CN101057305A - Transformer and related assembly method - Google Patents

Abstract

A transformer (1) comprising: a bobbin (2) defining a primary winding chamber (21) and a secondary winding chamber (22), a magnetic core (4) mounted on the bobbin, and a primary winding (31) and a secondary winding (32) wound in the primary winding chamber (21) and the secondary winding chamber (22). The core (4) is an EFD/EFF type core and is directed towards the secondary winding chamber (22). The spool (2) includes a cover (52). The bobbin (2) including the cover (52) is configured to provide enhanced insulation of the secondary winding (32) relative to the primary winding (31) and the magnetic core (4) such that the transformer (1) is SELV compliant with EN 61347 and UL 1310 compliant.

Description

Transformer and related assembly method

technical field

The invention relates to (electrical) transformers.

Background technique

Transformers are used in a variety of fields, for example, in power supply units for halogen lamps, where the input line voltage (e.g. a mains voltage of typically 220-240 volts used in most European countries and typical values used in most American countries 100-120 volts) is converted to an output voltage of 6 volts, 12 volts or 24 volts. Transformers are also commonly used as output isolation transformers in electronic converters for halogen lamps to produce an output voltage of 12 volts.

So-called "toroidal" transformers are often used for the above applications. In these state-of-the-art transformers, for operating voltages up to 250 Volts, the walls of the bobbin must have a thickness of approximately 1 mm in order to meet the requirements SELV (Safety Additional low pressure).

Currently, thick copper wires are also used for the secondary winding. These wires exhibit sufficient rigidity to allow automatic placement during assembly of the transformer. The ends of the primary windings are instead soldered to the associated pins. This is especially true for electronic transformers for halogen lamps. These electronic transformers are actually step-down transformers, that is, transformers in which the voltage in the secondary winding is lower than the voltage in the primary winding, resulting in a higher current in the secondary winding relative to the current in the primary winding. This results in thinner wires on the primary side that must be soldered to the pins in order to have sufficient rigidity. A bobbin of fairly simple construction is usually chosen, mainly to allow the pins and secondary wires to be fixed and to support the magnetic part of the transformer. In some cases this unavoidably includes the need to use triple insulated wire for the secondary winding and an additional layer of insulating tape. In order to meet the requirements of standards such as EN 61347 on SELV, since the secondary winding is external, all parts involved in the primary side must be placed 3 mm from the secondary winding if they are not insulated, if they are basically insulated , then these components are very close to the secondary winding. In the remaining cases, all parts involved in the primary side must be placed at a distance of at least 3 mm from the winding if they have basic insulation, and at least 6 mm if no such insulation is provided.

Additionally, these transformers are critical from an electromagnetic compatibility (EMC) point of view and require complex thermal protection, all of which are factors that hinder the cost-effectiveness of the final solution.

Additional key factors to consider relate to:

- mount the transformer on a very small substrate such as a narrow printed circuit board or PCB,

- if other components arranged on the same board (such as those involving the primary side of the transformer) have basic insulation, place the transformer very close to these components, or if these components do not have basic insulation, place these components in the at a distance of less than 1 mm, and such that the conductive copper tracks connected to the secondary winding are laid on the so-called component side of the board, that is to say on the side of the board on which the component is mounted,

- reduce the height of the final product, and

- Allows for fully automatic assembly of transformers.

Typical requirements to be met in today's applications, for example, are a transformer with a maximum height of less than 14.5 mm from the base, a power handling capacity of 75 watts at nominal operating conditions (European mains voltage), and the ability to secure the ends of the winding wires, In order to avoid possible problems during the assembly phase of the transformer during manufacture.

Contents of the invention

Although the transformer disclosed in EP-B-0 793 243 has solved these problems and needs well, still need to explore the structure of further improvement, forms the preamble part of claim 1 according to document EP-B-0 793 243 .

It is therefore an object of the present invention to provide a transformer which is adapted to fulfill the above requirements fully satisfactorily.

According to the invention, this object is achieved by a transformer having the characteristics set forth in the subsequent claims. The invention also relates to a corresponding assembly method. These claims form an integral part of the disclosure of the invention provided herein.

A particularly preferred embodiment of the invention uses an EFD type (or EFF type) core instead of a ferrite core. The EFD type core is an E-type core, but it is "flatter" than the E-type core, and the effective area is equal. In that way, a power of the order of 75 watts can be delivered under nominal European halotronic operating conditions, with a maximum height from the substrate of less than 14.5 mm.

A particularly preferred embodiment of the invention provides the presence of a bobbin around such a magnetic core, which allows the entire transformer component to be compatible with SELV (Safety Extra Low Voltage) insulation requirements.

Components related to the primary side can be placed very close to the transformer ground, while the secondary tracks can be routed on the component side of the board, since the secondary winding is reinforced insulation (according to EN61347) from the magnetic core, which is the more external conductive part of the component.

Preferably, the transformer of the present invention comprises two winding chambers which are isolated from each other and placed close to each other. Also preferably, the structure is an asymmetric structure, that is, insulation is provided in one chamber (usually the secondary winding chamber), and the form of insulation is reinforced insulation, which is opposite to the structure of EP-B-0793 243, in EP-B -0793 243 has double insulation in its construction. It is also preferable to provide insulation on the outside of the magnetic core (for example, by an extension of the cover associated with the bobbin), rather than just on the inside.

The transformers described herein can be automatically mounted (ie positioned on a substrate such as a PCB) even though the secondary wires are fairly flexible stranded wires. The wire-end terminations of the secondary windings are preferably soldered to the pins, thus creating thick pins that are stable and can be fixed as vertical pins. The operation of soldering the winding wires to the pins is facilitated by preferably providing holes, eg in the form of recesses (blind holes), in the transformer housing. These grooves hold the wires securely in place possibly in cooperation with the beak formation provided in the housing itself.

Preferably, the spool exhibits additional adhesive pins for securing the spool to the printed circuit board. These additional pins are necessary for additional fixing of the bobbin to the printed circuit board when the electrical connection pins with the soldered ends of the secondary winding become too thick.

Description of drawings

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a general perspective view of a transformer of the type described here,

Figure 2 is a side perspective view of the transformer described herein with one of the elements included in the transformer removed for clarity of illustration,

Figure 3 is a front perspective view of the transformer shown in Figure 2 with the same parts shown again removed for clarity, and

Figure 4 is a bottom perspective view of the bobbin of the transformer described herein.

Detailed ways

In the figures, reference numeral 1 generally designates a transformer suitable for use, for example in connection with a halogen lamp. Such transformers are usually intended to operate in a frequency range above the mains frequency and consist of the following basic components:

- a bobbin, usually consisting of a molded insulating material such as polyethylene, and comprising a primary winding body and a secondary winding body defining respective primary winding chambers 21 and secondary winding chambers 22,

- a primary winding 31 and a secondary winding 32 wound around the primary winding body and the secondary winding body in the primary winding chamber 21 and the secondary winding chamber 22 respectively; and

- A ferromagnetic core arranged above the bobbin 2 in such a way as to cooperate with the primary winding 31 and the secondary winding 32 to create a transimpedance structure when the transformer is in operation.

The two winding chambers 21 and 22 are thus separated and placed next to each other, that is to say next to each other.

In FIGS. 1 to 3 , the windings 31 , 32 and the magnetic core 4 are shown schematically with dashed/dot-dash lines only for the sake of a clearer representation and understanding of the features of the bobbin 2 . In FIG. 4, only the bobbin 2 is shown.

The transformer 1 described here is particularly adapted to work with EFD-type cores (sometimes also referred to as EFF-type cores by some manufacturers in the Far East).

A particularly preferred embodiment of the structure described here provides the magnetic core 4 as a ferrite core of the EFD 25 type. This core type is slightly different from the standard E-core in that it has a reduced profile while maintaining the same effective area (Ae). Adopting such a magnetic core is advantageous because it results in a lower overall thickness (height) of the transformer 1 while maintaining a good power handling capacity.

The construction described here employs this core type while ensuring full compatibility with insulation requirements as specified by standards such as EN 61347, which deals with so-called SELV (Safety Additional Low Voltage) insulation requirements.

In essence, the structure described here provides this level of insulation by providing reinforced insulation of the secondary winding 32 with respect to the primary winding 31 and the ferrite core 4 . That way, all electronic components related to the primary winding 31 can be mounted very close to the ferrite core 4 (this fact can be best understood in FIG. 2 ). As used herein, "very close" refers to a distance of less than 1 millimeter, or even just enough to prevent these parts from directly contacting each other (ie, "touching") each other.

By direct comparison, it will be appreciated that the structure described here differs from that disclosed in EP-B-0 793 243, generally because such prior art structures include double insulation between the primary and secondary windings. This is provided by basic insulation between the ferrite core and the primary winding plus additional insulation between the ferrite core and the secondary winding.

In the structure described here, a reinforced insulation level (SELV) is achieved with an insulation thickness of at least 1 mm between the primary winding 31 and the secondary winding 32 and between the secondary winding 32 and the ferrite core. Furthermore, an air "creepage" and a given gap are provided between the primary winding 31 and the secondary winding 32 on the one hand and between the secondary winding 32 and the ferrite core 4 on the other hand.

Such a clearance has a value of at least 6 mm, while an added value of 6.4 mm is taken into account when complying with the American standard UL 1310 (safety standard for class 2 power supply units). The standard is claimed by American Subject UL 897A to cover LED (Light Emitting Diode) kits for field installation, in which transformers1 are used.

In particular, a protective cover of electrically insulating material 52 is provided and adapted to be associated with the secondary winding chamber. The cover 52 is generally composed of the same electrically insulating material including the other components of the bobbin. This cover 52 is therefore considered in all respects to be part of the bobbin 2 of the transformer.

The thickness of all the walls surrounding the secondary chamber, in which the secondary winding 32 is wound, is therefore chosen to have a minimum thickness of not less than 1 mm, while the side walls of this chamber preferably have a thickness of 1.5 mm, since the following Labyrinths to be explained further. In the exemplary embodiment shown here, the cover 52 as a whole has an approximate "omega" or "mesa" shape (which is best understood in the front view of FIG. 3 ), and the sides of the secondary winding chamber The walls are thus defined by the side walls 520 of the cover 52 .

The thickness of the wall between the primary and secondary windings 31 , 32 , the wall of which is indicated by 320 in FIG. 2 , is typically 0.5 mm.

In order to ensure the desired creepage of 6-6.4mm, at both ends of the secondary winding 32 there is a through wall which establishes those parts of the bobbin structure which are at the end of the secondary winding chamber and which are provided in the cover 52. Complementary wall configurations, creating labyrinths.

In particular, a first set of through-walls is created by having the walls 320 between the primary winding 31 and the secondary winding 32 equipped with slots 321 adapted to be through-connected with those provided inside the cover 52 (ie the closest ) side of the protruding wall 521. The presence of wall 521 and its through relationship to slot 321 provided in wall 320 is best understood in the plan view of FIG. 4 .

By providing an end wall 322 having a central recess or slot 323 adapted to co-operate therethrough with another wall 522 protruding from the inner surface of the cover 52 at the opposite side of the secondary winding 32 The outer (ie, distal) end provides a completely similar labyrinth configuration, as best understood in FIG. 3 .

The labyrinth associated with the central wall 320 has a typical nominal length of about 6.7 millimeters, which is sufficient to provide an effective creepage distance between the primary and secondary windings 31 , 32 in excess of 6.4 millimeters. The labyrinth associated with wall 322 is typically arranged to provide a nominal creepage distance of approximately 6.5 millimeters.

Another factor for ensuring the desired insulation level is given by controlling the distance between the magnetic core 4 associated with the secondary winding and the connection pin 6 . These pins usually protrude from the lower surface of the bobbin 2 . These pins may be provided by any standard technique known for this purpose, eg in the form of metal pins which are inserted into the plastic mold of the bobbin. Alternatively, the pins 6 can be provided as an integral part of the bobbin plastic mold over which the wire ends of the secondary winding are wound and soldered. This structure is described in EP 04425772.3, which was filed on 13.10.2004 by the same applicant of the present application.

A similar structure can be adopted for the connection pins associated with the primary winding 21 . In the exemplary embodiment shown here, the number of primary pins indicated overall by 7 is four and the number of secondary pins 6 is two. This can be simply stipulated by the fact that the primary winding 22 can actually be arranged to comprise different taps/contacts in order to adapt the transformer 1 to work with different input voltages while providing the same output voltage. If that is not necessary, the placement of the primary pins can be conveniently chosen.

Returning again to the distance between the secondary pin 6 and the magnetic core 4, a typical value in the structure shown is at least 7.2 mm, which seems to fully satisfy the application conceived and to ensure SELV insulation.

Another factor that plays a role in determining the horizontal insulation of the transformers described here is given by the path between the secondary pin 6 (and secondary winding 32 ) and the ferrite core 4 within the transformer.

This again includes a labyrinth configuration substantially defined by two transverse L-shaped extensions 524 extending on either side of the cover 52 . These extensions are covered by grooves 324 when the cover is inserted into its position. The path length through the labyrinth is much greater than 6.4mm, which is established between the secondary winding and the ferrite core or possibly the secondary track on the component side of the board and the ferrite core .

From the view of FIG. 2 , it will be further understood that the vertical height of the cover 52 has a maximum value consistent with the central wall 320 and gradually decreases toward the distal end of the cover 52 to reach a height consistent with the end wall 322. minimum, thus avoiding any sticking out effect with respect to secondary pin 6.

Considering that it may be desirable to have secondary connection tracks on the component side of the PCB on which the transformer 1 is mounted, in order to ensure full SELV compatibility of the transformers described here for any possible application.

Cover 52 is designed in this manner to have an overall "omega" or "mesa" shape. As best understood in the views of Figures 1 and 2, the extension ("beak") 523 does not merely extend in line with the cover length 52, but also extends longitudinally of the transformer to envelop or enclose The magnetic core 4 of its entire extension thus supports the magnetic core at its "secondary" end.

That way, a possible air path between the secondary track and the ferrite core that is well over 6.4 mm can be easily obtained, for example by selectively varying the height of the vertical portion of the lateral protrusion 523 . It is possible to easily make the beak 523 into two walls which surround the entire outer leg of the ferrite core in order to shield the ferrite from components involved in the secondary side which are placed near the transformer. In this way, the extensions 523 will have a substantially C-shaped shape, these extensions 523 surrounding the outer legs of the magnetic core 4 .

With regard to connecting the ends of the windings 31, 32 to the respective pin sets 6, 7, this description implicitly assumes that the case in question is a buck application. In a buck application, the wires of the primary winding 31 are relatively thin and thus flexible, while being adapted to retain a given shape. For such wires, the construction preferably takes a standard construction, eg by winding and soldering the wire to the pin 7 .

In boost applications (that is, those applications in which the voltage of the secondary winding is higher than the voltage of the primary winding, thus resulting in a lower current in the secondary winding relative to the current in the primary winding), when the secondary winding 32 is used as The above generally applies to the same transformer for the primary winding and vice versa for winding 31.

In a typical step-down application, the wires of the secondary winding 32 (and this would be the wires of the primary winding in the case of a boost application) can be of three types: copper wire covered with insulating enamel, stranded wire, or A conductor consisting of a braid of very thin wires.

The copper wire covered by the enamel is very rigid in itself, unlike individual pins, whereby the respective ends can pass through the essential part of the bobbin 2, as well as function as the pin 6 itself, thus allowing automatic assembly of the transformer 1 .

In contrast, the braid of stranded wire, or even (worse) very fine wire, which is not inherently stiff, must require it to be wound around, or in any case secured to, the pins in order to produce a suitable A rigid member (pin) that penetrates a mounting hole in a printed circuit board. However, this can lead to a situation where high currents flow through very thin pins, thus raising the temperature of the components in that area. Connecting the stranded wires directly to the rails/ribbons set in the board reduces the temperature of the secondary winding by 5-10 degrees Celsius. Currently, this results in manual assembly of most transformers with litz wire secondary windings by operators.

In the structure described here, two holes 8 in the form of grooves (ie blind holes) are provided in the winding shaft in the vicinity of the secondary pin 6 .

Preferably, the holes 8 are produced directly during the molding process of the bobbin 2 .

The ends of the secondary winding 32 , indicated schematically by W and assumed to consist of litz wires or a braid of thin wires, thus pass through the groove 8 to be soldered to the pin 6 . The holes/grooves 8 allow the ends W to be held securely in a fixed position while the wire ends W are soldered to the pins 6, resulting in a very strong structure.

In the latter respect, it should be understood that the soft solder is a highly polar liquid once it enters the usual soldering temperatures used by the winding machine connecting the winding ends to the pins. It inherently tends to fill gaps between wires, such as those including, for example, stranded wire or wire braids. When the wire ends are removed from the soldering slots, these wire ends will thus in fact be compact and integral with the metal pin 6 .

By adopting such a structure, it is possible to manufacture a transformer 1 suitable for easy assembly in an automatic manner. This also benefits from the mentioned advantages according to the reduction of the temperature at the connection point with the use of the PCB strip and components positioned nearby.

Accordingly, without prejudice to the basic principles of the invention, changes may be made in the details and embodiments with respect to what has been merely illustrated and shown, without departing from the scope of the invention as defined by the appended claims .

Claims (24)

1. A transformer (1) comprising: - a winding shaft (2) defining a primary winding chamber (21) and a secondary winding chamber (22), - a magnetic core (4) mounted on said bobbin, and - a primary winding (31) and a secondary winding (32) respectively wound in said primary winding chamber (21) and secondary winding chamber (22), wherein the bobbin (2) comprises a connection with said secondary winding ( 32) associated cover (52), It is characterized by: - said magnetic core (4) is an EFD/EFF type magnetic core, and - said bobbin (2) comprising said cover (52) is configured to provide reinforced insulation of said secondary winding (32) with respect to said primary winding (31) and said magnetic core (4). 2. Transformer according to claim 1, characterized in that the transformer is SELV compliant to EN 61347 and UL 1310 compliant. 3. The transformer according to claim 1 or 2, characterized in that the primary winding chamber (21) and the secondary winding chamber (22) are arranged side by side. 4. Transformer according to any one of claims 1 to 3, characterized in that the cover (52) and thus the reinforced insulation mainly extends over the secondary winding (32) without extends to the primary winding (31). 5. Transformer according to any one of the preceding claims, characterized in that the cover (52) extends over the whole of the secondary winding (32). 6. Transformer according to any one of the preceding claims, characterized in that said cover (52) has a transverse extension (523) which surrounds said magnetic core at least laterally (4) and possibly extend around the entire outer leg of the ferrite core (4). 7. Transformer according to any one of the preceding claims, characterized in that the bobbin (2) including the cover (52) consists of insulating material. 8. Transformer according to any one of the preceding claims, characterized in that the bobbin (2) comprises at least 1 mm between the primary winding (31) and the secondary winding (32) A partition wall (320) having a wall thickness of 1 mm and a partition wall (520, 22, 322) having a wall thickness of at least 1 millimeter between the secondary winding (32) and the magnetic core. 9. Transformer according to any one of the preceding claims, characterized in that the bobbin (2) including the cover (52) is arranged to connect the primary winding (21) and the secondary An air creepage and clearance of at least 6.4 mm is provided between the primary windings (22) and between said secondary winding (22) and said magnetic core (4). 10. Transformer according to any one of the preceding claims, characterized in that the secondary winding chamber (22) is completely surrounded by a wall having a thickness of at least 1 mm. 11. Transformer according to any one of the preceding claims, characterized in that the cover (52) has an omega or mesa-like overall shape. 12. The transformer according to any one of claims 1 or 11, characterized in that the cover (52) has a lateral extension (523) extending from the cover (52) ) start extending to enclose the longitudinal legs of said EFD/EFF type magnetic core (4). 13. The transformer according to claim 12, characterized in that said laterally extending portion (523) has a substantially L-shaped shape with a distal portion covering said magnetic core (4 ) to create an air path outside the transformer (1) between if the secondary track (32) and said ferrite core (4). 14. Transformer according to claim 12, characterized in that said laterally extending portion (523) has a substantially C-shape surrounding the outer legs of said ferrite core (4). 15. A transformer according to any one of claims 12 to 14, characterized in that said transversely extending portion (523) of said cover (52) is That part of the magnetic core (4) extends longitudinally over the entire length of the upper part. 16. Transformer according to any one of the preceding claims, characterized in that the bobbin (2) comprises a partition between the primary winding chamber (21) and the secondary winding chamber (22) a wall (320) cooperating with said cover (52) in a labyrinth-like structure comprising: - a slot (321 ) in one (320) of said partition wall (320) and said cover (52), and - A wall formation (521 ) adapted to penetrate into said groove (321 ), said wall formation protruding from the other (52) of said partition wall (320) and said cover (52). 17. Transformer according to any one of the preceding claims, characterized in that the bobbin (2) is comprised on the distal side of the secondary winding chamber (22) exposed to the magnetic core (4) An end wall at the end having an associated labyrinth-like structure comprising: - a slot (323) in one (322) of said end wall (322) and said cover (52), and - A wall formation (522) adapted to pass into said groove (323), said wall formation protruding from the other (52) of said end (322) and said cover (52). 18. Transformer according to any one of the preceding claims, characterized in that the bobbin (2) is comprised on the distal side of the secondary winding chamber (22) exposed to the magnetic core (4) An "L" shaped groove (324) at the end with an associated labyrinth-like structure comprising: - the groove (324) itself, and - an "L" shaped wall formation (524) adapted to pass into said groove (324), said wall formation protruding from said cover (52), and - The inner vertical portion of the transversely extending portion (523) exposed to said bobbin (2). 19. Transformer according to any one of the preceding claims, characterized in that the bobbin (2) has associated electrical connection pins (6) for the ends of the secondary winding (32) , and wherein the creepage and clearance distance between said ferrite core (4) and said secondary pin (6) is at least 7.2 mm. 20. Transformer according to claim 1, characterized in that the bobbin (2) exhibits additional fixing pins for fixing the bobbin (2) to a printed circuit board. 21. Transformer according to any of the preceding claims, characterized in that the cover (52) has a direction from the area between the primary chamber (21) and the secondary chamber (22) towards the transformer ( 1) External reduced height. 21. A method of assembling a transformer (1), comprising: - a winding shaft (2) defining a primary winding chamber (21) and a secondary winding chamber (22), - a magnetic core (4) mounted on said bobbin (2), - a primary winding (31) and a secondary winding (32) wound in said primary winding chamber (21) and secondary winding chamber (22), respectively, and - at least one set of contact pins (7, 6) in contact with an end of at least one of said primary winding (31) and said secondary winding (32) ( W) associated, the method comprises the following steps: - in the vicinity of said at least one set of pins (6), said winding mandrel (2) is provided, said winding mandrel (2) having a set of holes (8) formed for said primary winding (31) and the guiding configuration of the end (W) of said at least one of said secondary windings (32), - arranging said end (W) of said at least one of said primary winding (31) and said secondary winding (32) in said set of holes (8), whereby said end a portion (W) located adjacent to said at least one set of pins (6), and - soldering said ends (W) to said at least one set of pins (6), while said ends (W) are retained on said at least one set of pins (6) through said holes (8) near. 22. A method according to claim 21, characterized in that the method comprises selecting said at least one of said primary winding (31) and said secondary winding (32) from litz wire and wire braid A step of. 23. A method as claimed in claim 22, comprising the step of causing the solder used for said soldering to fill the gaps between adjacent ones of said stranded or braided wires.

Images