WO2012131350A2

WO2012131350A2 - Resonant converter

Info

Publication number: WO2012131350A2
Application number: PCT/GB2012/050667
Authority: WO
Inventors: Russell Jacques; David Robert Coulson
Original assignee: Redisem Ltd
Priority date: 2011-03-25
Filing date: 2012-03-26
Publication date: 2012-10-04
Also published as: GB201105700D0; GB201105077D0; GB2489532A; WO2012131350A3

Abstract

The invention provides a novel design of transformer having a bobbin formed in two parts which may have their windings wound separately. The design also incorporates an EMI shield formed within the bobbin to provide EMI shielding between the windings on each of the bobbin halves. The transformer may also include a divider to provide improved electrical isolation of the windings and between the windings and the magnetic core of the transformer.

Description

RESONANT CONVERTER

The invention relates to transformers and bobbins therefore, in particular for resonant power converters.

Resonant converters are typically only used for high power applications because of their high cost. However, recent developments have meant the cost of these converters means they are no longer restricted to use with high power applications and are being considered for lower power applications. This means that resonant power conversion can now be used at lower powers than before. However, there are still challenges in terms of the transformer design which leads to this new invention in power transformer construction.

In high power resonant converter applications it is typical to use an E-type transformer construction such as in Figure 1 . The primary 12 and secondary 13 windings are separated on the bobbin and insulated from each other. This type of converter has high leakage between the primary and secondary windings which can be beneficial in converters such as the LLC converter which consists of a resonant circuit using a series resonant inductor (L_Si) 20, a parallel resonant inductor (L_P) 21 and a series resonant capacitor(C_R) 22, shown in figure 2. A typical LLC resonant converter utilises the transformer to construct the series and parallel inductances. L_P is formed by the magnetising inductance of the transformer and the leakage inductance forms all or part of

In reality, the leakage inductance does not appear only in the primary side of the transformer. It is typically shared between the primary and secondary sides of the transformer and can be represented by L_Si 20 and L_S2 24. This means that the actual leakage inductance is lower than the measured leakage inductance. In a typical E-type transformer construction the leakage inductance is split equally between L_Si and L_S2-

At high loads in an LLC converter it is not important whether the leakage inductance appears in the primary or secondary side of the converter. Whether the leakage inductance is in the primary or the secondary has little effect on the operating frequency. At low loads however, L_Si and L_P are used as a voltage divider to limit the output voltage and L_S2 is relatively insignificant. If L_Si is low, then the operating frequency has to be higher to regulate the output voltage. The operating frequency can be considerably higher if Lsi becomes too low. This can be a significant problem when designing a low cost LLC converter where the leakage inductance is derived from the main transformer. It is a particular problem in LLC converters with bipolar switches where a high operating frequency is undesirable.

Leakage can be further increased in the primary winding by adding a magnetic path between the primary and secondary windings. Such a path would not couple both windings to increase the leakage inductance. This improves the leakage distribution, but it also increases the transformer complexity, which makes it less suitable for low cost, high volume designs.

A typical E-type transformer construction is shown in figure 1. This construction has an airgap 1 1 in the centre limb which increases the leakage but tends to share the leakage inductance equally between the primary 12 and secondary 13.

Normally, the dimensions and material used for the magnetic path must be chosen to ensure that core saturation does not occur. Normally, magnetic cores are designed to provide the same cross-sectional area underneath the primary and secondary windings. Referring to the transformer in figures 10a to 10c, there are three flux paths 101 , 102 and 103. Flux path 101 couples the primary winding 13 to the secondary winding 12, and is the path by which most of the power is transferred from the primary to the secondary winding. The primary leakage inductance path 102 is only coupled to the primary winding 13, and is the means by which the resonant energy of the converter is stored in the primary inductance, which is essential for most resonant converters, particularly the LLC.

Flux path 103 is only coupled to the secondary winding 12, providing energy storage for the secondary leakage inductance, which is minimal in the LLC converter. It may be apparent that the greatest flux density occurs in the core underneath the primary winding, as this carries the sum of the major flux paths 101 , 102. Consequently, in resonant converters such as the LLC converter, the cross-sectional area of the core is typically chosen to avoid saturation of the core under the primary winding, leaving the core under the secondary winding under-utilised. Typically, in an LLC converter, the peak flux density under the primary winding is some 50% greater than the flux under the secondary winding. As a result, the core area under the secondary is larger than it needs to be. Consequently the length of the turns in the secondary winding are is greater than needed. This leads to increased losses in the secondary winding and greater overall transformer cost. There is therefore a need to provide an improved transformer design to ameliorate the problems with the prior art.

The present invention therefore aims to provide a transformer assembly, in particular a transformer assembly for use in a resonant power converter, and a resonant power converter having an improved transformer assembly, the transformer assembly having a first bobbin for winding one or more first windings on, the first bobbin comprising a first hollow central portion; a second bobbin for winding one or more second windings on, the second bobbin comprising a second hollow central portion; and a U-shaped magnetic element for conducting magnetic flux, wherein said first bobbin is engageable with said second bobbin such that the first and second hollow central portions are aligned each for receiving respective limbs of said U-shaped magnetic element.

The first and second bobbins are preferably removably attachable. The first bobbin and the second bobbin preferably include corresponding alignment means for relatively positioning each other in a defined orientation. The alignment means may include attachment means for attaching the first bobbin to the second bobbin. The attachment means preferably includes one or more projections and corresponding recesses in said respective first and second bobbins. In this way, the projections can engage the respective recesses to prevent relative movement of the bobbins in at least the direction of the axes of the hollow central portions.

Preferably, the projections engage the recesses by sliding the first and second bobbins relative to each other in the first direction so insertion of the limbs of said magnetic elements into the first and second hollow central portions prevents further relative movement of said bobbins in said first direction.

Preferably, the projections further engage the recesses with a latching action or interference fit so preventing further relative movement of said bobbins in any direction. The second bobbin preferably comprises a plurality of tongue portions extending from the edges of a first wall and a second wall on the first bobbin further comprises a plurality of corresponding grooves into which the tongues may be received to engage said first bobbin with said second bobbin.

The tongues may engage with the grooves by sliding the first and second bobbins together in a first direction perpendicular to the axis of the first and second hollow central portions and wherein insertion of said limbs of said magnetic element into the first and second hollow central portions prevents further relative movement in said first direction.

The U-shaped magnetic element may form part of a magnetic core element along with a second magnetic element. The magnetic core element preferably has a generally rectangular ring shape having a length, a breadth and a height and a thickness from the outer part of the ring to the inner part of the ring. Preferably, the ratio of the length divided by the breadth is in the range 1 to 3. The ratio of the breadth divided by the thickness is preferably in the range 3 to 5. Also, the preferred ratio of the thickness divided by the height is in the range 0.5 to 5.

The second magnetic element is preferably generally U-shaped. In this way, the first and second magnetic elements, each having an end portion and first and second limbs extending therefrom, wherein the length of the limbs are such that as the magnetic elements are inserted into the first and second bobbins, the first limb of the first magnetic element abuts the first limb of the second magnetic element. This would prevent the first and second magnetic elements from being inserted further. The lengths of the limbs are such that the other pair of limbs do not meet within the other bobbin, such that a gap remains between the end of second limb of the first magnetic element and the end of the second limb of the second magnetic element.

The present invention also provides a transformer assembly comprising: a first bobbin for winding one or more first windings on, the first bobbin comprising a first hollow central portion; a second bobbin for winding one or more second windings on, the second bobbin comprising a second hollow central portion; and first and second magnetic elements for conducting magnetic flux, each having an end portion and first and second limbs extending therefrom in a generally U-shaped configuration, wherein said first bobbin and said second bobbin are arranged beside each other such that the first and second hollow central portions are aligned for receiving the first and second limbs of each of said first and second magnetic elements; and wherein the length of the limbs are such that as the magnetic elements are inserted into the first and second bobbins, the first limb of the first magnetic element abuts the first limb of the second magnetic element preventing the first and second magnetic elements from being inserted further and such that a gap remains between the end of second limb of the first magnetic element and the end of the second limb of the second magnetic element.

Preferably, a non-magnetic spacer is interposed between the ends of the first and second element to at least partially fill the gap. The spacer may be mounted on one or the other of the magnetic elements, have parts on each magnetic element or be provided as a separate part.

The transformer assembly may further comprise a divider which is engageable with at least one of the first and second bobbins such that the divider is retained between the first winding and the second winding. The divider may be engageable with the first bobbin and the second bobbin, to bring the bobbins in registration with each other such that the first and second hollow central portions are aligned for receiving the limbs of said magnetic elements.

Preferably, the divider and said one or more bobbins include one or more inter-engaging flange and slot mechanisms such that the flanges on one enter into respective slots on the other. The slots and flanges may be on the divider or bobbins respectively or the divider and bobbins may include both flanges and slots.

Preferably, the flanges are engaged with the slots by sliding the divider between the first and second bobbins in a first direction perpendicular to the axes of the first and second hollow central portions and wherein insertion of the divider prevents relative movement of the bobbins in the direction perpendicular to the first direction and the axes of the first and second hollow central portions. The transformer assembly preferably further comprises a conductive screen provided between a winding on the first bobbin and a winding on the second bobbin, the screen having a connection means for grounding the screen. The screen preferably includes a conductive layer around the windings on one of the first and the second bobbins.

The screen may include a conductive sheet positioned between the first and the second bobbins.

The connection means may comprise a pin extending from the periphery of said conductive sheet.

The screen can include a conductive coating on a surface positioned between the primary and secondary bobbins. The transformer assembly may further comprise a conductive engaging member connected to a ground point and arranged to engage the connection means to provide the grounding of the screen.

The transformer assembly may further comprise an auxiliary winding. The first winding and the auxiliary winding are wound on the first bobbin. Preferably, the wire turns of the auxiliary winding form a complete single layer on the first bobbin underneath the first winding. The layout of the winding is preferably similar to that of the secondary winding.

The transformer assembly may further comprise a sense winding. The second winding and the sense winding are wound on the second bobbin. Preferably, the wire turns of the sense winding form a complete single layer on the second bobbin underneath the second winding. The layout of the winding is preferably similar to that of the auxiliary winding.

Preferably, the ratio of magnetising inductance to leakage inductance is in the range from one to ten.

The present invention further provides a transformer assembly comprising: a first bobbin for winding one or more first windings on, the first bobbin comprising a first hollow central portion; a second bobbin for winding one or more second windings on, the second bobbin comprising a second hollow central portion; and a magnetic element for conducting magnetic flux, wherein the magnetic element has a generally rectangular ring shape having a length, a breadth and a height and a thickness from the outer part of the ring to the inner part of the ring. In this assembly, the ratio of the length divided by the breadth is in the range 1 to 3, the ratio of the breadth divided by the thickness is in the range 3 to 5; and the ratio of the thickness divided by the height is in the range 0.5 to 5.

The present invention further provides a transformer assembly comprising: a first bobbin for winding one or more first windings on, the first bobbin comprising a first hollow central portion; a second bobbin for winding one or more second windings on, the second bobbin comprising a second hollow central; a U-shaped magnetic element for conducting magnetic flux; and a divider, wherein said divider is engageable with said bobbins such that the first and second hollow central portions are aligned for receiving limbs of said U-shaped magnetic element. The transformer assembly of the present invention is particularly suited to use with resonant power converters incorporating them and provides a number of advantages over the prior art. A better way of constructing the transformer is to try and imbalance the leakage inductances such that more inductance appears on the primary side than on the secondary. Leakage between primary and secondary is primarily caused by flux not coupling to the secondary. By placing a break (gap) in the magnetic path underneath the secondary a high reluctance path is formed underneath the secondary with a comparatively lower reluctance path underneath the primary. Positioning this high reluctance path underneath the secondary means that the secondary leakage inductance also sees the high reluctance and so the secondary leakage inductance is lower. Likewise, because there is a low reluctance path underneath the primary winding the leakage inductance in the primary is higher. If the airgap is placed under the secondary winding, then this increases the leakage inductance of the primary winding, but decreases the leakage inductance in the secondary winding. Placing the airgap under the secondary winding of an E-type transformer is generally undesirable because this means the core halves 14 will not be the same shape. This can add considerably to the cost of such transformers, which are typically low value items. This also complicates construction, as the two different cores must be paired up with each other rather than using two identical core halves 14.

Changing the core geometry has many benefits for the LLC converter leakage inductance distribution. Using a rectangular shaped core with a winding on each side of the core length creates a higher leakage inductance, which is preferable in LLC converters. Figure 4 shows a schematic layout of the arrangement and dimensions of a magnetic core. Referring to figure 4, the leakage inductance is increased by decreasing the length L, which shortens the flux leakage path or by increasing the breadth B, which broadens the flux leakage path.

Unlike some converters, LLC converters may need to operate over a large range of operating frequencies. This may mean that it has to operate at higher frequencies which can lead to excessive losses in the transformer due to eddy currents and similar high frequency effects in the transformer windings. Where an LLC converter is operating from a regulated high voltage bus, the supply is better defined and less variable and so the operating frequency range can be minimised. However, with off-line LLC converters operating with supplies provided directly from the rectified input line voltage, the operating frequency range is much higher. This can lead to higher transformer losses.

LLC and resonant converter transformer constructions typically use one bobbin with windings placed adjacent to each other and use an E-type or U-type core construction. An example of an E-type construction can be seen in figure 1 . Windings with this type of bobbin tend to be very long resulting in many layers on the bobbin, which means they tend to be deep. This large number of layers can create high winding (copper) losses, particularly at high frequencies. It is for this reason that it is desirable to construct transformers with shallower windings to reduce the overall depth.

An alternative approach is to change the winding structure so that the winding area on the bobbin is longer such that the number of layers on the winding is reduced and so the winding is less deep. Consequently, the winding losses are also reduced. Using a U-type core construction, it is possible to place the windings on separate bobbins, adjacent to each other. This allows the windings to be longer than if they are arranged co-axially, as is normal with E-type cores.

In the present invention, using a pair of U-type cores (UU-core) with two bobbins can help to reduce the average length of each turn when compared to the single bobbin arrangement. This means that the windings are not as long in the new arrangement and therefore the corresponding copper losses are also reduced. Capacitive coupling between primary and secondary windings are a typical source of Electro Magnetic Interference (EMI) in a power adapter. In a standard E-type construction the windings are placed either on top of each other or adjacent to each other. This usually means that there is a large capacitance between the primary and secondary windings.

By using a UU-core two-bobbin arrangement, the primary and secondary windings are physically separated. This means that the capacitive coupling is low because of the separation of the windings. This results in a naturally lower EMI in the transformer. Capacitive coupling between each of the primary and secondary windings and the magnetic core can also be problematic. The windings are naturally quite close to the magnetic core and so there is a significant capacitance between them and the core can couple the primary and secondary. Grounding the core can mitigate this. As an alternative to grounding the magnetic core, the EMI performance may be optimised by positioning an auxiliary winding underneath the primary winding. By approximately matching the number of turns on the auxiliary winding to the number of turns on the secondary winding the common mode current flowing capacitively through the core from primary circuits to the secondary circuits can be minimised, thus minimising EMI. Alternatively, if a sense winding is wound underneath the secondary winding, then the EMI performance may be optimised by matching the number of turns on the auxiliary and sense windings.

A typical method of reducing noise coupling between primary and secondary windings is to cover the noisy part with a conductive screen. This usually means fitting a screen over the primary winding and connecting it to a benign point in the primary circuit. In an E-type transformer construction it is difficult to fit an EMI screen between the primary and secondary windings because the windings are adjacent to each other and there is little space for a screen. In a UU-core two-bobbin arrangement, fitting a screen over the primary winding is simple, as the required space is available and easily accessible.

A further advantage of using the UU-core two-bobbin arrangement is that for the same power the bobbin can be flatter than E-type transformers. This is advantageous in many applications such as LED TV's where maintaining a low component height is essential. The UU-core two-bobbin arrangement allows for a low profile transformer without any cost or performance disadvantages, which is unusual compared to most transformer systems.

As well as providing a power conversion function, one of the main functions of the transformer in a power supply is providing isolation from the dangerously high voltage by maintaining isolation between the primary and secondary windings. To create the isolation either: adequate thickness insulation material should be used; multiple layers of insulation should be used; or the primary and secondary winding should be separated by an adequate clearance distance; or the creepage along any surface should be of adequate length.

In a standard transformer the primary and secondary windings are constructed in close proximity to maintain good coupling, so it is difficult to attain an adequate clearance distance between the windings. Isolation is normally attained by using triple insulated wire, margin construction, insulating tape or a combination of these.

Using a UU-core two-bobbin arrangement, the primary and secondary windings are on physically separate bobbins. This creates an inherently safer transformer construction. However, the windings may still not be sufficiently spaced apart to meet safety standards. Isolation can be achieved by utilising triple insulated wire, margin construction, insulating tape or a combination of these or by adding a further insulating non-conductive divider between the windings to achieve the required insulation level.

Using a 2-bobbin transformer construction, it is possible to utilise a divider arrangement to attain adequate creepage distance and insulation to provide adequate insulation between the primary and secondary windings such that the safety requirement can be met without relying on wire insulation. The divider can additionally provide adequate creepage distance between one of the windings, typically the secondary winding, and the magnetic element such that the magnetic element can be grounded to a benign point for EMI reduction purposes without compromising safety creepage distances. By using a divider arrangement, both the primary and secondary windings can be made from enamelled copper wire. It is preferable to use this type of wire because of price and performance characteristics. Using a UU-core two-bobbin arrangement, production of the transformer is simpler. It is possible to manufacture the primary and secondary bobbins in different locations at different times and to assemble the two halves after they are completed. It is also possible to pre-manufacture different primary and secondary winding and to combine these in different configurations to make different transformers with different turns ratios.

A problem that can occur in closely wound transformers is that it is difficult to maintain isolation between the primary and secondary windings during high voltage surges. Using a UU-core two-bobbin arrangement, the windings are physically separate from each other. This makes it possible to make an insulation system that does not break down during high voltage surges.

The combination of features in the present invention set out to provide a transformer and power converter which overcome some of the problems associated with the prior art.

A specific embodiment of the present invention will now be described with reference to the following drawings in which:

Figure 1 shows an E-type transformer used in resonant applications;

Figure 2 shows a partial schematic of a typical resonant converter;

Figure 3a shows a plan view of a UU-core two-bobbin arrangement according to the present invention;

Figure 3b shows a cross-sectional through the UU-core two-bobbin arrangement of Figure 3a;

Figure 4 shows the dimensions of the two UU-core arrangement;

Figures 5a-5e show various views of one bobbin arrangement;

Figure 6 shows two of the bobbins of figures 5a-5e joined together, with windings on them; Figures 7a-7d show a bobbin configured to allow for an isolating divider to be provided; Figures 8a to 8e show the isolating divider used with the bobbin in figures 7a-7d;

Figures 9a-9d show a partially constructed transformer including the isolating divider and bobbin from figures 7 and 8;

Figure 9e shows a cross section through the transformer of Figures 9a to 9d;

Figures 10a to 10c show the flux paths through a transformer similar to that of Figure 1 ; and

Figures 1 1 a to 1 1 d show a modified version of the embodiment of Figure 3. Figures 3a and 3b show an exemplary embodiment of a transformer according to the present invention, comprising two bobbins 36,37 and two U-shaped magnetic elements 34a, 34b. The first bobbin 37 comprises a central portion and end walls to define an area on which a primary winding 33 is provided. A second bobbin 36 which is similar to the first bobbin has a secondary winding 32 wound on to it. It is not however necessary that the bobbins are similar. Two U-shaped magnetic elements 34a, 34b are inserted into the hollow central portions of each core for conducting magnetic flux. Figures 5a to 5e show a modified bobbin 5 of the present invention. The bobbin has a similar structure with a central hollow portion into which the magnetic core is inserted and around which the windings are provided. The bobbin also includes a base from which a plurality of connection pins project. The bobbin is also provided with a tongue 51 and a complementary groove 52. The transformer will include two similar bobbins with both bobbins including the tongue and groove portions. The tongue 51 extends from the edge of the bobbin so that it may be received in the corresponding groove on the other bobbin forming the transformer. By providing these tongues 51 and grooves 52, the bobbins can be slideably engaged with each other.

The tongues are engaged with the grooves by sliding the first and second bobbins together in a first direction perpendicular to the axis of the first and second hollow central portions. This can be seen in figure 6. In this way, insertion of the magnetic element into the first and second hollow central portions prevents further relative movement in the first direction. Although not shown in the embodiment, the tongue 51 and groove 52 may have a positive retention feature, such as a latching action or interference fit, so that once engaged, the bobbins resist relative movement, facilitating handling in production. Furthermore, the insertion of the magnetic element into the bobbin assembly causes the limbs to pass through the first and second bobbins so that they cannot move laterally relative to each other, thus preventing them from sliding apart again. Thus even if the tongue and groove arrangement does not have a positive retention feature, the magnetic element helps to retain them in position. These measures also ensure that the pins extending from the bottom of the bobbins align, facilitating easy fitment into a printed circuit board.

With this configuration, the bobbins will remain attached without the need for further attachment means during assembly and then once assembled with the magnetic elements they cannot be slid apart. Figure 6 shows the two bobbins attached to each other with the windings also shown. The windings would normally be applied to each bobbin prior to connecting the bobbins together. As before, once the bobbins are joined together into this sub-assembly, as in Figure 6, the two legs of the U-shaped magnetic elements can be inserted. They align with the first and second hollow central portions to receive the U-shaped magnetic elements. This provides a unique transformer assembly which allows the two bobbins with their windings to be produced separately and then joined together once the windings have been formed on each bobbin. The first bobbin will typically include the primary winding(s) and the second bobbin might include the secondary winding(s). Both bobbins can include sensing or auxiliary windings.

The magnetic element will normally be created from a pair of equal U-shaped core halves. The dimensions of the magnetic core elements are selected according to the operating parameters and the desired performance characteristics to optimise performance without making the transformer unduly large or degrading performance. Figure 4 shows the two magnetic elements that make up the core and the respective dimensions. The core is preferably in the proportion such that the length (L) is in the region of two times its breadth (B). It is reasonable to reduce the length to width ratio such that the length equals the breadth and also to increase the ratio such that the length is three times the breadth. The thickness (T) is preferably approximately one quarter of the breadth. However, thickness may vary, preferably down to one fifth of the breadth for lower power applications but up to one third for higher powers. The height (H) of the magnetic element shall preferably be equal to the thickness of the core although it can be reduced down to half of the thickness to create a low profile transformer or increased up to five times the thickness to create a transformer that occupies a small area on the circuit board. These dimensional preferences can be varied in order to create a peculiar transformer shape for certain applications, however this may cause degradation in performance or a higher cost. The magnetic elements together form a core which includes an airgap in the magnetic path. The elements are arranged so that the gap lies underneath the secondary winding. Figure 3 shows an assembled transformer with airgap 31 underneath the secondary winding 32. For LLC-type power converters, the ratio of magnetising inductance to leakage inductance generally ranges between one and ten to provide optimal balance of degree of control and conversion efficiency.

The embodiments shown in Figures 3a, 3b and 5a to 5e are simple embodiments of the invention. However, the bobbins may include additional features to enhance the functionality of the transformer. Figure 7 shows a bobbin arranged for use in a transformer where a divider is provided to provide isolation between the windings, usually to enhance the electrical isolation and specifically the creepage distance. Transformers need to meet certain standards defining how close the winding elements can be to each other both directly through air (clearance) but also along surfaces between them (creepage).

A transformer can be constructed either including or not including an insulating divider without compromising its performance. If the transformer is to be constructed using a divider as in the embodiment of figures 7a to 7d, the bobbins include a slot mechanism whereby a divider can be inserted into the slots to create adequate creepage distance and through insulation between the primary and secondary windings to meet the required safety isolation. In addition the divider provides adequate creepage distance and through insulation between a winding and the magnetic element. The divider may additionally be used to join the two bobbins together rather than using the tongue and groove arrangement described above.

Figures 7a to 7d show a bobbin 70 with slots 71 , 72 that are designed to accept a divider. Figures 8a to 8e show a suitable divider 81 that fits into the bobbins. A transformer assembly that includes the two bobbins, divider and magnetic elements is shown in figures 9a to 9d. The bobbin 70 includes vertical slots 71 into which the divider can be inserted from above. As the divider is slid into the bobbin, the portions 80 enter the slots 72 on the top of the bobbin.

The tongue like projection increases the effective creepage distance of the transformer. For example, to follow a path along the surfaces between the two bobbins, it is necessary to pass into the slot and back out again around the projection on the divider. Thus the creepage path is at least twice the depth of the slot. The divider also provides isolation through the air.

EMI is a significant consideration for transformers of this type and minimising EMI is a significant design consideration. A major source of EMI is caused by capacitive coupling between the primary and secondary windings. The design of this transformer is such that the windings are physically separated rather than being wound on top of each other or adjacent to each other. This helps to reduce the capacitive coupling, however EMI can still be significant.

One way to reduce this is to provide a shield or screen between the windings. This can be achieved in a number of ways. For example, a conductive winding or foil screen can be provided over the primary or secondary windings. Figures 3a, 3b shows a possible screen 35, wound over the top of the primary winding 33. This screen preferably has a connection means (not shown) for grounding the conductor to a benign EMI point in the application. The connection means provides a convenient attachment for a providing an electrical connection to the screen. This allows the screen to be grounded to minimise any EMI. The first or second bobbin has conductive pins to facilitate connection of the screen to the main converter circuit on the PCB.

In the embodiment of the design shown in figures 9a to 9e, the screen may be a conductive sheet 91 inserted between the two bobbins 70a and 70b. This screen preferably has a connection (not shown) for grounding the conductor to a benign EMI point in the application to minimise any EMI. The connection means can comprise a pin extending from the periphery of the conductive sheet for the purpose of effecting an electrical connection to the printed circuit board. In the embodiment shown in figure 9e, the conductive sheet 91 has a projection 94 extending from the conductive sheet. The projection engages with the transformer earthing clip 95 which provides a separate connection to the printed circuit board. The earthing clip 95 extends down to a pin 93 which acts like one of the connecting pins 92. In this way, the pin 93 can be inserted into the PCB along with the other pins 92 to provide a connection to earth or some other benign connection point.

Instead or in addition to the separate sheet 91 shown in Figures 9c, a shield can be provided by having a conductive coating 81 on the outer surface of the divider 80, as shown in figure 8b. The conductive coating 81 is applied to a surface of the divider 80. When the coated divider is inserted into the transformer assembly, the screen is located between the primary and secondary bobbins, thus providing effective RF shielding to minimise EMI. Preferably, the conductive coating extends such that an electrical connection is provided by an interference contact between the coating and the transformer clip. For example, referring to figure 8, the conductive coating 81 extends up to the top surface 82. The upper surface is then maintained in electrical contact with the transformer clip 95, as shown in Figure 9d. The transformer clip 95 has a compressive contact to the top 82 of the divider 80, providing an electrical connection through the clip to the printed circuit board.

An auxiliary winding (not shown) can also be provided underneath the primary winding. The auxiliary winding wire gauge is selected to achieve a single full layer on the primary- side bobbin. This helps to further optimise the EMI performance of the transformer. A sense winding (not shown) can also be provided underneath the secondary winding. The sense winding wire gauge is optimally selected to achieve a single full layer on the primary- side bobbin. This helps to further optimise the EMI performance of the transformer. It will be appreciated that the auxiliary and sense windings are not essential but aid in improving EMI performance.

Figures 1 1 a to 1 1 c show the flux paths through the transformer, similar to those shown in Figures 10a to 10c, but for a UU-core transformer similar to that shown in Figure 3. However, in the embodiment shown in Figures 1 1 a to 1 1 c, the core has a modified shape with the portion of the core under the secondary winding 32 having a smaller cross-section than that of the core under the primary 33.

There are three main flux paths 1 1 1 , 1 12, 1 13 through the core. Flux path 1 1 1 couples the primary winding 33 to the secondary winding 32, and is the path by which most of the power is transferred from the primary to the secondary winding. The primary leakage inductance path 1 12 is only coupled to the primary winding 33, and is the means by which the resonant energy of the converter is stored in the primary inductance, which is essential for most resonant converters, particularly the LLC.

Flux path 1 13 is only coupled to the secondary winding 32, providing energy storage for the secondary leakage inductance, which is as indicated above usually minimal in LLC converters. The greatest flux density occurs in the core underneath the primary winding, as this carries the sum of the major flux paths 1 1 1 , 1 12.

The dimensions and materials used for the magnetic path must be chosen to ensure that core saturation does not occur. Conventionally, the cross-sectional area of the core is uniform throughout and is selected to avoid saturation of the core under the primary winding, since it is this portion of the core which carries the greatest flux and therefore most likely to saturate. However, as mentioned above, this effectively over specifies the core under the secondary where the flux is less.

The magnetic element may therefore be shaped asymmetrically to minimise cost and achieve maximum core utilisation. Therefore in the embodiment of Figure 1 1 , the core under the secondary has a reduced cross section, which is preferably selected to take into account this reduced flux. The core pieces 1 14a, 1 14b shown in figures 1 1 a-1 1 d are shaped so that the cross-sectional area underneath the primary winding 33 is larger than the cross-sectional area underneath the secondary winding 32. This can be seen in cross- sectional view of Figure 1 1 d. This feature allows the secondary bobbin to have a smaller hollow central portion which is smaller than that of the primary bobbin, thereby reducing the winding length and associated winding losses. The cross section of the secondary can therefore be selected independently of the primary, to avoid saturation of the secondary core without over specifying it.

As a result, the core area under the secondary is smaller. Consequently the length of each of the turns in the secondary winding can be shorter. This leads to reduced losses in the secondary winding and hence better performance and reduced overall transformer cost.

Claims

CLAIMS:

1 . A resonant power converter comprising a transformer assembly having:

a first bobbin for winding one or more first windings on, the first bobbin comprising a first hollow central portion;

a second bobbin for winding one or more second windings on, the second bobbin comprising a second hollow central portion; and

a U-shaped magnetic element for conducting magnetic flux, wherein

said first bobbin is engageable with said second bobbin such that the first and second hollow central portions are aligned each for receiving respective limbs of said U- shaped magnetic element.

2. A power converter according to claim 1 wherein said first bobbin is removeably attachable to said second bobbin.

3. A power converter according to claim 2 wherein said second bobbin further comprises a plurality of tongue portions extending from a first wall, and a second wall on the first bobbin further comprises a plurality of corresponding grooves into which the tongues may be received to engage said first bobbin with said second bobbin.

4. A power converter according to claim 3 wherein said tongues are engaged with said grooves by sliding the first and second bobbins together in a first direction perpendicular to the axis of said first and second hollow central portions and wherein insertion of said limbs of said magnetic element into said first and second hollow central portions prevents further relative movement in said first direction.

5. A power converter according to any one of the preceding claims, wherein said transformer assembly further comprises a second magnetic element which together with the U-shaped magnetic element form a magnetic core element, the core element having a generally rectangular ring shape having a length, a breadth and a height and a thickness from the outer part of the ring to the inner part of the ring.

6. A power converter according to claim 5, wherein the ratio of length to breadth of the magnetic core element is in the range 1 to 3.

7. A power converter according to claim 5 or 6, wherein the ratio of breadth to thickness of the magnetic core element is in the range 3 to 5.

8. A power converter according to claim 5, 6 or 7 wherein the ratio of thickness to height of the magnetic core element is in the range 0.5 to 5.

9. A power converter according to any one of claims 5 to 8, wherein said second magnetic element is U-shaped, the first and second magnetic elements each having an end portion and first and second limbs extending therefrom, wherein

the length of the limbs are such that as the magnetic elements are inserted into the first and second bobbins, the first limb of the first magnetic element abuts the first limb of the second magnetic element preventing the first and second magnetic elements from being inserted further and such that a gap remains between the end of the second limb of the first magnetic element and the end of the second limb of the second magnetic element.

10. A power converter according to any one of the preceding claims, said transformer assembly further comprising a divider, wherein said divider is engageable with at least one of said first and second bobbins such that the divider is retained between the first winding and the second winding.

1 1 . A power converter according to claim 10 wherein said divider is engageable with said first bobbin and said second bobbin, to bring the bobbins in registration with each other such that the first and second hollow central portions are aligned for receiving the limbs of said magnetic element.

12. A power converter according to claim 10 or 1 1 , wherein said divider and said one or more bobbins include one or more inter-engaging flange and slot mechanisms such that the flanges on one enter into respective slots on the other.

13. A power converter according to claim 12, wherein said flanges are engaged with said slots by sliding the divider between said first and second bobbins in a first direction perpendicular to the axes of said first and second hollow central portions and wherein insertion of said divider prevents relative movement of the bobbins in a second direction perpendicular to said first direction and said axes of said first and second hollow central portions.

14. A power converter according to any one of the preceding claims, said transformer assembly further comprising a conductive screen provided between a winding on the first bobbin and a winding on the second bobbin, the screen having a connection means for grounding the screen.

15. A power converter according to claim 14 wherein the screen includes a conductive layer around the windings on one of the first and the second bobbins.

16. A power converter according to claim 14 wherein the screen includes a conductive sheet positioned between the first and the second bobbins.

17. A power converter according to any one of claims 14 to 16 wherein the connection means comprises a pin extending from the periphery of said conductive sheet.

18. A power converter according to claim 14 wherein the screen includes a conductive coating on a surface positioned between the primary and secondary bobbins.

19. A power converter according to any one of claims 14 to 16, said transformer assembly further comprising a conductive engaging member connected to a ground point and arranged to engage the connection means to provide said grounding of the screen.

20. A power converter according to any one of the preceding claims, said transformer assembly further comprising an auxiliary winding wherein said first winding and said auxiliary winding are wound on the first bobbin with the wire turns of the auxiliary winding forming a complete single layer on the first bobbin underneath the first winding.

21 . A power converter according to any one of the preceding claims wherein the ratio of magnetising inductance to leakage inductance of said transformer assembly is in the range from one to ten.

22. A resonant power converter comprising a transformer assembly having:

a second bobbin for winding one or more second windings on, the second bobbin comprising a second hollow central portion; and a magnetic element for conducting magnetic flux, wherein the magnetic element has a generally rectangular ring shape having a length, a breadth and a height and a thickness from the outer part of the ring to the inner part of the ring, wherein:

the ratio of length to breadth is in the range 1 to 3;

the ratio of breadth to thickness is in the range 3 to 5; and

the ratio of thickness to height is in the range 0.5 to 5.

23. A resonant power converter comprising a transformer assembly having:

a second bobbin for winding one or more second windings on, the second bobbin comprising a second hollow central portion;

a U-shaped magnetic element for conducting magnetic flux; and

a divider, wherein said divider is engageable with said bobbins such that the first and second hollow central portions are aligned for receiving limbs of said U-shaped magnetic element.

24. A resonant power converter comprising a transformer assembly having:

first and second magnetic elements for conducting magnetic flux, each having an end portion and first and second limbs extending therefrom in a generally U-shaped configuration, wherein said first bobbin and said second bobbin are arranged beside each other such that the first and second hollow central portions are aligned for receiving the first and second limbs of each of said first and second magnetic elements; and wherein