Method for operating reverse converter for charging intermediate circuit capacitor

Abstract

The invention relates to a method (100) for operating a reverse converter (250) for charging an intermediate circuit capacitor (210), comprising the following steps: ascertaining (120) a voltage (Ux) characterizing a voltage at the intermediate circuit capacitor (210); specifying (140) a target charging current value (IL) for charging the intermediate circuit capacitor (210) and specifying a corresponding duty cycle (Tx) for actuating the first power switch (252) as long as the ascertained voltage (Ux) falls below the first voltage value (U1); the target charging voltage value (UL) is specified (150) as long as the ascertained voltage (Ux) corresponds to the first voltage value (U1) or the ascertained voltage (Ux) exceeds the first voltage value (U1) and the ascertained voltage (Ux) falls below the second voltage value (U2).

Description

Technical Field

[0001] The invention relates to a method for operating a flyback converter for charging an intermediate circuit capacitor and to a flyback converter for charging an intermediate circuit capacitor. The invention also relates to a drive train with a corresponding device and a vehicle with a drive train and to a computer program and a machine-readable storage medium. Background Art

[0002] Methods and devices for charging an intermediate circuit capacitor in a high-voltage power grid are known from the prior art. A vehicle preferably having an electric drive system comprises an intermediate circuit capacitor in the high-voltage power grid, which is arranged between an energy source, preferably a DC voltage energy source and / or a high-voltage energy source and a power switch of a pulse inverter. Preferably, the intermediate circuit capacitor is arranged on the DC voltage side inside the pulse inverter. The energy source, preferably a traction battery, is used to supply electrical energy to the motor. The electrical energy is converted for this purpose by means of a pulse inverter. The DC voltage of the energy source is converted into an AC voltage for supplying a multi-phase motor. If the vehicle is cut off or turned off, the energy source is disconnected from the high-voltage power grid by means of at least one switch and the high-voltage power grid is discharged so that no danger is caused from the high-voltage power grid itself when contacting conductive parts. When the vehicle is restarted, the energy source must be connected to the high-voltage power grid in an electrically conductive manner. Due to the voltage difference between the discharged high-voltage power grid and the energy source, an inadmissibly high compensation current may occur when the at least one switch is switched on. In order to reduce the voltage difference before the energy source is connected, the intermediate circuit capacitor in the high-voltage power grid is precharged or charged to a voltage that is almost corresponding to the voltage of the energy source. When at least one switch is subsequently connected or closed, there is no longer a significant voltage difference, and thus no relevant compensation current will occur. The powering, charging or precharging of the intermediate circuit capacitor is usually carried out by means of an additional circuit, which provides a current path in parallel with at least one switch and also includes a switch and a resistor, a precharge resistor or a series resistor. The switch of the parallel current path is closed for charging. The compensation current formed is limited to a reliable value by means of a resistor. If the charging process ends, at least one switch between the high-voltage power grid and the energy source is closed. Then, the drive system is ready for use again. The parallel current path with a switch and a precharge resistor requires additional components and space and causes additional weight and cost. Therefore, there is a need for an alternative solution, which can realize the charging of the intermediate circuit capacitor without a parallel current path. A method for charging an intermediate circuit capacitor in a high-voltage power grid is known from DE 102020206987. Summary of the invention

[0003] A method for operating a flyback converter for charging an intermediate circuit capacitor is proposed. The flyback converter can be connected to a low-voltage grid on the input side and to a high-voltage grid on the output side. The high-voltage grid includes an intermediate circuit capacitor to be charged. Preferably, the flyback converter is connected to the low-voltage grid on the input side and to the high-voltage grid on the output side during operation. Preferably, connection or connectable in this respect means conductive or electrical connection, coupling or common connection. The flyback converter includes a series circuit composed of a first power switch and a primary winding of a transformer between input terminals on the input side and a series circuit of a secondary winding of a transformer and a first diode connected in a cut-off direction between output terminals on the output side. On the output side, a first capacitor is connected between the output terminals. Preferably, a second capacitor is connected between the input terminals on the input side. The method includes the following steps: determining a voltage that characterizes the voltage on the intermediate circuit capacitor. As long as the determined voltage is lower than a first voltage value, a target charging current value for charging the intermediate circuit capacitor and a corresponding duty cycle for controlling the first power switch are predefined. As long as the ascertained voltage corresponds to a first voltage value or exceeds the first voltage value and the ascertained voltage is below a second voltage value, a target charging voltage value for charging the intermediate circuit capacitor and a corresponding duty cycle for actuating the first power switch are predefined. The first voltage value is smaller than the second voltage value.

[0004] The voltage is determined by a voltage measuring device and can be arranged inside or outside the flyback converter, preferably on the output side or input side of the flyback converter. The voltage is determined in this way so that the voltage applied to the intermediate circuit capacitor is determined, estimated or calculated. Preferably, a target charging current value is determined and predefined by means of a control device, preferably by means of a current regulator, preferably based on the determined voltage and the predefined target charging current value, and a duty cycle for actuating the first power switch is determined and predefined so that a charging current close to or corresponding to the target charging current value flows to the intermediate circuit capacitor on the output side of the flyback converter. Preferably, a target charging voltage value is determined and predefined by means of a control device, preferably by means of a voltage regulator, preferably based on the determined voltage and the predefined target charging voltage value, and a duty cycle for actuating the first power switch is determined and predefined so that a charging voltage close to or corresponding to the target charging voltage value occurs on the output side of the intermediate circuit capacitor to be charged.

[0005] In order to charge the intermediate circuit capacitor, the flyback converter is operated as a step-up chopper. Preferably, a lower input voltage, such as the vehicle voltage of about 12-14 volts or 48 volts of the vehicle, is continuously increased and the intermediate circuit capacitor is charged thereby until an output voltage corresponding to the energy source to be connected to the high-voltage power grid, such as 200, 400, 800 or also 1000 volts, is applied to the intermediate circuit capacitor. For this purpose, the first power switch on the input side is controlled with a predetermined duty cycle. Preferably, the intermediate circuit capacitor should be pre-charged or charged to the high voltage voltage to be reached as quickly as possible. Preferably, in order to avoid excessive current and / or excessive voltage on the first power switch, the operation of the flyback converter for charging the intermediate circuit capacitor is divided into at least two stages. First, a current-regulated stage is implemented and then a voltage-regulated stage is implemented. Preferably, in the current-regulated stage corresponding to the predetermined maximum target-charging current value, the duty cycle for controlling the first power switch is determined according to the determined voltage and the first power switch is controlled accordingly. If the determined voltage rises due to the operation of the flyback converter for charging the intermediate circuit capacitor above a predeterminable first voltage value, for example 90% of the high voltage to be achieved, the current-controlled operation is terminated and the voltage-controlled phase is carried out. Preferably, in the voltage-controlled phase corresponding to the predetermined target charging voltage value, the duty cycle for actuating the first power switch is determined as a function of the determined voltage and the first power switch is actuated accordingly. If the determined voltage rises due to the operation of the flyback converter for charging the intermediate circuit capacitor above a predeterminable second voltage value corresponding to, for example, the high voltage to be achieved, the voltage-controlled operation is terminated. Advantageously, this allows a rapid charging of the intermediate circuit capacitor while limiting the maximum current and voltage at the first power switch.

[0006] In another refinement, the pulse duty factor during the period in which the target charging current value is predefined is greater than a predefined first pulse duty factor.

[0007] During the period of pre-determining the target charging current value, the duty cycle is always pre-determined to be greater than the pre-determined first duty cycle or the minimum duty cycle. This reversal of the energy flow is reliably avoided. With the reversal of the energy flow, overcurrents and overvoltages may occur at the first power switch. Advantageously, a possible overload of the power switch is avoided with an increase in the duty cycle.

[0008] In a further refinement, the target charging current is reduced before the switch from the specification of the target charging current value to the specification of the target charging voltage value.

[0009] Before the transition from the current-regulated phase to the voltage-regulated phase, for example when 90% of the first voltage value is reached, the target charging current is reduced, for example to 10% of the predefined target charging current. This measure advantageously avoids large voltage overshoots by means of current regulation when the first voltage value is reached.

[0010] In a further refinement, the pulse duty factor is continuously recorded during the transition from the specification of the target charging current value to the specification of the target charging voltage value.

[0011] When changing from the current-regulated phase to the voltage-regulated phase, the duty cycle last present in the current-regulated phase is taken over and used at the beginning of the voltage-regulated phase, in particular by initializing the voltage regulator. The initialization of the voltage regulator causes a continuous change, thereby preferably avoiding abrupt changes in fixed operating states. Advantageously, overcurrents and overvoltages at the first power switch are thereby avoided.

[0012] In another refinement, the charging current is continuously reduced while the target charging voltage value is specified.

[0013] Advantageously, a large voltage overshoot is thereby avoided by means of voltage control when the second voltage value is reached.

[0014] In a further refinement, a current is ascertained that characterizes the current flowing through the first diode, and the method for operating the flyback converter is terminated after the ascertained current falls below a predefinable first current value, preferably while a target charging voltage is being provided.

[0015] If the current, preferably the current flowing through the output side of the current transformer, preferably during the voltage-regulated phase, falls below a predeterminable first current value, the method for operating the flyback converter is terminated. Advantageously, an interruption criterion is provided which reliably prevents overcurrents and overvoltages at the first power switch despite a possible reversal of the energy flow.

[0016] In a further refinement, the method comprises, before the predefinition of the target charging current value, a further step of actuating the first power switch for a predefinable period of time by means of a predefinable first pulse duty factor.

[0017] In order to ensure the non-destructive operation of the flyback converter, an active clamping circuit is preferably provided on the output side of the flyback converter. Preferably, the active clamping circuit comprises a series circuit of a third capacitor and a second power switch, wherein the series circuit is connected in parallel with the secondary winding of the transformer. The capacitance of the active clamping circuit is charged in a targeted manner by means of actuating the first power switch with a predeterminable duty cycle for a predeterminable duration. Here, the duty cycle is predetermined in such a way that an excessive current and voltage never occur on the first power switch. Accordingly, the duty cycle is selected to be very small, for example, 1 to 10% of the maximum selected duty cycle when providing the target charging current. The duration is selected in such a way that the third capacitor is reliably charged. After the third capacitor is charged, the intermediate circuit capacitor can be quickly charged without excessive current and voltage occurring on the first power switch. Advantageously, a method is provided that enables rapid and non-destructive charging of the intermediate circuit capacitor despite the active clamping circuit.

[0018] In one refinement, the method comprises, before providing the target charging current, a further step of initializing a current regulator of the flyback converter.

[0019] The initialization of the current regulator preferably includes the determination of a duty cycle to be predefined for actuating the first power switch as a function of the determined voltage and a target charging current to be set for charging the intermediate circuit capacitor. A method is advantageously provided which allows for rapid, non-destructive charging of the intermediate circuit capacitor.

[0020] Furthermore, the invention relates to a computer program which comprises instructions which, when the program is executed by a control device, cause the program to carry out the steps of the described method.

[0021] Furthermore, the invention relates to a machine-readable storage medium which comprises instructions which, when executed by a control device, cause the control device to carry out the steps of the described method.

[0022] The invention also relates to a flyback converter for charging an intermediate circuit capacitor in a high-voltage power grid. The flyback converter can be coupled to a low-voltage power grid on the input side and to a high-voltage power grid on the output side. The flyback converter comprises a control mechanism which is configured to carry out the method described above.

[0023] A flyback converter for charging an intermediate circuit capacitor in a high-voltage power grid is provided. The flyback converter comprises a control mechanism which is designed to carry out the described method. For this purpose, the control mechanism preferably comprises inputs and outputs which are designed to enable the determination of the voltage and / or current, preferably by means of voltage and / or current measuring mechanisms, and preferably comprises a μ-controller for current and voltage regulation and for actuating a first power switch.

[0024] Advantageously, a flyback converter is provided for quickly charging an intermediate circuit capacitor.

[0025] The invention also relates to a drive train having the described flyback converter and preferably having power electronics and / or an electric drive. Such a drive train is used, for example, to drive an electric vehicle. With the aid of the method and the flyback converter, efficient operation of the drive train can be achieved.

[0026] The invention further relates to a vehicle having a drive train as described. Advantageously, a vehicle is thus provided which comprises a flyback converter with which a fast charging of an intermediate circuit capacitor is possible.

[0027] It goes without saying that the features, properties and advantages of the method according to the invention are correspondingly applicable or can be applied to the device or the drive train and the vehicle, and vice versa.

[0028] Further features and advantages of embodiments of the present invention emerge from the following description with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The present invention will be explained in more detail below with reference to some drawings, for which:

[0030] Figure 1 shows a first schematic diagram of a flyback converter for charging an intermediate circuit capacitor,

[0031] Figure 2 shows a second schematic diagram of a flyback converter for charging an intermediate circuit capacitor,

[0032] Figure 3 A schematically illustrated vehicle with a drive train is shown,

[0033] Figure 4 shows a schematic diagram with exemplary profiles of voltage and current during a method for charging an intermediate circuit capacitor,

[0034] Figure 5 A schematically illustrated flow chart of a method for charging an intermediate circuit capacitor is shown. DETAILED DESCRIPTION

[0035] Figure 1 A first schematic diagram shows a flyback converter 250 or a possible circuit topology for charging an intermediate circuit capacitor 210. Preferably, the intermediate circuit capacitor 210 is connected to the high-voltage network 205 via output connections 242_p, 242_n for charging. Figure 1A flyback converter 250 is shown, which is configured to carry out a method for operating a flyback converter 250 for charging an intermediate circuit capacitor 210 in a high-voltage network 205. The flyback converter 250, also called a flyback converter, is connectable, preferably connected, to a low-voltage network 295 on the input side and connectable, preferably connected, to a high-voltage network 205 on the output side. The high-voltage network 205 comprises an intermediate circuit capacitor 210. The flyback converter 250 comprises a series connection between input connections 244_p, 244_n on the input side, which consists of a first power switch 252 and a primary winding 254 of a transformer 260. On the output side, the flyback converter comprises a series connection between output connections 242_p, 242_n, which consists of a secondary winding 256 of a transformer 260 and a first diode 258 connected in the reverse direction, wherein on the output side a first capacitor 262 is connected between the output connections 242_p, 242_n. Preferably, the primary winding 254 and the secondary winding 256 of the transformer 260 are wound in opposite directions in order to define the current direction on the primary side and on the secondary side. Preferably, the second capacitor 264 is connected between the input terminals 244_p, 244_n on the input side to smooth the voltage between the input terminals even when the flyback converter is operated in reverse. Preferably, the first diode 258 is configured as a body diode of the third power switch 268. Preferably, the flyback converter can transfer electrical energy from the output side to the input side by means of the control of the third power switch 268. Preferably, the DC voltage of the high-voltage power grid 205 is converted into an AC voltage by means of the third power switch 268 so that the transformer 260 transmits the AC voltage. Preferably, the leakage inductance 265 of the transformer 260 between the secondary winding 256 of the transformer 260 and the first diode 258 is shown in order to fully display the line topology. Preferably, the flyback converter 250 comprises an active clamping circuit or overvoltage protection circuit, which is designed as a series circuit of a third capacitor 263 and a second power switch 261. The series circuit of the active clamping circuit is connected in parallel with the secondary winding 256 of the transformer 260, preferably in parallel with the secondary winding 256 and the leakage inductance. The active clamping circuit reduces the voltage peaks on the primary winding 254 of the transformer 260 due to the leakage inductance 265 by means of the third capacitor 263. A voltage, preferably a voltage Ux, drops between the output connections 242_p, 242_n, which voltage represents, by way of example, the voltage at the connected intermediate circuit capacitor 210. In order to charge the intermediate circuit capacitor 210, a current Ix flows through the flyback converter 250 and into the capacitor via the output connections 242_p, 242_n.

[0036] Figure 2A second schematic diagram of a flyback converter 250 for charging an intermediate circuit capacitor 210 is shown. The high-voltage grid 205 can be connected to an energy source 220 by means of at least one switch 230. The high-voltage grid 205 is coupled to a low-voltage grid 295 by means of a DC voltage converter or flyback converter 250. The flyback converter 250 includes an inductive coupler or a transformer and thereby electrically separates the high-voltage grid 205 from the low-voltage grid 295. Preferably, electrical energy is supplied from the high-voltage grid 205 to the low-voltage grid 295. Preferably, the flyback converter 250 is reversely controlled and the step-down chopper is controlled in the operating mode. The low-voltage grid 295 preferably includes a low-voltage energy source 222, such as a battery or a storage battery and other (not shown) loads, such as a control device. The control mechanism 255 is designed and or set up to determine the voltage Ux, predetermine a target charging current value I_L and predetermine a corresponding duty cycle Tx and predetermine a target charging voltage value U_L. Preferably, the control device 255 includes a current regulator 270 or a voltage regulator for determining the corresponding duty cycle according to the target charging current value or the target charging voltage value. In this illustration, the control device 255 and the current regulator 270 are shown to be outside the flyback converter 250, wherein the control device 255 and the current regulator 270 can also be arranged inside the flyback converter 250.

[0037] Figure 3 A schematically illustrated vehicle 400 with a drive train 300 is shown. The illustration shows by way of example a vehicle 400 which can also be used on land, on water and in the air. The drive train 300 comprises a flyback converter 250 and preferably power electronics, a pulse-controlled inverter 272. Preferably, the drive train also comprises an energy source 220, an intermediate circuit capacitor 210, an electric machine 280 and / or a low-voltage energy source 222. The pulse-controlled inverter 272 is preferably used to supply the electric machine 280 with electrical energy from the energy source 220. Preferably, the pulse-controlled inverter 272 comprises an intermediate circuit capacitor 210, so that the flyback converter 250 is connected to the pulse-controlled inverter 272 during operation.

[0038] Figure 4A schematic diagram is shown with an exemplary curve of the voltage during operation of the flyback converter 250 for charging the intermediate circuit capacitor 210. The voltage U at the output terminal of the flyback converter 250 or at the intermediate circuit capacitor 210 and the current I flowing through the flyback converter 250 or into the intermediate circuit capacitor 210 are shown in the diagram on the y-axis with respect to the time t on the x-axis. Before the method starts, a low voltage Ux is applied to the output terminal. The intermediate circuit capacitor has not yet been charged. There is also no current Ix worth mentioning flowing through the flyback converter 250 or the intermediate circuit capacitor 210. At time t1, the control mechanism 255 receives a signal for charging the intermediate circuit capacitor 210. Preferably, the control mechanism 255 controls the first power switch 252 by means of a predeterminable first duty cycle T1 in a predeterminable duration Zx until time t2 in order to precharge the third capacitor 263 of the active clamping circuit. In this time period, the charging current Ix and the voltage Ux rise slightly. Subsequently, as long as the ascertained voltage Ux is below the first voltage value U1, the control device predetermines a target charging current value I_L in order to charge the intermediate circuit capacitor 210 by means of a predetermined duty cycle Tx for actuating the first power switch 252. By means of current regulation, the charging current Ix quickly rises to the level of the predetermined target charging current value I_L and the voltage Ux at the output connection rises accordingly. At time t3, as long as the ascertained voltage Ux corresponds to the first voltage value U1 or the ascertained voltage Ux exceeds the first voltage value U1 and the ascertained voltage Ux is below the second voltage value U2, the control device predetermines the target charging voltage value U_L. If the ascertained voltage Ux reaches the second voltage value U2, the intermediate circuit capacitor is charged and time t4 occurs. By means of voltage regulation, in the time period between t3 and t4, the voltage Ux continues to rise until it reaches the level of the predetermined target charging voltage. The charging current Ix accordingly drops again. Method 100 for operating flyback converter 250 is preferably terminated after the ascertained current Ix falls below a predefinable first current value I1.

[0039] Figure 5A schematically illustrated flow chart for a method 100 for operating a flyback converter 250 for charging an intermediate circuit capacitor 210 is shown. The method starts with step 105. In step 120, a voltage Ux is determined, which characterizes the voltage across the intermediate circuit capacitor 210. Preferably, in step 130, a first power switch 252 is actuated for a predeterminable duration Zx with the aid of a predeterminable first duty factor T1. Preferably, in step 132, a current regulator 270 of the flyback converter 250 is initialized. As long as the determined voltage Ux is below the first voltage value U1, a target charging current value I_L for charging the intermediate circuit capacitor 210 and a duty factor Tx for actuating the first power switch 252 are predetermined in step 140. As long as the ascertained voltage Ux corresponds to the first voltage value U1 or the ascertained voltage Ux exceeds the first voltage value U1 and the ascertained voltage Ux is below the second voltage value U2, a target charging voltage value U_L is predefined in step 150. Preferably, in step 160, a current Ix is ascertained, which characterizes the current flowing through the first diode 258, wherein the method 100 for operating the flyback converter 250 is terminated after the ascertained current Ix falls below the predefinable first current value I1. The method ends with step 175.

Claims

1. A method (100) for operating a flyback converter (250) for charging an intermediate circuit capacitor (210), in, The flyback converter (250) is connectable to a low-voltage grid (295) on the input side and to a high-voltage grid (205) on the output side. The high-voltage power grid (205) comprises an intermediate circuit capacitor (210), The flyback converter (250) comprises, on the input side between input connections (244_p, 244_n), a series circuit consisting of a first power switch (252) and a primary winding (254) of a transformer (260), and, on the output side between output connections (242_p, 242_n), a series circuit consisting of a secondary winding (256) of the transformer (260) and a first diode (258) connected in a blocking direction, wherein, on the output side, a first capacitor (262) is connected between the output connections (242_p, 242_n), The method comprises the following steps: determining (120) a voltage (Ux) characterizing the voltage across the intermediate circuit capacitor (210), As long as the determined voltage (Ux) is below a first voltage value (U1), a target charging current value (I_L) for charging the intermediate circuit capacitor (210) and a corresponding duty cycle (Tx) for actuating the first power switch (252) are predefined (140), As long as the determined voltage (Ux) corresponds to the first voltage value (U1) or the determined voltage (Ux) exceeds the first voltage value (U1) and the determined voltage (Ux) is below a second voltage value (U2), a target charging voltage value (U_L) for charging the intermediate circuit capacitor (210) and a corresponding duty cycle (Tx) for actuating the first power switch (252) are predefined (150), Wherein, the first voltage value (U1) is smaller than the second voltage value (U2).

2. The method according to claim 1, in, While the target charging current value (I_L) is being predefined (140), the duty cycle (Tx) is greater than a predefined first duty cycle (T1).

3. The method according to any one of the preceding claims, in, The target charging current (I_L) is reduced before the changeover from a specification (140) for a target charging current value (I_L) to a specification for a target charging voltage (U_L).

4. The method according to any one of the preceding claims, The pulse duty factor (Tx) is continuously received during the transition from a specification (140) for a target charging current value (I_L) to a specification for a target charging voltage value (U_L).

5. The method according to any one of the preceding claims, In this case, the charging current (Ix) is continuously reduced while the target charging voltage value (U_L) is predefined (150).

6. The method according to any one of the preceding claims, The method comprises the following steps: determining (160) a current (Ix) indicative of a current flowing through the first diode (258), in, The method (100) for operating the flyback converter (250) is terminated after the determined current (Ix) falls below a predeterminable first current value (I1).

7. The method according to any one of the preceding claims, Before the target charging current value (I_L) is predefined (140), the following steps are performed: The first power switch (252) is actuated (130) for a predeterminable time period (Zx) using a predeterminable first pulse duty factor (T1).

8. The method according to any one of the preceding claims, Before the target charging current value (I_L) is predefined (140), the following steps are performed: A current regulator (270) of the flyback converter (250) is initialized (132).

9. Computer program comprising instructions which, when the program is executed by a control device (255), cause it to carry out the steps of the method / method (100) according to claims 1 to 8.

10. A computer-readable storage medium comprising instructions which, when executed by a control unit (255), cause the control unit (255) to perform the method / steps of the method (100) according to claims 1 to 8.

11. A flyback converter (250) for charging an intermediate circuit capacitor (210) in a high-voltage network (205), in, The flyback converter (250) is coupled to a low-voltage grid (295) on the input side and to a high-voltage grid (205) on the output side. The flyback converter (250) comprises a control device (255) and is configured to carry out the method according to any one of claims 1 to 8.

12. Drive train (300) having a flyback converter (250) according to claim 11.

13. A vehicle (400) having a drive train (300) according to claim 12.

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