US3273099A

US3273099A - Transformer

Info

Publication number: US3273099A
Application number: US374925A
Authority: US
Inventors: Floyd M Minks
Original assignee: Kiekhaefer Corp
Current assignee: Kiekhaefer Corp
Priority date: 1964-06-15
Filing date: 1964-06-15
Publication date: 1966-09-13
Anticipated expiration: 1983-09-13

Description

Sept. 13, 1966 M s 3,273,099

TRANSFORMER Filed June 15, 1964 z /9 W; 1 ,5 IO

I I '1 INTERNAL 1,: ,i i [1H COMBUSTION W g" inl% If 1 ENGINE I I 1 H 1/ 3 ELECTRICAL g [I] 1 6 PULSE MZS/ 22! 1 SOURCE 2 INVENTOR- FLOYD M. MINKS Z7 3/ BY 33 qndrus Star/ e United States Patent 3,273,099 TRANSFORMER Floyd M. Minks, Chester, N.H., assignor to Kiekhaefer Corporation, Fond du Lac, Wis, a corporation of Delaware Filed June 15, 1964, Ser. No. 374,925 14 Claims. (Cl. 33683) This invention relates to a transformer construction and particularly to a high voltage pulse transformer which may be employed in internal-combustion engine ignition systems and the like.

Transformers are often employed to step up a low voltage to a relatively high voltage. The present invention is particularly directed to a transformer which is adapted to take an incoming low voltage pulse and provide a substantially higher voltage pulse of relatively short duration. For example, in ignition systems for automobiles and other internal-combustion engines, a voltage pulse may be fed through a primary coil having a secondary coupled thereto to provide a high voltage pulse to the spark plugs or other firing systems. If the voltage of the output pulse can be increased, the problems of misfiring can be substantially reduced. Of great practical significance is the rise time of the pulse; i.e. the time required for the ignition pulse to increase from a low value to the value at which a spark occurs across a spark plug. If this time is short, less energy is lost in shunt resistance. Thus, considering the shunt resistance that eventually develops in a practical system, with two pulses of equal energy, the one with the faster rise time will have a substantially greater ability to produce a spark and thus ignite the mixed fuel-air mixture.

The present invention is directed to a transformer having the characteristics of a high frequency system to provide a fast rise pulse and therefore provide a transformer which is particularly adapted to be employed in connection with an internal-combustion ignition system or other pulse system where a fast rise time and high voltage is important.

In accordance with the present invention, a core of a substantially continuous magnetic path is provided defining a winding opening with a low voltage primary winding and a high voltage secondary winding wound in stacked relation on a portion thereof. A secondary insulating and winding bobbin for the secondary encircles a portion of the path with the bobbin engaging the adjacent core portion toprovide maximum coupling of the secondary winding carried thereby. In assembly, the outer face of the bobbin may be provided with a winding groove within which the high voltage winding is wound, a multiple part bobbin may be assembled with a prewound secondary or the bobbin maybe molded or cast around the secondary winding. In the latter two constructions, the winding might be wound on a winding tube of insulation, a semi-conductor or a conductor having a narrow slit to prevent formation of a shorted turn. The end of the tube maybe rounded to reduce the adjacent dielectric stress.

The high .vo-ltagewinding in accordance with the present invention is generally a conductive tape member having a width substantially corresponding to the width of the winding groove and also having an insulating coating on at least one surface thereof. The tape is wound within the groove upon itself to form a multipleturn spiral winding substantially filling the groove. The outer end of the winding is grounded and the high voltage connection is made to an inner turn of the winding. The positioning of the high voltage connection at the inner convolution reduces the capacitance to ground because the inner convolution is of a minimum area and at the highest voltage. Further, in accordance with the present invention, a pancake type primary winding is wound about the core immediately adjacent the ends or sides of the bobbin. The primary coil is preferably a multipleturn conductor having all of the turns in a single plane to provide a planar coil. If desired, a similar primary coil may be mounted on opposite sides of the winding bobbin to minimize leakage inductance. The structure of this invention provides, except along the relative long path adjacent the side of the secondary winding, a continuous insulating mass between the high voltage end of the secondary and all grounded surfaces. Also, the winding of one turn per layer reduces the thickness of interlayer insulation in the total secondary by a factor of approximately two and thus contributes to a small transformer with low leakage inductance and distributed capacitance. Consequently, very high voltages and good pulse fidelity can be generated in the secondary winding in a relatively small and compact transformer.

The drawing furnished herewith illustrates preferred constructions of the present invention clearly disclosing the above features and advantages of the present invention as well as others which will be clear to those skilled in the art.

In the drawing:

FIG. 1 is a side elevational view of a transformer constructed in accordance with the present invention and connected in an ignition system;

FIG. 2 is a plan view of the transformer shown in FIG. 1;

FIG. 3 is an enlarge-d view with the portions in vertical section;

FIG. 4 is a fragmentary partial view of the winding tape employed in the transformer of FIGS. 1-3 and typically shows a preferred construction therefor;

FIG. 5 is a vertical section similar to FIG. 4 of an alternative transformer construction in accordance with the present invention; and

FIG. 6 is a bottom view of FIG. 5.

Referring to the drawings and particularly to FIG. 1, an ignition system is diagrammatically shown for an internal-combustion engine or the like having a spark plug 1 forming a part of the engine, which is otherwise shown in block diagram 2. The spark plug 1 is mounted within an engine block and in particular includes a spark gap 3 which is disposed within a firing cylinder, not shown. A pulse transformer 4 constructed in accordance with the present invention has its output connected across the spark gap 3 which will be through a distributor, not shown, for multiple cylinder engines and has its input connected to a suitable pulse source 5 for firing the engine. The pulse source 5 may be of any suitable or desired construction and is preferably a suitable circuit which provides a rapidly rising pulse to the pulse transformer 4 which in turn provides a rapid rising pulse to the spark plug 3 for igniting of an explosive mixture.

- Although the pulse transformer 4 is shown in an ignition system because of its highly practical adaptation to such application, it may be used in other circuits which requirea reliable pulse transformer having the characteristics described herein.

Generally, the illustrated pulse transformer 4 includes a magnetic core assembly 6 defining a winding window 6a within which a high voltage secondary winding 7 is wound. In the illustrated embodiment of the invention, secondary winding 7 is wound within an insulating and winding bobbin 8 which has a central opening encircling a coupling portion of the magnetic core assembly 6. A pancake type primary winding 9 encircles the corresponding portion of the magnetic core assembly 6 immediately adjacent to one side of the bobbin 8 and is connected to the pulse source 5. The structure provides a close coupling of the primary winding 9 to the secondary winding 7 with a very efiicient transfer of the energy into the secondary winding 7.

More particularly, the magnetic core assembly 6 includes a pair of U-shaped similar cores and 11 having side portions of circular cross sections and formed of a high frequency ferrite or other suitable high frequency core material such as is well known in the transformer art and the like. The cores 1t) and 11 are mounted with the ends of the side legs in alignment and adjacent one another with a slight spacing therebetween provided by suitable non-magnetic spacers 12. Suitable clamping means such as nut and bolt units 13 extend through edge slots or grooves in the sides of

cores

10 and 11 and are provided for interconnecting

cores

10 and 11 and the non-magnetic spacers 12 provide a continuous magnetic path interrupted only by the air gaps defined by spacers 12. The spacers 12 may be eliminated and the cores K10 and 11 clamped in touching engagement. A bias or reset winding may then advantageously encircle the core assembly and be connected to a suitable current source. The connected

cores

10 and 11 define a winding Window through which the secondary winding 7 and the primary winding 9 are wound; in the illustrated embodiment about the left side portion of the generally rectangular core assembly. The air gap, formed by spacers 12, in the magnetic path eliminates the necessity of a reset winding and the like for demagnetization of the core between pulses.

In accordance with the illustrated embodiment of the invention, the insulating-winding bobbin 8 is generally a doughnut shaped or annular member having an inner diameter corresponding substantially to the corresponding circular configuration of the

cores

10 and 11. The bobbin 8 encircles the side portions and spans the air gap forming spacer 12 and is formed with a generally U-shaped cross section defining an outwardly opening win-ding groove 14 on its outer periphery. Groove 14 extends radially inwardly toward the core portions and completely encloses the secondary winding 7 on three sides. The bobbin 8 is formed of a suitable insulating thermoplastic or other insulating material which is particularly adapted for insulating the high voltages generated between the secondary and ground, as more fully discussed hereinafter.

The secondary winding 7 is a multiple-turn winding formed from continuous lengths of conductive tape 15 and insulating tape 16, as shown most clearly in FIG. 4. The width of the

tapes

15 and 16 substantially corresponds to the width of the winding groove 14 and the tapes are wound in a spiral manner .to substantially and completely fillthe groove 14. The groove 14 is filled with a suitable impregnating material 17 such as an epoxy resin to essentially eliminate all voids within the groove and the secondary winding. The width of the secondary may be slightly less than the groove 14 to permit ready introduction of the impregnating material. The secondary winding 7 includes a substantially greater number of turns than the primary winding 9' for increasing or stepping up the voltage of the pulse fed to the gap 3 of spark plug 1. An outer covering or coating 18 is preferably applied to the outer surface of the bobbin'8. The coating may be a semi-conductor or other resistive material or a non-continuous conductive material to form a ground plane for the distributed capacity associated with secondary winding 7. This will essen tially eliminate and prevent external corona.

In practice, winding 7 may be formed, for example, from

separate tapes

15 and 16, from a conductive foil base as the conductive strip with an insulating layer coated on one or both sides, or from an insulating base coated on one or both sides with a conducting layer. The conductive tape may for example be formed of an insulating Mylar of .00025 inch thick and 7 inch wide 4 and an aluminum foil of .00017 inch thick and inch wide thereon. The lesser width of the conducting layer is preferably to protect against a short circuited turn. The secondary winding may includes 1000 turns with three turns or the like in the primary winding.

An integral insulating tower 19 projects axially from the bobbin 8 adjacent the inner convolutions or turns of the secondary with an insulated high voltage lead 20 extending therefrom. The conductor within lead 20 is connected to a wire or strip 21 connected to an inner turn or convolution of the secondary winding 7 within the groove 14 and wedged or otherwise held within the opening of tower 19 for connection with lead 20. Thus, in practice a couple of inner turns of the secondary winding 7 may be formed with the wire 21 inserted and the subsequent turns completed, generally in accordance with known practice in forming of condensers. The outer end of the winding 7 is connected by a lead 22 to one side of the primary '7 which is connected to ground to complete the connection of the secondary winding 7 in the circuit of spark plug 1, as schematically shown in FIG. 1.

Materials and winding techniques employed in connection with the secondary transformer winding will generally be similar to known techniques employed in winding capacitors. Generally, the high voltage and ground connection of the secondary winding 7 can be made in "accordance with known condenser concepts.

The insulation of the secondary winding 7 from ground is provided by the bobbin 8 which provides a continuous homogeneous mass of insulating material between the high voltage inner end of the secondary winding 7 and all of the grounded surfaces. The bobbin 8 is formed of suitable material such as thermoplastic or epoxy resin which can withstand the high voltages generated in the secondary winding 7 and with which the impregnating material bonds, shown along the edges as at 17 of secondary winding 7 for purposes of illustration. The impregnating material, as previously noted is to fill all voids within the groove. The insulation thus provides maximum insulating characteristics and for any given size provides maximum protection against breakdown and thus permits higher secondary voltages. The present invention is particularly directed to the use of a transformer in a pulse system generally similar to high frequency voltage systems. Dielectric failure of a continuous insulating mass is minimized and therefore provides a highly satisfactory and reliable operation for rapidly rising pulses.

In the selected applications generally where size and weight considerations are paramount, the inner corners of the groove 14- may be rounded as at 23 shown substantially enlarged for illustrative purposes and provided with a high resistivity or non-continuous conductive coating in any desired or well known manner. The rounding of the corners 23 and the provision of the coating will prevent corona effects and reduce concentration of dielectric stress and thereby increases the reliability of the total system. A similar result can be'obtained'by employing a thicker conductive tape with rounded corners for the innermost turn or turns or winding of the secondary on a tubular form which serves as the initial turn and which has the ends rounded.

As most clearly shown in FIG. '3, the axial depth of the bobbin 8 is slightly less than the depth of the winding window formed by the

U-shaped cores

10 and 11 and defines a small ring-like winding space to the one side of the bobbin within which the primary winding 9 is wound. The primary winding 9 is wound from a continuous conductor which is illustrated as having a rectangular cross section in FIG. 3 although round or other shapes may be employed. A suitable low voltage covering 24 such as a cloth tape encloses the winding 9 to prevent grounding. A pair of integral input terminals or input leads 25 projects axially of bobbin 8 and is connected to the pulse source 5 for selective energization of the primary winding 9 and transfer of energy to the high voltage secondary winding 7. The one lead is grounded and connected by lead 22 to the ground side of the secondary Winding 7 in the illustrated embodiment of the invention.

In operation, the illustrated ignition system has an ignition pulse which is applied to the primary Winding 9 as a low voltage, rapidly rising pulse and thereby generates a high voltage pulse in the secondary winding 7. The high voltage pulse is impressed across the spark gap 3 of the spark plug 1 and results in a spark for igniting a fuel mixture.

The present invention is particularly unique in providing a very high step-up voltage in a small compact unit which can be employed in an internal-combustion engine for automobiles, outboard motors and the like where it is subject to substantial vibration and rough usage. The bobbin 8 provides homogeneous insulation as previously noted to minimize breakdown of the insulation and loss of firing signal pulses. The voltage rises very rapidly with the present invention as a result of relatively close coupling of the primary to the secondary and low secondary distributed capacitance. The secondary windings may be wound of two insulated tapes producing a bifilar winding which is particularly useful in systems for exciting magnetrons and the like. The connection to the high voltage lead 18 at an inner convolution provides maximum insulation to all grounded surfaces and also minimizes the capacitance of the high voltage end of the winding 7 to ground. Leakage inductance can be further reduced, if necessary or desired, by placing a second primary winding on the opposite side of the bobbin and connecting it in series or parallel with the'illustrated primary winding 9. Similarly, a pair of secondary windings may be disposed on opposite sides of a primary winding.

Although shown in connection with a generally rectangular core structure, the present invention may employ any other suitable core structure providing close coupling of the primary and secondary. In some applications, the core structure may be eliminated. For example, in FIGS. 5 and 6, an alternative core construction is shown wherein a pot type core assembly 26 is provided for supporting primary and secondary windings, as follows.

A pancake type primary winding 27 and a high voltage multiple turn secondary winding 28 carried within a bobbin 29 are provided generally as in the structure previously described with respect to FIGS. 1-4. In FIGS. 5 and 6, however, the core assembly includes a pot type core 30 which is generally a cup-shaped member open at one end and having a central pole 3 1 integrally formed and projecting outwardly in accordance with the thickness of the core such that a vertical cross section, as

shown in FIG. 5, defines anE-coniiguration. A core' cover 32 closes the open end of the pot core 30 and is interconnected to provide a unitary unit construction by a bolt and nut attachment unit 33 which extends through aligned apertures in the cover i3 2 and the pole 31. The cross section of the annular winding space generally corresponds to a winding Window of FIGS. l4 such that the bobbin 29 and the primary winding =27 substantially fill the winding space. A terminal aperture 34 is provided in the base of the core 30 with a terminal tower 35 projecting outwardly from the bobbin 29 for winding 2-8. The primary terminals 36 of winding -27 project downwardly through suitable apertures in the core cover 32 for interconnection with the ignition circuit or the like.

In operation, the embodiment of FIGS. 5 'and 6 functions substantially in the same manner as the previous embodiment to provide a compact and reliable transformer for transforming of low voltage, rapidly rising pulses to high voltage, rapidly rising pulses. Thus, the output voltage pulse may have a peak value in the order of 30 to 100 kilovolts or more and a period of microseconds or less. A 1000 or more turns may be used in the secondary to reduce the voltage diiference per turn. The

solid, insulating bobbin provides adequate insulation to ground, particularly between the highest voltage innermost convolution and ground. i

As previously noted, the bobbin may constitute a winding form within which the secondary is wound, it may be preformed in two or more parts and assembled with a previously wound secondary or it may be molded or otherwise cast about the secondary within the scope of the present invention as defined in the accompanying claims.

The present invention thus provides an efiicient, high frequency pulse transformer which is particularly useful in producing firing pulses of high voltage and short duration.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

I claim:

1. A transformer structure comprising,

a magnetic core unit having a winding opening and an adjacent magnetic winding portion,

a secondary winding insulator of a high voltage insulating material encircling the core unit winding por tion at the opening and having a secondary Winding groove within the insulator and opening generally outwardly of the winding portion,

a secondary winding within the groove and having a high voltage terminal at the base of the groove, and

'a primary winding means encircling a portion of the core unit immediately adjacent at least one side of the Winding insulator.

2. The transformer of claim 1, having said secondary winding in the form of a tape conductor of a substantially greater width than thickness and coated with an insulating material on at least one face of the tape conductor, the width of the tape substantially corresponding to the width of the groove, and said tape condu'ctor located within the groove to form a spiral shaped winding essential-1y completely filling the groove with the high voltage turn adjacent the base of the groove and the low voltage turn adjacent the entrance ofthe groove.

3. The transformer of claim 2 wherein the winding insulator includes a high voltage terminal tower projecting axially of the insulator adjacent the base of the groove and a high voltage terminal secured to an inner convolution of the winding.

4. A transformer structure comprising,

a magnetic core unit having a winding Window defined in part by a magnetic winding portion,

a preformed secondary winding insulator of insulating material encircling said winding portion and having a Winding groove in an exterior surface,

a conductive tape means wound upon itself with insulation between the adjacent convoluti-ons to form a spiral secondary winding and wound within the groove with an axis parallel to the Winding port-ion and having a high voltage terminal at the base of the groove, and

a generally fl'at, pancake-type primary winding encircling the Winding portion immediately adjacent one side of the winding insulator, said winding having a relatively few turns compared to the secondary winding and extending generally coextensively of the secondary Winding.

5. The structure of claim 4 wherein the magnetic core unit includes a pair of U-shaped cores secured together to form a rectangular core unit.

6. The structure of claim 5 having a non-magnetic gap between at least one pair of adjacent ends of the cores and having the insulator encircling the air gap and both portions of the cores adjacent the encircled air gap.

7. The structure of claim 5 wherein the insulator and cores have corresponding mating dimensions.

l 8. The structure of claim 4 wherein the magnetic core unit includes a pot core open at one end and having a base with a central pole and an outer annular wall extending from the base and defining an annular opening substantially corresponding to the diameters of the insulator and primary Winding, and

a cover secured to the open end of the pot core with the primary winding disposed between the cover and the insulator.

9. The transformer of claim 4 for transmitting a high voltage pulse having,

said secondary Winding tape being of a substantially greater Width than thickness and coated with an insulating material on at least one surface, the width of the tape being slightly smaller than the width of the groove, and an impregnating material within the groove to essentially completely eliminate all voids within the groove. e 10. A high voltage pulse transformer for transmitting a pulse with a rapid rise time, comprising a magnetic core defining a substantially continuous magnetic path about a winding opening,

an annular winding bobbin having a central opening corresponding to a winding portion of the core at the winding opening and being assembled in mating relation therewith, the bobbin being extended outwardly from the winding portion for the width of the window and having a generally rectangular winding groove in the outer periphery thereof,

a primary winding encircling the winding portion of the core,

a secondary Winding located within and substantially tfilling the groove in the form of a continuous spirally wound conductive toil having insulation between eachturn to provide a multiple tur-n spiral winding of a single conductor width, said winding being wound I about the bobbin with the inner turns at the base of a the groove and the outer turns at the entrance to the groove, and

a high voltage terminal secured to an inner turn of the secondary winding.

11. A high voltage pulse transformer for transmitting a pulse with a rapid rise time, comprising i a magnetic core unit defining "a substantially continuou magnetic path about a winding opening and having 4 a Winding portion contiguous said opening,

a pancake primary winding including a spirally wound conductor encircling the core adjacent one end of the winding opening,

a winding bobbin having a central opening corresponding to the winding portion of the core and arranged in mating relation therewith, said bobbin extending outwardly from the winding portion across the opening to the opposite portion of the 'core and having a generally rectangular Winding groove in the outer portion, the inner corner's of the groove being curved and covered with an integral high resistivity coating to reduce local concentration of dielectric stress, the depth of the bobbin being generally less than the depth of the Winding opening by the thickness of the pancake primary coil,

a secondary winding located within and filling the groove in the form of a continuous conductive toil having continuous insulation on at least one "face, said fOll having -a width substantially corresponding to the groove to dill the groove with a multiple turn winding of a single conductor width baving inner turns at the base of the groove, and

a high voltage terminal secured to an inner secondary turn.

12. The structure of claim 11 wherein,

the magnetic core unit includes a pair of U-shaped cores secured together to form a generally rectangular core unit with a non-magnetic gap between at least one pair of adjacent ends of the 'U-shaped cores and having the bobbin encircling the gap and both portions of the cores adjacent the gap.

13. The structure of claim '11 wherein the bobbin is an annular member and the magnetic core unit includes a pot core having 'a central pole and an outer annular Wall defining an annular chamber having one and open, said chamber substantially corresponding to the bobbin and of a somewhat greater depth, and

a cover secured to the pot core overlying said open end of the chamber and having the primary winding disposed between the cover and the bobbin.

14. The structure of claim 13 wherein,

the outer surface of the bobbin is covered with an integral high resistivity coating.

References Cited by the Examiner UNITED STATES PATENTS 2,064,771 12/1936 VOgt 336178 X 2,762,019 9/1956 Short et al 336l78 X 2,867,785 1/1959 Crumbliss 3'36-178 2,998,583 8/ 1961 Worcester '336-223 X LEWIS H. MYERS, Primary Examiner.

ROBERT K. scHAEFE-R, Examiner.

C. TORR'ES, Assistant Examiner.

Claims

1. A TRANSFORMER STRUCTURE COMPRISING, A MAGNETIC CORE UNIT HAVING A WINDING OPENING AND AN ADJACENT MAGNETIC WINDING PORTION, A SECONDARY WINDING INSULATOR OF A HIGH VOLTAGE INSULATING MATERIAL ENCIRCLING THE CORE UNIT WINDING PORTION AT THE OPENING AND HAVING A SECONDARY WINDING GROOVE WITHIN THE INSULATOR AND OPENING GENERALLY OUTWARDLY OF THE WINDING PORTION, A SECONDARY WINDING WITHIN THE GROOVE AND HAVING A HIGH VOLTAGE TERMINAL AT THE BASE OF THE GROOVE, AND A PRIMARY WINDING MEANS ENCIRCLING A PORTION OF THE CORE UNIT IMMEDIATELY ADJACENT AT LEAST ONE SIDE OF THE WINDING INSULATOR.