GB2197544A

GB2197544A - Toroidal transformers

Info

Publication number: GB2197544A
Application number: GB08627558A
Authority: GB
Inventors: Frank Charles Mose
Original assignee: Individual
Current assignee: Individual
Priority date: 1986-11-18
Filing date: 1986-11-18
Publication date: 1988-05-18
Also published as: GB8627558D0

Abstract

A transformer consists of a toroidal core 9 wound with a primary winding 10; a pair of interengaging shells 7,8 which enclose the core and winding; and a secondary winding around the outside of the shells. An electrostatic screen around the primary may be provided by coating the inside of the shells or by inserting an aluminium shell. A further screen (13) may be provided by coating the outside of the shells or by wrapping with aluminium foil beneath the secondary. To provide for printed circuit mounting the lower shell is moulded integrally with a pin carrying frame (14) (Fig. 4). For computer use. <IMAGE>

Description

SPECIFICATION Improvements to toroidal transformers Recent safety requirements for instrinsic safety in the electrical isolation of the inputs and outputs of computer-based industrial electronic measuring and control equipment are difficult or impossible to fulfil using established design and construction techniques for the toroidal transformers which are used to collect and transfer the data and power to and from the logic modules and this has led to the extensive use of electrostatically screened optical isolators which are costly.In particular, the requirement for a specific insulation surface creepage path length which is large by comparison with the transformer dimensions coupled with design requirements for effective electrostatic screening between windings raises constructional difficulties which if not insuperable are not solvable in a cost-effective way by the methods and equipment in current use in the industry. The purpose of this invention is to provide a technique based on a new type of former which enables the safety and screening specifications to be met while using existing toroidal winding technology.

In this document the term "primary" is used to define the winding(s) closest to the core and the term "secondary" to define the winding(s) put on over the primary. They have no significance as to their use in electrical circuits.

The specification for the type of transformer covered by the invention can be understood by referring to Fig. 1. 1 is the primary winding which can be wound directly on the insulated toroidal core 2. Both are covered by an electrostatic screen 3 connected to one end of the primary or to the earth of the logic circuit which the primary feeds. By way of example, this assembly must be isolated under Intrinsic Safety Specification (I.S.) requirements by one millimetre of solid insulation 4 with a minimum creepage distance between electrical conductors on primary and secondary of ten millimetres.Since the secondary winding 6 is protected from capacitive coupling with the primary winding, the core and the logic module by a further electrostatic screen 5 this insulation requirement 4 applies also between screens as well as windings and isolates up to several thousand volts the data collection world on one side from the data processing world on the other except by electromagnetic coupling. Potting in a resin compound is not considered safe because of the possibility of bubbles or inclusions and the alternative would be to injection mould a plastic layer round the toroidal core, the primary winding and the screen as an assembly, subsequently applying a screen and secondary winding to the assembly, but this is an expensive procedure and it is difficult to ensure accurate positioning of the assembly in the mould to give 100% integrity of the insulation.

Also its manufacure in this manner by subcontractors would be impracticable because the combination of facilities does not exist.

A new type of former can provide a solution to these problems and forms the basis of this invention. The former consists of two open toroidal half shells which fit together to enclose the toroidal core and the primary winding, the edges of the shells being formed to produce a labyrinth which can be made sufficiently long to satisfy the creepage requirement whilst the material thickness is chosen to satisfy the solid insulation specification.

The primary electrostatic screen can be made integrel with this former by coating sections of the inner surface with conductive material or by inserting thin metallic components, -e.g.

an impact extruded aluminium shell and an aluminium washer, one making contact with the other at a single point to prevent circulating currents. Fig. 2 shows the preferred arrangement. The upper shell 7 fits into the lower shell 8, enclosing the toroidal core 9 with its primary winding 10 as shown in detail in the sectional view. Fig. 3 shows an enlarged section of the area ringed in Fig. 2. Depending on dimensions and creepage requirements the labyrinth may be extended if required.

The secondary winding is wound round and through the bobbin by means of a toroidal winder into which the bobbin will fit. In the case of a pulse transformer, which will be a major application of this invention for industrial data processing and control, the primary winding will frequently occupy only a sector of the toroid and to keep the leakage inductance low the secondary winding would be positioned over the primary and occupy a similar sector.

In such a case where two-way transmission takes place it will be desirable to include a second electrostatic screen underneath the secondary winding, because a large voltage pulse between the primary electrostatic screen and the secondary winding can induce potentially damaging voltages across the winding due to variations in the distributed capacitance. This screen could be a piece of insulated metal foil placed round the sector of the shell before the secondary winding is applied, but it is better to apply a conducting coating to the outside of the shells over the required area in a similar manner to the primary screen.

The gap between the shells prevents circulating current and contact is made to both halves.

In order to provide terminals and a structure suitable for mounting on a printed circuit board the lower shell is moulded integrally with a frame on which are mounted terminal pins and a section to protect the lower part of the secondary winding and provide a lengthened creepage path. This is done in such a way that the assembly can be handled by a normal toroidal winding machine, and is shown typically in Fig. 4. The primary terminal pins 10 are positioned so that the "earthy" one can penetrate inside the shell making con tact with the lower section of the electrostatic screen directly and with the upper section by means of a springy cantilever extension. The primary wires 11 are brought out through holes and soldered directly to the pins. 14 is the anti-creepage shield. The shaded areas 13 denote the external secondary screens coated on the outside of the shells contact with which can be made by positioning one of the pins 12 suitably in a similar manner to the internal screens by a springy extension 15, contact with the other section being made di rectly as shown. The secondary winding 16 is applied on top of this screen with suitable insulation in between. The wire insulation is normally sufficient. The whole component may be dipped in a suitable protective resin, which seals the labyrinth and protects the secondary winding.

Claims

1. A toroidal transformer in which the insu lation between primary and secondary wind ings is provided by a pair of insulating inter secting toroidal shells which enclose the core and its primary winding(s) the secondary wind ing(s) being wound outside the shell.

2. A toroidal transformer as claimed in Claim 1 in which the junction between the intersecting toroidal shells is formed as a laby rinth to provide a long creepage path.

3. A toroidal transformer as claimed in Claims 1 and 2 in which the electrostatic screen(s) are formed as part of the insulating shells by suitable conducting coatings.

4. A toroidal transformer as claimed in Claims 1 and 2 in which the electrostatic screen is formed as an insert or inserts fitted into the insulating shells.

5. A toroidal transformer as previously claimed in which the insulating shell mouldings are provided with mountings and terminals which do not interfere with handling in a nor mal toroidal winding machine.