US2699530A - High-frequency electric transformer - Google Patents

Description

Jan. 11, 1955 K. E. LATIMER 2,699,530

HIGH-FREQUENCY ELECTRIC TRANSFORMER Filed Nov. 2, 1950 1 2% 42 V M ht 5 2% H W 'i 26 27 Copper ZZ-PermanQmf Magnezf 2a ,Q 23 im- T 4 INVENTOR. KENNETH E. LATIMER 3/ AGENT United States Patent Office 2,699,530 Patented Jan. 11, 1955 2,699,530 HIGH-FREQUENCY ELECTRIC TRANSFORMER Kenneth Eric Latimer, London, England, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application November 2, 1950, Serial No. 193,583

Claims priority, application Great Britain November 7, 1949 6 Claims. (Cl. 336-73) This invention relates to high frequency electromagnetic transformers comprising a circuit of ferromagnetic material.

Transformers of the foregoing type are used, for example, to produce short, rectangular high-tension pulses to supply magnetrons in a radar installation, which may have, for example, a primary amplitude from kv. to 10 kv., a pulse duration of 1 microsecond, a pulse repetition frequency of at least 50 per second and a secondary amplitude of up to 500 kv.

To prevent substantial distortion in the output rectangular wave form of the transformer, it is necessary to operate along the linear portion of the magnetisation curve of the ferromagnetic core material. As is wellknown, the magnetisation curve of a ferromagnetic material is like the symbol I, having a top bend, a bottom 1 bend and a substantially linear portion in the region of the origin of the magnetising field (H) and the magnetic induction (B) axes.

A transformer carrying sinusoidal current will normally make use of both the positive and negative portions of the magnetisation curve. A transformer carrying a pulse current, however, will normally use only one half of the curve, since the pulse current is unidirectional. Accordingly, it has been suggested to provide a constant magnetising field in the magnetic circuit so that, in the absence of any input current, the magnetic core material is substantially saturated in one direction. This may be accomplished by providing an additional winding on the core carrying a direct current or by means of a permanent magnet. The direction of the pulse current is then arranged to produce a magnetising field in the opposite direction to this constant magnetising field.

h lilrench patent specification No. 990,545 describes a electric pulses having a ferrite magnetic circuit in which a constant magnetising field tending toward saturation of the magnetic circuit is produced by a permanent magnet and an opposing field is developed by the pulse current, a closed electrical conductor being arranged between the magnetic circuit and the permanent magnet to prevent losses due to the high frequency components of the field existing in the material of the permanent magnet.

The present invention is an improvement in or modification of the invention described in the French patent specification No. 990,545.

According to the present invention, a high frequency electric transformer for electric pulses has a closed magnetic circuit of ferrite material comprising a main portion in which a constant magnetising field is pro duced by a permanent magnet and an opposing field is developed by the pulse current and a shunted portion in which the constant magnetising field and the field developed by the pulse current are in the same direction, the magnetic constants of the shunted portion being chosen so that the magnetic material in this shunted portion is not saturated by the resultant of the constant field and that due to the pulse current. Furthermore, the transformer is provided with a closed circuit of good electrical conducting material positioned at the region of the junction between the permanent magnet and the magnetic circuit to prevent excessive eddy current losses in the permanent magnet due to the high frequency components in the pulse current.

When a closed magnetic circuit is used in such a frequency electric transformer for transmitting transformer and it is desired to polarize a portion of. it by means of a constant magnetising. field produced by a permanent magnet, it follows that there must be two oppositely charged poles and that the magnetising. field in the magnetic circuit between these poles will have one direction in one part of the magnetic circuit and the opposite direction in the other part. It further follows that if the pulse current is arranged to set up an opposing held in that part of the magnetic circuit which is called the main portion herein, it must tend to increase the magnetising field operating in the other part which is called the shunted portion. Accordingly, it is important that the shunted portion should be designed so that the constant magnetising field together with the magnetising field due to the pulse current does not saturate the core material in this shunted portion of the magnetic circuit and, preferably, that the combined fields produce a flux density below the maximum working value corresponding to a point below or above thetop or bottom bends, respectively, on the magnetisation curve of the material.

According to one form of the invention the shunted portion has a core with at least three times the cross sectional area of the main portion of the core and an air gap which gives the shunted portion approximately the same reluctance as the main portion of the core.

However, it may bepreferable to make the crosssectional area of the shunted portion somewhat larger than the minimum figure of three times the crosssectional area of the main portion so that the flux density does not quite reach the maximum working value. For example, the area of the shunted portion may be made four times that of the main portion of the core and, in this case, the length of the air gaps therein is increased to make the reluctance of the shunted portion again approximately equal to that of the main portion. In this case, the maximum flux density in the shunted portion is of the maximum working value chosen.

In a preferred embodiment, the shunted portion of the magnetic circuit is shaped in the form of a disc with one surface abutting a surface of the permanent magnet and the opposite surface abutting a surface of the main portion of the core so that the flux in the shunted portion is in a radial direction. The area of the surface of the permanent magnet is at least double that of the main portion and the thickness of the disc is somewhat greater than half the diameter of the main portion. In this embodiment, the airgap is arranged outside the boundaries of the permanent magnet face and, preferably, at as great a radius as possible. In such an arrangement, the flux in the main portion is substantially constant throughout its length while the stray flux is negligibly small and does not cause appreciable eddy currents in the Winding or in an outer case enclosing the transformer.

Preferably, the magnet is provided with an inverted cup shaped yoke, the depending part of which is a sliding fit over the shunted portion so that a firm butt Contact can be provided between the permanent magnet and the main portion of the magnetic circuit,

In order that the invention may be more clearly understood, the feature whereby saturation of the magnetic material in the shunt path is avoided will be described with reference to Figure l of the accompanying drawings.

Figure l is a schematic circuit diagram showing an electric circuit corresponding, element for element. with a magnetic circuit according to the invention in which the cross sectional area of the shunted portion is three times that of the main portion of the magnetic circuit and the reluctance of the main portion and shunted portion are the same. In Figure 1, the reluctance of the main portion of the core of ferrite material is re resented by resistance 1 of value R and the maguctomotive force due to the pulse current is represented by the electromotive force of value E developed by a generator 2 connected in series with the resistance 1. The magnetomotive force of the permanent magnet is represented by the electromotive force of value E developed by a battery 3 connected in series with a resistance 4 and a choke 5 inseries with the elements 1 and 2. The value R' of the resistance 4 represents the reluctance of the branch in which the permanent magnet is placed and the choke represents the closed circuit of conducting material arranged in the region of the junction between the permanent magnet and the magnetic circuit. The shunted portion is represented generally at 6 and comprises three series combinations of resistances 7, 8; 9, 10; and 11, 12. The resistances 8, 10 and 12 each have a resistance of R so that together they represent the reluctance of the shunted portion having three times the cross sectional area of the main portion of the circuit. The resistances 7, 9 and 11 each have a value of 2R and together represent the air gap in the shunted portion.

The constant magnetising field due to the permanent magnet is then represented by a current flowing through both the main portion of the circuit and the shunted portion. In the main portion, this current 4) corresponds to the maximum working fiux density of the core ma terial. It can be readily seen that the current flowing through each of the series of combinations of resistance 7, 8; 9, 10; and 11, 12; will be The magnetising field due to the pulse current is represented by a current 2 in the circuit of Figure l and this similarly flows through both the main portion and through the shunted portion so that that part flowing through each of the series of combinations of resistance is In the main portion of the circuit the constant magnetising field and the field due to the pulse current are in opposition so that the resultant flux in this part of the core is a value corresponding to a flux density equal and opposite to the constant magnetising field due to the permanent magnet. In the shunted portion the steady magnetising field and the field due to the pulse current are additive producing a total flux of 3 through the shunted portion, which is represented in the circuit of Figure 1 by a current through each of the series combinations of resistances referred to above. Consequently, although the total fiux in the shunted portion is three times that in the main portion, the cross sectional area is also three times as great so that the flux density in this part of the core is the same as for the main portion and saturation beyond the maximum working flux density is thereby avoided.

Since in the mass production of such transformers the magnetomotive force developed by the permanent magnets may vary from one magnet to another, it is desirable that some means should be adopted for maintaining the constant magnetising field at a value close to the permitted saturation value.

According to a preferred form of the invention, a high frequency transformer has a part of the magnetic path in the branch containing the permanent magnet arranged to have a saturation value approximately twice the maximum working flux of the main portion of the ferrite material used. This may be accomplished by limiting the cross sectional area of the magnetic material in the branch containing the permanent magnet. In the corresponding electrical. circuit of Figure 1, this has the effect of replacing the resistance 4 by a barretter or similar non-ohmic resistance.

In order that the invention may be readily carried into effect. a practical form of high frequency transformer will now be described in detail with reference to Figure 2 of the accompanying drawings, which is a cross-sectional view of a preferred form of high frequency transformer.

Referring to Figure 2, a central cylindrical core of ferrite material has a lower part 15 of diameter D and an upper part 16 of diameter D slightly larger than D /2: and a thickness of D n C/ On the part 15 is wound the primary and secondary windings 17 of the transformer which are wound on a cylindrical bobbin 18. The part 15 of the central core abuts against a cylindrical core element 19 which has a thickness D/4 and a cutaway 20 to provide room for connecting leads, not shown, to the transformer windings 17. The windings 17 are surrounded by a hollow cylindrical core member 21 having a length equal to that of the core part 15 and having a wall thickness such that \/D12-D22=D, where D1 is the outside diameter and D2 the inside diameter of the core member 21. The part 16 of the central core abuts against a cylindrical permanent magnet 22 which is magnetized axially and has a diameter slightly less than D but slightly larger than D /2. Surrounding the part 16 of the central core is an annular core member 23 having an axial thick ness less than that of the part 16, an outside diameter equal to that of the core member 21 and an inside diameter slightly greater than that of the part 16 so that an annular air-gap 24 exists between the core members 16 and 23.

Two annular copper washers 25 are arranged on top of the core member 23 and in the region of the junction between the permanent magnet 22 and the magnetic circuit including the central core 15 and 16, and having a. combined thickness such that, together with the part 23, they have the same thickness as the part 16. An inverted cup-shaped steel yoke 26, having a depending cylindrical central portion 27, fits over the permanent magnet 22 and the washers 25 and extends downward to fit into a cylindrical groove 28 formed in the upper and the outer edge of the core member 23. The depending center portion 27 abuts the upper surface of the permanent magnet 22. The core member 19 rests on a hollow cylindrical Bakelite spacing ring 29 and the entire assembly fits inside a container 30 closed at its lower end by a cylindrical disc 31. As used, the container 30 is filled with oil so that the transformer windings 17 are oil immersed.

In the construction described with reference to Figure 2, the core elements or the main portion of the magnetic circuit 15, 16, 19 and 21 and the core element 23 are all of ferrite material and the elements 15, 19 and 21 comprise that part of the core in which the flux changes direction due to the pulse currents flowing in the windings 17. The greater part of the elements 16 and 23, i. e., the shunted portion, carry the flux due to the pulse currents while the remaining parts of these elements carry the uni-directional flux of the permanent magnet 22. The permanent magnet 22 and the yoke 26 form the branch of the circuit carrying the constant magnetizing field. The dimensions of the yoke 26, particularly the dimensions of the depending portion 27, are chosen so that the yoke material saturates at twice the maximum working flux of the main portion 15, 19, 21 of the circuit. The copper washers 25 serve to prevent the high frequency components of the magnetic field due to the pulse currents from passing through the permanent magnet 22 resulting in heavy losses by providing a low conduction path for eddy currents generated within the washers 25 While maintaining very low losses.

What I claim is:

1. A high frequency electric pulse transformer comprising a first closed magnetic circuit including a main portion of ferrite material, a second closed magnetic circuit including a member common to said first and second circuits shunting said main portion, primary and secondary windings wound about said main portion, said second closed magnetic circuit including a yoke member and a permanent magnet in series to produce in said main portion of said first magnetic circuit a magnetic field opposed to the field produced in the main portion by a pulse current in said primary winding and in the same direction as the field produced in said shunting member by said pulse current, and an electrically conductive closed circuit member disposed adjacent and in close proximity to said permanent magnet and said common shunting member, said shunting member having a cross-sectional area and an air-gap which will pro vide a reluctance at which the combined field effect produced by said permanent magnet and said pulse current in said shunting member is below the saturation field strength of said shunting member.

2. A high frequency electric pulse transformer comprising a first closed magnetic circuit including a main portion of ferrite material, a second closed magnetic circuit including a member common to said first and second circuits shunting said main portion, primary and secondary windings wound about said main portion, said second closed magnetic circuit including a yoke member and a permanent magnet in series to produce in said main portion of said first magnetic circuit a magnetic field opposed to the field produced in the main portion by a pulse current in said primary Winding and in the same direction as the field produced in said shunting member by said pulse current. and an electrically conductive closed circuit member disposed adjacent and in close proximity to said permanent magnet and said common shunting member, said shunting member having a cross-sectional area and an air-gap which will provide a reluctance at which the combined field effect produced by said permanent magnet and said pulse current in said shunting member is below the saturation field strength of said shunting member, said yoke having a reluctance and a cross-sectional area at which said yoke saturates at twice the field strength required to saturate said main portion.

3. A high frequency electric pulse transformer comprising a first closed magnetic circuit including a main portion of ferrite material having a given cross-sectional area, a second closed magnetic circuit including a member common to said first and second circuits shunting said main portion, primary and secondary windings wound about said main portion, said second closed magnetic circuit including a yoke member and a permanent magnet in series to produce in said main portion of said first magnetic circuit a magnetic field opposed to the field produced in the main portion by a pulse current in said primary winding and in the same direction as the field produced in said shunting member by said pulse current, and an electrically conductive closed circuit member disposed adjacent and in close proximity to said permanent magnet and said common shunting member, said shunting member having a cross-sectional area at least three times said given cross-sectional area and an air-gap which will produce a reluctance at which the combined field effect produced by said permanent magnet and said pulse current in said shunting member is below the saturation field strength of said shunting member, said yoke having a reluctance and a cross-sectional area at which said yoke saturates at twice the field strength required to saturate said main portion.

4. A high frequency electric pulse transformer comprising a first closed magnetic circuit including a cylindrical main portion of ferrite material having a given diameter D, a second closed magnetic circuit including a member common to said first and second circuits shunting said main portion, primary and secondary windings wound about said main portion, said second closed magnetic circuit including a yoke member and a flat cylindrical disc-shaped permanent magnet having a diameter at least equal to D /2 in series to produce in said main portion of said first magnetic circuit a magnetic field opposed to the field produced in the main portion by a pulse current in said primary winding and in the same direction as the field produced in said shunting member by said pulse current, and an electrically conductive closed circuit member disposed adjacent and in close proximity to said permanent magnet and said common shunting member, said shunting member having a cross-sectional area and an air-gap which will provide a reluctance at which the combined field effect produced by said permanent magnet and said pulse current in said shunting member is below the saturation field strength of said shunting member.

5. A high frequency electric pulse transformer comprising a first closed magnetic circuit including a cylindrical main portion of ferrite material, a second closed magnetic circuit including a portion common to said first and second circuits shunting said main portion, said shunting portion including a disc-shaped member disposed at one end of said cylindrical main portion and engaging the latter with one of its surfaces and a second member defining with said disc-shaped member an annular airgap, said disc-shaped member having a thickness at least equal to primary and secondary windings Wound about said main portion, said second closed magnetic circuit including a yoke member and a disc-shaped permanent magnet engaging the other surface of said disc-shaped member in series to produce in said main portion of said first magnetic circuit a magnetic field opposed to the field produced in the main portion by a pulse current in said primary winding and in the same direction as the field produced in said shunting portion by said pulse current, and an electrically conductive closed circuit member disposed adjacent and in close proximity to said permanent magnet and said disc-shaped member, said shunting portion having a cross-sectional area and the air-gap providing a reluctance at high the combined field efiect produced by said permanent magnet and said pulse current in said shunting portion is below the saturation field strength of said shunting portion, said yoke having a reluctance and a cross-sectional area at which said yoke saturates at twice the field strength required to saturate said main portion.

6. A transformer as set forth in claim 5 wherein the shunting portion has a cross-sectional area of about three times the cross-sectional area of said main portion, said shunting portion including said air-gap having about the same reluctance as said main portion.

References Cited in the file of this patent UNITED STATES PATENTS 2,395,881 Klemperer Mar. 5, 1946

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