GB2075272A

GB2075272A - Pulse transformer

Info

Publication number: GB2075272A
Application number: GB8106744A
Authority: GB
Original assignee: Doduco GmbH and Co KG Dr Eugen Duerrwaechter
Current assignee: Doduco Solutions GmbH
Priority date: 1980-03-06
Filing date: 1981-03-04
Publication date: 1981-11-11
Also published as: DE3008583C2; GB2075272B; FR2477760A1; DE3008583A1; FR2477760B3

Abstract

A pulse transformer comprises primary and secondary windings 2,3 and a magnetic core 1 which is a bistable magnetic element, such as Wiegand wire. As shown, one embodiment contains a third DC winding and all three windings surround the core. The fluxes of the DC and AC windings are combined, and the direct current is variable to vary the phase angle at which the magnetisation of the core is reversed to produce a pulse firing a thyristor 5. The alternating current may be sinusoidal or sawtooth. A movable permanent magnet may replace the third winding. <IMAGE>

Description

SPECIFICATION Pulse transformer The invention relates to a pulse transformer.

Previously proposed pulse transformers are intended to produce, from an alternating voltage supplied to the transformer input, a train of pulses available at the transformer output, which pulses should be as steep, high and short as possible.

A preferred particular but not exclusive field of application is the ignition of thyristors and triacs, provided galvanic separation is necessary here for reasons of safety (contact protection).

Various constructions of pulse transformers have been proposed. In the case of one previously proposed type, a sinusoidal voltage is supplied to a transformer at the terminals of the primary winding. The secondary winding is provided at a part of the magnetic circuit which is of especially narrow cross-section, and which is thus quickly saturated. A further increase in magnetic flux can therefore occur only by way of the stray path in the air.

In the case of another previously proposed pulse transformer, a core is employed which has outstanding saturation, e.g. a core of Mu-metal. Its primary winding is connected to a sinusoidal net voltage by way of an iron choke coil.

In both cases there occurs in the secondary winding an approximately trapezoidal flux course and the secondary induced voltage consequently shows marked pulses.

Such transformers are by their nature very expensive. An object of the present invention is to provide a pulse transformer with galvanic separation, which produces, by simple means, short, high pulses which are largely free from interference components.

According to the present invention there is provided a pulse transformer, especially for the ignition of thyristors, triacs or the like with galvanic separation of the transformer input from the transformer output, comprising an electrical primary winding, an electrical secondary winding, and also a ferromagnetic core which couples the primary and the secondary windings magnetically together, characterised in that said core is a bistable magnetic element (hereinafter referred to as BME).

As bistable magnetic elements, also referred to as bistable magnetic switch cores (and hereinafter and in the claims referred to as BMEs), it is recommended in particular that so-called Wiegand wires be employed, whose structure and manufacture are described in DE-OS 2,143,326. Wiegand wires are homogeneous, ferromagnetic wires (e.g. of an alloy of iron and nickel, preferably 48% iron and 52% nickel; or of an alloy of iron and cobalt; or of an alloy of iron with cobalt and nickel; or an alloy of cobalt with iron and vandium, preferably 52% cobalt, 38% iron and 10% vanadium) which, due to special mechanical and heat treatment, possess a soft magnetic core and a hard magnetic outer surface, i.e. the surface possesses higher coercive force than the core. Typical Wiegand wires have a length of 5mm to 50mm, preferably 20mm to 30mm.If a Wiegand wire, in which the direction of megnetisation of the soft magnetic core coincides with that of the hard magnetic surface, is introduced into an external magnetic field whose direction coincides with that of the axis of the wire, but is opposed to the direction of magnetisation of the Wiegand wire, on exceeding a field strength of approximately 16 A/cm, the direction of magnetisation of the soft core of the Wiegand wire is reversed. This reversal is also referred to as resetting. On further reversing of the direction of the external magnetic field, and on the external magnetic field exceeding a critical field strength, the direction of magnetisation of the core is again reversed, so that the core and the surface are again of parallel magnetisation.This reversal of the direction of magnetisation occurs very abruptly and is accompanied by a correspondingly notable change in magnetic flux per unit of time (Wiegand effect). In an induction coil, this alternation of magnetic flux may induce a short and very high voltage pulse, (according to the number of turns and to the load resistance of the coil), upto approximately 12v.) known as a Wiegand pulse.

Also on returning of the core, a pulse is produced in an induction coil, which is however of much lower amplitude and of a different sign from the case of the reversal from the anti-parallel to the parallel direction of magnetisation.

If, as external magnetic field, an alternating field is selected, which is capable of reversing magnetisation firstly of the core and then also of the surface layer and of bringing these to magnetic saturation, Wiegand pulses occur, due to the reversal of the direction of magnetisation of the soft magnetic core, alternately of positive and negative polarity, which is termed symmetrical excitation of the Wiegand wire. For this purpose, field strengths of approximately - (80 to 120 A/cm) to + (80 to 120 A/cm) are required. The change of magnetisation of the surface also occurs abruptly and also produces a pulse in the induction coil, which is however much smaller than the pulse induced in the reversal of the core and is generally not evaluated.

If however an external magnetic field is selected which is capable of reversing only the soft core but not the hard surface layer in direction of magnetisation, the high Wiegand pulses occur only with unchanged polarity which is referred to as asymmetrical excitation of the Wiegand wire. For this purpose, a field strength is required in one direction of at least 16 A/cm (for the resetting of the Wiegand wire) and in the opposite direction a field strength of approximately 80 to 120 A/cm.

It is characteristic of the Wiegand effect that the amplitude and width of the pulses it produces are largely independent of the speed of change of the external magnetic field and that they possess a high signal-to-noise ratio.

Also suitable for the purpose of the invention are differently constructed bistable magnetic elements which possess two zones of differing magnetic hardness (coercive force) magnetically coupled to each other, and may be employed in the same manner as Wiegand wires for producing pulses by an induced, abrupt reversal of the soft magnetic zone. Thus, a bistable magnetic switch core in the form of a wire has previously been proposed in, for example, DE-PS 2,514,131, which comprises a hard magnetic core (e.g. of nickel-cobalt), an electrically conductive intermediate layer (e.g. of copper) deposited thereon, and a soft magnetic layer (e.g. of nickel-iron) deposited thereon.Another variant additionally employs a core of a magnetic, non-conductive metal inner conductor (e.g. of beryllium-copper), on to which the hard magnetic layer is deposited, then on this the intermediate layer, and on this the soft magnetic layer. This bistable magnetic switch core does, however, produce smaller switch pulses than a Wiegand wire.

Depending upon the particular application concerned, a pulse transformer of the present invention may operate with symmetrical or asymmetrical excitation of the BME. With symmetrical excitation it is most convenient to supply a primary winding (exciter winding) with a sinusoidal alternating current. It is one of the advantages of the invention that the exciter alternating current may assume any other desired curve form. The only prerequisite for symmetrical excitation is that the magnetic field produced by the primary winding at the position of the BME attains values sufficiently great to ensure symmetrical excitation of the BME: with Wiegand wires, field strengths H3 of approximately + (80 to 120) A/cm are required for this purpose.Whereas, with symmetrical excitation the pulse transformer delivers a train of pulses with alternating polarity, with asymmetrical excitation, it delivers only pulses of constant polarity. Also, with asymmetrical excitation, a magnetic alternating field is required atthe position of the BME, which must however be only strong enough in one direction to provide the field strength HR (with Wiegand wires approximately -16 A/cm) necessaryforthe magnetic resetting of the BME (pole-reversal of the soft magnetic portion of the BME from parallel orientation - with reference to the direction of megnetisation of the hard magnetic portion - to anti-paralel orientation).In the other direction, field strengths are required which are sufficientto bring the BME once again into parallel orientation of the direction of magnetisation by reversing the poles of its soft magnetic portion; with Wiegand wires, a field strength of approximately H3 = 80 to 120 A/cm is required for this purpose.The asymmetrical magnetic alternating field can be created, either by superimposing on the exciter alternating current a direct current component which is weaker than the peak value of the alternating current component, or the primary winding is supplied with a more or less symmetrical alternating current and, on the magnetic alternating field produced thereby at the position of the BME, is superimposed a magnetic direct field which is parallel to but weaker than said alternating field, e.g. by arranging close to the BME a suitably dimensioned permanent magnet.

The employment of a BME as core of a pulse transformer has the further advantage that the pulse width and height are almost independent of the form and frequency of the exciter alternating current. In addition, ambient influences, especially temperature, have no noticeable effect on the production of pulses. When employing Wiegand wires as BMEs, the half-value width of the pulses is 20us.

In most cases therefore, the pulse produced by the reversal of the direction of magnetisation of the soft magnetic zone from the anti-parallel direction to the parallel direction can be employed directly without further preparation for control purposes. With asymmetrical excitation, the pulse produced by the reversal of magnetisation direction of the soft magnetic portion to anti-parallel orientation of both portions, is notably smaller than that produced by the reversal of the magnetisation of the soft magnetic portion to parallel orientation. With symmetrical excitation, the pulse produced by the reversal of the hard magnetic portion is notably smaller than the pulse occurring during reversal of the soft magnetic portion. The smaller pulse in each case can, if necessary, be suppressed by a simple discriminator circuit.

In addition to these advantages, the pulse transformer of the invention is of very simple construction. A typical Wiegand wire is 5mm to 50mm, preferably 20mm to 30mm in length. With the two windings, it has a diameter of aproximately 1 mm to 2mm. This arrangement is then embedded in a suitable synthetic resin, thus providing a very compact, robust and efficient structural element In order to achieve a high signal yield, a Wiegand wire is preferably employed as BME, which is enclosed in the primary and secondary windings.

The pulse transformer according to the invention is also eminently suitable for changing of the phase position of the pulses produced, which is of essential importance in the directing of thyristors and triacs. In previously proposed systems for the control of thyristors and triacs, phase displacement of the pulses is effected for example by the employment of a phase bridge or rotary transformer in conjunction with a pulse transformer. This type of phase displacement of pulses is expensive.

The present invention offers an ingenious solution to phase displacement whereby, as third winding, a D.C. winding is provided which is coupled magnetically with the BME. Said third winding produces a magnetic direct field which is superimposed at the position of the BME upon the magnetic alternating field produced by the primary winding. Since the field strengths, which are necessary for symmetrical or asymmetrical excitation of the BME for pulse production, have a predetermined value, the phase position of the pulses produced in the BME is displaced, in relation to the pure alternating field, within each period of the exciter alternating current, by the addition or subtraction of a static (in relation to the cycle duration of the exciter alternating current) or quasi-static magnetic directfield, whereby the relation between the phase position and the strength of the direct current supplied to the third winding is linear, if the alternating current employed is of saw-tooth form.However, in the case also of a sinusoidal alternating current for excitation of the BME, the relation between the phase position of the pulses and the strength of the direct current in the third winding is still simple.

In order to obtain a firm coupling between the BME and the third winding, said winding is also preferably placed around the BME.

The static or quasi-static magnetic direct field at the position of the BME might also be produced by a permanent magnet, in which case the change in strength of the direct field at the position of the BME can be effected by moving the permanent magnet closer or further away or, where the permanent magnet is stationary, by moving a ferromagnetic object closer or further away, which deforms the field of the permanent magnet.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which: Figure 1 shows a pulse transformer according to the invention, with Wiegand wire in the control circuit of a thyristor: Figure 2 is a representation similar to Figure 1 but with a third winding of the Wiegand wire for phase displacement; Figure 3 is a diagram illustrating the phase displacement of the pulses of a Wiegand wire with sinsuoidal exciter current; and Figure 4 is a diagram corresponding to Figure 3, with saw-tooth type exciter current.

In Figure 1 a thyristor 5 is shown, in the load circuit of which is the resistance Rv. A gate 6 and cathode 7 of the thyristor 5 are connected to the secondary winding 2 of a pulse transformer in the form of a Wiegand wire 1 which is enclosed firstly, by a primary winding 3, and secondly, by the secondary winding 2. The primary winding 3 is supplied from a source of alternating current 4 which excites the Wiegand wire 1 symmetrically and, in each half-wave of the alternating current in the secondary winding 2, produces a Wiegand pulse.

The embodiment shown in Figure 2 corresponds to that of Figure 1 except for an additional D.C. winding 8 on the Wiegand wire 1, which is connected to an adjustable D.C. source 9 and produces a static or quasi-static magnetic field which is superimposed on the magnetic field of the primary winding 3. The effect achieved thereby is explained with reference to Figures 3 and 4.

Figures 3 and 4 illustrate the process of phase displacement of Wiegand pulses 10 with asymmetrical excitation. At the position of the Wiegand wire, the primary winding (exciter winding) produces the magnetic alternating field (I) H = Ho sin cot, (Figure 3) in which H is the magnetic field strength, Ho is its amplitude, to is the frequency of cycle, and t is time.By superimposition with a magnetic direct field of strength -H= a resulting magnetic field is obtained with the time course: (II) H = He . sin ot - H The production of a Wiegand pulse remains constant at the field strength: (Ill) H = Hz, hereinafter also referred to as ignition field strength Hz. The corresponding phase position cotz = cpz of the Wiegand pulse 10 is obtained on combining (II) and (ill) to give: Hz+H=.

(IV) sin q)z = Hz + H= .

o By employing a saw-tooth form for the exciter current according to Figure 4 with vertical edge, a linear connection is obtained, on the other hand, between the phase position çz of the Wiegand pulse 10 and the field strength of the magnetic direct field H=, which, in turn, is proportional to the direct current flowing through the third winding 8.The pure, magnetic alternating field in Figure 4 has the form: (V) H=H, . cot By superimposition with the magnetic direct field of strength - H= this results in an alternating field with the course: (Vl) H=Ho- -H 2n The production of a Wiegand pulse occurs once again according to (III) with H = Hz, so that we obtain for the corresponding phase position cpZ = cote the linear expression: (VII) cpZ ~ Hz 2n - so that the phase position of the Wiegand pulses 10 can be displaced within the period 2::/cho proportional to the field strength H= of the direct field and thus proportional to the current strength of the direct current exciting the magnetic direct field.

According to the invention, therefore, by means of a compact, robust and inexpensive structural unit, it is possible to achieve not only the production of pulses which are immediately suitable for the ignition of thyristors and the like, but also their phase displacement and galvanic separation.

Claims

1. A pulse transformer, especially for the ignition of thyristors, triacs or the like with galvanic separation of the transformer input from the transformer output, comprising an electrical primary winding, an electrical secondary winding, and also a ferromagnetic core which couples the primary and the secondary windings magnetically together, characterised in that said core is a bistable magnetic element.

2. A pulse transformer according to Claim 1, in which the BME is a Wiegand wire.

3. A pulse transformer according to Claim 1 or 2, in which the primary winding encloses the BME.

4. A pulse transformer according to any one of the foregoing claims, in which the secondary winding encloses the BME.

5. A pulse transformer according to any one of the foregoing claims, in which a third winding is provided which is coupled magnetically with the BME and is connected to a variable D.C. source.

6. A pulse transformer according to claim 5 in which the third winding encloses the BME.

7. A pulse transformer according to any of claims 1 to 4, in which a permanent magnet is provided, having a magnetic field, the strength of which is variable in relation to the movement of an object at the position ofthe BME.

8. A pulse transformer according to any claims 5 to 7, in which the primary winding is connected to an A.C. source which produces a sinusoidal alternating current.

9. A pulse transformer according to any of claims 5 to 7, in which the primary winding is connected to an A.C. source producing a saw-tooth form alternating current.

10. A pulse transformer according to claim 9, in which the saw-tooth form of the alternating current has a vertical edge.

11. A pulse transformer substantially as hereinbefore described with reference to any of Figures 1 to 4 of the accompanying drawings.