EP4009338A1 - Transformer for a switching power supply - Google Patents

Abstract

A transformer (150) for a switched-mode power supply (110) is described. The transformer (150) comprises a coil core (201) and a primary coil (211) and a secondary coil (221), each of which encloses a central region of the coil core (201). Furthermore, the transformer (150) comprises a multiply insulated shielding layer (324), which is arranged between the central area of the coil core (201) and the primary coil (211) and/or the secondary coil (221) and is electrically conductive with the primary coil (211) or the secondary coil (221) is connected.

Description

The invention relates to a transformer for a switched-mode power supply that can be used, for example, in a household appliance.

A domestic appliance, such as a washing machine or a dishwasher, typically has a switched-mode power supply with which the domestic appliance can be connected to an AC mains supply. Switching power supplies typically produce relatively large common-mode or common-mode interference, which is usually blocked by one or more filter measures (e.g. common-mode choke, Y-capacitors, common-mode capacitors, etc.). Such common-mode interference can be caused in particular by parasitic capacitive coupling between the primary and secondary coils of the transformer of the switched-mode power supply, with energy being overcoupled by parasitic capacitive coupling and being transferred into the common-mode system.

Installing filter measures in a switched-mode power supply to block common-mode interference is relatively expensive. This document deals with the technical task of preventing common-mode interference in a switched-mode power supply in a particularly efficient and reliable manner.

The object is solved by the subject matter of the independent patent claim. Advantageous embodiments are defined in particular in the dependent patent claims, described in the following description or shown in the attached drawing.

According to one aspect of the invention, a transformer for a switched-mode power supply is described. The switched-mode power supply can be designed to be connected to an electrical supply network (for example with an AC voltage of 230 V and a network frequency of 50 Hz). The switched-mode power supply can also be designed to provide a DC voltage (eg 12V) for an electrical device, in particular for a household appliance. The transformer can be used to provide galvanic isolation between the device and the supply network. Furthermore, the Transformer are used to bring about a voltage transformation from the voltage level of the supply network to the voltage level of the device.

The transformer comprises a coil core, the coil core comprising, for example, a ferromagnetic and/or a soft magnetic material (e.g. ferrite). The coil core can have the shape of the number "eight" and/or the shape of two opposite letters "E". In particular, the coil core can have a central area, for example a central leg. Furthermore, the coil core can have an outer area, in particular an outer leg, on each of two opposite sides of the central area. The central area can be connected to an outer area via transverse parts of the coil core.

In addition, the transformer may have a bobbin made of plastic, for example. The coil body can have a central hollow cylinder which encloses the central area or the central leg of the coil core. The one or more coils of the transformer may be wound around the central hollow cylinder of the bobbin. The central hollow cylinder of the coil former can have a circular, oval, rectangular or octagonal cross-section, for example. In other words, the trajectory of the hollow cylinder can be circular, oval, rectangular or octagonal.

The transformer further includes a primary coil enclosing the central portion of the coil core. In particular, the coil wire of the primary coil can be wound around the central area, in particular around the central leg, of the coil core. Furthermore, the transformer includes a secondary coil that encloses the central area of the coil core. The secondary coil can enclose the primary coil at least partially or completely. Alternatively or additionally, the primary coil can enclose the secondary coil at least partially or completely.

In particular, the transformer can be designed to transform a primary voltage applied to the primary coil to a secondary voltage applied to the secondary coil that is reduced compared to the primary voltage. In this case can the primary coil at least partially or completely encloses the secondary coil. In particular, the coil wire of the primary coil can be wound at least partially or completely around the secondary coil, in particular around the windings of the secondary coil. Alternatively, the transformer can be designed to transform the primary voltage present at the primary coil to a secondary voltage present at the secondary coil that is increased compared to the primary voltage. In this case, the secondary coil can enclose the primary coil at least partially or completely. In particular, the coil wire of the secondary coil can be wound at least partially or completely around the primary coil, in particular around the windings of the primary coil. A particularly cost-efficient transformer can thus be provided.

If appropriate, a mixed form can also exist in which the secondary coil encloses part of the turns of the primary coil and in which another part of the turns of the primary coil encloses the secondary coil.

An insulating layer can be arranged between the windings of the primary coil and the windings of the secondary coil.

The transformer can thus have a coil core with a central leg. The central leg can be arranged around the central axis of the transformer. The windings of the primary coil and/or the secondary coil can be arranged (radially) around the central axis of the transformer. The windings of the primary coil can (at least partially or all) be arranged closer to the central axis than the windings of the secondary coil. Alternatively, the windings of the secondary coil may (at least partially or all) be located closer to the central axis than the windings of the primary coil.

The transformer also includes (at least) a multiply insulated, in particular a triple insulated, shielding layer (also referred to as shielding foil), which is arranged between the central area of the coil core and the primary coil and/or the secondary coil and is electrically conductively connected to the primary coil or the secondary coil can. In particular, the transformer can have at least one multiply insulated Have shielding layer, which is arranged between the two coils, and which is electrically conductively connected to the primary coil or to the secondary coil.

The multiply insulated shielding layer can be designed to shield an electric field formed by the primary coil and/or by the secondary coil from the respective other coil. For this purpose, the multiply insulated shielding layer can have an electrically conductive central layer, in particular an electrically conductive foil.

By providing a shield layer that is insulated multiple times, in particular three times, a transformer with a relatively low parasitic capacitive coupling between the primary coil and the secondary coil can be provided in a (cost and/or installation space) efficient manner. This makes it possible to reduce or completely eliminate filter measures in a switched-mode power supply for suppressing common-mode interference, as a result of which the costs and the space required for a switched-mode power supply can be reduced.

As explained above, the multiply insulated shielding layer can have an electrically conductive central layer (e.g. a metal foil). Furthermore, the multiply insulated shielding layer can include a first insulating jacket that completely encloses the central layer. The shielding layer insulated multiple times (in particular twice or more) can also have a second insulation sheathing that completely encloses the first insulation sheathing. In addition, the multiple (particularly the triple or higher) insulated shielding layer can have a third insulating jacket which completely encloses the second insulating jacket.

The individual insulation sheaths can each be formed separately. As an alternative or in addition, the individual insulating casings can at least partially have a different structure (eg using different materials and/or material compositions) and/or can be structured differently. The use of several, in particular three, different insulating sheaths enables a particularly space-efficient suppression of the parasitic capacitive coupling between the primary coil and the secondary coil of the transformer.

The primary coil and/or the secondary coil typically each have a specific winding width of (coil wire) windings around the central axis of the transformer, in particular around the central area of the coil core. The width of the primary coil and/or the secondary coil extends in the direction of the central axis. The primary coil and the secondary coil can have (substantially) the same winding width.

The multiply insulated shielding layer can extend (essentially) over the entire winding width of the primary coil and/or the secondary coil. In other words, the multi-insulated shielding layer may have a width (in the direction of the central axis) equal to or greater than the winding width of the primary coil and/or the secondary coil. In this way, the parasitic capacitive coupling between the primary coil and the secondary coil can be reduced or eliminated in a particularly reliable manner.

As already explained above, the multiply insulated shielding layer can be arranged between the primary coil (in particular between the windings of the primary coil) and the secondary coil (in particular the windings of the secondary coil).

The multiply insulated shielding layer can be designed to essentially completely enclose the lateral surface of the primary coil formed by the windings of the primary coil or the lateral surface of the secondary coil formed by the windings of the secondary coil. The outer surface can enclose the central axis of the transformer. By completely enclosing the lateral surface of the primary coil or the secondary coil, the parasitic capacitive coupling between the primary coil and the secondary coil can be reduced or eliminated in a particularly reliable manner.

As already explained above, the coil core can have two transverse parts which, starting from the central region of the coil core, extend away from the central axis of the transformer on opposite sides of the primary coil and/or the secondary coil. The transverse parts can be arranged perpendicularly to the central axis. Furthermore, the transverse parts can connect the central area to an outer area of the coil core.

Correspondingly, the coil former can have two transverse walls which, starting from the central hollow cylinder of the coil former, extend away from the central axis of the transformer on opposite sides of the primary coil and/or the secondary coil. The transverse walls can be arranged perpendicular to the central axis.

The windings of the primary coil and/or the secondary coil can be routed directly up to at least one transverse part or both transverse parts of the coil core or directly up to at least one transverse wall or both transverse walls of the coil body. The available area for windings between the transverse parts of the coil core or between the transverse walls of the coil body can thus be fully utilized.

Alternatively or additionally, the transformer can be designed in such a way that between the primary coil and/or the secondary coil and at least one transverse part (or both transverse parts) of the coil core, apart from the insulating layer of the coil wire of the respective coil and/or apart from a wall of the coil body, no further (electrically isolating) insulation element is arranged. Alternatively or additionally, the transformer can be designed in such a way that no further (electrically insulating) insulating element is arranged between the primary coil and/or the secondary coil and at least one transverse wall (or both transverse walls) of the coil former, apart from the insulating layer of the coil wire of the respective coil . The available area for windings between the transverse parts of the coil core or between the transverse walls of the coil body can thus be fully utilized (without having to use an additional insulating element between the coil wire and a transverse part of the coil core or a transverse wall of the coil body). A particularly space-efficient transformer can thus be provided.

The multiply insulated shielding layer can touch at least one transverse part of the coil core and/or a transverse wall of the coil body, in particular both transverse parts of the coil core and/or both opposite transverse walls of the coil body. In other words, the multiply insulated shielding layer can extend from a transverse part of the coil core or from a transverse wall of the coil body along the direction of the central axis to the opposite transverse part of the coil core or extend to the opposite transverse wall of the bobbin. In this way, the parasitic capacitive coupling between the primary coil and the secondary coil can be reduced or eliminated in a particularly reliable manner.

The coil (i.e. the primary coil or the secondary coil) to which the multiply insulated shielding layer is electrically conductively connected may have a reference point which, in comparison to an AC point of the coil, has a less fluctuating, in particular a (substantially) constant, voltage potential having. The reference point can be located at a first end and the AC point can be located at an opposite second end (of the coil wire) of the coil. The reference point can, for example, be at a (constant) reference potential (about 0V). On the other hand, the potential of the AC point can fluctuate over time, e.g. between the reference potential and a certain maximum voltage.

The multiply insulated shielding layer can be electrically conductively connected to the reference point. In this way, the parasitic capacitive coupling between the primary coil and the secondary coil can be reduced or eliminated in a particularly reliable manner.

In one example, the multi-insulated shield layer is arranged between the central area of the coil core and the primary coil. Furthermore, in the example, the multiply insulated shielding layer is electrically conductively connected to the secondary coil. The arrangement of the multiply insulated shielding layer on the secondary coil allows the parasitic capacitive coupling between the primary coil and the secondary coil to be reduced or eliminated in a particularly reliable manner.

The transformer can also include an at least singly insulated shielding layer between the central area of the coil core and the primary coil. The at least simply insulated shielding layer can be electrically conductively connected to the primary coil. By using a further shielding layer, the parasitic capacitive coupling between the primary coil and the secondary coil can be further reduced.

The primary coil and/or the secondary coil can each have a coil wire insulated multiple times, in particular triple insulated. A multiply insulated coil wire can be a metallic and/or electrically conductive strand or wire have, which is enclosed by several layers of insulation. In particular, the multiply insulated coil wire can comprise an electrically conductive wire, as well as a first insulation covering that completely encloses the wire, a second insulation covering that completely encloses the first insulation covering, and possibly a third insulation covering that completely encloses the second insulation covering. The space required for the transformer can be further reduced by using a coil wire with multiple insulation.

The transformer can be constructed in such a way that the primary coil at least partially or completely encloses the secondary coil. In this case, at least the secondary coil can have a multiply insulated coil wire. Alternatively, the transformer can be designed in such a way that the secondary coil at least partially or completely encloses the primary coil. In this case, at least the primary coil can have a multiply insulated coil wire. A particularly cost-efficient transformer can thus be provided.

The primary coil can thus have a multiply insulated coil wire. In this case, the transformer can have a multiply insulated shielding layer which is electrically conductively connected to the primary coil. Alternatively or additionally, the secondary coil can have a multiply insulated coil wire. In this case, the transformer can have a multiply insulated shielding layer which is electrically conductively connected to the secondary coil. In this way, a particularly reliable reduction in the parasitic capacitive coupling of the transformer can be brought about.

According to a further aspect, a switched-mode power supply for a domestic appliance is described. The switched-mode power supply includes a transformer that is designed as described in this document. The use of the transformer described in this document enables the switched-mode power supply to not include a filter unit for suppressing common-mode and/or common-mode interference. In this way, a switched-mode power supply that is particularly cost-effective and space-efficient can be provided.

According to a further aspect, a household appliance (e.g. a washing machine, a dishwasher, an oven, a stove, a food processor, a vacuum cleaner, a dryer, a refrigerator, etc.) that includes the switched-mode power supply described in this document.

It should be noted that any aspect of the transformer or switched-mode power supply described in this document can be combined in many different ways. In particular, the features of the patent claims can be combined with one another in many different ways.

• Figur 1a ein Blockdiagramm eines beispielhaften Hausgeräts mit einem Schaltnetzteil;
• Figur 1b ein Blockdiagramm eines beispielhaften Schaltnetzteils;
• Figur 2a ein Schaltbild eines Transformators für ein Schaltnetzteil;
• Figur 2b beispielhafte Wicklungen eines Transformators;
• Figur 2c einen beispielhaften Transformator mit Abschirmungen;
• Figur 3a ein Schaltbild eines Transformators mit einer mehrfach isolierten Abschirmung;
• Figur 3b beispielhafte Wicklungen eines Transformators mit einer mehrfach isolierten Abschirmung; und
• Figur 3c einen beispielhaften Aufbau einer mehrfach isolierten Abschirmung (d.h. Schirmschicht).

The invention is described in more detail below with reference to exemplary embodiments illustrated in the attached drawing. show it

• Figure 1a a block diagram of an exemplary household appliance with a switched-mode power supply;
• Figure 1b a block diagram of an exemplary switched-mode power supply;
• Figure 2a a circuit diagram of a transformer for a switched-mode power supply;
• Figure 2b exemplary windings of a transformer;
• Figure 2c an example transformer with shields;
• Figure 3a a circuit diagram of a transformer with a multiply insulated shield;
• Figure 3b exemplary windings of a transformer with a multiply insulated shield; and
• Figure 3c an exemplary structure of a multi-insulated shielding (ie shielding layer).

As explained at the beginning, this document deals with the efficient and reliable avoidance of common-mode or common-mode interference in a switched-mode power supply. In this context shows Fig. 1a an exemplary household appliance 100, for example a washing machine, with a switched-mode power supply 110, via which the household appliance 100 can be connected to an AC power supply network 101.

Fig. 1b shows a block diagram of an exemplary switched-mode power supply 110. The switched-mode power supply 110 has a rectifier 112 at an input, which is set up to convert an AC and/or mains voltage 121 (e.g. a mains voltage of 230V with a frequency of 50Hz) provided by the supply network 101 into to convert a DC voltage (eg a DC voltage with a nominal value of 325V). At this time, a smoothing capacitor 113 may be used to smooth the rectified voltage. Furthermore, the switching power supply 110 at the input (between the Mains connection and the rectifier 112) comprise a filter unit 111 which is set up to block interference caused by the switched-mode power supply 110, in particular common-mode interference.

The switched-mode power supply 110 also includes a transformer 150, which is designed to bring about a galvanic isolation between the mains connection and the device 100 in which the switched-mode power supply 110 is installed. Furthermore, a voltage transformation from the rectified mains voltage 124 (e.g. 325V) to the operating voltage 126 (e.g. 12V) of the device 100 can be effected by the transformer 150 .

The switched-mode power supply 110 comprises a switching element 114, in particular a transistor, which is set up to generate a switched voltage 123 on the basis of the mains-side DC voltage 124, the switched voltage 123 having a frequency between 15 and 300 kHz, for example. The switched voltage 123 is then converted into a transformed (switched) voltage 125 by means of the transformer 150, the transformed (switched) voltage 125 being smaller than the switched voltage 123 by a factor of 10 or more, for example. The transformed (switched) voltage 125 can be rectified by means of a rectifier 115 in order to provide the operating voltage 126 of the device 100 .

The switching element 114 can be operated, in particular clocked and/or switched, by means of a control loop depending on the value of the generated operating voltage 126 in order to cause the value of the operating voltage 126 to have a specific desired value (e.g. 12V).

Figure 2a 12 shows a circuit diagram of an exemplary transformer 110. The transformer 110 has a primary side 210 with a primary coil 211 that is coupled to the (line-side) switched voltage 123. FIG. In this case, the AC point 212 is loaded with the maximum value of the switched voltage 123 in a pulsed manner. Furthermore, the transformer 110 has a secondary side 220 with a secondary coil 221 at which the transformed (switched) voltage 125 is provided. In this case, the AC point 222 is loaded with the maximum value of the transformed (switched) voltage 125 in a pulsed manner. A coil core 201 is arranged between the primary coil 211 and the secondary coil 221 .

Figure 2b FIG. 12 illustrates exemplary windings of transformer 150. In particular, primary coil 211 and secondary coil 221 may be wound around a common center portion of coil core 201. FIG. In this case, the primary coil 211 can lie on the inside and the secondary coil 221 can enclose the primary coil 211 . In an alternative example (not shown), the secondary coil 221 is internally disposed and is surrounded by the primary coil 211 . The coil wire of the primary coil 211 and the secondary coil 221 is electrically insulated in each case. Furthermore, an insulation layer 202 can be arranged between the primary coil 211 and the secondary coil 221 .

The primary coil 211 and/or the secondary coil 221 are typically arranged on a non-conductive bobbin 203 . The coil body 203 can have a central hollow cylinder around which the windings of the primary coil 211 and/or the secondary coil 221 are arranged. The central area of the coil core 201 can be arranged inside the hollow cylinder of the coil body 203 . The bobbin 203 may have transverse walls extending radially away from the central hollow cylinder. The windings of the primary coil 211 and/or the secondary coil 221 can be arranged between the transverse walls of the bobbin 203 .

In order to avoid or extend a creepage distance between the windings of the primary coil 211 and the windings of the secondary coil 221, there is typically a respective (Electrically insulating) insulation element 213, 223 arranged. As a result, the windings of the coils 211, 221 cannot be brought up to the transverse part of the coil core 201 running transversely to the longitudinal or central axis of the coils 211, 221 or up to the transverse walls of the coil body 203, so that the required installation space of the Transformer 150 is increased.

As explained at the outset, the common-mode interference in the switched-mode power supply 110 can be caused in particular by a capacitive coupling between the primary coil 211 and the secondary coil 221 . To reduce capacitive coupling One or more shielding layers 214, 224, in particular shielding foils, can be arranged between the primary coil 211 and the secondary coil 221. In particular, a primary-side shielding layer 214 and a secondary-side shielding layer 224 can be used, as exemplified in FIG Figure 2c shown.

A particularly reliable reduction or avoidance of the capacitive coupling between the primary coil 211 and the secondary coil 221 can, as in Figure 3a shown, can be brought about by the use of at least one shielding layer 324 that is (electrically) insulated several times, in particular three times. The use of at least one multiply insulated shielding layer 324 also makes it possible (as exemplified in Figure 3b shown), to dispense with insulation elements 213, 223 between the windings of the coils 211, 221 and a transverse part of the coil core 201 or a transverse wall of the coil former 203, so that the windings reach the transverse part of the coil core 201 or up to the transverse wall of the Bobbin 203 can be introduced.

A shielding layer 214, 234 for a coil 211, 221 can be electrically conductively connected to a reference potential of the respective coil 211, 221, as exemplified in FIG Figure 3a shown. The reference potential can be the potential at the respective coil 211, 221, which has the lowest possible AC (alternating current) or alternating component. The reference potential can in particular be present at the opposite reference point 215, 225 to the AC point 212, 222 with the maximum value of the switched voltage 123, 125 of the respective coil 211, 221. The primary-side shielding layer 214 can thus be coupled to the primary-side reference point 215 . The secondary side shield layer 324 may be coupled to the secondary side reference point 225 .

in the in Figure 3a In the example shown, the secondary-side shielding layer 324 is designed as a shielding layer that is insulated multiple times, in particular as a triple insulated one. The shielding layer 214 on the primary side is designed as a single-insulated shielding layer. The use of a single multi-insulated shielding layer 324 on the secondary side 220 is typically sufficient to bring about a reliable reduction or avoidance of the capacitive coupling between the secondary coil 221 and the primary coil 211 .

3c shows an exemplary structure of a multiple, in particular triple, (electrically) insulated shielding layer 324. The shielding layer 324 comprises an electrically conductive and/or metallic central layer 300. The central layer 300 is covered by a plurality of, in particular by three, non-conductive insulation coatings 301, 302 , 303 surrounded. In this case, the first insulating casing 301 encloses the central layer 300. The second insulating casing 302 encloses the first insulating casing 301 with the central layer 300. Furthermore, a third insulating casing 303 can enclose the second insulating casing 302. By using a multiply insulated shielding layer 324, the capacitive coupling between the secondary coil 221 and the primary coil 211 can be avoided or reduced in a particularly space-efficient manner. As a result of this, the installation of a filter unit 111 in a switched-mode power supply 110 may be dispensed with.

There is thus described a switched mode power supply transformer 150 having two foil shields (i.e. shield layers) 214, 324 to avoid common mode interference. In this case, at least one shielding film 324, preferably the shielding film 324 on the secondary side 220, is designed as a multiple, in particular triple, insulated shield. The shielding foil 214 on the primary side can be connected to the intermediate circuit at a reference point 215 with the lowest possible AC load (e.g. 0V or + intermediate circuit voltage). The shielding foil 324 on the secondary side can be connected to a reference point 225 with a potential of 0V or the output voltage on the secondary side.

The capacitive coupling between the primary circuit 210 and the secondary circuit 220 of the transformer 150 can be prevented by using the shielding foils 214, 324, and no common-mode interference arises as a result. The two shielding foils 214, 324 are preferably designed in such a way that the shielding foils 214, 324 fill the winding width of the respective coil 211, 221 as completely as possible.

The measures described in this document make it possible (since common-mode interference is no longer generated) to one or more filter elements 111, such as a common-mode choke, a Y-capacitor and/or a common-mode capacitor , partially or completely, whereby the costs and / or the space of a switched-mode power supply 110 can be reduced. Furthermore, through the The use of a multiply insulated shielding layer 324 means that specified insulation values of the transformer 150 (possibly by law) can be met in an efficient and reliable manner.

The present invention is not limited to the exemplary embodiments shown. In particular, it should be noted that the description and the figures are only intended to illustrate the principle of the proposed transformer and/or switched-mode power supply.

Claims (15)

Transformer (150) for a switched-mode power supply (110); wherein the transformer (150) comprises - a coil core (201); - A primary coil (211) and a secondary coil (221), each enclosing a central region of the coil core (201); and - A multiply insulated shielding layer (324), which is arranged between the central area of the coil core (201) and the primary coil (211) and/or the secondary coil (221) and is electrically conductively connected to the primary coil (211) or the secondary coil (221). . The transformer (150) of claim 1, wherein the multi-insulated shield layer (324) comprises - an electrically conductive central layer (300); - a first insulating coating (301) completely enclosing the central layer (300); - a second insulating jacket (302) which completely encloses the first insulating jacket (301); and - In particular, a third insulation jacket (303) which completely encloses the second insulation jacket (302). Transformer (150) according to any one of the preceding claims, wherein - the primary coil (211) and/or the secondary coil (221) each have a winding width of windings around a central axis of the transformer (150); and - The multiply insulated shielding layer (324) extends essentially over the entire winding width of the primary coil (211) and/or the secondary coil (221). Transformer (150) according to any one of the preceding claims, wherein - the multiply insulated shielding layer (324) is arranged between the primary coil (211) and the secondary coil (221); and - the multiply insulated shielding layer (324) is designed to essentially completely enclose a lateral surface of the primary coil (211) formed by windings of the primary coil (211) or a lateral surface of the secondary coil (221) formed by windings of the secondary coil (221). Transformer (150) according to any one of the preceding claims, wherein - the coil core (201) has two transverse parts which, starting from the central region of the coil core (201), extend away from a central axis of the transformer (150) on opposite sides of the primary coil (211) and/or the secondary coil (221); and - between the primary coil (211) and/or the secondary coil (221) and at least one transverse part of the coil core (201), apart from an insulating layer of a coil wire of the respective coil (211, 221) and/or apart from a transverse wall of a coil body (203 ), no further insulation element (213, 223) is arranged. Transformer (150) according to any one of the preceding claims, wherein - the coil (211, 221), to which the multiply insulated shielding layer (324) is electrically conductively connected, has a reference point (215, 225) which, in comparison to an AC point (212, 222) of the coil (211, 221) has a less fluctuating, in particular a substantially constant, voltage potential; - The reference point (215, 225) is arranged in particular at a first end and the AC point (212, 222) at an opposite second end of the coil (211, 221); and - The multiply insulated shielding layer (324) is electrically conductively connected to the reference point (215, 225). Transformer (150) according to any one of the preceding claims, wherein - The multiply insulated shielding layer (324) is arranged between the central area of the coil core (201) and the primary coil (211); and or - The multiply insulated shielding layer (324) is electrically conductively connected to the secondary coil (221). The transformer (150) of claim 7, wherein - The transformer (150) comprises an at least single-insulated shielding layer (214) between the central area of the coil core (201) and the primary coil (211); and - The shielding layer (214), which is insulated at least once, is electrically conductively connected to the primary coil (221). Transformer (150) according to one of the preceding claims, in which the primary coil (211) and/or the secondary coil (221) each have a multiply insulated coil wire. The transformer (150) of claim 9, wherein the multiply insulated coil wire comprises - an electrically conductive wire; - a first insulation covering completely enclosing the wire; - a second insulating jacket completely enclosing the first insulating jacket; and - In particular, a third insulation jacket which completely encloses the second insulation jacket. Transformer (150) according to any one of the preceding claims, wherein - the primary coil (211) has a multiply insulated coil wire, and - The transformer (150) has a multiply insulated shielding layer (214) which is electrically conductively connected to the primary coil (211); and or - the secondary coil (211) has a multiply insulated coil wire, and - The transformer (150) has a multiply insulated shielding layer (324) which is electrically conductively connected to the secondary coil (211). Transformer (150) according to any one of the preceding claims, wherein - The primary coil (211) encloses the secondary coil (221) at least partially or completely; and - The secondary coil (221) has a multiply insulated coil wire; or - The secondary coil (221) encloses the primary coil (211) at least partially or completely; and - The primary coil (211) has a multiply insulated coil wire. Transformer (150) according to any one of the preceding claims, wherein - the transformer (150) is designed to transform a primary voltage applied to the primary coil (211) to a secondary voltage applied to the secondary coil (221) which is reduced compared to the primary voltage; and - The primary coil (211) encloses the secondary coil (221) at least partially or completely; or - the transformer (150) is designed to transform a primary voltage applied to the primary coil (211) to a secondary voltage applied to the secondary coil (221) which is increased compared to the primary voltage; and - The secondary coil (221) encloses the primary coil (211) at least partially or completely. Switching power supply (110) for a household appliance (100); wherein the switched-mode power supply (110) comprises a transformer (150) according to any one of the preceding claims. The switched-mode power supply (110) according to claim 14, wherein the switched-mode power supply (110) does not include a filter unit (111) for suppressing common-mode and/or common-mode interference.

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