC converter will use an internal toroidal or bobbin transformer, which consists of a toroidal ferrite core and primary and secondary windings wound with varnished copper wire. The standard polyurethane varnish is applied to the copper wire, which is no more than 0.1mm in conductor diameter (remember: we are talking about a low power converter of 1W or 2W here) and the polyurethane plastic film is only 0.005mm. However, even without this extremely thin insulating coating, the dielectric strength of the varnished copper wire can easily exceed 1000VDC. If the primary and secondary windings were wound directly together without any additional insulation layer, the electrical isolation would still be 1kVDC + 1kVDC = 2kVDC.

Therefore, even if the insulation on one winding fails, has a pinhole defect, 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 together without compromising electrical isolation (even if there is a possibility of a problem with the insulation on one winding or the other). (See Figure 1) This type of isolation is called working or functional isolation.

Figure 1: Functional isolation transformer structure

However, while transformers with functional isolation are safe and reliable for most industrial and commercial applications, stringent safety applications or those with isolation levels above 4kVDC do not allow the input and output windings to be wound directly together. These windings must be isolated. But how do you determine the spacing?

Underwriters Laboratories (UL) defines the required spacing based on the transformer's operating voltage and three levels of isolation: basic, supplementary, and reinforced. Physical spacing is further broken down into creepage and clearance.

Basic isolation, supplementary isolation and reinforced isolation

The definitions of basic, supplementary and reinforced isolation are not clear at present. Basic isolation is "insulation required to provide basic protection against electric shock", supplementary isolation is "supplementary insulation in addition to basic insulation to ensure protection against electric shock in the event of a failure of the basic insulation", and reinforced isolation is "a single insulation system providing a degree of protection against electric shock equivalent to double insulation (double insulation is the combination of basic insulation and supplementary insulation)". For transformers used in DC/DC converters, these definitions are recursive. When does a transformer design need to have basic isolation or only functional isolation? Can adding a plastic strip between the windings turn a functional isolation transformer into a supplementary isolation transformer? Can adding two layers of plastic strips achieve a reinforced isolation transformer? In practice, these formal definitions of transformer isolation levels only apply when there are creepage and electrical clearance requirements.

Creepage distance and clearance

Creepage distance is the shortest distance between two points along the surface (tracking distance). Clearance is the shortest distance between two points in space (discharge distance).

Figure 2: Definition of creepage distance and clearance

If creepage distance is very small, then clearance is often used for both measurements. This is similar to how CTI (Comparative Tracking Index) is defined. CTI is a measure of the voltage at which isolation fails due to either a conductor (partial conductive path along or through the surface of an insulating material) or a sparkover (a spark across an air gap). Again, in cases where creepage distance is very small, this isolation failure can occur through either a conductor or a sparkover, so in these cases, creepage distance = clearance.

Transformer Isolation Level

Based on these definitions of creepage distance and clearance, UL defines the minimum spacing required to meet the following three isolation levels: