EP0293617B1 - High-frequency power transmitter - Google Patents

Description

The invention relates to a high-frequency power transformer for switching power supplies.

Switched-mode power supplies are known to be used for the energy supply of electrical devices, such as personal computers, video monitors and the like, in which usually several different consumers have to be supplied with different voltages, in particular direct voltages. In order to keep the size of these switching power supplies as small as possible, the highest possible frequencies are used to operate these power supplies. So-called clocked power supplies have been found to be particularly advantageous, but increasing the operating frequency usually results in an increase in switching losses in the rectifier components, which can be associated with a reduction in efficiency.

This problem can usually be overcome by using sinusoidal currents and voltages in order to avoid the disadvantage of high rates of rise in current and voltage. In this way it is possible to increase the operating frequency up to the MHz range. The switching power supplies can be operated in series or parallel resonance. A particular problem for the switching power supplies working according to the resonance principle is that a defined leakage inductance must be maintained in the power transformers.

In a high-frequency power transformer known from DE-OS 35 42 103, the problem of leakage inductance is solved in that one of the middle legs of two symmetrically arranged ferrite pot cores is shortened accordingly. The shortening of the middle leg - due to the resulting air gap - has a very large leakage inductance. This measure is intended to ensure that the output voltages and the resonance frequency of the circuit remain almost constant when the load changes. Here, the primary winding and secondary winding created in conventional winding technology are each arranged on a middle leg of a pot core half. The leakage inductance can only be changed by changing the air gap or by changing the length of the middle leg. When used in switching power supplies, the inductive part of the resonant circuit is implemented by the transformer itself.

The article "Power Transformer Design for 1 MHz Resonant Converter" in the magazine HFPC Proceedings, May 1986, pages 36 to 54, makes a high-frequency power transformer known that has a type of sandwich design. As shown in FIGS. 1 and 2 of this article, two ferrite cores are used in the known power transformer, the core shown in FIG. 1 having an EI combination and the core shown in FIG. 2 having an EE combination. As can be seen from Figure 2 of this publication, the primary windings on a dielectric base material are spirally arranged conductor tracks.

The secondary windings consist of stacked circuit boards with insulators in between. To ensure galvanic isolation between the primary and secondary side, a specially designed coil former was provided. To reduce both the leakage inductance and the space requirement, the transmitter is integrated into the circuit board of the switching power supply. A disadvantage of this embodiment is that copper-coated boards are used as carrier material for the primary winding, which only allow a limited height of the conductor tracks.

Since minimum distances must be maintained between the individual spirally arranged windings, only a relatively small copper cross section is available for the primary current. If a higher output power is required, this requires larger copper cross sections on the primary and secondary side, which can only be accommodated on the primary side with a larger number of circuit boards and thus a larger design. It has been shown that the losses of the power transformers used at high frequencies arise essentially through current displacement effects (skin effect, proximity effect) in the primary winding. These effects cannot be minimized with the given winding structure. A further disadvantage arises from the board-integrated structure, since - as can be seen in Figure 1 - the power transformer must be assembled directly on the board, which means higher assembly costs. Furthermore, a change in the leakage inductance in an already existing design is not readily possible.

From DE-OS 26 45 536, which forms the closest prior art, a broadband high-frequency power transformer in sandwich construction is known, which is intended for use in a device for inductive heating. The transformer should have very low leakage losses. With a core made of two ferrite parts, scatter losses that are practically zero can be achieved. A sandwich-type transformer is also known from "Patent Abstracts of Japan", Volume 10, No. 217 (E-423) (2273), JP-A-61 54 607.

The invention has for its object to provide a high-frequency power transmitter for switching power supplies working on the resonance principle, with which not only a particularly small but also a precisely defined leakage inductance can be achieved. This object is achieved by a high-frequency power transformer with the features of claim 1. Further details and features of the invention emerge from the following description and the subclaims.

Fig. 1

die explosionsartig auseinandergezogenen Teile eines Hochfrequenz-Leistungsübertragers,

Fig. 2

verschiedene Wicklungskombinationen,

Fig. 3

die Abhängigkeit der Streuinduktivität L vom Abstand a und

Fig. 4

die Schnittansicht eines in Fig. 2b schematisch dargestellten Hochfrequenz-Leistungsübertragers.

Fig. 5

die Schnittansicht eines schematisch dargestellten Hochfrequenz-Leistungsübertragers mit verkürztem Spulenträger.

The invention is explained in more detail with reference to the drawing, in which several exemplary embodiments are shown; show it:

Fig. 1

the exploded parts of a high-frequency power transformer,

Fig. 2

different winding combinations,

Fig. 3

the dependence of the leakage inductance L on the distance a and

Fig. 4

the sectional view of a high-frequency power transformer shown schematically in Fig. 2b.

Fig. 5

the sectional view of a schematically illustrated high-frequency power transformer with a shortened coil carrier.

Fig. 1 shows the mechanical structure of the high-frequency power transformer according to the invention, which is constructed according to the sandwich principle. The illustrated embodiment illustrates one of the many possibilities for nesting the individual windings with one another, with a particularly small leakage inductance being achieved in this form. Additional insulation of the cable leads, especially of the secondary winding parts, has been omitted because it is easier to present.

With 1 the middle leg of a soft magnetic core part 2 is designated, which together with a core part 3 with middle leg 4 forms the actual core of the power transformer. The core parts 2 and 3 are of the E-type, the middle legs 1 and 4 being round and the spaces 5 and 6 accommodating the coils in the region of the core parts 2 and 3 being correspondingly cylindrical. The cylindrical shape of the middle leg is particularly advantageous, since the components to be stacked can be annular, which considerably simplifies their manufacture. However, it is understandable that core parts can also be used in which the central limbs can be square or rectangular in cross-section, with appropriate design of the spaces 5 and 6 surrounding them.

On the middle leg 1, a sleeve-shaped coil carrier 7 can be pushed on, which has a flange 8 with which it is supported against the core part 2. The winding parts 9 of the primary windings or the bobbins 10 carrying them and the winding parts 12 of the secondary windings as well as insulation disks 13 in between can be stacked on the coil carrier 7. In the exemplary embodiment according to FIG. 1, the coil formers 10 for the winding parts 9 of the primary winding are of circular design, the winding 9 being inserted in a preferably U-shaped cross section.

Fig. 1 shows a particularly advantageous embodiment of the secondary winding parts 12, which are designed as flat, single-winded, annular stamped parts with terminal lugs 12a and 12b. The terminal lugs 12a and 12b are normally led out in isolation. It is pointed out that a plurality of winding parts 12 of the secondary winding can be connected in series, in which case connecting lugs 12b which belong together are then electrically connected to one another. This connection can be routed to the outside and then provides a center tap of the winding parts connected in series.

After the winding parts 9 or their bobbins 10 as well as the winding parts 12 and the insulation washers 13 are placed appropriately on the bobbin 7, the bobbin 7 is pushed onto the middle leg 1 of the core part 2. Then the core part is from above 3 placed, the middle leg 4 engages in the interior of the sleeve-shaped coil carrier 7, whereby all parts are secured to each other. The core parts 2 and 3 can be pressed against one another with the aid of resilient clamps, which are not shown. The connecting lugs 12a and 12b and the connections of the winding parts 9 are led out through the lateral recesses in the core parts 2 and 3.

According to the invention, with the basic structure shown in FIG. 1, a defined leakage inductance can be set with the insulation disks 13 between the primary winding parts 9 and the secondary winding parts 12, specifically by the number of insulation disks and their thickness.

2 shows some of the basic winding combinations with which a large leakage inductance range can be covered depending on the distance a. 12 again shows the secondary winding parts and 9 the primary winding parts. The thin lines 13 represent insulating washers or insulating spacing washers.

2a, a very small, fixed leakage inductance can be achieved with good coupling. The arrangement according to FIG. 2b allows small stray inductance values to be easily adjusted, in contrast to the exemplary embodiment according to FIG. 2c, with which relatively large stray inductances can be predetermined. FIG. 2d shows an embodiment according to FIG. 2a, but with additional primary and secondary winding parts.

3 shows the relationship between the leakage inductance L and the distance a, generated by the spacer insulation washers 13, for different winding combinations detect. With increasing distance a, there is inevitably a slight deterioration in the coupling factor between the primary and secondary winding parts, but this has a negligible influence on the transferability of the power. The embodiment according to FIG. 2d further shows that an adjustable leakage inductance can also be realized with more than two primary part windings.

A division of the primary winding into several partial windings on several bobbins has the advantage that smaller current displacement effects (copper losses) than in the exemplary embodiments according to FIGS. 2a to 2c can be achieved due to the better spatial distribution of the windings. In addition to solid copper wires, it is also possible to use HF strands with any cross-section and number of strands on the individual bobbins. This applies equally to the primary winding as well as to the secondary windings. In the exemplary embodiment, solid copper sheets are used on the secondary side, which can be replaced at higher frequencies to minimize the current displacement effects by several thin copper sheets which are insulated from one another.

The different output powers of the transformers required in the switching power supply applications require different copper cross sections of the windings. This is easily possible with the winding principle according to the invention. The examples shown are designed with four secondary windings, which can be connected as required according to the respective requirements, such as output voltage and output current. The number of winding parts can be increased or reduced at any time.

FIG. 4 shows a sectional drawing of the high-frequency power transformer, which is constructed in accordance with the winding combination according to FIG. 2b. The after the The clearance and creepage distances required by VDE guidelines can be easily complied with if the primary winding diameter and the secondary copper sheet diameter are selected accordingly. Insulation between the primary winding parts 9 and the secondary winding parts 12 can be dispensed with in this particular embodiment, since the coil carriers 10 are pushed over the coil carrier 7. The lead wires for the primary and secondary side are led out by a spatial separation of 180 °. The core shapes with a round middle leg, such as the RM, PM and ETD types, are best suited for this.

Fig. 5 shows a sectional drawing of the high-frequency power transformer with a shortened common coil carrier (7, 8). In this embodiment, the common coil carrier (7, 8) envelops only a part of the middle leg (1, 4) in the assembled state and a further coil former (11) is plugged directly onto the part of the middle leg (4) that has been left free. This embodiment can be used particularly advantageously if one of the windings - in the example shown the primary winding (9) - requires a high number of turns, or a winding with a larger conductor cross section is required.

The use of simple stamped parts for the secondary windings, spacer insulation washers and insulation washers, as well as uncomplicated injection molded parts for the coil formers enables cost-effective production. Depending on the application, the individual components can be assembled by simply stacking them. From a manufacturing point of view, this leads to flexible automation of high-frequency power transformers. The components manufactured in this way can be cast in order to achieve higher mechanical strength and better network insulation. Installation in the printed circuits happens in the usual technology, but particularly small leakage inductance values can be achieved if the rectifier diodes are connected directly to the secondary-side connecting plates and not via the conductor tracks of the circuit boards.

Claims (7)

1. A high-frequency power transformer comprising

   - soft-magnetic core components (2, 3) provided with at least one central limb (1),

   - primary- and secondary windings (9, 12) which can be constructed in the form of a plurality of sub-windings,

   - a tubular coil carrier (7, 8) which is positioned on the central limb (1) and on which the primary- and/or secondary windings (9, 12), or parts thereof, can be stacked and can be fixed and retained following the assembly of the core components (2, 3),

   - insulating-spacing washers (13), which have the form of circular rings and are stacked on the central limb (1) and the tubular coil carrier (7, 8), between the primary- and secondary windings (9, 12) and where applicable between the sub-windings of the primary- and/or secondary windings (9, 12),
   characterised in that

   - the high-frequency power transformer is a high-frequency power transformer of a switch-mode power supply unit operating in accordance with the resonance principle, wherein the inductive part of the oscillating circuit is formed by the power transformer itself and that

   - the defined leakage inductance, which is provided for the oscillating circuit and which differs from zero, is adjusted by means of the thickness and the number of the insulating-spacing washers (13) between the primary- and secondary windings (9, 12).
2. A high-frequency power transformer as claimed in Claim 1, characterised in that the winding components (9) are accommodated on coil bodies (10) composed of insulating material which have the form of circular rings and preferably are U-shaped in cross-section.
3. A high-frequency power transformer as claimed in Claims 1 and 2, characterised in that individual winding components (12) are designed as flat, single-turn punched parts which have the form of circular rings and are provided with terminal lugs (12a, 12b).
4. A high-frequency power transformer as claimed in Claim 3, characterised in that a plurality of punched parts are combined to form winding components (12) comprising a plurality of turns and/or central tappings.
5. A high-frequency power transformer as claimed in one of Claims 1 to 4, characterised in that the tubular coil carrier (7, 8), which is common to all the winding components (9, 12), can be positioned on the central limb (1) of the first core component (2) in such manner that in the assembled state it also encases and centres the central limb (4) of the other core component.
6. A high-frequency power transformer as claimed in one of Claims 1 to 4, characterised in that the tubular coil carrier (7, 8), which is common to all the winding components (9, 12), can be positioned on the central limb (1) of the first core component (2) in such manner that in the assembled state it encases only a part of the central limbs (1, 4), and that at least one coil body (11) can be positioned directly onto the exposed part of the central limbs (1, 4).
7. A high-frequency power transformer as claimed in Claims 5 or 6, characterised in that the coil bodies (10) of the individual winding components (9) and/or the tubular coil carrier (7, 8) are produced as injection moulded parts.

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