DE102005006175A1 - Switching power supply with independent of the operating state of the power unit supply of facilities - Google Patents

Abstract

The invention relates to a switched-mode power supply, comprising a transformer with at least one secondary winding (5) which can be connected to a load via a rectifier arrangement, and at least one primary winding (4) which has a first winding terminal which has a switching element (2) A terminal for a reference potential is connected, and a further terminal which can be connected to a DC voltage (U1) and a control device (6) for controlling the switch-on and switch-off times of the switching element (2), DOLLAR A being used for the independent supply of devices ( 7) at least one auxiliary secondary winding (17) of the transformer is provided which can be connected via an auxiliary rectifier arrangement to the devices to be supplied (7), and a clocking auxiliary device (19) for generating a pulsed current (I2) from a DC power source and at least one Hilfssprimärwicklung (15) of the transformer connected to the clocked DC power source is connected. The advantage thus achieved consists in particular in utilizing the transformer core (3) present in the switched-mode power supply also for the transmission of the auxiliary power for the supply of secondary equipment (7), without influencing the function of the main supply.

Description

The The invention relates to a switching power supply consisting of a transformer with at least one secondary winding, the above a rectifier arrangement can be connected to a load and at least one primary winding, which has a first winding connection, via a Switching element connected to a terminal for a reference potential is and another connection, which is connected to a DC voltage, and a control device for controlling the on and off times of the switching element. The invention further relates to a method for operating such a switching power supply.

Switching Power Supplies are well known. One knows such ones, which after the river converter principle work and those that work as a flyback principle. In the latter is during a Einspeicherphase by a through the primary winding a transformer flowing Electricity stored magnetic energy. This will be during a Discharge phase after rectification and smoothing to a secondary side connected load delivered. A switching element that is in series with the primary winding of the transformer, is load-dependent by a control device. and off.

at Switching power supplies with high requirements in terms of efficiency, thermal load and failure safety and telecontrol It is increasingly necessary, additional, mostly secondary-sided Provide control electronics, regardless of the current main output voltage of the switching power supply must be powered. clocked Power supplies have the property that their transmitted Leistungsmomentanwert depends directly on the duty cycle. Should the power supply be temporary be operated without main output voltage, which is e.g. at remote switching off or overload occurs at the output, reduce the turn-on of the switching element almost to zero. On the secondary side the transformer located electronic components can then only using more expensive Circuits continue to be supplied.

To It is therefore known in the prior art to provide a separate auxiliary supply to provide with its own auxiliary transformer. This auxiliary transformer must have the same safety distances between the turns the windings have the same as the main transformer. This is the size of this Auxiliary transformer not from the transferred - usually low - performance dependent, but from the creepage distances. It inevitably results relatively size Constructions with corresponding space requirements on the layout and related high costs.

Of It is also known, an additional secondary winding provide on the transformer and so the stray inductances in Flyback converter or the smoothing effect to use the output choke on the flux and push-pull converter, in order to be able to derive an auxiliary voltage even in the event of a short circuit. at Remote switching, however, it is necessary, one switchable base load at the output to activate. In this standby mode Therefore, the power unit is operated with a very low duty cycle. Due to the base load, the output voltage is loaded almost to zero, while still energy for the supply of the control electronics on the secondary side via the additional secondary winding can be disconnected. With modern switching power supplies, it is often required, the secondary-side Control electronics already supply before the power unit to starts to clock. Therefore this type of auxiliary supply generation is not suitable.

Of the The present invention is therefore based on the object for secondary side Devices of a switching power supply one of the operating state of the Power unit independent Supply voltage without an additional Provide auxiliary transformer.

The solution the task is carried out by means of a switching power supply according to claim 1. The advantage thus achieved is especially in the switching power supply existing transformer core also for the transmission of auxiliary power to supply additional, mostly secondary-sided To use facilities without doing the function of main supply to influence. On the other hand, the thus implemented auxiliary power transmission not influenced by the function of the main supply. The auxiliary power transmission can also do this from the secondary side to the primary page of the power unit. Applications for this transfer direction can be Need to be low auxiliary voltages on the primary side of the power unit (for example for charging batteries). For switching power supplies that are idle to be able to require a base load at the output of the power unit by retransmission an auxiliary power over the auxiliary windings on the primary side of the power unit (for example to power the control equipment) an additional internal base load omitted.

With the switching power supply according to the invention with independent transmission of the auxiliary power and a simple standby operation of devices (eg TV sets) can be realized. In this case, the power unit can remain switched off in standby mode and the required secondary side auxiliary power is transmitted exclusively via the auxiliary windings.

at an advantageous embodiment of the invention is provided that the transformer has a three-legged closed core and the primary winding as well as the secondary winding in Middle thighs are arranged and the auxiliary primary winding as well as the auxiliary secondary winding on an outer thigh are arranged. The middle leg can have an air gap.

Advantageous is it, the auxiliary primary development to two auxiliary primary windings connected in series and the auxiliary secondary winding split up into two series of auxiliary secondary windings and one each of these auxiliary primary windings and one of these auxiliary secondary developments on a common outer thigh of the core next to each other. The distance between the auxiliary primary winding and the auxiliary secondary development an outer thigh is under consideration minimized creepage distances, resulting in a good coupling with low transmission losses the auxiliary power is reached.

By this arrangement comes for it the series connection of the auxiliary windings to a complete compensation the induced voltages of the power section with those at the center leg attached windings. The associated additional thermal load the core is very low, because the transmitted auxiliary power only a fraction of the main power (e.g., 1%).

to Provision of a supply voltage for the control device for Control of the on and off times of the switching element in the primary circuit of Power unit, it is recommended to use a buck converter, the rectified AC voltage of the DC link in Power section on one for the supply voltage reduces required value.

These Step-down converters are circuits, which are usually a high-voltage switching transistor integrated and according to the buck converter principle from the DC link (e.g., 320 VDC at 230 VAC mains voltage) provides a low voltage floating voltage from e.g. Generate 15 V against the intermediate circuit ground. For this they need ICs only an additional throttle and also usually not integrated freewheeling diode. By this circuit is the primary one Supply voltage from the presence of DC link voltage to disposal; this also independent from the duty cycle of the power unit and whether it is currently switched off or not.

By the big one Input voltage range, which a buck converter conceptually may be the primary Supply voltage already at values of the DC link voltage be ensured where the power transfer is not working yet can. Therefore, when the minimum operating voltage is reached of the power unit, the supply voltage is already present. These Circuits usually have an output power of about 3-5 W.

In Similarly, it is also advantageous to supply voltage for the Auxiliary primary circuit by means of buck converter from the rectified AC voltage to provide the DC link in the power section. It can in both cases the same buck converter are used, which simplifies of the layout and cost reduction leads. It is a fixed, independent and reliable supply voltage available.

A preferred embodiment of the invention is that the clocking Auxiliary device on the primary side a low voltage switching transistor, wherein its gate connected to the power output of a frequency generator and wherein the auxiliary primary windings with a winding connection over the low voltage switching transistor to the reference potential and with connected to the further connection to the supply voltage are. Here are the auxiliary secondary windings and the auxiliary primary windings wrapped in the same direction and those to the auxiliary secondary windings connected auxiliary rectifier arrangement is as a one-way rectifier circuit with a smoothing Capacitor formed.

Of the Frequency generator is thereby from the above-described primary supply voltage fed. The low voltage switching transistor then switches the two Auxiliary primary windings in time with the frequency generator to the primary supply voltage. The transfer The auxiliary power is carried out according to the principle of a flyback converter, the no smoothing choke needed. The output voltage of the auxiliary power transmission according to the invention becomes like this certainly. On the one hand, the duty cycle of the primary auxiliary switching element set by the frequency generator to a fixed value. The Output voltage is then determined by the amount of primary supply voltage as well the turn ratio between the auxiliary primary windings and the auxiliary secondary developments and determined by the load on the output of the auxiliary supply. To catch the fluctuations while a longitudinal regulator can be followed.

The number of turns of the auxiliary windings is chosen so that on the one hand by a small number of turns, the clock frequency of Hilfsversor On the other hand, the undesirable increase in temperature of the core by a sufficient number of turns (eg, four turns on the primary and on the secondary side) is limited to a multiple (eg, 6 times) the clock frequency of the power unit.

It is also appropriate, the Current through the output of the frequency generator with a current detector circuit to capture and if exceeded set limit values to switch off the output. This is before especially when charging the smoothing capacitor on the secondary side important.

A other expression The invention provides, the clocking auxiliary device as a frequency generator form, wherein the power output via a pair of transistors and a coupling capacitor with a winding connection in series switched auxiliary primary windings connected is. The further winding connection of the auxiliary primary windings is then connected to the reference potential. The auxiliary secondary windings and the auxiliary primary windings are wound in the same direction and those to the auxiliary secondary windings connected auxiliary rectifier arrangement is as a two-way rectifier with a smoothing Capacitor formed.

Of the Frequency generator can be designed as a switching power supply drive IC, the one by the primary Supply voltage is fed. The gate drive signal of ICs are amplified with the transistor pair and passed through the coupling capacitor, thereby a push-pull voltage is generated, which has no DC component.

The transfer the auxiliary power is carried out according to the principle of a push-pull flow converter, which needs no smoothing chokes because of the peak charge. The Output voltage is determined by the level of the primary supply voltage as well the turns ratio between Auxiliary primary windings and auxiliary secondary developments. In this circuit, the duty cycle between primary power and auxiliary circuit as well the load at the output of the auxiliary supply only a subordinate Influence on the output voltage. To catch the slight fluctuations can also be a longitudinal regulator here be followed.

to simple installation of the auxiliary windings provides the invention, the Form auxiliary windings as plastic-molded parts. Thus, the necessary safety insulation to the windings of the Achieved power part. Furthermore, by this construction the Isolation specification between auxiliary primary and auxiliary secondary development easier fulfilled become. The interconnection of the respective auxiliary windings in series takes place on a printed circuit board.

A further execution For easy installation of the auxiliary windings provides, the auxiliary primary windings and the auxiliary secondary windings as well as the connecting lines between the series-connected Auxiliary windings as a uniform plastic-molded part train. The overlaps the turns and connecting conductors as well as the connections of the Auxiliary primary windings and secondary windings are arranged on the side facing away from the core, outer side to the distance of the auxiliary windings to the windings of the power unit to maximize. Such an embodiment has only four led out of the molding Connections, those on the primary and the secondary side connected to the circuit. In the manufacture of the transformer is this "clasp-shaped" molding in front of the Gluing the core halves to impose on a core half.

Cheap for the production and mounting the auxiliary windings is further an embodiment, at the auxiliary primary windings and the auxiliary secondary windings are designed as multi-core ribbon cables in such a way that a first ribbon cable loop an auxiliary primary winding and an auxiliary secondary winding forms and a second ribbon cable loop another Hilfssprimärwicklung and another auxiliary secondary development forms and each one of these ribbon cable loops around the outer legs the core are looped, with the individual wires of the ribbon cable are connected at the ends so that the wires for each auxiliary winding whole turns around the outer thighs of the core form and the auxiliary primary wraps and the auxiliary secondary wraps each connected in series. The advantage is that no investment contrary to the embodiments described above are necessary in tools for plastic extrusion.

at The choice of ribbon cable is based on the thickness of the insulation to comply with the standard requirements. Since these are mainly from the potential differences between the primary and secondary windings depend, no general statement can be made.

conveniently, become the ends of the auxiliary primary and auxiliary secondary windings Ribbon cable loops with their ends in cutting bushings on attached to a printed circuit board, wherein the individual wire ends of the Ribbon cable over Tracks on the circuit board connected to whole turns are.

As mentioned above, the inductions caused by the auxiliary windings toward the core In addition, it may be favorable to connect the devices to be supplied on the secondary side of the transformer with both the auxiliary rectifier arrangement and with the rectifier arrangement of the power unit. The circuit designed in this way made it possible to supply the secondary-side devices both by the auxiliary supply and by the voltage derived from the secondary side by the power unit.

The Switching power supply is then operated so that the facilities on the secondary side of the transformer are only fed by the auxiliary voltage, when the voltage at the output of the rectifier arrangement in the power section under the for the secondary side Facilities required value is and that when exceeded this value the secondary side Facilities over the rectifier arrangement are supplied in the power section. The Auxiliary power can then be turned off and it comes to none additional Strain on the core.

The Invention will now be described by way of example with reference to FIG on the attached figures explained. In a schematic representation:

1 : a switching power supply with buck converter

2 a switching power supply according to the invention with independent auxiliary supply according to the principle of a flyback converter

3 a switching power supply according to the invention with independent auxiliary supply according to the principle of a push-pull current transformer

4 : the arrangement of the auxiliary windings on the core of the transformer

5 : the arrangement of the auxiliary windings formed from ribbon cable at the core of the transformer

1 shows an exemplary arrangement of a switching power supply circuit including a buck converter 1 , which derives a supply voltage U2 from a DC voltage U1 in the DC link. The DC voltage U1 in the DC link can be, for example, a rectified AC voltage. By means of a rectifier bridge, for example, after smoothing out an AC voltage of 230 V, 320 V DC voltage is produced as DC voltage U1 in the DC link. But it can also be used a different DC voltage source. About a switching element 2 is a primary winding 4 that around the core 3 is wound, applied to this DC voltage U1 in the DC link. The on and off times of the switching element 2 be via a control device 6 governed with the base of the control 2 connected is. The control device is supplied 6 with the supply voltage U2. The supply voltage U2 is the output voltage of the buck converter 1 which consists, for example, of the following elements: An LNK switch 10 (eg LNK 306 , Manufacturer Power Integrations), which is connected to the DC voltage U1 in the DC link. The output of the LNK switch 10 , which has a freewheeling diode 11 is connected in parallel, is via a smoothing throttle 12 and a subsequent smoothing capacitor 13 guided. Such circuits usually have an output power of 3-5 W. The supply voltage U2 provided is, for example, 15 V.

Secondary side is a secondary winding 5 around the core 3 wound. The with the secondary winding 5 connected rectifier arrangement comprises a diode 8th and a smoothing capacitor 9 , The output voltage U3 results in a smoothed DC voltage, with secondary-side facilities 7 take over additional control tasks (eg for temperature control).

In 2 is an inventive embodiment of the independent auxiliary supply to see, with the in 1 described circuit is supplemented by other elements. The supply voltage U2 for the auxiliary primary circuit corresponds to the supply voltage U2 for the control device 6 for controlling the switching element 2 , The supply voltage U2 for the Hilfssprimärkreis but can also be derived from a completely independent - not shown here - galvanically isolated source.

The clocking auxiliary device 19 is in series with the auxiliary primary windings 15 and 16 arranged and is exemplified by a switching element 14 , a frequency generator 20 and an intermediate resistor 22 educated. Here is the frequency generator 20 (eg LMC 555 ) is connected to the supply voltage U2 and its output via the resistor 22 with the base of the switching element 14 connected. The switching element 14 may for example be a low voltage transistor STN2NE10 manufacturer STM. Connected is the switching element 14 to the source terminal via a shunt resistor 28 to the reference potential of the DC link. The drain connection is located above the two auxiliary primary windings 15 and 16 at the supply voltage U2. Through the auxiliary primary windings 15 and 16 the clocked primary auxiliary current I2 flows. Parallel to the auxiliary primary developments 15 and 16 are two Zener diodes for demagnetization 24 and 25 and a diode 23 connected.

The current through the output of the frequency generator 20 is monitored by a current detector circuit to turn off the output when set limit values are exceeded.

The current detection circuit is formed of the shunt resistor 28 to which a transistor 21 is connected in parallel with the base and the collector terminal. The collector terminal is connected to the reference potential of the intermediate circuit, the emitter terminal of the transistor 21 is with the over the resistance 22 guided output of the frequency generator 20 connected.

On the secondary side are the two auxiliary secondary windings 17 and 18 in the opposite direction to the auxiliary primary developments 15 and 16 are wound. In 2 This is indicated by the start of the winding marked with a dot. The secondary-side auxiliary rectifier circuit consists of a diode 26 and a smoothing capacitor 27 and supplies as auxiliary output voltage U4 a smoothed DC voltage for the supply of the secondary-side devices 7 ,

3 shows a further exemplary embodiment of the independent auxiliary power supply according to the invention. In turn, the forms in 1 illustrated circuit the base. The supply of the primary auxiliary supply elements again takes place with the supply voltage U2, which, however, can also come from a completely independent source (not shown here).

The clocking auxiliary device 19 is back in line with the auxiliary primary windings 15 and 16 arranged and consists of a frequency generator 20 and a transistor pair 30 . 31 , Here is the frequency generator 20 (For example, a switching power supply IC type UC3842) connected to the supply voltage U2. The gate drive signal of the frequency generator 20 comes with a pair of transistors 30 and 31 amplified and then via a coupling capacitor 32 guided. The two series of auxiliary primary windings connected in series 15 and 16 are connected to this coupling capacitor 32 and connected to the reference potential of the DC link.

The serially connected auxiliary secondary windings 17 and 18 are in the same direction as the auxiliary primary windings 15 and 16 around the core 3 wound and with a two-way rectifier 29 with smoothing capacitor 27 connected (the winding starts are marked with a dot). This push-pull current transformer does not need a smoothing choke because of the peak charge. The secondary-side auxiliary output voltage U4 supplies the secondary-side devices 7 , which are additionally connected to the rectifier arrangement in the power section. The supply of the facilities 7 then takes place via the power section, when the voltage at the output of the rectifier arrangement (U3) in the power section above that for the secondary-side facilities 7 required value is.

The current through the output of the frequency generator 20 is being supervised. This is the I _SENSE connection of the frequency _generator 20 with the collector terminal of the second transistor 31 of the amplifying transistor pair 30 . 31 connected. This collector connection is via a shunt resistor 28 connected to the reference potential.

In 4 is the core 3 of the transformer with the windings 4 . 5 . 15 . 16 . 17 and 18 shown schematically. It is a three-limbed ferrite transformer in ETD design. Around the middle leg of the core 3 inside is the primary winding 4 and separated by an insulating layer above the secondary winding 5 of the power unit wound. On the first outer leg is the first auxiliary primary winding 15 and next to it the second auxiliary secondary development 18 , These two windings are connected via connecting lines with the arranged at the other outer leg auxiliary windings 16 and 17 connected, wherein the auxiliary primary windings 15 and 16 are connected in series and the auxiliary secondary windings 17 and 18 are connected in series. The winding sense of the auxiliary windings 15 . 16 . 17 and 18 in the example shown construction corresponds to a flyback converter.

In 4 Furthermore, the idealized course of the magnetic flux of the power section Φ _L and the magnetic flux of the auxiliary supply Φ _H can be seen. The magnetic flux of the power section Φ _L in this case runs in closed field lines by the central limb and is branched on the two outer legs of the core 3 , The magnetic flux of the auxiliary supply Φ _H runs on the outside and thus superimposes itself in the outer leg with the magnetic flux of the power section Φ _L. The magnetic flux of the auxiliary supply Φ _{H is} caused by a variable current I 2 passing through the auxiliary primary windings 15 and 16 flows. Closed by secondary side devices circuit flows through the auxiliary secondary windings 17 and 18 a current induced by an induced voltage I4.

A core 3 with built-up of ribbon cable auxiliary windings 15 . 16 . 17 and 18 is in 5 shown schematically. In the middle leg of the three-limbed nucleus 3 inside is the primary winding 4 and above that the secondary winding 5 of the power unit attached. A first ribbon cable is through performed the gap between the first outer leg and the winding on the middle leg. Shown is a four-core ribbon cable, which is spliced at both ends in two times two wires. Two of these wires now form on a circuit board through a series connection 33 two turns of the first auxiliary primary winding 15 , The adjacent two wires are also on the circuit board 33 to two turns of the second auxiliary secondary winding 18 connected.

On the circuit board 33 the connecting lines are located to the second ribbon cable mounted in the same way on the second outer leg. Here are the auxiliary primary windings 15 and 16 as well as the auxiliary secondary windings 17 and 18 connected in series. The winding sense again results in a flyback converter.

The wiring of the wires on the circuit board 33 is done by tracks, via ribbon cable connector 34 contacted with the wires of Fachbandkabel. It is in the exemplary representation of four 4-pin connector (eg IDT sockets, Fa. Molex).

Claims (13)

Switching power supply with - a transformer with at least one secondary winding ( 5 ), which can be connected to a load via a rectifier arrangement, - at least one primary winding ( 4 ) having a first winding terminal connected via a switching element ( 2 ) is connected to a terminal for a reference potential and a further terminal which is connectable to a DC voltage (U1), - a control device ( 6 ) for controlling the on and off times of the switching element ( 2 ), characterized in that for the independent supply of facilities ( 7 ) the following elements are provided: - at least one auxiliary secondary development ( 17 ) of the transformer, which via an auxiliary rectifier arrangement to the devices to be supplied ( 7 ) is connectable, - a clocking auxiliary device ( 19 ) for generating a pulsed current (I2) from a DC power source and - at least one auxiliary primary winding ( 15 ) of the transformer connected to the clocked DC source. Switching power supply according to claim 1, characterized in that the transformer has a three-legged closed core ( 3 ) and the primary winding ( 4 ) as well as the secondary winding ( 5 ) are arranged in the middle leg and the auxiliary primary winding ( 15 ) as well as the auxiliary secondary development ( 17 ) are arranged on an outer thighs. Switching power supply according to Claims 1 and 2, characterized in that the auxiliary primary winding ( 15 ) to two auxiliary primary windings connected in series ( 15 . 16 ) and the auxiliary secondary development ( 17 ) to two auxiliary secondary windings connected in series ( 17 . 18 ) and that one of each of these auxiliary primary developments and one of these auxiliary secondary developments is distributed on a common outer leg of the core ( 3 ) are next to each other. Switching power supply according to Claims 1 to 3, characterized in that a supply voltage (U2) for the control device ( 6 ) for controlling the on and off times of the switching element ( 2 ) by means of buck converter ( 1 ) is derived from the DC voltage (U1) of the DC link in the power section. Switching power supply according to claims 1 to 4, characterized in that the auxiliary primary circuit to the means of buck converter ( 1 ) from the DC voltage (U1) of the intermediate circuit in the power unit derived supply voltage (U2) is connected. Switching power supply according to claims 1 to 3, characterized in that the clocking auxiliary device ( 19 ) a low voltage switching transistor ( 14 ) whose gate connection to the power output of a frequency generator ( 20 ) and that the auxiliary primary windings ( 15 . 16 ) with a winding connection via the low-voltage switching transistor ( 14 ) are connected to the reference potential and with the further connection to the supply voltage U2 and that further the auxiliary secondary windings ( 17 . 18 ) and the auxiliary primary developments ( 15 . 16 ) are wound in the same direction and that they are linked to the auxiliary secondary 17 . 18 ) connected auxiliary rectifier arrangement as a disposable rectifier circuit ( 26 ) with smoothing capacitor ( 27 ) is trained. Switching power supply according to claims 1 to 3, characterized in that the clocking auxiliary device ( 19 ) a frequency generator ( 20 ) whose power output via a pair of transistors ( 30 . 31 ) and a coupling capacitor ( 32 ) to a winding terminal of the auxiliary primary windings ( 15 . 16 ) and that the further connection of the auxiliary primary windings ( 15 . 16 ) is connected to the reference potential and that, in addition, the auxiliary secondary developments ( 17 . 18 ) and the auxiliary primary developments ( 15 . 16 ) are wound in the same direction and that the auxiliary secondary windings ( 17 . 18 ) connected auxiliary rectifier arrangement as a two-way rectifier ( 29 ) with smoothing Capacitor ( 27 ) is trained. Switching power supply according to Claims 1 to 3, characterized in that the auxiliary primary windings ( 15 . 16 ) and the auxiliary secondary developments ( 17 . 18 ) are formed as plastic-molded parts, that the overlaps of the windings and the connections on the core ( 3 ) side facing away and that the interconnection of the auxiliary windings takes place in series on a printed circuit board. Switching power supply according to Claims 1 to 3, characterized in that the auxiliary primary windings ( 15 . 16 ) and the auxiliary secondary developments ( 17 . 18 ) and the connecting lines between the series-connected auxiliary windings ( 15 . 16 and 17 . 18 ) are formed as a uniform plastic-coated molded part and the overlaps of the windings and the connecting lines and the connections on the core ( 3 ) side facing away. Switching power supply according to Claims 1 to 3, characterized in that the auxiliary primary windings ( 15 . 16 ) and the auxiliary secondary developments ( 17 . 18 ) are designed as multicore ribbon cables in such a way that a first ribbon cable loop an auxiliary primary winding ( 15 ) and an auxiliary secondary development ( 18 ) and a second ribbon cable loop forms another auxiliary primary winding ( 16 ) and another auxiliary secondary development ( 17 ) and one each of these ribbon cable loops around the outer legs of the core ( 3 ) are wound and that the individual wires of the ribbon cables are connected at the ends so that they for each auxiliary winding whole turns around the legs of the core ( 3 ) and the auxiliary primary developments ( 15 . 16 ) and the auxiliary secondary developments ( 17 . 18 ) are each connected in series. Switched-mode power supply according to Claims 1 to 3 and 10, characterized in that the auxiliary primary windings ( 15 . 16 ) and auxiliary secondary developments ( 17 . 18 ) forming ribbon cable loops with their ends in insulation displacement bushes ( 34 ) on a printed circuit board ( 33 ) are attached and the individual wire ends of the ribbon cable via traces on the circuit board ( 33 ) are connected to whole turns. Switched-mode power supply according to Claims 1 to 3, characterized in that the devices ( 7 ) are connected on the secondary side of the transformer with both the auxiliary rectifier arrangement and the rectifier arrangement of the power unit. Method for operating a switched-mode power supply according to Claims 1 to 3 and 12, characterized in that the devices ( 7 ) are only fed by the auxiliary voltage (U4) on the secondary side of the transformer when the voltage at the output of the rectifier arrangement (U3) in the power unit is lower than that for the secondary equipment ( 7 ) value is required and that when exceeding this value, the secondary-side devices are supplied via the rectifier arrangement in the power section.