Switching power supply with coupled bass reducer stages

Description

[0001] The invention relates to a galvanically isolated switching power supply with two coupled step-down stages for reducing the isolation requirements between input voltage and output voltage.

[0002] In the area of ​​safe isolation, power supplies are responsible for adapting the voltage to the respective load. Depending on the power source, the input voltage may be in the hazardous range above a certain voltage limit. For example, an industrial control cabinet is typically powered by a hazardous voltage of 120VAC or 230VAC, or low voltage up to 1000VAC / 1500VDC. Electrical safety requirements such as insulation distances or materials, and touch protection must be observed. The loads in the control cabinet, such as a controller (PLC), sensors, or actuators, are powered by a safe extra-low voltage (SELV) so that they do not have to meet special electrical safety requirements in the load itself or its connections.

[0003] Therefore, one of the tasks of a power supply is to safely separate the potentially hazardous input voltage from the accessible output voltage.

[0004] Within a power supply system, normative requirements for minimum clearance and creepage distances (Cd) are established, depending on the specific application, to separate the potentially hazardous input voltage from the accessible output voltage. Fundamental requirements for clearance and creepage distances are defined, for example, in the IEC 60664 series of standards: "Insulation coordination for electrical equipment in low-voltage installations". Depending on the product and application, the relevant product standards, such as IEC 62109: "Safety of inverters for use in photovoltaic energy systems", IEC 60950: "Information technology equipment - Safety", IEC 61010: "Safety requirements for electrical measuring, control, regulating and laboratory equipment", or IEC 62368: "Audio / video, information and communication equipment - Part 1: Safety requirements", are applicable.

[0005] The publication "Qian T. et al.: "Input-Series Two-Stage DC-DC Converter with Inductor Coupling", Power Electronics Specialists Conference, 2007. IEEE, PISCATAWAY, NJ, USA, June 17, 2007, pages 326-330, XP031218310 ISBN 978-1-4244-0654-8" concerns 2-stage DC / DC converters with inductive coupling.

[0006] Publication JP 2008 187817 A concerns a DC power supply with a plurality of flyback DC / DC converters.

[0007] Figure 1 Figure 1 shows a schematic diagram of the isolation in a galvanically isolated power supply 100 and illustrates the isolation voltages VIS1, VIS2, and VIS3 that occur across the galvanic isolation gap 103. For this purpose, switches S1 and S2 are closed, and the input voltage VIN and the output voltage VOUT are each grounded at a specific point. The highest voltage across the isolation gaps IS1 to IS5 is typically measured at the maximum input voltage VIN.

[0008] Figure 2 Figure 2 shows a schematic diagram of the isolation in a galvanically isolated switched-mode power supply. Essentially, isolation requirements arise across the transformer Tr1 and across the feedback loop, i.e., fault control of the output variables via the optocoupler OC1. The output variables can be, among others, the output voltage or the output current, possibly for multiple output voltages.

[0009] Not shown here is any input-side rectification and smoothing for an AC-powered switched-mode power supply; only the actual galvanically isolated DC / DC converter is depicted. Likewise, the schematic diagram is independent of the DC / DC converter's operating principle. It can be implemented according to known circuit principles, for example, as a flyback converter, forward converter, half-bridge converter, full-bridge converter, or resonant converter. Especially at high input voltages, circuit concepts such as two-transistor converters or a series connection of the above concepts are also used. The control principle used for the power switch S3 can be, for example, hard-switching with pulse-width modulation (PWM) or resonant with frequency modulation (PFM).

[0010] Similarly, the power switch S3 can be implemented as a power switch that can be switched on and off at will, regardless of the technology used, and can be implemented, for example, as a MOSFET or bipolar transistor or IGBT or GAN-FET or SiC-FET.

[0011] The Figures 3a and 3b Figures 300a and 300b show schematic diagrams of the reference potential for the primary-side control circuitry and the optocoupler OC1 in a galvanically isolated switched-mode power supply. The primary-side control circuitry of the power switch S3 and the feedback optocoupler OC1 typically uses the negative input voltage -VIN as its reference potential. This allows the power switch S3 to be controlled directly.

[0012] The OC1 optocoupler can also be implemented as both an actual optocoupler and a magnetic coupler.

[0013] The isolation voltages result at maximum input voltage. If, for example, the supply voltage is DC, it is grounded at either +VIN or -VIN. At low output voltages, it is only of minor importance whether +VOUT or -VOUT is considered grounded. The highest isolation voltages occur across the isolation gap at transformer Tr1 with VIS1 and VIS2 when -VIN is grounded. Since the optocoupler OC1 is usually referenced to -VIN, the highest isolation voltage VIS3 results when +VIN is considered grounded. The isolation voltage VIS2 is effectively lower than the input voltage VIN, but depending on the switching power supply principle, its peak value can be significantly higher than VIN.

[0014] If the switched-mode power supply is to be used universally, it can be grounded at either the positive or negative input voltage. These different grounding options result in very high demands on the insulation across the separation gap.

[0015] The object of the present invention is therefore to provide a switched-mode power supply with reduced insulation requirements.

[0016] By extending the basic circuit design, the insulation requirements across the isolation gap are to be significantly reduced. The insulation voltages across the components of the isolation gap are to be reduced independently of the grounding of the input and output voltages.

[0017] This problem is solved by the items with the features according to the independent claims. Advantageous embodiments are the subject of the dependent claims, the description, and the drawings.

[0018] A basic concept of the invention is to connect a step-down converter to the galvanically isolated DC / DC converter in both the positive and negative input voltage stages. A voltage drop V11 and V12 occurs across the two step-down converter stages (see Figure 4 This reduces the insulation voltage across the isolation gap V IS1 independently of the grounding. It is advantageous to combine or couple the coils of the two buck converter circuits and wind them on a ferrite core, which automatically symmetrs both buck converter stages and eliminates tolerances between the coils. This requires simultaneous activation of the power switches of both buck converters.

[0019] According to a first aspect, the problem is solved by a switching power supply according to claim 1.

[0020] With such a switched-mode power supply, the technical advantage is achieved that a voltage drop occurs across the two step-down stages, which reduces the insulation voltage across the separation gap independently of the grounding.

[0021] In an advantageous embodiment of the switching power supply, the two step-down stages are designed to cause a voltage drop in the input voltage and thus reduce the isolation voltage across the galvanic isolation element.

[0022] This achieves the technical advantage that the specified safety requirements regarding isolation between the input voltage and the output voltage can be more easily met, i.e., the requirements for the galvanic isolation element can be reduced.

[0023] According to the invention, the two bass-depressor stages each comprise a coil wound on a common core.

[0024] This achieves the technical advantage that the two bass-shift stages automatically symmetry and tolerances between the coils are irrelevant.

[0025] According to the invention, each of the two buck converter stages comprises a power switch, a freewheeling diode, and coils coupled via the common core. The freewheeling diodes can also be implemented as power switches.

[0026] The identical structure of the two step-down stages provides the technical advantage that the voltage drop across both step-down stages is the same and the insulation requirements across the galvanic isolation element are always reduced by the corresponding voltage drop.

[0027] According to the invention, the two buck converter stages can be controlled together via a control signal.

[0028] By controlling the two step-down stages together, the technical advantage is achieved that the voltage drop across both step-down stages is the same and the insulation requirements across the galvanic isolation element are always reduced by the corresponding voltage drop.

[0029] In an advantageous embodiment of the switching power supply, the input circuit comprises a pulse transformer or a driver circuit configured to adapt the control signal of the two buck converter stages to a reference potential of the two power switches.

[0030] According to the invention, the input circuit comprises a control circuit configured to generate the control signal, wherein a reference potential of the control circuit is connected to a center tap between the two buck converter stages.

[0031] This achieves the technical advantage of halving the input voltage at the control circuit, thereby reducing the insulation voltage to approximately half, regardless of the grounding.

[0032] In an advantageous embodiment of the switching power supply, the two buck converter stages are coupled to each other on the input side via two input capacitors connected in series, the center tap of which forms the center tap between the two buck converter stages.

[0033] The step-down converter halves the input voltage, while the capacitors smooth the AC component.

[0034] In an advantageous embodiment of the switching power supply, the two input capacitors are connected in series between the positive input voltage and the negative input voltage.

[0035] This achieves the technical advantage that the two input capacitors implement a voltage divider whose center tap is easily accessible.

[0036] In an advantageous embodiment of the switching power supply, the two buck converter stages are coupled to each other on the output side via a single output capacitor.

[0037] This achieves the technical advantage of easier implementation of the step-down stages, as no center tap is required on the output side.

[0038] In an advantageous embodiment of the switched-mode power supply, the galvanic isolation element comprises a transformer which can be controlled via a power switch by the control circuit.

[0039] By controlling the transformer via the control circuit, whose reference potential is connected to a center tap between the two step-down stages, the technical advantage is achieved that the insulation voltage across the transformer is reduced to half of the positive input voltage when grounded at the positive input voltage.

[0040] In an advantageous embodiment of the switched-mode power supply, the input circuit comprises a transformer connected between the control circuit and the power switch, which galvanically isolates the control circuit from the power switch.

[0041] A transformer adapts the control signal to the reference potential of the circuit breaker.

[0042] According to the invention, the galvanic isolation element comprises a feedback element for controlling the input circuit, in particular an optocoupler or a magnetic coupler. The optocoupler is controlled by the secondary-side control unit 202 (see Fig. 2 ).

[0043] The output of the feedback element into the primary-side control circuit and its reference potential is connected to the center tap between the two buck converter stages, thereby achieving the technical advantage that the isolation voltage across the feedback element is reduced to approximately half.

[0044] In an advantageous embodiment, the switching power supply is grounded at the positive input voltage or at the negative input voltage.

[0045] According to a second aspect, the invention relates to a method according to claim 11 for reducing the insulation requirement of a switched-mode power supply.

[0046] This method achieves the technical advantage that a voltage drop occurs across the two step-down stages, thereby reducing the insulation voltage across the separation gap, i.e. the galvanic isolation element, independently of the grounding.

[0047] Further examples of implementation are explained with reference to the accompanying drawings. These show: Fig. 1 a schematic diagram of the isolation in a galvanically isolated power supply 100; Fig. 2 a schematic diagram of the isolation in a galvanically isolated switched-mode power supply 200; Fig. 3a / b / c schematic diagrams of the reference potential of the primary-side control and the optocoupler OC1 in a galvanically isolated switched-mode power supply 300a, 300b and of a buck converter 300c; Fig. 4 a schematic diagram of a switched-mode power supply 400 with reduced isolation requirements; Fig. 5 a schematic diagram of a switched-mode power supply 500 with lower voltage stress on the optocoupler by changing the reference potential of the primary-side control; Fig. 6 a schematic diagram of a switched-mode power supply 600 with coupled buck converter stages according to one embodiment; and Fig. 7 a schematic representation of a method 700 for reducing the isolation requirements of a switched-mode power supply according to one embodiment.

[0048] The switching power supplies presented below include buck converters with coupled buck converter stages.

[0049] A corresponding subtractor Fig 3c A step-down converter (also called a buck converter) is a switching DC voltage converter in which the output voltage V OUT is always smaller than the magnitude of the input voltage V IN.

[0050] The step-down converter comprises a switch S connected in series with a diode D between the positive +V IN and negative input voltages -V IN. Switch S is further connected in series with an inductor L between the positive input voltage +V IN and the positive output voltage +V OUT. Finally, a capacitor C is connected between the positive +V OUT and negative output voltages -V OUT.

[0051] The switch S (usually a transistor) is regularly switched on and off by a controller; typically, several hundred to several million switching cycles are performed per second. This transfers electrical energy from the input voltage source VIN to the output load. The buck converter comprises two energy storage devices: the inductor L and the capacitor C, which supply power to the load during the phases when the switch is open. The inductance of the inductor L keeps the higher input voltage away from the load. The output voltage can be adjusted by controlling the on and off times of the switch S. This control is usually achieved by a regulator to maintain the output voltage or current at a desired value.

[0052] During the on-time, the load current flows through the inductor and the load; diode D is reverse-biased. During the off-time, the energy stored in the inductor is dissipated: the current through the load continues to flow, but now through diode D and from capacitor C.

[0053] The inductor L and the capacitor C form a second-order low-pass filter. Effectively, the step-down conversion is achieved by filtering out the DC component from the square wave voltage. The amount of this remaining DC component can be adjusted by the duty cycle.

[0054] Fig. 4 shows a schematic diagram of a 400 series switched-mode power supply with reduced insulation requirements.

[0055] The switching power supply 400 comprises an input circuit 401 supplied by an input voltage VIN; an output circuit 402 coupled to the input circuit 401 for providing an output voltage VOUT; and a galvanic isolator 103 between the input circuit 401 and the output circuit 402, which is configured to meet a specified safety requirement regarding isolation between the input voltage VIN and the output voltage VOUT. The input circuit 401 comprises two coupled buck converter stages 403, of which a first buck converter stage 404 is connected in the positive input voltage +VIN and a second buck converter stage 405 is connected in the negative input voltage -VIN upstream of the galvanic isolator 103.

[0056] The two step-down stages 403 cause a voltage drop in the input voltage V IN and thus reduce an isolation voltage V IS1 , V IS2 , V IS3 across the galvanic isolation element 103.

[0057] The galvanic isolation element 103 comprises a transformer Tr1, which decouples a power path 406 of the input circuit 401 from the output circuit 402 of the switched-mode power supply 400. Isolation voltages VIS1 and VIS2 drop across the transformer Tr1 between the input circuit 401 and the output circuit 402. The galvanic isolation element 103 further comprises an optocoupler OC1, which decouples a control circuit 407 of the input circuit 401 from the output circuit 402 of the switched-mode power supply 400. Isolation voltage VIS3 drops across the optocoupler OC1 between the input circuit 401 and the output circuit 402.

[0058] Across the two buck converter stages 404 and 405, a voltage drop of V11 and V12 occurs, respectively, which reduces the insulation voltage across the isolation gap VIS1 independently of the grounding. It is advantageous to combine or couple the coils of the two buck converter circuits 404 and 405 and wind them on a ferrite core. This automatically symmetrs both buck converter stages 404 and 405, rendering tolerances between the coils irrelevant. Simultaneous activation of the power switches of both buck converters 404 and 405 is necessary for this.

[0059] A reference potential of the control unit 407 of the input circuit 401 is connected to a center tap 409 between the two buck converter stages 404, 405. This results in a halving of the isolation voltage V IS3 across the optocoupler OC1 to approximately half the input voltage V IN / 2.

[0060] Fig. 5This diagram shows a schematic of a 500-watt switching power supply with reduced voltage stress on the optocoupler achieved by changing the reference potential of the primary-side control. The two buck converter stages are not shown in the 500-watt switching power supply; this diagram serves only to simplify the principle of changing the reference potential of the primary-side control.

[0061] The control of the input circuit 501 is connected to the midpoint 509 between the two step-down stages, which corresponds to the center tap of the two input capacitors C11 and C12, instead of to the negative input voltage potential -V IN, thereby halving the input voltage V IN to V IN / 2 and reducing the isolation voltage Viss to about half, independent of the grounding.

[0062] Fig. 6 shows a schematic diagram of a 600 series switched-mode power supply with coupled buck converter stages according to one embodiment.

[0063] The switching power supply 600 comprises an input circuit 601 supplied by an input voltage VIN; an output circuit 602 coupled to the input circuit 601 for providing an output voltage VOUT; and a galvanic isolating element 103 between the input circuit 601 and the output circuit 602, which is configured to meet a specified safety requirement regarding isolation between the input voltage VIN and the output voltage VOUT. The galvanic isolating element 103 corresponds to the one described in the following document: Figure 4 described galvanic isolating element 103. The input circuit 601 comprises two coupled buck converter stages 603, of which a first buck converter stage is connected in the positive input voltage +V IN and a second buck converter stage is connected in the negative input voltage -V IN upstream of the galvanic isolating element 103.

[0064] The two step-down stages cause a voltage drop in the input voltage V IN and thus reduce an isolation voltage V IS1 , V IS2 , V IS3 across the galvanic isolation element 103.

[0065] The two buck converter stages 603 each comprise an inductor L11 wound on a common core. Each buck converter stage 603 also includes a power switch S11, S12, a freewheeling diode D11, D12, and the inductors L11 coupled via the common core. The two buck converter stages 603 can be jointly controlled via a control signal 604. The input circuit 601 comprises a pulse transformer Tr11 or, alternatively, a driver circuit (not shown), configured to adapt the control signal 604 of the two buck converter stages 603 to a reference potential of the two power switches S11, S12.

[0066] The input circuit 601 comprises a drive circuit 201, which is configured to generate the drive signal 604. A reference potential 605 of the drive circuit 201 is connected to a center tap 606 between the two buck converter stages 603. The two buck converter stages 603 are coupled to each other on the input side via two input capacitors C11 and C12 connected in series, the center tap 606 of which forms the center tap 606 between the two buck converter stages 603. The two input capacitors C11 and C12 are connected in series between the positive input voltage +VIN and the negative input voltage -VIN. The two buck converter stages 603 are further coupled to each other on the output side via a single output capacitor C13.

[0067] The transformer Tr1 of the galvanic isolation element 103 can be controlled via a power switch S3 by the control circuit 201.

[0068] The input circuit 601 comprises a transformer Tr2 or a driver stage (not shown) connected between the control circuit 201 and the power switch S3, which adapts the control circuit 201 to the reference potential of the power switch S3.

[0069] The galvanic isolation element 103 further comprises a feedback element OC1 for controlling the input circuit 601, which functions as an optocoupler OC1 (as in Fig. 6 (shown) or as a magnetic coupler (not in Fig. 6 (as shown) can be implemented. The feedback element can be controlled by the secondary-side control unit 202.

[0070] The 600 switching power supply can be grounded at the positive input voltage +V IN or at the negative input voltage -V IN.

[0071] Figure 6Figure 1 shows an embodiment of a switched-mode power supply 600, to which two coupled buck converter stages 603 are connected. Each buck converter stage 603 consists of a switchable power switch S11 or S12 (manufactured using any technology), a freewheeling diode D11 or D12 (which can also be a switchable power switch), and a coupled coil L11. The output coils of the buck converter stages 603 are wound on a core. The buck converter stages 603 are controlled by a signal 201. The control signal 604 can be adapted to the reference potential of the two power switches S11 and S12 via a pulse transformer Tr11. Alternatively, adaptation via a driver circuit (not shown) is possible. Fig. 6 (shown) possible.

[0072] The shared coil and control circuitry ensure that the voltage drop V11 and V12 across both step-down converter stages 603 is equal, and the insulation requirements VIS1 and VIS2 across transformer Tr1 are always reduced by the voltage drop V11 or V12, depending on the input grounding. This also reduces the voltage load on the downstream switching power supply, particularly on the power semiconductors.

[0073] With input grounding at +V IN, and with the primary-side control 201 or regulation referenced to -V IN as usual, the isolation voltage V IS3 across the feedback or optocoupler OC1 is equal to the maximum input voltage V IN. By changing the reference of the primary-side control 201 to the center tap 606, V IN / 2 between the two step-down converters 603, the isolation voltage V IS3 is halved to V IN / 2 when grounded at +V IN.

[0074] The OC1 optocoupler can be implemented as both an actual optocoupler and a magnetic coupler.

[0075] The 600 switching power supply Figure 6 Only one control signal 604 is required for both buck converter circuits 603, which can be supplied to the power switches S11 and S12 via a pulse transformer Tr11 with two output windings.

[0076] One advantage of the 600 switching power supply is the reduction of the subsequent insulation voltages across the galvanic isolating element 103.

[0077] The midpoint 606, VIN / 2 of the two buck converter circuits or buck converter stages 603 can be used to reduce the isolation voltage, especially for the control and the feedback optocoupler OC1.

[0078] Another advantage is that instead of two capacitors, only one output-side capacitor C13 is used.

[0079] Similarly, slightly different switching times of the semiconductor switches result in a larger tolerance. For example, if switch S11 is switched on first, the current flows through both coils of L11, C13 or Tr1, D12 and C12 to the mains connection. Due to the doubled number of turns N, the inductance is quadratically larger, since L = N² < * AL. With identical capacitors, the time constant t = L * C is thus quadrupled.

[0080] The invention presented is particularly suitable for use in a switched-mode power supply with a high input voltage of, for example, 1500VDC with unknown grounding and therefore very high insulation requirements.

[0081] Fig. 7 shows a schematic representation of a method 700 for reducing the insulation requirements of a switched-mode power supply according to one embodiment.

[0082] The switching power supply can be a 400, 500, or 600 series switching power supply, as described above. Figures 4 to 6described. In particular, the switching power supply comprises an input circuit 401 supplied by an input voltage V IN; an output circuit 402 coupled to the input circuit 401 for providing an output voltage V OUT; and a galvanic isolating element 103 between the input circuit 401 and the output circuit 402, which is configured to comply with a specified safety requirement regarding isolation between the input voltage V IN and the output voltage V OUT.

[0083] The method 700 comprises the following steps: Switching 701 two coupled buck converter stages 403 into the input circuit 401 of the switching power supply 400, 500, 600, of which a first buck converter stage 404 in the positive input voltage +V IN and a second buck converter stage 405 in the negative input voltage -V IN is connected upstream of the galvanic isolating element 103.

Claims

1. A switched-mode power supply (400, 500, 600), comprising: an input circuit (401) powered by an input voltage (VIN); an output circuit (402) coupled to the input circuit, for providing an output voltage (VOUT); and a DC isolating element (103) between the input circuit (401) and the output circuit (402), which DC isolating element (103) is configured to observe a prescribed safety requirement in regard to isolation between the input voltage (VIN) and the output voltage (VOUT), wherein the input circuit (401) comprises two coupled step-down converter stages (403), of which a first step-down converter stage (404) in the positive input voltage (+VIN) and a second step-down converter stage (405) in the negative input voltage (-VIN) are connected, wherein the two coupled step-down converter stages (404, 405) are connected to the input circuit (401) upstream of the DC isolating element (103), wherein the two step-down converter stages (603) each comprise a coil (L11) which are wound on a common core, wherein each of the two coupled step-down converter stages (603) comprises a power switch (S11, S12), a free-wheeling diode (D11, D12) and the coils (L11) which are wound on the common core, wherein the two step-down converter stages (603) are configured to be controlled jointly via a control signal (604), wherein the input circuit (601) comprises a control circuit (201) configured to generate the control signal (604), wherein a reference potential (605) of the control circuit (201) is associated with a center tap (606) between the two step-down converter stages (603), wherein the DC isolating element (103) comprises a feedback element (OC1) configured to regulate the input circuit (601), wherein the feedback element (OC1) is configured to be controlled by a secondary-side regulator (202).

2. The switched-mode power supply (400, 500, 600) according to claim 1, wherein the two step-down converter stages (403) are configured to cause a voltage drop of the input voltage (VIN) and thus reduce an isolation voltage (VIS1, VIS2, VIS3) across the DC isolating element (103).

3. The switched-mode power supply (600) according to claim 1 or 2, wherein the input circuit (601) comprises a pulse transformer (Tr11) or a driver circuit configured to adapt the control signal (604) of the two step-down converter stages (603) to a reference potential of both power switches (S11, S12).

4. The switched-mode power supply (600) according to any of the preceding claims, wherein the two step-down converter stages (603) are coupled to one another on an input side via two input capacitors (C11, C12) connected in series, wherein a center tap (606) of the two input capacitors (C11, C12) forms the center tap (606) between the two step-down converter stages (603).

5. The switched-mode power supply (600) according to claim 4, wherein the two input capacitors (C11, C12) are connected in series between the positive input voltage (+VIN) and the negative input voltage (-VIN).

6. The switched-mode power supply (600) according to any of the preceding claims, wherein the two step-down converter stages (603) are coupled to one another on an output side via a single output capacitor (C13).

7. The switched-mode power supply (600) according to any of the preceding claims, wherein the DC isolating element (103) comprises a transformer (Tr1) configured to be controlled by the control circuit (201) via a power switch (S3).

8. The switched-mode power supply (600) according to claim 7, wherein the input circuit (610) comprises a transformer (Tr2) connected between the control circuit (201) and the power switch (S3), wherein the transformer (Tr2) galvanically isolates the control circuit (201) from the power switch (S3).

9. The switched-mode power supply (600) according to any of the preceding claims, wherein the feedback element (OC1) configured to regulate the input circuit (601) is an optocoupler (OC1) or a magnetic coupler10. The switched-mode power supply (400, 500, 600) according to any of the preceding claims, wherein the switched-mode power supply (400, 500, 600) is grounded to the positive input voltage (+VIN) or to the negative input voltage (-VIN).

11. A method (700) for reducing an insulation requirement on a switched-mode power supply (400, 500, 600) comprising an input circuit (401) powered by an input voltage (VIN); an output circuit (402) coupled to the input circuit (401), for providing an output voltage (VOUT); and a DC isolating element (103) between the input circuit (401) and the output circuit (402), which DC isolating element (103) is configured to observe a prescribed safety requirement in regard to isolation between the input voltage (VIN) and the output voltage (VOUT), the method (700) comprising: switching two coupled step-down converter stages (403) in the input circuit (410) of the switched-mode power supply (400, 500, 600), wherein a first step-down converter stage (404) of the two coupled step-down converter stages (403) is connected upstream of the DC isolating element (103) in the positive input voltage (+VIN) and a second step-down converter stage (405) of the two coupled step-down converter stages (403) is connected upstream of the DC isolating element (103) in the negative input voltage (-VIN), wherein the two step-down converter stages (603) each comprise a coil (L11) which are wound on a common core, wherein each of the two coupled step-down converter stages (603) comprises a power switch (S11, S12), a free-wheeling diode (D11, D12) and the coils (L11) which are wound on the common core, wherein the two step-down converter stages (603) are configured to be controlled jointly via a control signal (604), wherein the input circuit (601) comprises a control circuit (201) configured to generate the control signal (604), wherein a reference potential (605) of the control circuit (201) is associated with a center tap (606) between the two step-down converter stages (603), wherein the DC isolating element (103) comprises a feedback element (OC1) configured to regulate the input circuit (601), wherein the feedback element (OC1) is configured to be controlled by a secondary-side regulator (202).