Method for operating a flyback converter for charging a DC link capacitor
Patent Information
- Application Number
- EP2023735322
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2022-08-26
- Filing Date
- 2023-06-27
- Publication Date
- 2025-07-02
AI Technical Summary
Existing methods for charging an intermediate circuit capacitor in a high-voltage network require additional components and space due to the need for a parallel current path with a switch and precharge resistor, leading to increased weight and cost, and there is a need for alternative solutions that eliminate this requirement.
A flyback converter is connected to a low-voltage network on the input side and a high-voltage network on the output side, using a series connection of a power switch and transformer winding, along with a diode and capacitors, to charge the intermediate circuit capacitor by controlling the power switch with specific duty cycles based on voltage and current measurements to achieve rapid charging without excessive currents or voltages.
The method enables rapid, efficient, and non-destructive charging of the intermediate circuit capacitor, limiting maximum currents and voltages, and avoiding overloads and overvoltages, thus reducing the need for additional components and improving operational efficiency.
Smart Images
Figure 1.1
Abstract
Description
[0001] Description
[0002] title
[0003] Method for operating a flyback converter for charging an intermediate circuit capacitor
[0004] The invention relates to a method for operating a flyback converter for charging an intermediate circuit capacitor, as well as to the flyback converter for charging the intermediate circuit capacitor. Furthermore, the invention relates to a drive train with a corresponding device and a vehicle with a drive train, as well as to a computer program and a machine-readable storage medium.
[0005] State of the art
[0006] Methods and devices for charging an intermediate circuit capacitor in a high-voltage network are known from the prior art. Preferably, vehicles with an electric drive train comprise an intermediate circuit capacitor in a high-voltage network, which is arranged between an energy source, preferably a direct current and / or high-voltage energy source, and the power switches of a pulse-controlled inverter. The intermediate circuit capacitor is preferably arranged within the pulse-controlled inverter on the direct current side. The energy source, preferably a traction battery, serves to supply an electrical machine with electrical energy. For this purpose, the electrical energy is converted by means of the pulse-controlled inverter. The direct current of the energy source is converted into an alternating current to supply a multi-phase electrical machine.When the vehicle is switched off or parked, the energy source is disconnected from the high-voltage network using at least one switch, and the high-voltage network is discharged to ensure that the high-voltage network does not pose a danger, even if conductive parts are touched. When the vehicle is restarted, the energy source must be conductively connected to the high-voltage network. Due to the voltage difference between the discharged high-voltage network and the energy source, switching it on using at least one switch would result in unacceptably high compensating currents. To reduce the voltage differences before switching on the energy source, the intermediate circuit capacitor in the high-voltage network is precharged or charged to a voltage that approximately corresponds to the voltage of the energy source.When the at least one switch is subsequently switched on or closed, there are no longer any significant voltage differences, meaning that no relevant compensating currents occur. The charging, recharging or pre-charging of the intermediate circuit capacitor is usually carried out by means of an additional circuit that provides a parallel current path to the at least one switch and also comprises a switch and a resistor, pre-charging resistor or series resistor. For charging, the switch of the parallel current path is closed. The resulting compensating current is limited to permissible values by the resistor. Once the charging process is complete, the at least one switch between the high-voltage network and the energy source is closed. The drive train is then ready for use again. The parallel current path with switch and pre-charging resistor requires additional components and space and leads to additional weight and costs.Therefore, there is a need for alternative solutions that enable charging of the DC link capacitor without the parallel current path. DE 10 2020 206987 A1 discloses a method for charging a DC link capacitor in a high-voltage network.
[0007] Disclosure of the invention
[0008] A method is provided for operating a flyback converter for charging an intermediate circuit capacitor. The flyback converter can be connected to a low-voltage network on the input side and to a high-voltage network on the output side. The high-voltage network comprises the intermediate circuit capacitor to be charged. During operation, the flyback converter is preferably connected to the low-voltage network on the input side and to the high-voltage network on the output side. In this context, "connected" or "connectable" preferably means electrically conductively or galvanically connected, coupled, or connected together. The flyback converter comprises, on the input side, between the input terminals, a series circuit comprising a first power switch and a primary winding of a transformer, and, on the output side, between the output terminals, a series circuit comprising a secondary winding of the transformer and a first diode connected in the reverse direction.On the output side, a first capacitor is connected between the output terminals. On the input side, a second capacitor is preferably connected between the input terminals. The method comprises the steps of: determining a voltage that characterizes the voltage at the intermediate circuit capacitor. Specifying a target charging current value for charging the intermediate circuit capacitor and specifying a corresponding duty cycle for controlling the first power switch as long as the determined voltage falls below a first voltage value. Specifying a target charging voltage value for charging the intermediate circuit capacitor and specifying a corresponding duty cycle for controlling the first power switch as long as the determined voltage corresponds to the first voltage value or the determined voltage exceeds the first voltage value and the determined voltage falls below a second voltage value.The first voltage value is smaller than the second voltage value.
[0009] The voltage is determined using 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 such a way that the voltage across the intermediate circuit capacitor is determined, estimated, or calculated. A target charging current value is preferably determined and specified using a control device, so that a duty cycle for controlling the first power switch is determined and specified, preferably by means of the control device, preferably by means of a current regulator, preferably as a function of the determined voltage and the specified target charging current value, so that a charging current that is close to or corresponding to the target charging current value is established on the output side of the flyback converter to the intermediate circuit capacitor.Preferably, a target charging voltage value is determined and specified by means of the control device, so that a duty cycle for controlling the first power switch is determined and specified preferably by means of the control device, preferably by means of a voltage regulator, preferably as a function of the determined voltage and the specified target charging voltage value, so that a charging voltage that is close to or corresponding to the target charging voltage value is established on the output side at the intermediate circuit capacitor to be charged. To charge the intermediate circuit capacitor, the flyback converter is operated as a boost converter. A lower input voltage is preferred, for example an on-board network voltage of a vehicle of approx.12 - 14 volts or 48 volts are continuously increased and the intermediate circuit capacitor is charged until an output voltage corresponding to an energy source connected to the high-voltage network, for example 200, 400, 800 or even 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 pre-charging or charging of the intermediate circuit capacitor to the desired high-voltage voltage should take place as quickly as possible. To avoid excessive currents and / or voltages at the first power switch, the operation of the flyback converter for charging the intermediate circuit capacitor is preferably divided into at least two phases. First, a current-controlled phase is carried out, followed by a voltage-controlled phase.Preferably, in the current-controlled phase, the duty cycle for controlling the first power switch is determined as a function of the determined voltage according to a predetermined maximum target charging current value, and the first power switch is controlled accordingly. If the determined voltage rises above a predeterminable first voltage value, for example, 90% of the achievable high-voltage voltage, due to the operation of the flyback converter for charging the intermediate circuit capacitor, the current-controlled operation is terminated and the voltage-controlled phase is carried out. Preferably, in the voltage-controlled phase, the duty cycle for controlling the first power switch is determined as a function of the determined voltage according to the predetermined target charging voltage value, and the first power switch is controlled accordingly.If the detected voltage rises above a predeterminable second voltage value due to the operation of the flyback converter for charging the intermediate circuit capacitor, which, for example, corresponds to the high-voltage to be achieved, the voltage-controlled operation is terminated. This advantageously enables rapid charging of an intermediate circuit capacitor while limiting the maximum currents and voltages at the first power switch. In another embodiment, the duty cycle is greater than a first predefined duty cycle while specifying the target charging current value.
[0010] When specifying the target charging current value, the duty cycle is always set higher than the first specified duty cycle or a minimum duty cycle. This reliably prevents a reversal of the energy flow. A reversal of the energy flow could cause overcurrents and overvoltages at the first circuit breaker. Increasing the duty cycle advantageously prevents potential overloads of the circuit breaker.
[0011] In another embodiment, the target charging current is reduced before the transition from specifying the target charging current value to specifying the target charging voltage value.
[0012] Before the transition from the current-controlled phase to the voltage-controlled phase, for example, when 90% of the first voltage value is reached, the target charging current is reduced, for example, to 10% of the specified target charging current. This measure advantageously prevents large voltage overshoots when the first voltage value is reached by means of current control.
[0013] In another embodiment, the duty cycle is continuously adopted during the transition from specifying the target charging current value to specifying the target charging voltage value.
[0014] During the transition from the current-controlled phase to the voltage-controlled phase, the duty cycle last present in the current-controlled phase is adopted and used at the beginning of the voltage-controlled phase, in particular by initializing the voltage regulator. Initializing the voltage regulator results in a smooth transition, thereby preferably avoiding an abrupt change in the steady-state operating state. This advantageously prevents overcurrents and overvoltages at the first circuit breaker. In another embodiment, the charging current decreases continuously while the target charging voltage value is being specified.
[0015] Advantageously, large voltage overshoots when the second voltage value is reached are thus avoided by means of voltage regulation.
[0016] In another embodiment, a current which characterizes the current through the first diode is determined and the method for operating the flyback converter is terminated after the determined current falls below a first predeterminable current value, preferably during the step of providing the target charging voltage.
[0017] If the current, preferably the output current through the current transformer, falls below a first preset current value, preferably during the voltage-controlled phase, the method for operating the flyback converter is terminated. Advantageously, a termination criterion is provided that reliably prevents overcurrents and overvoltages at the first circuit breaker despite a possible reversal of the power flow.
[0018] In another embodiment, the method comprises a further step before specifying the target charging current value: controlling the first power switch by means of a predefinable first duty cycle for a predefinable period of time.
[0019] To ensure non-destructive operation of the flyback converter, an active clamping circuit is preferably provided on the output side of the flyback converter. The active clamping circuit preferably comprises a series connection of a third capacitor and a second power switch, wherein the series connection is connected in parallel to the secondary winding of the transformer. By controlling the first power switch using a predeterminable duty cycle for a predeterminable period of time, the capacitance of the active clamping circuit is specifically charged. The duty cycle is specified such that excessive currents and voltages never occur at the first power switch. Accordingly, the duty cycle is selected to be very small, for example 1 to 10% of the maximum duty cycle selected when providing the target charging current. The period of time is selected such that the third capacitor is charged reliably.After the third capacitor has been charged, the intermediate circuit capacitor can be charged quickly without excessive currents and voltages occurring at the first circuit breaker. Advantageously, a method is provided that enables rapid, non-destructive charging of the intermediate circuit capacitor despite the active clamping circuit.
[0020] In one embodiment, the method comprises a further step before providing the target charging current: initializing a current regulator of the flyback converter.
[0021] Preferably, initializing the current controller includes determining the specified duty cycle for controlling the first power switch as a function of the determined voltage and the set target charging current for charging the intermediate circuit capacitor. Advantageously, a method is provided that enables rapid, non-destructive charging of the intermediate circuit capacitor.
[0022] Furthermore, the invention relates to a computer program comprising instructions which, when the program is executed by a control device, cause the control device to carry out the steps of the described method.
[0023] Furthermore, the invention relates to a computer-readable storage medium comprising instructions which, when executed by a control device, cause the control device to carry out the steps of the described method.
[0024] The invention further relates to a flyback converter for charging an intermediate circuit capacitor in a high-voltage network. The flyback converter can be coupled to a low-voltage network on the input side and to a high-voltage network on the output side. The flyback converter comprises a control device configured to carry out a method as described above. A flyback converter for charging an intermediate circuit capacitor in a high-voltage network is provided. The flyback converter comprises a control device configured to carry out the described method. For this purpose, the control device preferably comprises configured inputs and outputs that enable the voltage and / or current to be determined, preferably by means of voltage and / or current measuring devices, and preferably a p-controller for current regulation and voltage regulation and for controlling the first power switch.
[0025] Advantageously, a flyback converter is provided for rapid charging of an intermediate circuit capacitor.
[0026] The invention further relates to a drive train with a described flyback converter and preferably with power electronics and / or an electric drive. Such a drive train serves, for example, to drive an electric vehicle. The method and the flyback converter enable efficient operation of the drive train.
[0027] The invention further relates to a vehicle with a described drive train. Advantageously, a vehicle is thus provided that includes a flyback converter that enables rapid charging of the intermediate circuit capacitor.
[0028] It is understood that the features, properties and advantages of the method according to the invention apply or are applicable accordingly to the device or the drive train and the vehicle and vice versa.
[0029] Further features and advantages of embodiments of the invention will become apparent from the following description with reference to the accompanying drawings.
[0030] Short description of the drawing
[0031] In the following, the invention will be explained in more detail with reference to some figures, in which: Figure 1 shows a first schematic representation of a flyback converter for charging an intermediate circuit capacitor,
[0032] Figure 2 shows a second schematic representation of a flyback converter for charging an intermediate circuit capacitor,
[0033] Figure 3 shows a schematic representation of a vehicle with a drive train,
[0034] Figure 4 is a schematic diagram with an example of the voltage and current curve during the process of charging an intermediate circuit capacitor.
[0035] Figure 5 is a schematic flow diagram for a method for charging an intermediate circuit capacitor.
[0036] Embodiments of the invention
[0037] Figure 1 shows a first schematic representation of a flyback converter 250 or a possible circuit topology for charging an intermediate circuit capacitor 210. The intermediate circuit capacitor 210 is preferably connected to the high-voltage network 205 via the output terminals 242_p, 242_n for charging. Figure 1 shows a flyback converter 250 configured to perform a method for operating the flyback converter 250 for charging the intermediate circuit capacitor 210 in a high-voltage network 205. The flyback converter 250, also called a flyback converter, can be connected, preferably connected, on the input side to a low-voltage network 295 and on the output side to a high-voltage network 205. The high-voltage network 205 includes the intermediate circuit capacitor 210. The flyback converter 250 includes, on the input side, a series circuit consisting of a first power switch 252 and a primary winding 254 of a transformer 260 between the input terminals 244_p, 244_n.On the output side, the flyback converter comprises a series connection of a secondary winding 256 of the transformer 260 and a reverse-biased first diode 258 between the output terminals 242_p, 242_n, with a first capacitor 262 connected between the output terminals 242_p, 242_n on the output side. Preferably, the primary winding 254 and the secondary winding 256 of the transformer 260 are wound in opposite directions to define the current direction on the primary and secondary sides. Preferably, a 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 during reverse operation of the flyback converter. Preferably, the first diode 258 is configured as a body diode of a third power switch 268. Preferably, the flyback converter can transfer electrical energy from the output side to the input side by controlling the third power switch 268.Preferably, the third power switch 268 converts the DC voltage of the high-voltage network 205 into an AC voltage so that the transformer 260 can transmit it. Preferably, to fully illustrate the circuit topology, the leakage inductance 265 of the transformer 260 is shown between the secondary winding 256 of the transformer 260 and the first diode 258. 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 to the secondary winding 256 of the transformer 260, preferably in parallel to the secondary winding 256 and the leakage inductance 265.The active clamping circuit uses the third capacitor 263 to reduce voltage peaks across the primary winding 254 of the transformer 260 caused by the stray inductance 265. A voltage drop occurs between the output terminals 242_p, 242_n, preferably the voltage Ux, which characterizes, for example, the voltage across a connected intermediate circuit capacitor 210. To charge the intermediate circuit capacitor 210, a current Ix flows through the flyback converter 250 and into the capacitor via output terminals 242_p, 242_n. Figure 2 shows a second schematic representation of a flyback converter 250 for charging an intermediate circuit capacitor 210. The high-voltage network 205 can be connected to an energy source 220 by means of at least one switch 230. The high-voltage network 205 is coupled to a low-voltage network 295 by means of the DC-DC converter or flyback converter 250.The flyback converter 250 comprises an inductive coupling or a transformer and thus galvanically separates the high-voltage network 205 from the low-voltage network 295. The low-voltage network 295 is preferably supplied with electrical energy from the high-voltage network 205. For this purpose, the flyback converter 250 is preferably controlled in reverse and in the buck converter operating mode. The low-voltage network 295 preferably comprises a low-voltage energy source 222, for example a battery or an accumulator, and other consumers (not shown), for example control units. A control device 255 is designed and / or configured to determine a voltage Ux, to specify a target charging current value l_L, to specify a corresponding duty cycle Tx, and to specify a target charging voltage value U_L.The control device 255 preferably comprises a current regulator 270 or a voltage regulator for determining the corresponding duty cycle as a function of 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 outside the flyback converter 250, although the control device 255 and the current regulator 270 can also be arranged within the flyback converter 250.
[0038] Figure 3 shows a schematically illustrated vehicle 400 with a drive train 300. The illustration shows, by way of example, a vehicle 400 which can be used equally on land, on water, and in the air. The drive train 300 comprises the flyback converter 250 and preferably power electronics, a pulse-controlled inverter 272. The drive train preferably further comprises an energy source 220, the intermediate circuit capacitor 210, an electric machine 280, and / or a low-voltage energy source 222. The pulse-controlled inverter 272 preferably serves to supply the electric machine 280 with electrical energy from the energy source 220. The pulse-controlled inverter 272 preferably comprises the intermediate circuit capacitor 210, so that during operation the flyback converter 250 is connected to the pulse-controlled inverter 272.Figure 4 shows a schematic diagram with an example of the voltage curve during operation of a flyback converter 250 for charging an intermediate circuit capacitor 210. The voltage U at the output terminals of the flyback converter 250 or at the intermediate circuit capacitor 210 and the current I through the flyback converter 250 or into the intermediate circuit capacitor 210 are plotted in the diagram on the y-axis versus time t on the x-axis. Before the start of the method, a low voltage Ux is present at the output terminals. The intermediate circuit capacitor is not yet charged. Also, no significant current Ix flows through the flyback converter 250 or the intermediate circuit capacitor 210. At time t1, the control device 255 receives the signal to charge the intermediate circuit capacitor 210.Preferably, the control device 255 controls the first power switch 252 by means of a predeterminable first duty cycle Tl for a predeterminable period of time Zx until time t2 to precharge the third capacitor 263 of the active clamping circuit. During this period, the charging current Ix and the voltage Ux rise slightly. Subsequently, the control device specifies a target charging current value l_L for charging the intermediate circuit capacitor 210 by specifying a duty cycle Tx to control the first power switch 252 as long as the determined voltage Ux falls below a first voltage value Ul. By means of the current regulation, the charging current Ix rapidly rises to the level of the predefined target charging current value l_L and the voltage Ux at the output terminals rises accordingly.At time t3, the control device specifies a target charging voltage value U_L as long as the determined voltage Ux corresponds to the first voltage value Ul or the determined voltage Ux exceeds the first voltage value Ul and the determined voltage Ux falls below a second voltage value U2. When the determined voltage Ux reaches the second voltage value U2, the intermediate circuit capacitor is charged, and time t4 occurs. By means of voltage regulation, the voltage Ux continues to rise in the period between t3 and t4 until it reaches the level of the specified target charging voltage. Accordingly, the charging current Ix drops again. Preferably, the method 100 for operating the flyback converter 250 is terminated after the determined current Ix falls below a first predefinable current value II. Figure 5 shows a schematically illustrated flow diagram for a method 100 for operating a flyback converter 250 for charging an intermediate circuit capacitor 210.The method begins with step 105. In step 120, a voltage Ux is determined which characterizes the voltage at the intermediate circuit capacitor 210. Preferably, in step 130, the first power switch 252 is controlled by means of a predefinable first duty cycle TI for a predefinable period of time Zx. Preferably, in step 132, a current regulator 270 of the flyback converter 250 is initialized. In step 140, a target charging current value l_L for charging the intermediate circuit capacitor 210 is specified, and a duty cycle Tx for controlling the first power switch 252 as long as the determined voltage Ux falls below a first voltage value Ul. In step 150, a target charging voltage value U_L is specified as long as the determined voltage Ux corresponds to the first voltage value Ul or the determined voltage Ux exceeds the first voltage value Ul and the determined voltage Ux falls below a second voltage value U2.Preferably, in step 160, a current Ix is determined that characterizes the current through the first diode 258, wherein the method 100 for operating a flyback converter 250 is terminated after the determined current Ix falls below a first predeterminable current value II. The method ends with step 175.
Claims
Claims 1. Method (100) for operating a flyback converter (250) for charging an intermediate circuit capacitor (210), wherein the flyback converter (250) is connectable on the input side to a low-voltage network (295) and on the output side to a high-voltage network (205), wherein the high-voltage network (205) comprises the intermediate circuit capacitor (210), wherein the flyback converter (250) comprises on the input side between the input terminals (244_p, 244_n) a series circuit of a first power switch (252) and a primary winding (254) of a transformer (260) and on the output side between the output terminals (242_p, 242_n) a series circuit of a secondary winding (256) of the 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 terminals (242_p, 242_n ) are connected, with the steps: Determining (120) a voltage (Ux) which characterizes the voltage at the intermediate circuit capacitor (210), Presetting (140) a target charging current value (l_L) for charging the intermediate circuit capacitor (210) and presetting a corresponding duty cycle (Tx) for controlling the first power switch (252) as long as the determined voltage (Ux) falls below a first voltage value (Ul), presetting (150) a target charging voltage value (U_L) for charging the intermediate circuit capacitor (210) and presetting a corresponding duty cycle (Tx) for controlling the first power switch (252) as long as the determined voltage (Ux) corresponds to the first voltage value (Ul) or the determined voltage (Ux) exceeds the first voltage value (Ul) and the determined voltage (Ux) falls below a second voltage value (U2), wherein the first voltage value (Ul) is less than the second voltage value (U2). Method according to claim 1, wherein during the specification (140) of the target charging current value (I_L), the duty cycle (Tx) is greater than a predetermined first duty cycle (TI). Method according to one of the preceding claims, wherein before the transition from the specification (140) of the target charging current value (I_L) to the specification of the target charging voltage (U_L), the target charging current (I_L) is reduced. Method according to one of the preceding claims, wherein during the transition from the specification (140) of the target charging current value (I_L) to the specification of the target charging voltage value (U_L), the duty cycle (Tx) is continuously adopted. Method according to one of the preceding claims, wherein during the specification (150) of the target charging voltage value (U_L), the charging current (Ix) continuously decreases. Method according to one of the preceding claims, comprising the step: Determining (160) a current (Ix) that characterizes the current through the first diode (258), wherein the method (100) for operating a flyback converter (250) is terminated after the determined current (Ix) falls below a first predeterminable current value (II). Method according to one of the preceding claims, comprising the step of, before specifying (140) the target charging current value (I_L): controlling (130) the first power switch (252) by means of a predeterminable first duty cycle (TI) for a predeterminable period of time (Zx). Method according to one of the preceding claims, comprising the step of, before specifying (140) the target charging current value (I_L): initializing (132) a current regulator (270) of the flyback converter (250). Computer program, comprising instructions which, when the program is executed by a control device (255), cause the control device (255) to carry out the method / the steps of the method (100) according to claims 1 to 8. Computer-readable storage medium, comprising instructions which, when executed by a control device (255), cause the control device (255) to carry out the method / the steps of the method (100) according to claims 1 to 8. Flyback converter (250) for charging an intermediate circuit capacitor (210) in a high-voltage network (205), wherein the flyback converter (250) is coupled on the input side to a low-voltage network (295) and on the output side to a high-voltage network (205), wherein the flyback converter (250) comprises a control device (255) and is configured to carry out a method according to one of claims 1 to 8. Drivetrain (300) with a flyback converter (250) according to claim 11. Vehicle (400) with a drivetrain (300) according to claim 12.