Active-clamp flyback converter and operating method therefor

Abstract

The invention relates to a method for operating a flyback converter (2) having an active-clamp circuit (4), a main switching branch (3) being connected to a primary-side coil (51) of a transformer (5) in series between a first supply potential (VH) and a second supply potential (VL), the active-clamp circuit (4) being connected to a central node (M) between the main switching branch (3) and the primary-side coil (51) and one of the supply potentials (VH, VL), the active-clamp circuit (4) having an active-clamp switching branch (42) in series with an active-clamp capacitance (41), the main switching branch (3) and the active-clamp switching branch (42) each having two or more series-connected semiconductor switches (31, 32, 43, 44), one or more flying capacitances (7) each being connected to an intermediate node (ZH, ZAC) between the series-connected semiconductor switches (31, 32) of the main switching branch (3) and to an intermediate node between the series-connected semiconductor switches (43, 44) of the active-clamp switching branch (42), wherein the semiconductor switches (31, 32, 43, 44) of the main switching branch (3) and the active-clamp switching branch (42) are each switched to open and close in chronological succession, wherein, in order to open or close the main switching branch (3) and the active-clamp switching branch (42), that semiconductor switch (31, 32, 43, 44) of the main switching branch (3) or active-clamp switching branch (42) in question that leads to a current path through a freewheeling diode of at least one of the semiconductor switches (31, 32, 43, 44) of the other, main switching branch (3) or active-clamp switching branch (42) is opened or closed first so that a voltage (UC) across the flying capacitance is changed in the direction of a predefined setpoint voltage (Usoll).

Description

[0001] Description

[0002] title

[0003] Active clamp flyback converter and operating procedures thereof

[0004] Technical field

[0005] The invention relates to flyback converters for a DC / DC converter, particularly for use in an electric vehicle to provide a high voltage in the range of, for example, 400 to 800 volts for a high-voltage electrical system. The invention further relates to measures for overvoltage protection of components of the flyback converter.

[0006] Technical background

[0007] To provide a constant low-voltage power supply in an electric vehicle, a DC / DC converter is typically used. The function of the DC / DC converter is to provide a constant-voltage output low-voltage power supply that is independent of the load or oscillations in the high-voltage electrical system. Specifically, the DC / DC converter is designed to generate a low voltage of, for example, 12 volts for the electric vehicle's electrical system from a high-voltage DC voltage in the range of, for example, 400 to 800 volts.

[0008] Such a DC / DC converter is often implemented as a flyback converter with an active clamp. This converter has a topology in which a main switching branch is connected in series with a primary-side coil of a transformer and connected to the DC supply voltage of the high-voltage electrical system. The main switching branch enables the primary-side coil to be energized with an AC voltage. A secondary-side coil of the transformer is connected via a rectifier diode to an output filter, such as an LC filter or CLC filter, with at least one output capacitance, from which the output voltage can be tapped.

[0009] An active-clamp circuit is formed by an active-clamp capacitor and an active-clamp switching branch connected in series. The active-clamp circuit is connected between a midpoint of the series connection of the main switching branch and the primary-side coil, and one of the supply potentials of the high-voltage electrical system. The active-clamp circuit serves to provide a current path for the transformer when the main switching unit is switched off, thus improving the efficiency of the converter.

[0010] One disadvantage of the flyback converter is the so-called reflected voltage emanating from the secondary-side coil of the transformer. This voltage acts on the disconnected (open) main switching branch after the main switching branch is switched off, and can cause voltage spikes to occur on the components of the main switching branch that exceed their dielectric strength. Therefore, this main switching branch must be dimensioned with a high breakdown voltage. The resulting voltages correspond to a reflected voltage and, when the power supply comes from the high-voltage electrical system, can be twice the voltage of the high-voltage supply. The voltage spikes even exceed this voltage.

[0011] This leads to a limited selection of suitable semiconductor components for use in such DC / DC converters, since semiconductor components with voltage ratings of high voltages, such as over 1,700 volts, are rare and expensive, and with increasing voltage ratings, the switching behavior deteriorates, especially with regard to the required switching energy, and the on-resistance increases.

[0012] It is therefore an object of the present invention to provide an improved flyback converter that can be implemented with semiconductor devices with lower voltage withstand capability. Disclosure of the invention

[0013] According to the invention, a method for operating a flyback converter with an active clamp circuit according to claim 1 and a flyback converter with an active clamp circuit according to the dependent claim are provided.

[0014] Further details are specified in the dependent claims.

[0015] According to a first aspect, a method for operating a flyback converter with an active-clamp circuit is defined, wherein a main switching branch is connected in series to a primary-side coil of a transformer between a first and a second supply potential, wherein the active-clamp circuit is connected to a mid-node between the main switching branch and the primary-side coil and one of the supply potentials, wherein the active-clamp circuit has an active-clamp switching branch connected in series to an active-clamp capacitor, wherein the main switching branch and the active-clamp switching branch each have two or more series-connected semiconductor switches, the intermediate nodes of which are pairwise connected to each other by a respective flying capacitor, wherein the semiconductor switches of the main switching branch and the active-clamp switching branch are switched sequentially in time for opening and closing, respectively.wherein, to open or close the main switching branch and the active-clamp switching branch, the semiconductor switch of the respective main switching branch or the active-clamp switching branch that leads to a current path through a freewheeling diode of at least one of the semiconductor switches of the other of the main switching branch and the active-clamp switching branch is opened or closed first, so that a voltage across the flying capacitance is changed in the direction of a predetermined setpoint voltage.

[0016] The reflected voltage induced by the secondary-side coil and the potential voltage spikes at the components of the main switching branch can cause the voltage withstand capability of the semiconductor switches to be exceeded. These overvoltages have previously been countered by appropriately dimensioning the semiconductor switches installed in the main switching branch to have a high voltage withstand capability. In contrast, the flyback converter with active clamp for the DC / DC converter according to the invention provides a multilevel converter topology based on a quasi-2-level (Q2L) operating principle. The Q2L operating principle stipulates that the converter operates in a 2-level mode and uses a flying capacitor as an active snubber.The Q2L operating mode for conventional converters is described, for example, in the following publications: “New 40kV / 300kVA Quasi-2-Level Operated 5-Level Flying Capacitor SiC “Super-Switch” IPM”, Piotr Czyz et al., 10th International Conference on Power Electronics - ECCE Asia, May 27-30, 2021, Busan, Korea, WO2022 / 128 134 A1; “Quasi-Two-Level Flying-Capacitor-Converter for Medium Voltage Grid Applications”, Stefan Mersche et al., IEEE, 2019, pages 3666-3673; and “Evaluation of Flying Capacitor Quasi 2-level Modulation for MV Applications”, Anatolii Tcai et al., PCIM Europe digital days 2021, May 3-7, 2021, ISBN 978-3-8007-5515-8, VDE Verlag, pages 199-203.

[0017] The flying capacitor multilevel converter architecture involves replacing a semiconductor switch in a switching unit with a series connection of two or more semiconductor switches, thus forming a high-side branch with multiple series-connected semiconductor switches and a low-side branch with multiple series-connected semiconductor switches. The intermediate nodes of the high-side branch and the intermediate nodes of the low-side branch are capacitively coupled in pairs using so-called flying capacitors. In the quasi-two-level operating principle, the series-connected semiconductor switches of a switching unit are considered a single switch, although the semiconductor switches are switched with a slight time offset from each other.One of the semiconductor switches switches first, and another semiconductor switch then switches when the voltage edge of the switched voltage, resulting from the current path, has essentially completed the entire voltage swing to be switched. This typically takes between 30 and 100 ns, depending on the type of semiconductor device.

[0018] The flying capacitor is sized to maintain the voltage within predefined limits for this short period and is typically very small, e.g., several hundred nF. The output voltage waveform is essentially a square wave with a short step in the rising and falling edges of the output voltage. Because the flying capacitor's capacitance is very small, high output currents can quickly discharge or charge it, leading to an overvoltage across one of the semiconductor switches. To prevent this, an active capacitance balancing method must be implemented to control the on / off signaling of the adjacent semiconductor switches.

[0019] Implementing the Q2L operating mode in a flyback converter is not straightforward, as current technology requires coupling the intermediate nodes of a high-side switching unit and a low-side switching unit of a half-bridge / inverter topology. This coupling is not used in a flyback converter.

[0020] For efficiency reasons, flyback converters are often equipped with an active clamp circuit to dissipate turn-off currents from the primary-side coil. The goal is to operate a flyback converter using a Q2L operating method.

[0021] The flyback converter is designed with a multilevel topology in which the main switching branch, which is connected in series to the primary-side coil of the transformer, is configured with a series connection of a first (oriented towards a higher voltage potential) and a second semiconductor switch (oriented towards a lower voltage potential). The main switching branch can also be configured with more than two series-connected semiconductor switches. The semiconductor switches of the main switching branch can be MOSFETs, IGBTs, IGCTs, and the like, and can include an intrinsic reverse-biased diode (freewheeling diode).

[0022] Furthermore, an active-clamp circuit is provided, comprising a series connection of an active-clamp capacitor and an active-clamp switching branch, and connecting the center node between the main switching branch and the primary-side coil of the transformer to one of the supply potentials. The active-clamp switching branch is configured with a third (oriented towards the first, e.g., higher voltage potential) and a fourth semiconductor switch (oriented towards the second, i.e., lower voltage potential). Analogous to the main switching branch, the active-clamp switching branch can also be configured with more than two series-connected semiconductor switches. The semiconductor switches of the active-clamp switching branch can be configured as MOSFETs, IGBTs, IGCTs, and the like, and may include an intrinsic reverse-biased diode.

[0023] The intermediate nodes between the semiconductor switches of the main switching branch and between the semiconductor switches of the active-clamp switching branch are coupled to each other via a flying capacitor. If there are more than two semiconductor switches in the main switching branch and more than two semiconductor switches in the active-clamp switching branch, the intermediate nodes between the semiconductor switches of the main switching branch and between the semiconductor switches of the active-clamp switching branch can be coupled to each other in pairs via a respective flying capacitor.

[0024] The flying capacitor acts as a snubber to equalize the voltage distribution between the semiconductor switches of the main switching branch and the semiconductor switches of the active-clamp switching branch. This circuit allows the voltage load resulting from overvoltages and voltage spikes coupled from the secondary side of the transformer to be distributed across the two or more semiconductor switches of the main switching branch.

[0025] To operate the flyback converter described above, a control unit can be provided that manages the semiconductor switches of the main switching branch and the semiconductor switches of the active-clamp switching branch according to a predetermined operating procedure. Using a voltage measurement unit, the operating procedure can be carried out depending on the voltage across the flying capacitor.

[0026] The flyback converter is operated by switching the main switching branch on and off, through which energy is transformed to the secondary side. To switch on, i.e., to energize the multi-sided coil of the transformer, the main switching branch must transition from an open to a closed state. This is achieved by opening the semiconductor switches of the active-clamp switching branch.

[0027] The following describes the operation of the flyback converter described above, using a main switching branch connected to or oriented towards the first supply potential (high supply potential) and a primary-side coil connected to or oriented towards the second supply potential (low supply potential). The active-clamp switching branch connects the center node to the second (low) supply potential. It should be noted that the operation of such a flyback converter can also be configured in reverse. The operation described below would then change accordingly, but the underlying principle remains the same.

[0028] Furthermore, the semiconductor switches of the relevant main switching branch and the active-clamp switching branch can be opened or closed sequentially, with the switching of one of the semiconductor switches being carried out each time the switching operation of the previously switched semiconductor switch of the relevant main switching branch or the active-clamp switching branch is completed.

[0029] To switch the main switching branch, the semiconductor switches are opened or closed sequentially. The selection of which semiconductor switch is switched first depends on the voltage across the flying capacitor, which is thereby regulated to a target voltage. Therefore, before a switching operation of the main switching branch, which is determined by a conventional, well-known operating procedure, the voltage across the flying capacitor is first measured.

[0030] It may be provided that one or more flying capacitors are designed to compensate for a voltage distribution between the semiconductor switches of the main switching branch and the semiconductor switches of the active-clamp switching branch.

[0031] Furthermore, the voltages across one or more flying capacitors can be measured, whereby the target voltage for each flying capacitor is chosen so that the voltages across the semiconductor switches are evenly distributed.

[0032] In the active-clamp flyback converter, the target voltage across the flying capacitor is: 2*k* VDC / (N), where VDC is the supply voltage between the supply potentials, N is the number of semiconductor switches connected in series, and k is the index of the flying capacitor. If two semiconductor switches are connected in series to a flying capacitor, the target voltage across the flying capacitor is 2* VDC / 2 = VDC. If three semiconductor switches are connected in series to two flying capacitors, the target voltage of the flying capacitor closer to the center node is 2*1* VDC / 3, and the target voltage of the other flying capacitor is 2*2* VDC / 3, and so on.

[0033] If the voltage across the flying capacitor is higher than the set voltage for switching on the main switching branch, the second semiconductor switch of the main switching branch connected to the center node, or the semiconductor switch of the main switching branch that applies a lower voltage potential to the flying capacitor when closed, is closed, so that a current flows through the flying capacitor from the center node via the freewheeling diode of the fourth semiconductor switch(es) of the active-clamp switching branch connected to the low supply potential, and the flying capacitor is discharged.

[0034] If the voltage across the flying capacitor is lower than the set voltage for switching on the main switching branch, the first semiconductor switch connected to the first, high supply potential is closed. This allows current to flow through the flying capacitor from the high supply potential via the freewheeling diode of the third semiconductor switch(es) or the semiconductor switch(es) connected to the center node of the active-clamp switching branch, charging the flying capacitor. Subsequently, after the switching process of the closed semiconductor switch is complete, the remaining open semiconductor switch of the main switching branch is closed, or (if there are more than two semiconductor switches in the main switching branch) the remaining open semiconductor switches of the main switching branch are closed successively according to the scheme described above.

[0035] The semiconductor switches of the active-clamp switching branch are switched in the opposite direction to the main switch. When the main switching branch is turned on, the active-clamp switching branch is turned off, and vice versa.

[0036] If, immediately before the main switching branch is switched off, the voltage across the flying capacitor is higher than a predetermined setpoint voltage, the first semiconductor switch of the main switching branch, connected to the high supply potential, is opened, so that a current flows through the second semiconductor switch and the flying capacitor from the center node via the freewheeling diode of the fourth semiconductor switch, discharging the flying capacitor in the process.

[0037] If, immediately before switching off, the voltage across the flying capacitor is lower than the set voltage, the second semiconductor switch, connected to the center node, opens. This allows current to flow through the flying capacitor from the high supply potential via the freewheeling diode of the third semiconductor switch, charging the flying capacitor in the process. Subsequently, the still-closed semiconductor switch of the main switching branch opens.

[0038] The operating procedure for switching the active clamp circuit is known from the prior art. Switching on or closing typically occurs immediately or after a specific dead time following the opening of the main switching unit, and vice versa.

[0039] The active-clamp switching branch is switched on in the corresponding manner. The voltage across the flying capacitor is equalized according to the specified setpoint voltage. To this end, if the voltage across the flying capacitor is lower than the setpoint voltage, the fourth semiconductor switch of the active-clamp switching branch, connected to the second, lower supply potential, is closed from a switched-off state of the main switching branch. This causes a compensating current to flow through the flying capacitor and the freewheeling diode of the second semiconductor switch of the main switching branch (oriented towards the second supply potential), which is connected to the center node.Conversely, if the voltage across the flying capacitor is higher than the set voltage, the third semiconductor switch of the active-clamp switching branch, coupled to the center node, is closed first, so that a current flows through the flying capacitor and the freewheeling diode of the first semiconductor switch of the main switching branch, which is connected to the first, e.g., high supply potential.

[0040] According to another aspect, a flyback converter with an active clamp circuit includes:

[0041] - a transformer with a primary-side coil;

[0042] - a main switching branch that is connected in series with the primary-side coil of the transformer between a first and a second supply potential,

[0043] - the active-clamp circuit, which is connected to a center node between the main switching branch and the primary-side coil and one of the first and second supply potentials, and which has an active-clamp switching branch connected in series to an active-clamp capacitor, wherein the main switching branch and the active-clamp switching branch each have two or more semiconductor switches connected in series,

[0044] - one or more flying capacitors connected to the intermediate nodes between the serially connected semiconductor switches of the main switching branch and the active-clamp switching branch, such that the same number of semiconductor switches of the main switching branch and the active-clamp switching branch are connected between the terminals of the respective flying capacitor and the center node,

[0045] - a control unit configured to switch the semiconductor switches of the main switching branch and the active-clamp switching branch sequentially to open and close them, wherein, to open or close the main switching branch and the active-clamp switching branch, the semiconductor switch of the respective main switching branch or active-clamp switching branch that leads to a current path through a freewheeling diode of one of the semiconductor switches of the other of the main switching branch and the active-clamp switching branch is opened or closed first, so that a voltage across the respective flying capacitance is changed in the direction of a predetermined setpoint voltage.

[0046] Furthermore, the multiple flying capacitors can be connected to the intermediate nodes of the serially connected semiconductor switches of the main switching branch and the intermediate nodes between the serially connected semiconductor switches of the active-clamp switching branch in such a way that the same number of semiconductor switches are arranged in series between the terminals of the respective flying capacitor and the center node.

[0047] Brief description of the drawings

[0048] The embodiments are explained in more detail below with reference to the accompanying drawings. These show:

[0049] Figure 1 shows a circuit diagram of a flyback converter for a DC / DC converter.

[0050] converter;

[0051] Figure 2 is a flowchart illustrating a procedure for operating the flyback converter; and

[0052] Figures 3a-3g show switching states for switching the switching element and switching the clamp switching element.

[0053] Description of embodiments

[0054] Figure 1 shows a circuit diagram of a DC / DC converter 1 with a flyback converter 2, which is built with a multilevel topology. The flyback converter 2 comprises a main switching branch 3 with a first 31 and a second semiconductor switch 32, which are preferably configured as MOSFETs, IGBTs or the like and each have a discrete or intrinsic freewheeling diode.

[0055] The main switching branch 3 is connected in series to a primary-side coil 51 of a transformer 5. The series connection of the main switching branch 3 and the primary-side coil 51 is connected to a supply voltage source 6, i.e., between a first, high supply potential VH and a second, low supply potential VL. The midpoint M between the main switching branch 3 and the primary-side coil 51 is connected to an active-clamp circuit 4. The active-clamp circuit 4 has a series connection of an active-clamp capacitor 41 and an active-clamp switching branch 42. The active-clamp circuit 4 is connected between the midpoint M and the second, low supply potential VL.

[0056] The secondary-side coil 52 is coupled via a diode 11 as a rectifier to an output capacitor 12 in order to produce an output voltage V ouTo smooth the signal, diode 11 rectifies the AC voltage signal from the secondary-side coil 52, and the output voltage V ou t is buffered in the output capacitor 12.

[0057] The active-clamp switching branch 42 is configured with a third and fourth semiconductor switch 43, 44, which are connected in series and can also be configured as MOSFETs, IGBTs, or the like, and each have a freewheeling diode. The semiconductor switches 31, 32, 43, 44 of the main switching branch 3 and the active-clamp switching branch 4 each have an intermediate node ZH, ZAC, which are connected to each other via a flying capacitor 7.

[0058] A control unit 10 is provided which operates the flyback converter 2 by switching the semiconductor switches 31, 32, 43, 44.

[0059] Furthermore, a voltage measuring unit 9 is provided, which measures the voltage Uc across the flying capacitance 7 and executes an operating procedure that depends on the voltage Uc across the flying capacitance 7.

[0060] The control unit 10 executes a procedure as illustrated by the flowchart in Figure 2 and the switching states of the flyback converter 2 according to Figures 3a to 3f. Figures 3a-3f illustrate the switching states of the semiconductor switches 31, 32, 43, 44 using current paths shown in bold (current-carrying) and dashed (not current-carrying). The flowchart illustrates the operating procedure, which is designed to switch the semiconductor switches 31, 32 of the main switching branch 3 or the active-clamp switching branch 42 using a Q2L operating procedure such that the voltage distribution during the two-stage switching process follows a specified predefined pattern.The Q2L operating method provides that when one of the switching branches 3, 4 is switched, the semiconductor switches 31, 32, 43, 44 are opened or closed in quick succession, whereby the switching of the last semiconductor switch 31, 32, 43, 44 occurs when the voltage edge of the switching process of the first opened or closed semiconductor switch 31, 32, 43, 44 has passed through or been completed. This can be time-controlled.

[0061] The flying capacitance (7) has a capacity to absorb a voltage that fluctuates within predefined limits during the time between switching operations. The flying capacitance can generally be calculated as a function of the converter's maximum current and the delay time between switching operations of a branch in Q2L operation. This current should not discharge / charge the flying capacitance by more than 5% within the delay time.

[0062] Starting from an open (non-conducting) main switching branch 3 in step S1, as shown in the state of figure 3a, the voltage Uc across the flying capacitance 7 is first measured with the voltage measuring unit 9 in step S2 before the main switching branch 3 is switched on.

[0063] In step S3, the voltage Uc is checked against a predetermined target voltage Usoii. If, in step S3, it is determined that the voltage across the flying capacitance 7 is lower than the predetermined target voltage (U), S0n) (Alternative: Yes), then in step S4 the state shown in Figure 3b is reached, in which the first semiconductor switch 31 is closed, so that current flows through the flying capacitor 7 and the freewheeling diode of the third semiconductor switch 43. The flying capacitor 7 is charged, and after the voltage swing has ended due to the switching process in step S5, the second semiconductor switch 32 is also closed, thus completely closing the main switching branch 3 and energizing the primary-side coil 51, as shown in Figure 3d.

[0064] If, in step S3, it is determined that the voltage Uc across the flying capacitor 7 is greater than the set voltage Usoii (alternative: No), then, according to the state shown in Figure 3c, the second semiconductor switch 32 is closed in step S6, allowing current to flow through the flying capacitor 7 and the freewheeling diode of the fourth semiconductor switch 44. The flying capacitor 7 is then discharged, and after the predetermined voltage is reached or the voltage swing caused by the switching process has ended, the first semiconductor switch 31 is closed in step S7 to completely close the main switching branch 3.

[0065] The process is then carried out in reverse order.

[0066] Starting from a closed (conducting) main switching branch 3 in step S8, as shown in the state of figure 3d, the voltage Uc across the flying capacitance 7 is first measured in step S9 before the main switching branch 3 is switched off.

[0067] In step S10, the voltage Uc is checked against a predetermined target voltage Usoii. If, in step S10, it is determined that the voltage across the flying capacitor 7 is lower than the predetermined target voltage (alternative: Yes), then in step S11 the state shown in Figure 3b is assumed, in which the second semiconductor switch 32 is opened, allowing current to flow through the flying capacitor 7 and the freewheeling diode of the third semiconductor switch 43. The flying capacitor 7 is then charged, and subsequently, i.e., after the voltage swing caused by the switching operation has ceased, in step S12 the first semiconductor switch 32 is also opened, thus fully opening the main switching branch 3 and interrupting the current flow through the primary-side coil 51, as shown in Figure 3a.

[0068] If, in step S10, it is determined that the voltage Uc across the flying capacitor 7 is greater than the set voltage Usoii (alternative: No), then, according to the state shown in Figure 3c, in step S13 the first semiconductor switch 32 is opened, allowing current to flow through the flying capacitor 7 and the freewheeling diode of the fourth semiconductor switch 44. The flying capacitor 7 is thereby discharged, and after the predetermined set voltage Usoii is reached, in step S14 the second semiconductor switch 32 is opened to fully open the main switching branch 3.

[0069] Starting from the state shown in Figure 3a after the main switching branch is switched off in step S15, the voltage across the flying capacitor 7 is first measured in step S16 to provide a current path for the switched-off transformer 5. In step S17, it is checked whether the capacitance voltage Uc of the flying capacitor 7 is greater than the set voltage. If the capacitance voltage Uc is greater than the set voltage Usoii, then in step S18 the fourth semiconductor switch 44 is closed, so that a current path is created via the fourth semiconductor switch 44 of the flying capacitor 7 and the freewheeling diode of the second semiconductor switch 32, corresponding to the state shown in Figure 3e.

[0070] Subsequently, or after the voltage swing has ended due to the switching process (or when the capacitance voltage equals the target voltage Usoii), the third semiconductor switch 43 is also closed in step S19 to provide the desired current path for the primary-side coil 51. This is shown in Figure 3f.

[0071] If, during the comparison in step S17, the voltage Uc is lower than the target voltage (alternative: No), then in step S20 the third semiconductor switch 43 is closed, creating a current path through the freewheeling diode of the first semiconductor switch 31, the flying capacitor 7, and the third semiconductor switch 43 to the center node M, as shown in Figure 3g. This charges the flying capacitor 7, and subsequently, in step S21, the still open fourth semiconductor switch 44 can also be closed to provide the current path.

[0072] The switching off of the Active-Clamp circuit 4 is carried out in an analogous manner.

[0073] The procedure is carried out cyclically to operate the converter, whereby the main switching branch 3 is alternately opened and closed at a predetermined cycle frequency.

Claims

Claims 1. Method for operating a flyback converter (2) with an active-clamp circuit (4), wherein a main switching branch (3) is connected in series to a primary-side coil (51) of a transformer (5) between a first (VH) and a second supply potential (VL), wherein the active-clamp circuit (4) is connected to a mid-node (M) between the main switching branch (3) and the primary-side coil (51) and one of the supply potentials (VH, VL), wherein the active-clamp circuit (4) has an active-clamp switching branch (42) connected in series to an active-clamp capacitor (41), wherein the main switching branch (3) and the active-clamp switching branch (42) each have two or more series-connected semiconductor switches (31, 32, 43, 44), wherein one or more flying capacitors (7) are each connected to an intermediate node (ZH, ZAC) between the serially connected semiconductor switches (31 ,32) of the main switching branch (3) and are connected to an intermediate node between the series-connected semiconductor switches (43, 44) of the active-clamp switching branch (42), wherein the semiconductor switches (31, 32, 43, 44) of the main switching branch (3) and the active-clamp switching branch (42) are switched sequentially for opening and closing, wherein, for opening or closing the main switching branch (3) and the active-clamp switching branch (42), the semiconductor switch (31, 32, 43, 44) of the respective main switching branch (3) or the active-clamp switching branch (42) is opened or closed first which leads to a current path through a freewheeling diode of at least one of the semiconductor switches (31, 32, 43, 44) of the other of the main switching branch (3) and the active-clamp switching branch (42) This results in a voltage (Uc) across the flying capacitance (7) being changed in the direction of a predetermined target voltage (llset).

2. Method according to claim 1, wherein the main switching branch (3) and the active clamp switching branch (42) are alternately closed and opened to operate the flyback converter (2).

3. Method according to claim 1 or 2, wherein the semiconductor switches (31, 32, 43, 44) are designed as SiC transistors, MOSFETs, IGBTs, IGCTs or GaN MOSFETs, in particular with intrinsic freewheeling diodes.

4. Method according to any one of claims 1 to 3, wherein the semiconductor switches (31, 32, 43, 44) of the relevant main switching branch (3) and the active-clamp switching branch (42) are opened or closed successively, wherein the switching of one of the semiconductor switches is carried out each time when the switching operation of the previously switched semiconductor switch (31, 32, 43, 44) of the relevant main switching branch (3) or of the active-clamp switching branch (42) is completed.

5. Method according to any one of claims 1 to 4, wherein the one or more flying capacitors (7) are configured to compensate for a voltage distribution between the semiconductor switches (31, 32) of the main switching branch (3) and the semiconductor switches (43, 44) of the active clamp switching branch (42).

6. Method according to any one of claims 1 to 5, wherein the voltages (Uc) across the one or more flying capacitors (7) are measured, wherein the target voltage (llsoll) for each flying capacitor (7) is selected such that the voltages across the semiconductor switches (31, 32, 43, 44) are evenly distributed.

7. Method according to claim 6, wherein a target voltage (llsoll) of one or more flying capacitors is determined by 2*k* VDC / N, where VDC is the supply voltage between the first and the second supply potential (VH, VL), N is the number of series-connected semiconductor switches of the main switching branch (3) or the active-clamp switching branch (42), and k is the index of the series-arranged semiconductor switches (7)) starting from the center node (M) in the direction of the respective supply potential (VH, VL).

8. Flyback converter (2) with an active clamp circuit (4), comprising: - a transformer (5) with a primary-side coil (51); - a main switching branch (3) which is connected in series to the primary-side coil (51) of the transformer (5) between a first and a second supply potential (VH, VL), - the active clamp circuit (4), which is connected with a center node (M) between the main switching branch (3) and the primary-side coil (51) and one of the first and second supply potentials (VH, VL) and which has an active-clamp switching branch (42) connected in series to an active-clamp capacitor (41), wherein the main switching branch (3) and the active-clamp switching branch (42) each have two or more than two series-connected semiconductor switches (31, 32, 43, 44), - one or more flying capacitors (7) each connected to an intermediate node between the serially connected semiconductor switches (31, 32) of the main switching branch (3) and to an intermediate node between the serially connected semiconductor switches (43, 44) of the active clamp switching branch (42), - a control unit (10) configured to switch the semiconductor switches (31, 32, 43, 44) of the main switching branch (3) and the active-clamp switching branch (42) sequentially to open and close them, wherein, for opening or closing the main switching branch (3) and the active-clamp switching branch (42), the semiconductor switch (31, 32, 43, 44) of the respective main switching branch (3) or the active-clamp switching branch (42) is opened or closed first which leads to a current path through a freewheeling diode of one of the semiconductor switches (31, 32, 43, 44) of the other of the main switching branch (3) and the active-clamp switching branch (42), so that a voltage across the flying capacitance (7) is changed in the direction of a predetermined setpoint voltage (Usoii).

9. Flyback converter (2) according to claim 8, wherein the multiple flying capacitors (7) are connected to the intermediate nodes of the serially connected semiconductor switches (31, 32) of the main switching branch (3) and the intermediate nodes between the serially connected semiconductor switches (43, 44) of the active-clamp switching branch (42) such that the same number of semiconductor switches (31, 32, 43, 44) are arranged in series between the terminals of the respective flying capacitor (7) and the center node.

10. DC / DC converter (1) with a flyback converter (2) according to claim 8 or 9, the transformer (5), wherein a secondary-side coil (52) of the transformer (5) is connected to a rectifier (11) and an output capacitor (12) for providing an output voltage (V ou t) are connected.

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